Highly Conductive Nanostructured C-TiO2 Electrodes with Enhanced

Jun 14, 2012 - As shown in Figure 5, we can fit the experimental data at low scan rates using an equivalent circuit model of a semi-infinite one-dimen...
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Highly Conductive Nanostructured C-TiO2 Electrodes with Enhanced Electrochemical Stability and Double Layer Charge Storage Capacitance Fraser Mole,† Jue Wang,† Daniel A. Clayton,† Cailing Xu,†,‡ and Shanlin Pan*,† †

Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336, United States Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu, China 730000

‡

S Supporting Information *

ABSTRACT: The present work reports the structural and electrochemical properties of carbon-modified nanostructured TiO2 electrodes (C-TiO2) prepared by anodizing titanium in a fluoride-based electrolyte followed by thermal annealing in an atmosphere of methane and hydrogen in the presence of Fe precursors. The C-TiO2 nanostructured electrodes are highly conductive and contain more than 1 × 1010 /cm2 of nanowires or nanotubes to enhance their double layer charge capacitance and electrochemical stability. Electrogenerated chemiluminescence (ECL) study shows that a C-TiO2 electrode can replace noble metal electrodes for ultrasensitive ECL detection. Dynamic potential control experiments of redox reactions show that the C-TiO2 electrode has a broad potential window for a redox reaction. Double layer charging capacitance of the C-TiO2 electrode is found to be 3 orders of magnitude higher than an ideal planar electrode because of its high surface area and efficient charge collection capability from the nanowire structured surface. The effect of anodization voltage, surface treatment with Fe precursors for carbon modification, the barrier layer between the Ti substrate, and anodized layer on the double layer charging capacitance is studied. Ferrocene carboxylic acid binds covalently to the anodized Ti surface forming a self-assembled monolayer, serving as an ideal precursor layer to yield C-TiO2 electrodes with better double layer charging performance than the other precursors.

1. INTRODUCTION We report the preparation of highly conductive carbon modified TiO2 electrodes with nanostructured surfaces, their electrochemical stability for redox reactions and electrogenerated chemiluminescence (ECL), and double layer charge storage performance. Nanostructured electrodes containing nanowires, nanoparticles, and other features in the nanometer domain are of great interest for the fundamental understanding of structural dependence of redox reaction activities at small sized electrodes and for many applications.1−20 First, mass transfer dynamics of redox species at the nanometer sized electrode surface differ greatly from that of a bulk planar electrode. This is because the redox diffusion layer thickness at a nanometer electrode is comparable to the dimensions of a nanostructured electrode. This thin diffusion layer produces a concentration profile of redox species independent of the scan rate of the electrode potential if diffusion layers of each individual nanoelectrode domain do not overlap with each other. Meanwhile, unstable intermediates produced at a working electrode can be electrochemically detected at fast potential scan rates. Contribution from double layer charging to the overall collected current is small due to the fast mass transfer of redox reaction and small surface area of a single nanoelectrode. When diffusion layers of individual nano© 2012 American Chemical Society

electrodes overlap with each other due to their close proximity, the collective response of the redox concentration profile will depend on the real surface area of the nanostructured electrode and other parameters such as size distribution and relative distance of the nanosized domains on the nanostructured electrode surface. Second, electrodes at the nanometer scale may serve as nanosized antennas to facilitate redox charge transfer when the electrode size is comparable to the size of a redox center.1 Because of these interesting electrochemical properties of nanostructured electrodes, they can be utilized in many applications. For example, nanoelectrodes are used as electrode materials for lithium batteries,2 double layer charge storage capacitors,3,4 elements for signal transduction of a sensor for detecting specific molecular recognition electrochemically,5,6 photoactive materials of photovoltaic devices,8−10 and electrode materials to enhance ECL.11−13 In the charge storage area, increased real surface area versus geometric area is an important factor that determines the double layer capacitance of a nanostructured electrode. In addition, reliable electrical contact to each individual nanostructure on the charge Received: February 29, 2012 Revised: June 9, 2012 Published: June 14, 2012 10610

