Electrochemical Impedance Spectroscopy of ... - ACS Publications

J. Phys. Chem. C 2010, 114, 9781–9790

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Electrochemical Impedance Spectroscopy of Porous TiO2 for Photocatalytic Applications Nir Baram and Yair Ein-Eli* Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel ReceiVed: December 10, 2009; ReVised Manuscript ReceiVed: March 9, 2010

High surface area immobilized TiO2 were grown via several electrochemical anodization methods for photocatalytic applications. Mesoporous TiO2 was grown in a molten salts electrolyte and in a sulfuric acid solution above the micro sparking potential. On the contrary, nanotubular TiO2 was grown in a sodium sulfate solution with the addition of fluoride ions, leading to the formation of fine elongated nanotubes with high surface area. The different types of photocatalysts were characterized by SEM and XRD in addition to electrochemical studies which include linear sweep voltammetry and open circuit potential relaxation. Electrochemical Impedance Spectroscopy (EIS) was used to study the impedance and capacitance of the TiO2 in the dark and under UV illumination together with Mott-Schottky analysis. The results of the EIS were correlated with the microstructural characterization and the photocurrents measurements along with photocatalytic degradation of Methyl Orange (MeO). The combined results led us to a better understanding of the electronic properties of n-type TiO2 and the effect of the growing method on its properties such as the surface area, crystal structure, charge carrier concentration, and charge transfer rate. The nanotubular structure possesses the highest surface area and higher charge carrier concentration, albeit the charge transfer rate is slower. Nevertheless, it is the most efficient photocatalyst toward degradation of MeO. The use of the described combined methods is a powerful tool toward predicting and understanding the ideal anode for photocatalytic process. 1. Introduction Nanostructured titanium dioxide has been attracting attention from both fundamental and applied perspectives, driven by its unique photoactivity in a broad range of applications, extending from photocatalysis of hazardous chemicals,1-4 photochromism,5 and self-cleaning surfaces6,7 to solar energy conversion,8 gas sensors,9,10 and hydrogen storage.11,12 Nevertheless, the low photoefficiency has still remained a technological challenge.13 Anodization of titanium is an attractive method due to a wide range of functional properties of the formed TiO2 layers, such as their high corrosion resistance14 and their ability to provide improved biocompatibility.15 Surface area is an important characteristic in these applications and it can be modified by the anodization method. The surface area of TiO2 may be increased by forming a mesoporous structure during the anodization.16 Further increase in the surface area can be achieved by anodization in fluoride-containing electrolytes to grow TiO2 with a nanotubular morphology.17-19 The TiO2 produced can be either amorphous or crystalline. An amorphous structure of the photocatalyst provides recombination centers, reducing the charge carrier concentration leading to a negligible photocatalytic effect, therefore it is extremely important for the TiO2 catalyst to be crystalline.20 Three main crystal structures of TiO2 can be identified: brookite and anatase are the metastable phases whereas rutile is the thermodynamically stable one; nevertheless the former phases are the most common. The difference in structure can exert influences on the properties and therefore on the photocatalytic behavior. Thus, the anatase form is considered more reactive for photocatalytic applications.21-24 * To whom correspondence should be addressed. E-mail: eineli@ tx.technion.ac.il.

The relevant properties of TiO2 for these applications (such as surface area and defect concentration) are directly related to its semiconducting behavior, which can be affected by the growing method. Usually the difference between different types of photocatalysts is attributed mistakably only to the difference in the surface area and does not take into account the difference in the semiconducting properties. Thus, the semiconducting properties of the mesoporous and nanotubular structures electrochemically produced need to be understood and correlated to their microstructure. The impedance behavior of these structures is a promising characterization method capable of providing the necessary insight.25,26 In our work, nanotubular and mesoporous TiO2 were grown via anodization of Ti via various methods. The anodization was performed in order to grow high aspect ratio, crystalline TiO2 catalyst on the Ti metallic surface. Single sine and Mott-Schottky experiments were conducted under different conditions in order to understand the semiconducting properties of the oxide layer. Photoelectrochemical properties of TiO2/Ti film electrodes were studied by anodic photocurrent response. Finally, the properties of the different oxides were compared and were correlated to their photocatalytic properties in regards to MeO photodegradation. 2. Experimental Section 2.1. Anodization of Ti. Anodization of Ti was carried out with titanium foils (99.2% purity, Alfa Aesar), 0.5 mm thick subsequent to mechanical polishing and chemical etching in a HF:HCl:HNO3 solution. Different anodization methods were used in order to grow a highly porous oxide layer. With the first method, nanotubular TiO2 (NT TiO2) was grown by anodization at a constant potential of 20 V, using a HewlettPackard 6035A system power supply. The anodization was performed for 2 h at room temperature in 1 M Na2SO4 + 0.5

10.1021/jp911687w  2010 American Chemical Society Published on Web 05/11/2010

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wt % NaF solution with a platinum counter electrode, as was previously described by Macak et al.27 Subsequent to the anodization process, the samples were thermally treated at 450 °C for 3 h in ambient air. With the second method, immobilized titania was grown by anodizing in 0.5 M sulfuric acid at a constant current density of 100 mA/cm2 until final potentials of 110 (S110 TiO2) and 150 V (S150 TiO2) were reached. Such formation was conducted subsequent to the cleaning procedure described above. From the last method, TiO2 was grown in a molten salts electrolyte as was described in ref 28. The electrolyte was composed of molten sodium nitrite and sodium nitrate (50:50 molar ratios) at a temperature of 280 °C. The TiO2 was grown with a constant current density of 50 mA/cm2 until a final potential of 80 V was reached (MS TiO2). 2.2. Electrode Characterization. The surface morphology of the formed TiO2, pore size, and distribution were characterized by using Scanning Electron Microscopy (SEM) on a LEO 982 Gemini microscope equipped with a field emission gun (FEG-SEM). X-ray diffraction measurements (XRD) were acquired in order to determine the TiO2 phase, using a conventional X-ray powder diffractometer (Philips X’Pert Diffractometer, Eindhoven, The Netherlands) with a Cu KR tube, operated at 40 mA and 40 kV. The surface area of the photocatalysts was evaluated with the dye adsorption method, as described earlier by Tirosh et al.29 The TiO2 was immersed in 10-5 M N3 dye (ruthenium dye) in an ethanol solution for 24 h. Subsequently, the TiO2 was dipped in a 1 M NaOH solution for 3 days. The NaOH solution was measured with a UV-vis spectrophotometer (Varian Cary 100 Scan) and the concentration of the dye was evaluated at a wavelength of 524 nm. The surface area was evaluated assuming surface occupation of 1.65 nm2 of each dye molecule and assuming monolayer coverage. These experiments were repeated five times for each sample. 2.3. Electrochemical Characterization. Potentiodynamic evaluation of the Ti/TiO2 electrodes (linear sweep voltammetry) in a 0.1 g/L NaCl solution was performed at a scan rate of 5 mVs-1, using a 263A (EG&G Princeton Applied Research) potentiostat/galvanostat. The potentials were measured relative to a standard calomel reference electrode (SCE). The experiments were carried out in the dark and under UV illumination (30 W UV lamp, λ ) 365 nm) with a light intensity of 3000 µW/cm2 mounted over the cell. Electrochemical Impedance Spectroscopy (EIS) was used to evaluate the properties of the different photocatalysts under AC polarization. EIS experiments were conducted with use of PARSTAT 2273 (Princeton Applied Research) power supply in a 1 M sodium sulfate (Na2SO4) solution and in a frequency range of 100 kHz to 100 mHz for an amplitude of 10 mV in DC potential of +0.2 VSCE after a 10 min delay. These studies were carried out in the dark and also under UV illumination (15 W UV lamp, λ ) 365 nm) with a light of 220 µW/cm2 intensity mounted over the cell. In the electrochemical cell, a platinum electrode was used as the counter electrode and a Calomel electrode as a reference. Bode plots were extracted from these experiments and the data were evaluated and fitted to the suggested equivalent circuit with use of the ZSimpWin 3.22d program (EChem Software). In addition, Mott-Schottky experiments were conducted to evaluate the capacitance behavior under DC potential polarization. The potential range was -0.8 to +1.5 VSCE with potential steps of 50 mV at a constant frequency of 100 Hz.

Baram and Ein-Eli Open Circuit Potential (OCP) Relaxation was conducted as well, using the same electrochemical cell as was used in the EIS experiments. OCP was measured for 30 min in the dark and then under UV illumination for 60 min. OCP relaxation was measured for an additional 2 h in the dark. 2.4. Photocatalytic Degradation of Methyl Orange (MeO). The photocatalytic process was carried out in a cell containing the TiO2 photocatalyst with an exposed surface area of 25 cm2 as a working electrode. Platinum and calomel electrodes were used as counter and reference electrodes, respectively. Degradation of Methyl Orange (MeO) was preformed with the same UV light that was described before but with a light intensity of 600 µW/cm2. A stirred 10-5 M MeO solution with addition of 0.2 g/L Na2SO4 was used as the solution (total volume of 200 mL). The photocatalytic experiments were conducted under anodic potential of +0.2 VSCE, using a PARSTAT 2273 (Princeton Applied Research) power supply. The degradation rate was monitored with a UV-vis spectrophotometer (Varian Cary 100 Scan) and the concentration of the MeO was evaluated at a wavelength of 460 nm. 3. Results 3.1. Electrode Characterization. Top view SEM micrographs of the various immobilized TiO2 are presented in Figure 1. Looking at the surface structures reveals that the most prominent difference between the samples is the surface morphology and surface area. Crystalline nanotubular TiO2 (termed as NT TiO2, Figure 1a) was grown at relatively low potential (below the micro sparking potential) and is composed of elongated and narrow nanotubes with a mean pore size of 75 nm. The micro sparking potential depends on the nature of the anodized metal, as well as on the composition and resistivity of the electrolyte and thus may possess a wide range of potential values. Other factors, such as the current density and surface topography, do not affect the micro sparking potential, noticeably.30 In this solution, the applied potential is below the micro sparking potential and the unique structure is formed due to the F- etching during anodization. Without the F- ions in this solution, a uniform and nonporous layer is formed. On the contrary, mesoporous oxide layers were grown via anodization at higher potentials (above the micro sparking potential) in two different solutions. The first mesoporous TiO2 was grown in a molten salts electrolyte containing sodium nitrite and sodium nitrate (termed as MS TiO2). The anodization process in molten salts results in the formation of a thick porous oxide layer with pore-size distributions of 50-200 nm, as can be seen in Figure 1b. With the second method, mesoporous TiO2 was grown in a 0.5 M H2SO4 solution at a constant current density of 100 mA/cm2 until final potentials of 110 and 150 V (we term these materials from now on S110 TiO2 and S150 TiO2, respectively) were reached. These two mesoporous TiO2 films are shown in the SEM micrographs presented in panels c and d of Figure 1c. The micrographs indicate that the surface is covered with pores with a wide diameter distribution of 50-500 nm. Increasing the potential from 110 to 150 V has led to increased pore density at the surface. The final potential, being reached during the anodization process in the molten salts electrolytes and in the sulfuric acid solutions, is above the micro sparking potential threshold, causing the formation of “wormlike” porous structure,31 as opposed to the nanotubular structure formation being formed due to a chemical etching by the fluoride ions. Since the formation of S150 material is longer, having higher final potential (150 V), the oxide layer has a denser pore structure than the S110 material, obtained at a lower potential of 110 V.