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Labview software, which allowed us to incrementally decrease the voltage from the initial anodization voltage. This barrier removal step used the same cell setup as in the previous anodization step. The program was set to decrease the voltage by 5% of its magnitude every 25 s starting from the anodization voltage to 0.1 V. Upon completion of the voltage step process, the TiO2 substrates were removed and washed with distilled water and acetone before being dried with compressed air to remove all traces of the electrolyte solution. After anodization and barrier removal, a small amount of high purity silver paste was applied to one side of the exposed Ti in order to maintain good electrical connection postannealing. The substrates were then annealed for an hour in a muffle furnace (Thermo Scientific) at 450 °C an hour. 2.3. Loading of Fe Precursor to Anodized TiO2 Template for Carbon Modification. To study the effects of carbon modification on electrochemical performance of the above anodized TiO2 electrodes, various Fe loading techniques were studied in an attempt to enhance the Fe coverage of the TiO2 substrate and improve carbon modification efficiency as Fe has been proven to be able to break down methane molecules into hydrogen and carbon. After annealing at 450 °C for 1 h, the substrates were subjected to various different Fe solutions: (1) 1 M Fe(NO3)3 for 20 min before washing with DI water, acetone and air drying as per original experiment; (2) 10 mM ferrocenecarboxylic acid solution in ethanol for 4 h before washing with DI water, acetone and air drying; or (3) submersed in 1 M Fe(NO3)3 under UV light for 30 min on each side before washing with DI water, acetone and air drying. The mass of the dried substrates was recorded prior to the next carbon modification step. 2.4. Carbon Modification of Anodized TiO2 Template To Form C-TiO2 Nanostructured Electrode. To transform the anodized TiO2 template into a useful electrode material, all Fe precursor treated TiO2 substrates were loaded into a tube furnace (X1100 MTIXTL) and thermally annealed in the presence of a gas mixture of 16% CH4, 20.51% H2, and balance N2. In order to load the substrates into the tube furnace for carbon modification, the substrates were placed into a quartz boat horizontally so that both the top and bottom of the substrates were exposed to air before placing a quartz plate above. N2 gas was passed through the furnace for around 20 min to purge any O2 within the chamber before several vacuum/N2 purge cycles were carried out. The furnace was set to heat up to a temperature of 1000 °C with a ramp rate of 50 °C/min and a dwell time of 1 h. At approximately 700−750 °C the N2 gas was turned off and the CH4/H2/N2 gas mixture was turned on at a flow rate of around 60 sccm. After the heating cycle had concluded the furnace was left to cool to room temperature. At approximately 700−750 °C the CH4/H2/N2 gas flow was turned off, and the nitrogen flow was started and remained on until the substrates had reached room temperature. 2.5. Electrochemical and Structural Characterizations. The electrochemical properties of C-TiO2 were first characterized in 1.0 M Na2SO4 containing 5.0 mM K3Ru(NH3)6 or 5.0 mM K3Fe(CN)6. The Ti substrates utilized in the above procedure for the fabrication of CTiO2 electrodes were studied by cyclic voltammetry (CV) at each stage of the fabrication process in 0.1 M NaOH using a potentiostat (CHI1207a, CHI Instruments). An electrochemical cell was set up with the C-TiO2 electrode as the working electrode, a platinum wire counter electrode, and Ag/AgCl reference electrode with 3.5 M KCl. Electrogenerated chemiluminescence (ECL) was obtained using a home-built ECL setup. Detailed description of this setup can be found in our recent publications.24,25 Microscopic morphology of obtained nanoelectrodes was confirmed using SEM (JEOL 7000 FE SEM). Transmission electron microscopy (TEM) samples were prepared by removing the C-TiO2 nanostructures from the Ti substrate by scraping with a razor blade and suspending them in DI water prior to being transferred onto a 200 mesh copper grid (Electron Microscopy Sciences, Hatfield, PA). The samples were then imaged using a FEI Tecnai F-20 TEM (FEI, Hillsboro, OR). XRD was taken using a Brukker D8 XRD (cobalt X-ray tube, Kα1, 1.78896 Å, 40 kV, and 35 mA) at room temperature in air.

collecting substrate is also critical to address the charge storage and collection effectively. Nanostructured electrodes such as nanotubes and nanowires have been used to enhance double layer charge storage performance.14−17 The function of the nanostructured electrode with nanowire structures is twofold: first, they can enhance the surface area of the charge storage electrode, and, second, they serve as reliable electrical contacts to the charge collector and as a scaffold for the attachment of redox active species (e.g., metal oxides) in the application of electrochemical energy storage systems. Such modification with nanostructured electrodes has shown remarkable contribution to charge storage properties because of the extremely high surface-to-volume ratios and the short ion diffusion path length. For example, Li and co-workers recently demonstrated the coating of carbon microfibers with ultrathin films of MnO2 and Zn2SnO4 for use as high performance supercapacitor electrodes.18 Dong and co-workers19 demonstrated that MnO2 coated titanium nitrite nanotube arrays displayed high performance charge storage. Similar structures were also reported recently by Leed and co-workers.20 More recently, functionalized nanostructured TiO2 electrodes have been used for electrochemical applications due to their high chemical stability, excellent functionality, nontoxicity, and relatively low price. Hu and coworkers21 used a carbon-doped TiO2 porous template to achieve excellent electrochemical catalytic performance of such electrodes for biomolecular sensing. Such carbon doping was obtained using a self-doping method that simply anneals asanodized TiO2 film in argon without using other carbon precursors. Other carbon doping methods such as annealing TiO2 under carbon monoxide22 have been used to obtain conductive and catalytic TiO2 electrodes for sensing. Schmuki and co-workers27 used acetylene as a carbon source to dope an anodized TiO2 template to obtain semimetallic TiO2 nanotubes for electrochemical catalytic reaction. In this report we present the structural and electrochemical characteristics of carbon modified anodized TiO2 electrodes prepared by thermal annealing an anodized TiO2 template in an atmosphere of methane and hydrogen in the presence of Fe precursor. Highly conductive nanostructured electrodes with high surface area are studied. Enhanced double layer charging performance and redox reaction at the new carbon modified electrode are addressed quantitatively using digital simulation. The structure and double layer capacitance of the carbon modified electrode are studied using various surface characterization tools and electrochemical methods (e.g., cyclic voltammetry and ECL) to learn the effect of carbon modification conditions on their double layer charge storage performance and redox reaction stability.