Electrochemical Impedance Spectroscopy of Porous TiO2

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Figure 1. Top view of SEM micrographs of anodized TiO2 layers: (a) NT TiO2, which was grown in 1 M Na2SO4 + 0.5 wt % NaF at a constant potential of 20 V for 2 h; (b) MS TiO2, which was grown in sodium nitrite and sodium nitrate (50:50 molar ratio) at a constant current density of 50 mA/cm2 until the final potential of 80 V; and (c, d) S110 TiO2 and S150 TiO2, which were grown in 0.5 M H2SO4 solution at a constant current density of 100 mA/cm2 until the final potentials of 110 and 150 V, respectively, were reached.

TABLE 1: Surface Area of the Different Photocatalystsa

It is clear that the main difference between the different photocatalysts is the surface morphology. By using different anodization conditions such as different solutions, potentials, and so on we can control the morphology of the oxide layer. While for the S150, S110, and MS TiO2 the structure is mesoporous due to the high potential being used during the anodization, the NT TiO2 has a nanotubular structure due to the addition of fluoride ions to the solution during the anodization. The different morphology affects the exposed surface area, which is a very important characteristic in photochemical cells and can directly affect the photocatalytic rate. The surface area of the NT TiO2 was experimentally evaluated with the use of the N3 ruthenium dye adsorption method, as was previously described.29,32,33 It was found that the nominal surface area of the NT TiO2 was increased by a factor of ∼25, in agreement with previously calculated data.34 Comparing the nanotubular structure to the mesoporous TiO2 reveals that the NT TiO2 has the largest surface area as was expected while S110 and MS TiO2 have the smallest exposed area (Table 1). In addition to the surface morphology, the anodization method can also influence the formation of a crystalline structure. Growth of the oxide layer below the micro sparking potential usually leads to an amorphous oxide structure formation. On the contrary, application of higher potentials leads to the formation of a crystalline structure; initially, anatase is being formed and then it is irreversible being transformed into the rutile phase (if the energy is sufficient).

photocatalyst

surface area [cm2]

NT S110 S150 MS

94.0 65.1 76.7 67.9

a The surface area was estimated by using adsorption of N3 ruthenium dye on a nominal surface area of 4 cm2 samples.

The formed NT structure is amorphous; however, following a thermal treatment for 3 h at 450 °C the microstructure of the formed NT TiO2 is crystalline anatase (XRD pattern Figure 2), which is the most reactive crystal structure of TiO2.22 Growing TiO2 at high potential resulted in the crystallographic structure without any additional thermal treatment. The XRD patterns, shown in Figure 2, reveal that the S110 TiO2 is anatase, while increasing the anodization potential to 150 V led to the formation of rutile in addition to the existing anatase. The MS TiO2 is in the form of rutile only due to the high potential and temperature being used during the anodization. 3.2. Electrochemical Characterization. The electrochemical behavior of the photocatalysts was studied under linear sweep voltammetry conditions in the dark and under UV illumination and is presented in Figure 3a. Polarization currents are being detected in the dark only above a potential of 2 VSCE, as a result of water electrolysis occurring at higher potential than the theoretical one (E0 ) 1.229V), due to the

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Figure 3. (a) Linear sweep voltammetry curves of all the photocatalysts in the dark and under illumination; (b) the dependence of the photocurrents on (E - Eonset)1/2 according to eq 4.

Figure 2. XRD patterns of the different photocatalysts: NT TiO2 (after thermal treatment); MS TiO2; S150 TiO2; and S110 TiO2.

high overpotential effect of the TiO2 semiconductor. These results demonstrate a typical characteristic of n-type semiconductors.35 Under UV illumination with energy larger than the band gap energy, electrons and holes are generated. Applying positive bias above the flat band potential leads to band bending. The electrons are transferred from the anode to the counter electrode via the external circuit while the holes are transferred to the surface of the TiO2 and react with the solution,36 therefore the current density is linearly increased until stabilizing at a potential of 2 VSCE. Further increase in the potential up to 4 VSCE is again due to water oxidation. Using the currents from experiments both with no illumination (dark) and under illumination enabled us to calculate the photocurrent values which were obtained by taking the difference between the total current under UV illumination and the dark current (eq 1).

iPh ) iTotal - iDark

(1)

Out of the four TiO2 electrodes, the NT TiO2 possesses the highest saturation current. This behavior is expected from

the measured oxide surface areas, compared with NT, S150, and S110 TiO2. On the contrary, unusual behavior is observed for MS TiO2 having a relatively lower photocurrent than expected, possibly due to factors (which will be discussed later in section 3.4) other than the surface area affecting the photocurrents. The detected photocurrent can be described by using eq 2:

Iph ) Γ0(1 - R)(1 - e-RW/1+RLP)

(2)

where Γ0 is the incident photon flux, R is the optical reflectivity of the solid, Lp is the hole diffusion length (i.e., the minority carrier diffusion length in n-type semiconductor), and R is the linear light absorption coefficient. W is the depth of the space charge region and can be described by eq 3:

W)

2εε0(E - Eonset) qND

(3)

where ε is the static dielectric constant (60), ε0 is the permittivity of free space (8.86 × 10-14 F · cm-1), E - Eonset is the potential drop in the space charge region (Eonset is the potential at which anodic current is first observed), ND is the concentration of donor impurities in a semiconductor (in the case of TiO2; ND ) ne), and q is the electron charge (1.6 × 10-19 C).


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Figure 4. EIS results for all the photocatalysts as was recorded in the frequency range of 100 kHz to 100 mHz under an applied DC potential of +0.2 VSCE and presented in the Bode form. The measured data are presented as symbols and the calculated data as continuous lines for each photocatalyst.

Figure 5. Suggested equivalent model for n-type TiO2. This model was fitted to the EIS results in Figures 4 and 6.

Therefore, the photocurrent dependence on the potential can be simplified as:

Iph ≈ A√E - Eonset

(4)

where A is a constant. The photocurrents dependence on (E Eonset)1/2 is plotted and presented in Figure 3b and unlike eq 4, the photocurrent does not increase linearly. Actually, the photocurrent curve can be divided into three different sections for all the photocatalysts studied. The first part is observed below a potential of 0.5 VSCE0.5. The second part has different potential ranges, depending on the photocatalyst (for NT TiO2 the second part is observed in the range of 0.5-1.5 VSCE0.5). The third and last part in the curve is observed at higher potentials (above 1.5 VSCE0.5 for NT TiO2) and can be correlated with the stabilization potential shown in Figure 3a. The nonlinear behavior of the photocurrent was observed previously by others as well37,38 and can be seen also in Mott-Schottky capacitance

measurements (presented in section 3.4). This nonlinear behavior can be explained by multiple donor states in the band gap which can promote an indirect tunneling of electrons through the oxide layer, resulting in slope changing. 3.3. EIS Experiments. 3.3.1. Dark Conditions. The behavior of the photocatalysts under AC response was investigated with Electrochemical Impedance Spectroscopy (EIS). This study was performed in order to evaluate the differences between the photocatalysts. The measured data of the TiO2 photocatalysts are presented in the Bode plot in Figure 4 (appear as symbols). The data was recorded in the frequency range of 100 kHz to 100 mHz under an applied DC potential of +0.2 VSCE. The Bode plot of the different photocatalysts demonstrates the IR drop in the oxide layer when the frequency is decreased. At these low frequencies the capacitive behavior is not ideal, with a sharp increase in the impedance, and the differences between the photocatalysts can be seen. The amorphous NT and MS TiO2 have the highest impedance while the crystalline NT TiO2 has the lowest one. Again, the correlation between the high surface area of the crystalline nanotubular structure and its electrochemical properties can be seen. The measured data were fitted to a suggested equivalent model, which is illustrated in Figure 5. This model is based on the model of Roberts and Crowell,39 and was used by others as well, applying it to n-type TiO2.34,40 This equivalent model is based on the space charge capacitance (C0) and the charge transfer resistance (R0). The values C1 and R1, C2 and R2,and C3 and R3 are the capacitances and resistances of discrete donor levels, corresponding to each parallel R-C pair. These trapping levels can emerge above the Fermi level in the semiconductor and contribute to the depletion layer charge, or on the other hand, may act as recombination centers for electrons and holes, depending on the frequency and the applied DC potential. Except for the electrical components of the oxide layer, the equivalent model also contains the solution resistance, Rs, the capacitance of the double layer, Cdl, and its resistance, respectively, that were added to the equivalent model. The modification to the model was performed in order to better represent the n-type semiconductor having an interface with an aqueous solution.41 This modification provides much better accuracy when fitting the model to the experimental results. The data calculated with use of the equivalent model also are presented in Figure 4, appearing as solid lines (connecting the experimental data, presented as symbols). The calculated model parameters are listed in Table 2 and are normalized by the nominal surface area and not by the actual exposed surface area. From Table 2 it can be seen that the solution resistance value is similar in all the experiments, as expected, and its value is very similar to the theoretical one. Comparing the total capacitance of the photocatalysts (composed from C0, C1, C2,

TABLE 2: Model Parameters of the Photocatalysts Based on the EIS Results in the Dark (Figure 4) photocatalyst 2