2. EXPERIMENTAL SECTION 2.1. Anodization of Ti. Ordered TiO2 nanotube templates were made by one-step anodic oxidation of 99% or 99.6% pure titanium substrates (Alfa Aesar). The electrochemical cell employed consisted of a double copper cathode on which the Ti substrates were attached by conductive tape. The Ti substrate was immersed in ethylene glycol containing 2% (w/w) H2O and 0.3% (w/w) NH4F. The solution was stirred constantly throughout the entire anodization process. The anodization voltage was supplied by a variable voltage dc source (Agilent Technologies N57550A) while the current density was recorded using a multimeter (Extech Instruments Multiview 110) connected in series. Typically, a 5 mm × 1 cm Ti plate was anodized at a voltage of 40 V and current of around 1 A for 1 h. 2.2. Removal of TiO2 Barrier Layer. Removal of the TiO2 barrier layer was achieved using a Keithley 2400 multimeter controlled using 10611

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Figure 1. (A) SEM image of a C-TiO2 electrode prepared by anodizing a 99.6% pure Ti plate electrode. (B) A zoom-in SEM image of carbon modified TiO2 nanostructured electrode. (C) SEM image of the cross section of a C-TiO2 electrode prepared from a 99% pure Ti starting material. (D) High-resolution TEM image of the nanostructured C-TiO2 top layer transferred onto a TEM grid.

3. RESULTS AND DISCUSSION 3.1. Anodization of Ti and Morphology Characterization. Anodization of a Ti substrate not only produces a nanoporous TiO2 structure containing nanochannels layered on top of the Ti substrate, which is important to maximize the surface area of the electrode, but also helps the formation of nanowire and nanotube features of C-TiO2 after the Fecatalyzed carbon modification reaction. The mechanism that leads to formation of nanochannels is believed to begin with water electrolysis at the Ti anode to produce a compact TiO2 layer.26 Soluble fluoride ions then start direct complexation reaction with Ti4+ at the oxide/electrolyte interface to chemically etch away part of the TiO2 surface.27 This chemical complexation reaction begins to compete with the anodic oxidation at the Ti−liquid interface under applied constant voltage to yield a thick oxide layer containing vertically aligned self-organized nanochannels.28,29 Pore size and length of the produced nanochannels of the TiO2 layer can be precisely controlled by the anodization conditions such as voltage, current density, and anodization time. Perfect uniformity of pore size and distribution was not achieved across the entire substrate using the one-step anodization. However, a highly ordered oxide template can be prepared by ultrasonically removing the first layer of TiO2 nanotubes in water and further repeating the anodization step under the same anodization condition as the first one as shown in recent work of Zhang and co-workers30 or using pattern-guided anodization.31 SEM

imaging of the porous TiO2 template after removing the barrier layer and annealing shows disordered pores can be observed because no special surface treatment is applied in our experiment (cf. Figure S1 of Supporting Information). The average pore size of the template anodized at 40 V is about 60 nm in diameter. This was believed to have occurred due to the impurities in the 99% pure Ti substrate which could lead to irregularities in the surface of the resulting substrate. 3.2. Carbon Modification of TiO2 Template and Structure Characterization. Microscopic structure changes and electrochemical performance of the anodized TiO 2 nanostructure with and without carbon modification are studied. As shown in Figure 1, the porous TiO2 morphology readily changes after anodization to bundles of nanowires of CTiO2 when a high-purity Ti substrate is anodized. The morphology change can be explained by the fact that the TiO2 template can melt at 1000 °C while the presence of hydrogen and C from CH4 can reduce the oxide to form doped TiO2, which presumably has higher melting temperature and hardness than anatase TiO2. The hardening process of carbondoped TiO2 layer has been observed previously by Schmuki and co-workers.27 The physical change of TiO2 at high temperature and the chemical reduction reaction work collectively to yield new nanostructured features that are different from that of anodized TiO2. Morphology of the CTiO2 electrode from pure Ti shows nanowire structures that form when the boundaries of nanochannels of the anodized TiO2 template collapse at high temperature. Element analysis of 10612

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a selected area of the C-TiO2 nanowire coated substrate shows 2.92% (atomic %) C is present in the titanium sample while having a lower percentage of oxygen than pure TiO2. The formed morphology after carbon modification is also dependent on the purity of the starting Ti substrate. For instance, the morphology of carbon modified TiO2 substrate made from 99% Ti does not show an extended nanowire shape because of the impurity present in the starting material that may increase the hardness of an anodized TiO2 template to maintain its morphology during the carbon modification process. SEM imaging of TiO2 substrates formed from 99% Ti contains pores with poor periodicity and ordering; therefore, the morphology obtained after carbon modification is less ordered than the pure Ti substrate. A cross-section view of the C-modified TiO2 in Figure 1C shows that the carbon modified TiO2 template prepared by anodizing a 99.0% Ti substrate shows nanotube structures and elemental analysis shows a large fraction of O is replaced by C. To further examine the morphology of the CTiO2 nanostructured electrode at the nanometer scale, we transferred a small amount of such carbon doped nanowires onto a TEM grid for high-resolution TEM imaging (Figure 1D). TEM images show the tubular nanostructure of the CTiO2 oxide sample as well as small nanoparticles which might be small graphite nanoparticles and/or catalytic precursors used for methane decomposition. There are no carbon nanotubes formed on such Ti oxide substrates even in the presence of Fe catalyst as would be expected for many other substrates.32,33 As stated above, the interesting C-TiO2 nanostructures of nanowires or nanotubes have to do with the TiO2 morphology change at high temperature and its collective response to the chemical reduction reaction. To further show more mechanistic evidence on how the carbon modification process in the presence of methane and hydrogen helps transform the nanochannels of the TiO2 template to a nanostructured electrode with nanowires and nanotubes, a control experiment with a bare TiO2 template treated at the same temperature under a nitrogen environment in the absence of methane and hydrogen was carried out to compare with C-TiO2. As shown in Figure 2, nanochannels on the anodized TiO2 template are completely transformed to large crystalline domains of TiO2 when the nanoporous TiO2 template melts (cf. Figure 2B) in comparison to C-TiO2 (cf. Figure 2A). The thermally treated TiO2 template in the absence of methane and hydrogen shows extremely high resistivity. We conclude that the carbon modification in the presence of hydrogen and methane helps the TiO2 template morphology transformation to provide new nanosized features with high surface area. Surface coverage of the nanowires (for high purity Ti) and nanotubes (for low purity Ti) is ∼1 × 1010/cm2, which is close to that of the nanopore density before being transformed to a new nanostructured surface of C-TiO2. We then used Raman to reveal the structural information on the C-TiO2 nanostructure. Figure 2C shows the comparison of Raman features of anodized TiO2 after annealing at 450 °C, with that of C-TiO2. The Raman features of anodized TiO2 indicate the crystalline structure of the oxide is anatase. The features of Ti oxide after C modification is not quite clear in the Raman spectra due to the dramatic changes in its electronic structure and composition. The red spectra indicating the anodized TiO2 has Raman lines at around 150, 400, 500, and 650 cm−1 which we believe correspond to the Eg, B1g, A1g, or B1g and Eg modes of the anatase phase of TiO2, respectively.34 After carbon modification, we can see that the main Raman lines correspond

Figure 2. (A) SEM image of a C-TiO2 electrode made by anodizing a 99% pure Ti plate electrode. (B) Image of a bare TiO2 template made by anodizing a 99% pure Ti plate and annealing under nitrogen protection at 1000 °C. (C) Corresponding Raman spectra of C-TiO2 electrode and thermal annealed bare TiO2 anodic template.