C0, µF/cm R0, KΩ · cm2 C1, µF/cm2 R1, KΩ · cm2 C2, µF/cm2 R2, KΩ · cm2 C3, µF/cm2 R3, KΩ · cm2 Ctotal, µF/cm2 Rs, KΩ · cm2 Cdl, µF/cm2 Rct, KΩ · cm2

amorphous NT

NT

S110

S150

MS

1.254 ( 0.055 47.13 ( 2.07 2.048 ( 0.090 10.96 ( 0.48 1.448 ( 0.064 0.416 ( 0.018 1.261 ( 0.055 0.017 ( 0.001 6.01 ( 0.14 0.0095 ( 0.0004 5.512 ( 0.242 448.9 ( 19.7

15.68 ( 0.59 1.137 ( 0.043 59.05 ( 2.22 0.028 ( 0.001 34.83 ( 1.31 0.007 ( 0.000 91.72 ( 3.45 0.640 ( 0.024 201.28 ( 4.34 0.0081 ( 0.0003 167.4 ( 6.3 88.5 ( 3.3

0.456 ( 0.037 6.36 ( 0.52 2.458 ( 0.201 0.260 ( 0.021 10.5 ( 0.9 1.082 ( 0.088 0.683 ( 0.059 0.040 ( 0.003 14.1 ( 0.9 0.0129 ( 0.0011 14.1 ( 1.2 485.7 ( 39.7

0.731 ( 0.048 0.834 ( 0.055 3.68 ( 0.24 0.177 ( 0.012 47.11 ( 3.12 0.348 ( 0.023 1.005 ( 0.066 0.033 ( 0.002 52.53 ( 3.12 0.0107 ( 0.0007 49.64 ( 3.27 182.4 ( 12.0

0.469 ( 0.049 775.1 ( 80.5 1.74 ( 0.18 112.9 ( 11.7 0.868 ( 0.090 0.071 ( 0.007 0.0 ( 0.0 894.7 ( 92.9 3.08 ( 0.21 0.0128 ( 0.0013 1.353 ( 0.141 1.462 ( 0.152

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Figure 6. EIS results for all the photocatalysts under UV light illumination with 220 µW/cm2 light intensity as was recorded in the frequency range of 100 kHz to 100 mHz under applied dc potential of +0.2 VSCE and presented in the Bode form. The measured data are presented as symbols and the calculated data as continuous lines for each photocatalyst.

and C3) yields the following order: NT > S150 > S110 > amorphous NT > MS. The crystalline NT TiO2 possesses the highest capacitance, which is ∼3.8 times higher than S150 material, ∼14 times higher than S110 material, ∼30 times higher than the amorphous NT TiO2, and ∼80 times higher than the MS TiO2. The double layer’s capacitance value (Cdl) correlates similarly to the total capacitance of the different oxides; higher capacitance of the photocatalyst leads to higher capacitance of the double layer. Increase in the oxide layer capacitance value may lead to higher charge density at the oxide-solution interface and therefore to an increase in the capacitance of the double layer and a lower resistance. The order of the capacitance, as was described, was expected from the photocurrents measurements and the difference in the surface area, but as evidenced from the results, there is no correlation between the different capacitance values, the difference in surface area, and the measured photocurrents. In addition, the MS TiO2 has almost the same surface area as the other mesoporous structures, such as S110 material, but its capacitance is significantly lower. Comparing the amorphous NT TiO2 to the crystalline form, one can see that the differentiation is not only due to the surface area. Therefore, it can be concluded that the surface area may affect the results; however, it is not the prime factor governing the electrochemical behavior of the oxide layer. Other differences, such as the nature of the structure (amorphous vs crystalline), may lead to the difference in the capacitance values. Such structural differences can be expressed in the level of recombination centers, thus affecting the photocatalytic rate.20

3.3.2. UV Illumination. EIS behavior of the photocatalysts was also studied under UV illumination, as can be seen in the Bode plot presented in Figure 6. The results presented in Table 3 were fitted to the same model previously discussed. In general, similar behavior is recorded in the Bode plot for the different photocatalysts as was in the dark. The only difference is the smaller impedance values under illumination as a result of electrons and holes generation. Under UV illumination, the total capacitance is increased by a factor of ∼1.5 for all photocatalysts. As expected, under UV illumination, electrons and holes are generated and the capacitance is increased, leading to increased conductivity of the TiO2 and to photo-oxidation of water.42 Therefore, not only was the total capacitance increased but the double layer capacitance, Cdl, increased as well. The increased capacitance of the oxide layer leads to increased charge transfer in the oxide-solution interface and, therefore, to an increased reaction rate at the surface of the photocatalysts. This results in increased capacitance in the double layer under UV illumination. This experiment demonstrates the ability of TiO2 to participate in photocatalytic reactions under UV light illumination. 3.4. Mott-Schottky Analysis. Mott-Schottky analysis was conducted in order to evaluate the effect of DC potential on the capacitance behavior of the various photocatalysts and to calculate the charge carrier concentration. The results of Mott-Schottky analysis for all photocatalysts materials are presented in Figure 7. Mott-Schottky analysis was measured at a constant frequency of 100 Hz in the DC potential range of -0.8 VSCE to +1.5 VSCE. The capacitance, Csc, was calculated according to eq 5:

Csc ) -

1 2πfZim

(5)

where f is the frequency in the experiment and Zim is the imaginary component of the impedance. The Mott-Schottky behavior is in accordance with the expected behavior of n-type semiconductor under increased anodic bias, i.e. a sharp decrease in the capacitance when the anodic bias is increased above the flat band potential until a plateau in the capacitance is reached.34,43 The reason for a decreased capacitance is the generation of electron-hole pairs; the electrons are repulsed to the bulk and the holes are attracted to the surface of the photocatalyst, leading to decreased capacitance of electrons under anodic bias. In all these studies, the Mott-Schottky curve contains two to three linear regions with a different slope. This behavior was observed in the photocurrents behavior presented in Figure 3b

TABLE 3: Model Parameters of the Photocatalysts Based on the EIS Results under UV Illumination (Figure 6) photocatalyst 2

C0, µF/cm R0, KΩ · cm2 C1, µF/cm2 R1, KΩ · cm2 C2, µF/cm2 R2, KΩ · cm2 C3, µF/cm2 R3, KΩ · cm2 Ctotal, µF/cm2 Rs, KΩ · cm2 Cdl, µF/cm2 Rct, KΩ · cm2

amorphous NT

NT

S110

S150

MS

0.0 ( 0.0 12.66 ( 0.94 6.534 ( 0.484 0.011 ( 0.001 4.064 ( 0.301 0.564 ( 0.042 0.283 ( 0.021 0.018 ( 0.001 10.88 ( 0.57 0.0075 ( 0.0006 7.34 ( 0.54 238.6 ( 17.7

20.41 ( 0.96 0.567 ( 0.027 141.1 ( 6.6 0.328 ( 0.015 75.98 ( 3.56 0.022 ( 0.001 43.73 ( 2.05 0.0059 ( 0.0003 281.22 ( 7.85 0.0060 ( 0.0003 239.1 ( 11.2 28.71 ( 1.35

1.146 ( 0.079 4.457 ( 0.306 1.936 ( 0.133 0.026 ( 0.001 15.85 ( 1.09 0.687 ( 0.047 7.556 ( 0.519 0.082 ( 0.006 26.49 ( 1.22 0.0075 ( 0.0005 23.79 ( 1.63 138.3 ( 9.5

1.401 ( 0.110 60.34 ( 4.74 6.776 ( 0.532 0.143 ( 0.011 1.836 ( 0.144 0.029 ( 0.002 61.95 ( 4.86 0.502 ( 0.040 71.96 ( 4.90 0.0066 ( 0.0005 84.61 ( 6.64 0.964 ( 0.076

0.354 ( 0.030 24.19 ( 2.08 0.590 ( 0.051 0.0104 ( 0.0009 2.127 ( 0.183 5.976 ( 0.514 0.688 ( 0.059 0.546 ( 0.047 3.76 ( 0.20 0.0055 ( 0.0005 7.039 ( 0.605 241.5 ( 20.6


J. Phys. Chem. C, Vol. 114, No. 21, 2010 9787 TABLE 4: Charge Carrier Concentration of the Different Photocatalysts As Was Derived with the Mott-Schottky Analysis (Figure 7)

Figure 7. Mott-Schottky analysis of the photocatalysts at a frequency of 100 Hz and DC potential range of -0.8 to +1.5 VSCE. The inset is the magnification of the selected area.

and was explained by the multiple donor states in the band gap. It must be emphasized that the equivalent model in Figure 5 contains four different capacitance levels: the space charge capacitance and three distinct donor levels; however, in the Mott-Schottky curve only two or three levels can be observed. This behavior is explained by the non-uniform changes in the slope (gradual change), which is a contribution of all donor levels and not necessarily one specific donor level at a time, until the capacitance is stabilized at a constant value. From the Mott-Schottky curve, the charge carrier density can be calculated according to the Mott-Schottky equation

Csc-2 )

(

)(

2 KT E - Efb εε0qNd q

)

(6)

where Csc is the total capacitance of the space charge region, E is the potential, Efb is the flat band potential, K is Boltzmann’s constant, and T is the temperature (room temperature, ∼298 K). The charge carrier density was calculated for all photocatalysts materials and is presented in Table 4. The charge carrier density was calculated according to the first linear slope of the curve (from the flat band potential until ∼0.5 VSCE). Comparing the S150 and S110 materials indicates that the charge carrier concentration is similar and the difference can be attributed to the surface area gap. This assumption is based on the normalized Mott-Schottky curve by the actual surface area. However, when the Mott-Schottky curve for the NT TiO2 is normalized by the oxide measured surface area, there is no correlation with the charge carrier concentration of S110 and S150. Even if this value is divided by a factor of 30, which is the estimation for the surface area difference between a nanotubular structure and a flat surface, the charge carrier concentration of NT TiO2 is higher than that of the other photocatalysts. These results hint again that the difference between the oxide layers is not affected by only the surface area. Previous reports have shown evidence of fluoride ions and complexes ([TiF6]2-) present in the tubes in this type of anodization.44 This may have led to an increased density of electronic defects and, therefore, to an increased charge carrier concentration. The S110 TiO2 has a similar actual surface area as the MS TiO2 but the charge carrier concentration and the capacitance with and without UV illumination are quite different. The capacitance of the MS Rutile TiO2 is much smaller, as was

photocatalyst

Nd × 1018 [1/cm3]

amorphous NT NT S110 S150 MS

7.4 ( 0.06 655 ( 0.3 21 ( 0.03 14.9 ( 0 0.03 1.6((3.19) × 10-4

previously evidenced in the literature.45 A possible reason for this behavior is the combination of the crystal structure with the anodization method. The anodization of the MS TiO2 was performed at a high temperature of 280 °C, which may lead to less oxygen deficiency defects in the lattice and therefore to a lower donor level of the semiconductor. This small capacitance may have led also to the lower current density of the MS TiO2. The effect of the surface area has an impact on the results but as was evident here it is not the only factor that affects the capacitance and the photocurrents. The different anodization methods lead to changes in the crystal structure and to differences in the electronic properties of the oxide layer. However, these results still do not explain why the nanotubular structure (NT TiO2) possesses lower photocurrents than expected, although it has a very high charge carrier concentration and, therefore, higher capacitance than the other photocatalysts. To further understand this, we evaluated the charge transfer to the solution by open circuit potential decay studies. 3.5. Open Circuit Potential Decay. Open Circuit Potential (OCP) decay study is a simple method capable of determining the differences between the oxide layers and, most importantly, it does not depend on the surface area. OCP of the different photocatalysts were studied under interrupted UV illumination. The OCP relaxation was measured in the dark, subsequent to a UV illumination, and is presented in Figure 8. Such a study enables a better understanding of the transfer process during the relaxation of the major charge carriers (electrons). As can be seen in Figure 8, the potential change upon UV illumination was ∼0.1-0.3 V for all the photocatalysts. The decrease in OCP is due to the generation of electron-hole pairs that lead to changes in the flat band potential and, therefore, to a decrease in the OCP down to more negative potentials. The OCP behavior upon the interrupted illumination is very similar for all the crystalline samples, where a sharp decrease in the potential upon illumination is followed by a fast

Figure 8. Open circuit potential dependence on time in the dark and under UV light illumination for amorphous NT, NT, S150, S110, and MS TiO2.

9788


Baram and Ein-Eli

TABLE 5: Kinetic Constants for OCP Decay of the Various Photocatalysts As Presented in Figure 8, Using Eq 9 photocatalyst

kct × 10-4 [s-1]


4.78 ( 0.0 1 5.83 ( 0.0 5 6.45 ( 0.0 3 6.03 ( 0.0 4 7.72 ( 0.0 8

stabilization. Only the amorphous NT TiO2 behaves differently, as the time for stabilization upon illumination is much longer and no potential stabilization is observed during the 1 h time period of this experiment. The difference between the amorphous and crystalline nanotubular TiO2 is not attributed to surface area but rather to the crystal structure. It is well-known that an amorphous structure facilitates electron-hole recombination due to a high concentration of defects.20,43 This can be evidenced also in the charge carrier concentration, which is 1 order of magnitude smaller for the amorphous NT TiO2 than for the crystalline NT TiO2. Nevertheless, the most interesting behavior is the OCP relaxation subsequent to illumination of the different oxides. During OCP decay, the electron concentration drops due to a first order reaction with oxidants in the solution.46 The dependence of the photocurrent on the potential can be calculated with the use of eqs 2, 3, and 4. In addition, we can replace the current with the charge carrier concentration in the interface ne(ph) that contributes to the photocurrents and calculate their dependence on the potential:

ne(ph) ≈ A√Vph

(7)

A is a constant and Vph is the photopotential in the semiconductor, which is the difference between the potential under illumination and the dark potential. The dependence of the kinetic charge transfer constant on ne(ph) can be described by eq 8:

kct ) -

1 dne(ph) ne(ph) dt

(8)

Use of these two equations (eqs 7 and 8) yields the dependence of the photopotential on time:

Vph ≈ -Ae-kctt

(9)

The kinetic charge transfer constant of the photocatalysts was fitted by using exponential regression and is presented in Table 5. The amorphous NT TiO2 has the slowest kinetic constant again due to its noncrystalline structure. Out of the four crystalline oxide layers, the NT TiO2 has the slowest charge transfer kinetics. In addition, the change in the OCP upon illumination for the NT TiO2 was the smallest (∼0.13 V). The smaller change in the OCP upon illumination indicates that the NT TiO2 is charged with fewer electrons than the other photocatalysts. These results are well correlated with the increase in the capacitance of the NT TiO2 upon illumination. Its capacitance is ∼1.3 times higher than the value under dark conditions (total capacitance; presented in Tables 2 and 3). This increment is smaller than that of most of the photocatalysts. The increase in the capacitance of the S110 material is ∼1.87