to the D and G bands of carbon. This implies that there is still a trace amount of carbon deposition onto the C-TiO2 substrate during thermal annealing in the presence of methane and hydrogen. XRD of the C-TiO2 (Figure 3) and a calculated standard show that the carbon modified Ti oxide has a composition of TiC0.32O0.46. The actual composition stoichiometry varies from sample to sample depending on the condition of carbon modification, Fe precursor loading method, and anodization conditions of Ti substrates. To evaluate the conductivity, surface area change, and electrochemical stability of C-TiO2 nanoelectrodes, we first investigated their performance by using dynamic control of electrode potential in the presence of two common reversible redox species and compared this with the redox behavior of bare gold disk electrode. Figure 4A shows the cyclic voltammetry (CV) of Fe(CN)63− at a C-TiO2 electrode in comparison with that of a bare gold electrode at a potential scan rate of 100 mV/s. It clearly shows that pronounced reversible redox behavior of Fe(CN)63− at the C-TiO2 surface. No strong adsorption of Fe(CN)63− ions on the surface of the carbon modified electrode is observed due to the large offset of the cathodic and anodic peak potentials, Epa and Epc, respectively. ΔE (the difference between Epc and Epa) of the CV is about 65 mV for both C-TiO2 and bare gold disk electrodes. The peak splitting at gold is not 56−57 mV as we expect for one electron transfer reversible reaction under purely mass transfer condition at room temperature because the Ohmic drop between reference and working electrode was not corrected for both gold and carbon modified TiO2 electrode. The above experimental result indicates that the carbon modified TiO2 electrode is highly conductive and the redox reaction of Fe(CN)63− at its surface is highly reversible. Other 10613

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Figure 3. XRD spectrum of a C-TiO2 electrode made from 99% Ti in comparison to a bare TiO2 template annealed at 1000 °C in a nitrogen atmosphere.

greater double layer charging capacitance than bare gold electrode. It is also shown that the double layer charge storage capacity of C-TiO2 is not due to positively charged Ru(NH3)63+ and negatively charged Fe(CN)63− because there is no surface absorption of redox species onto the C-TiO2 nanoelectrode to enhance the overall current density. Therefore, the enhanced double layer charging capacitance at C-TiO2 is mainly from the response of Na+ and SO42− ions. To address the charge storage performance at the C-TiO2 quantitatively, we used digital simulation to fit the redox reaction behavior, and double layer charging performance at various scan rates. As shown in Figure 5, we can fit the experimental data at low scan rates using an equivalent circuit model of a semi-infinite one-dimensional planar electrode by including a double charging capacitor, redox reaction, and Ohmic drop correction. Double layer charging capacitance is found to be 4800 μC/cm2 as calculated from the geometric area of the C-TiO2 electrode. This charge density is about 48−480fold of that of an ideal fully charged planar surface and can be explained by the 48−480-fold increase in real surface area in comparison to the planar electrode. We also notice that the equivalent model does not work well at high scan rates as we oversimplified the system by considering the nanostructured electrode as a planar system. This is because thinner diffusion layers can be developed for the redox reaction at fast scan rates of electrode potential and detailed consideration of the real surface area and local geometries are needed to explain the discrepancy of calculated results and experimental data. 3.3. Effect of Anodization Voltage and Fe Precursor Loading on Double Layer Charging Capacitance of CTiO2 Electrode. Carbon growth on various substrates is widely known to be catalyzed by Fe under appropriate carbon modification conditions.35 However, there was no carbon nanotube present on the surface of the Fe precursor-treated TiO2 templates under our thermal annealing treatment conditions in the presence of methane and hydrogen. This has to do with the physical changes in the morphology of TiO2 and reduction of the oxide substrate by carbon and hydrogen. It should be noted that nanostructures, such as carbon nanotube

controls performed on a bare Ti electrode and nanostructured TiO2 on Ti substrate with the same electrode size as C-TiO2 show no redox reaction behavior observed due to their inert surface and slow charge transfer kinetics. A positively charged redox ion Ru(NH3)63+ is then used to probe the electrochemical activities of the C-TiO2 electrode. As shown in Figure 4B, highly reversible CV of Ru(NH3)63+ can be obtained at carbon modified TiO2 electrodes with ΔE around 65 mV, which is close to that of the bare gold disk electrode. The other two controls of bare Ti and anodized Ti coated with TiO2 show no redox reaction of Ru(NH3)63+, indicating that reaction of Ru(NH3)63+ at bare Ti and TiO2 electrodes is sluggish. The double layer charging current at the C-TiO2 is much greater than the gold electrode. This is due to the high surface area of the nanostructure electrode as shown in SEM imaging studies. To compare mass transfer behavior of redox species and double layer charging effect at bare gold electrode and C-TiO2, we show the scan rate dependence of the cathodic peak current in Figure 4C,D at the C-TiO2 electrode. C-modified TiO2 electrodes show their peak current linearly increase with the square root of the scan rate, indicating the reversible reaction characteristics of the redox reaction and linear mass transfer features of redox species at the C-TiO2 nanoelectrodes throughout the range of applied scan rates. It should be noted that the faradaic current density at the C-TiO2 electrode is found to be only slightly higher than that of the bare gold disk electrode. This can be explained by the fact that the geometric surface area plays a major role at slow scan rates that produce a thick redox diffusion layer of redox molecules so that the nanostructured surface has no contribution to the overall mass transfer process. Figure 4C,D also shows the dramatic increase in double layer charging current density at the C-TiO2 nanoelectrode in comparison to the bare gold disk electrode. This can be explained by fast ion diffusion and migration near the nanostructured electrode with much greater surface area than the planar gold electrode under applied potential in a strong electrolyte. The nanostructured electrode causes much thinner diffusion layers of ions than the gold planar electrode under condition of high ionic strength which can produce much 10614

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Figure 4. CVs of a bare Ti plate, anodized TiO2 plate, C-TiO2 in 1.0 M Na2SO4 containing (A) 5.0 mM K3Fe(CN)6, and (B) 5.0 mM K3Ru(NH3)6 in comparison to redox behavior at a gold disk electrode. Scan rate: 100 mV/s. (C) Scan rate dependence of CVs at a C-TiO2 in 1 M Na2SO4 containing 5.0 mM K3Fe(CN)6. (D) 1.0 M Na2SO4 containing 5.0 mM K3Ru(NH3)6. Insets of (C, D) are the cathodic peak current plotted against the square root of scan rate data and linear fitting results. Gold disk electrode diameter: 2.0 mm.

supported TiO2,36 TiO2−carbon nanotube nanocomposite,37 and self-standing carbon nanotubes grown on top of anodized TiO2 template,38 have been reported in literature for photocatalytic applications because of the attractive photoelectrochemical activity of TiO2 upon UV light absorption.39 Carbondoped TiO2 has also been demonstrated in literature as we discussed earlier.21−23 However, the conductivity of TiO2 was not improved through these modifications, and few experiments have been carried out to demonstrate the charge storage behavior of such doped electrodes. Our results show substantial improvement of the conductivity of the TiO2 film after transforming its morphology by incorporating carbon into the oxide nanoelectrode to provide a high surface area. To study the effect of sample preparation procedures on double layer capacitance and electrode conductivity, CV was used to measure the charging/discharging characteristics of the substrate at each stage of the fabrication process (cf. Figure S2 in Supporting Information). The specific capacitance of the Cdoped TiO2 electrodes before and after carbon modification was 0.29 and 11.91 F/g, respectively. The pronounced increase

Figure 5. Simulated CV of C-TiO2 nanotube electrode with geometric surface area of 0.25 mm2 in comparison to experimental CVs collected in 5.0 mM K3Ru(NH3)6 at various scan rates from 10 mV/s to 2 V/s. Simulation parameters: DO = DR = 8.0 × 10−6 cm2/s; K0 = 100 cm/s; A = 0.70 cm2; Ru (Ohmic drop) = 10 ohms; Cdl = 0.0012 F.

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fully understood why the mass of carbon loaded is so small; it may simply be due to the difference in the retention time for gas molecules caused by the substrate positioning within the crucible. Our results suggest that the optimal anodization voltage of 40−60 V is the best for maximizing the double layer charging capacitance. Selection of the Fe precursor is one of the factors that should be controlled to optimize the double layer charging capacitance and redox reaction activities. Submersion of the anodized TiO2 substrate in 1 M Fe(NO3)3 solution would be satisfactory to allow physical adsorption of Fe ions onto the surface of the TiO2 template. Fe ions get reduced by hydrogen and converted to Fe nanoparticles to help break down CH4 to C and H2. Carbon modification in the CH4 and H2 mix gas was expected to be quite efficient because the mass transfer of gas molecules from the gas mixture was expected to be efficient in the porous TiO2 substrate. Carbon atoms and hydrogen are expected to be used to dope the anodized TiO2 template while excess C remains attached to the surface of TiO2. However, optimal growth still relies on control of the TiO2 pore size and template thickness. In addition, it was believed that the presence of eddies and turbulent flow on the bottom of the sample lead to a greater retention time of the gas mixture on the bottom compared to the top of the sample. This was resolved by arranging the substrates horizontally so that both sides of the substrate were exposed to the atmosphere before covering the quartz boat with two quartz plates. This forced turbulent flow by the incoming gas mixture within the quartz boat on both sides, leading to a greater retention time of gas within the boat and ensured ample time for carbon doping into both sides of the TiO2 substrate. We studied the double layer charge storage response of each of the substrates treated under various conditions for Fe catalyst loading. The largest current response and subsequently the largest specific capacitance per gram were achieved by the substrate immersed in 10 mM carboxyferrocene solution. The specific capacitance in these samples appreciably outperforms the other techniques of catalyst loading. The presence of the carboxylic acid group provides a way for the catalyst to covalently bond to the surface of the TiO2, which increases the chance of a uniform monolayer of Fe catalyst forming. The uniformity of the Fe layer improves the mass transfer during carbon modification and leads to more uniform carbon deposition on the surface of the substrate. This has the effect of having a larger double layer area and, subsequently, a larger specific capacitance. Irradiating the Fe(NO3)3-dipped electrodes has the effect of photoreducing the Fe2+ ions into solid Fe particles on the surface of the TiO2 template. This is a more reliable method of Fe deposition than submersion alone as it ensures that Fe particles are deposited uniformly over the surface of the substrate provided the UV lamp can fully cover the substrate. Table 1 summarizes the specific capacitance we measured for C-doped Ti oxide samples under various conditions, including the anodization voltage of the TiO2 template and the different methods used for Fe catalyst loading. We calculated the specific capacitance using both anodic current and cathodic current of their CVs and the mean specific capacitance was obtained. The best voltage for charge storage was found to be 40 V; that might provide best porosity and excellent electrode conductivity after carbon modification so that both charge storage capacity and mass transfer are optimal. Ferrocenecarboxylic acid is the best precursor for Fe catalyst loading because of the formation of a self-assembled monolayer of this ferrocene derivative on the