times and therefore the OCP upon illumination is changed to a higher potential range (∼0.3 V). The differences in the relaxation rate of the different oxides indicate that the photocatalysts differ from each other not only in the surface area and the charge carrier concentration but also in the charge transfer rate. The potential decay allows us to evaluate the hole and electron reactivity and, therefore, the efficiency of the photocatalyst. The faster the OCP decays, the faster the electron decays, and thus the charge transfer to the solution is faster.47 It must be emphasized that the charge transfer kinetics is not dependent on the double layer resistance, as this resistance points on the amount of charge that is concentrated at the interface of the oxide/electrolyte. However, this value does not indicate the flux of charge to the solution, i.e. the kinetic transfer. Therefore, although the NT TiO2 has the highest charge carrier concentration and the highest capacitance (total and double layer capacitance), its charge transfer rate is the slowest. On the contrary, the MS TiO2 has the opposite values: lowest charge carrier concentration and highest charge transfer rate. Therefore, a possible explanation for this behavior is the correlation between the charge carrier concentration and the charge transfer rate. Higher charge carrier concentration due to a defective structure may lead to small changes in the band gap and affect also the kinetic charge transfer.47,48 In the case of S110 and S150 TiO2 it is difficult to define the differences in the behavior, since their charge carrier concentration, surface area, and relaxation rates are very similar. Nevertheless, we can conclude that the difference in the surface area as was estimated by the dye adsorption fit well with the difference in the photocurrents measurements (except for the MS TiO2 and the reason was explained before) but not with the difference in the charge carrier concentration and the capacitance, measured by using the equivalent model. The NT TiO2 has a higher charge carrier concentration and therefore its capacitance is very high compared with that of other photocatalysts. However, due to a slower charge transfer, the difference in the photocurrents is smaller than expected. This behavior might be the result of the growing methods, leading to changes in the defect concentration. The addition of F- ions during the anodization may have led to a more defective structure of the NT TiO2 and, therefore, to different properties of the oxide layer. Nevertheless, its photocurrents are still higher than those for the other photocatalysts and thus it is expected to be a better photocatalyst. 3.6. Photocatalysis of MeO. Photocatalytic degradation of the MeO molecule was conducted by using the various photocatalysts in order to evaluate their photocatalytic behavior. The experiments were conducted under +0.2 VSCE anodic potential to correlate the electronic properties with the degradation ability of hazardous materials. The results of MeO degradation with the different photocatalysts are presented in Figure 9. The concentration profile follows the pseudo-first-order model and the kinetic degradation constant, k, is calculated and presented in Table 6. The results demonstrate the neglected effect of MeO degradation with use of the amorphous NT TiO2 as expected due to the amorphous structure. An insignificant effect was achieved also by using the MS TiO2 and this may be attributed to its small capacitance but also to the Rutile crystalline structure, which is known to be less reactive than that of Anatase. The crystal structure might also affect the photocatalytic rate of the S150 material, which has higher capacitance and higher photocurrents than S110 material, but due to the Rutile fraction in its structure, the degradation rate was almost the same. The highest degradation rate was achieved with the


J. Phys. Chem. C, Vol. 114, No. 21, 2010 9789 slower photocatalytic rate of MeO degradation than was expected. Nevertheless, the NT TiO2 with the highest surface area is still the most efficient photocatalytic anode with the highest degradation rate. The combination of microstructural characterization with photocurrents transient measurements and EIS data was proven to be an efficient tool for understanding the electronic properties of the TiO2. Measuring the photocurrents may predict the efficiency of a photocatalytic process; however, in order to understand the difference between the photocatalysts, analysis of the electronic properties is needed, using EIS measurements and OCP relaxation experiments. Acknowledgment. This work was financially supported by the Technion’s Russell Berrie Nanotechnology Institute.

Figure 9. Photocatalysis of MeO, using the different types of photocatalysts under anodic potential of +0.2 VSCE.

TABLE 6: Photocatalytic Kinetic Rate Constants of MeO Degradation, Using the Different Types of Photocatalysts As Shown in Figure 9 photocatalyst

k × 10-4 [min-1]


0.9 ( 0.3 52.1 ( 0.8 16.5 ( 0.2 15.4 ( 0.3 3.6 ( 0.5

NT TiO2 (at least ∼3.15 times faster). These results are in agreement with the expected results from the measured photocurrents but not with the difference in the Mott-Schottky and EIS results. Again, this difference is attributed to the charge transfer rate in the solution, as was discussed earlier. The photocurrent measurements allow us to estimate which photocatalyst is the most efficient and gauge the difference in the photocatalytic rate between the different samples. However, in order to understand the photocatalytic behavior, the electronic and crystallographic properties should be investigated. Nevertheless, the NT TiO2 is still the most efficient photocatalyst with the highest elimination rate and the fastest elimination time. We have demonstrated before that the NT TiO2 is the most efficient photocatalyst on E. coli bacteria inactivation as well.49 From these results the powerful ability of the TiO2 to eliminate contaminations, such as MeO and E.coli bacteria, using UV light is clearly demonstrated. It must be emphasized that the same solution of MeO was evaluated under UV illumination without any photocatalyst and no degradation was spotted. 4. Conclusions Various electrochemical anodization techniques were evaluated as methods of growing high surface area TiO2 photocatalysts. The use of high potentials (above the micro sparking potential) in both a molten salts electrolyte and a sulfuric acid solution leads to the formation of a mesoporous oxide layer. On the contrary, the use of a low potential (20 V) with the addition of fluoride ions leads to the formation of a fine and elongated nanotubular structure. Evaluation of the microstructure of the different oxide layers indicates that the main difference is the surface area. This yields the highest photocurrents and capacitance of the NT TiO2 in addition to the higher charge carrier concentration, as was revealed by Mott-Schottky analysis. However, the surface area is not the only difference: slower charge transfer kinetics of the NT TiO2 leads to a more moderate differentiation of the photocurrents and therefore to a

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