in current response of the C-TiO2 substrate indicates that carbon modification can dramatically improve the surface area and conductivity. The carbon modification yielded black-coated substrates which showed a vast improvement to their current response and specific capacitance when compared to the same substrates without being modified with carbon. The current response was measured using CV at incremental scan rates. The steady state current can be given by i = νCd where ν is the potential sweep rate in V/s.40 Under ideal conditions a symmetric graph above and below the zero current would be obtained, indicating perfect charging and discharging cycles. The current would increase and reach a steady state at which the double layer capacitance (Cd) could be calculated for a given scan rate. CVs obtained for the C-TiO2 substrates did not display this ideal behavior as they were asymmetric above and below zero current. The presence of the cathodic current peak shows that there was some influence from redox reactions, notably the oxidation of carbon. The cathodic and anodic current at the zero potential are plotted against the scan rate to give a linear graph with slope, Cd, for the charging and discharging cycles. The specific capacitance for each charging and discharging cycle was determined and the mean taken as an estimation of the specific capacitance. The mass of each C-TiO2 electrode was measured before and after carbon modification. This gave an approximation of the mass of active material which permitted calculation of the specific capacitance per gram of active material. This method, however, assumed that all the mass gained during carbon modification was from the doped carbon. Given that the actual surface of the electrode which is used for the electrochemical test is smaller than the anodized portion of the electrode, the mass of active material is in fact being overestimated, which in turn leads to an underestimation of the specific capacitance. Our measurement shows that the average specific capacitance of the carbon modified TiO2 electrode is as high as 11.91 F/g in 0.1 M NaOH. This is a large improvement over the 0.29 F/g calculated for the annealed TiO2 substrate before carbon modification. Equivalent charge storage performance and improved electrode stability can be obtained in organic solvent in comparison with aqueous electrolyte (cf. Figure S3 in Supporting Information). Further experiments were carried out in order to determine the effect of anodization voltage on the double layer charging capacitance of C-TiO2. C-TiO2 prepared from an anodized template at 60 V is found to have a far larger current response than the 20 or 40 V samples. However, a lower specific capacitance for the 60 V sample than the 40 V sample is obtained because a large mass of carbon is incorporated into the 60 V sample. The 20 V sample showed a poor current response and poor capacitance in comparison to 40 and 60 V because of a thin layer of TiO2 and small pore size after anodization of Ti, yielding low loading carbon and poor ion transport. The visual appearance of the substrate anodized at 20 V showed little coverage of carbon across the entire substrate. This could imply that the initial anodization voltage is too low for structured and uniform nanopores and would limit the carbon modification from enhancing the electrode conductivity and structural transformation. Carbon depositing out with these sections would not be particularly useful in charge storage applications which may explain the large mass difference and small specific capacitance per gram. The 40 V sample, while having a smaller current response than the 60 V sample, showed the greatest specific capacitance. This may be due to the small mass of active material deposited, however. It is not 10616

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starts to take place at 0.9 V vs Ag/AgCl reference electrode and peaks at 1.4 V, while no ECL observed at the anodized Ti plate due to the poor conductivity of the thick TiO2 layer. There is no ECL generation at the Ti planar electrode because of the sluggish kinetics of the coreactant TrPA. The observed ECL turn-on potential is close to that of a gold electrode and Pt electrode. The ECL intensity per geometric area at C-TiO2 is presumably higher than planar gold and Pt electrodes because of the efficient mass transfer of redox species and high real surface area of a nanostructured electrode. In comparison to CV, which has a large background current due to double layer charging, the ECL response curve at a C-TiO2 shows zero background as the collected ECL signal is only sensitive to the specific redox reaction of ECL active species (e.g., Ru(bpy)32+) at the nanostructured electrode without being obscured by double layer charging current. This experiment indicates that our C-TiO2 could be used to replace noble metal electrode materials as a new platform for ultrasensitive sensing based on ECL techniques. Pulsed ECL response at the C-TiO2 electrode shows stable light emitting features, as shown in Figure S4 of Supporting Information.

Table 1. Mass by Difference and the Resulting Capacitance per Gram of Substrates Anodized at Different Voltages and Fe Precursor Loading Methods anodization voltage (V)

Fe precursor

mass (mg)

+Ca (F/g)

−Cb (F/g)

mean C (F/g)

20 40 60 40 40 40

Fe(NO3)2 Fe(NO3)2 Fe(NO3)2 Fe(NO3)2 carboxyferrocene Fe(NO3)2+ UV

4.7 0.9 2.8 0.8 2.3 0.8

0.25 10.03 8.14 4.25 12.96 4.63

0.38 13.79 11.71 5.57 18.58 6.88

0.32 11.91 9.92 4.91 15.77 5.76

a

Calculated from cathodic current and devided by mass. bCalculated from anodic current and devided by mass.

metal oxide surface. Such attachment is important to stabilize the Fe during rinsing of the substrate and to decrease the aggregation of Fe clusters during carbon modification so that even surface coating/doping can be achieved. 3.4. Double Layer Charge Storage Performance of CTiO2 Electrodes with a Symmetric Geometry. To test how C-TiO2 would behavior in its double layer charging in a symmetric double layer configuration, two C-TiO2 electrodes were placed in 1 M NaOH solution in parallel so that the charge storage performance of the two electrodes could be tested. As shown in Figure 6, we scanned the voltage at a rate of 250 mV/s and the current remained stable between 0.1 and 1.2 V. Further increases in the applied potential are found to cause the oxidation of the electrodes and electrolysis of the electrolyte. Therefore, the C-TiO2 electrode has a maximum operating voltage of 1.2 V, which is better than that for a pure carbon electrode; this is due to the high electrochemical stability of these C doped Ti oxide electrodes. 3.5. Electrogenerated Chemiluminescence (ECL) at CTiO2 Nanostructured Electrode. To avoid large background current while having redox properties detected at high sensitivity using the highly conductive C-TiO2 nanostructured electrode, ECL of Ru(bpy)32+ was generated at the new electrode in the presence of a coreactant. As shown in our earlier publication, ECL of Ru(bpy)32+ can be generated at a working electrode by Ru(bpy)33+, generated through oxidation of Ru(bpy)32+, and Ru(bpy)31+ from reducing Ru(bpy)32+ in the presence of highly reducing species produced from the coreactant TrPA. Figure 7 shows CV and ECL response of Ru(bpy)32+ obtained at a C-TiO2 electrode. ECL of Ru(bpy)32+

4. CONCLUSIONS In summary, we presented the fabrication and electrochemical performance of a highly conductive nanostructured C-TiO2 electrode. Our study shows improvements to the redox reaction activity and double layer charge storage capacitance can be obtained by eliminating the TiO2 barrier layer between the Ti substrate and solution and modifying the oxide substrate with carbon. Optimal double layer capacitance can be obtained at anodization voltage of 40 V. The specific capacitance of C-TiO2 made with ferrocene carboxylic acid was found to have optimal conductivity and charge storage capacity. We believe that ferrocene carboxylic acid binds more strongly to the TiO2 substrate forming a more even Fe layer improving mass transfer during thermal reduction of TiO2 in the presence of hydrogen and carbon. A symmetric double layer charge storage device is formed by combing two C-TiO2 electrodes and the device has an operation voltage up to 1.2 V. ECL studies show that CTiO2 is an electrochemically stable electrode for ECL generation, and it can be used to replace noble metal electrodes without being obscured by double layer charging current for ultrasensitive ECL sensors.

Figure 6. (A) Schematic of a pair of C-TiO2 electrodes in a symmetric configuration for testing their double layer charging performance. (B) Double layer charging storage performance of a pair of C-TiO2 electrode parallel to each other with distance of 0.4 mm to each other in 1.0 M NaOH. 10617

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Figure 7. (A) CV and (B) electrogenerated chemiluminescence (ECL) at a nanostructured C-TiO2 electrode in phosphate buffer (pH = 7.0) containing 25.0 μM Ru(bpy)32+ and 0.1 M coreactant tripropylamine (TrPA), in comparison to that of a bare Ti plate electrode, and an anodized Ti template annealed at 450 °C in air. Scan rate: 100 mV/s.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of anodized Ti substrate, CV of a C-TiO2 substrate at each stage of its fabrication procedures; CV of CTiO2 in 0.1 M tetrabutylammonium hexafluorophosphate (TBAHFP) acetonitrile solution; pulse ECL response in PBS buffer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is supported by the Department of Energy under Award DE-SC0005392. F.M. acknowledges the support of the ACS International Research Experience for Undergraduates (IREU) program.

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