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KINETICS, CATALYSIS, AND REACTION ENGINEERING Investigation of Physicochemical Parameters That Influence Photocatalytic Degradation of Methyl Orange over TiO2 Nanotubes York R. Smith,† Archana Kar,‡ and Vaidyanathan (Ravi) Subramanian†,* Departments of Chemical and Metallurgical Engineering and Electrical and Biomedical Engineering, UniVersity of NeVada, Reno, 1664 North Virginia Street, Reno, NeVada 89431
The photocatalytic degradation of a model textile dye, methyl orange (MO), using anodized titanium dioxide (TiO2) nanotubes has been investigated. The effects of light intensity, dye concentration, external bias, pH, and nanotube dimensions (length, diameter, and wall thickness) on MO photodegradation have been examined. The application of a minimal bias of +0.0 versus saturated calomel electrode (SCE) can enhance the dye degradation at least 10 times compared to unbiased conditions for dye concentrations between 20 and 100 µM. The overall initial dye degradation rate demonstrates three types of dependence on dye concentration over a range from 2.5-100 µM. For lower dye concentrations (2.5-40 µM) and natural pH (∼6.0) conditions, Langmuir-Hinshelwood (LH) kinetics was observed. The nanotubes diameter, calcination condition, and the anatase-to-rutile ratio in the crystalline TiO2 nanotubes together influence the photocatalytic and photoelectrochemical properties of the TiO2 nanotubes. 1. Introduction Wastewaters containing organic pollutants such as azo-dyes from the textile industries raise much concern because of the harmful environmental and toxic effects they cause on ecological systems.1 Recent developments in advanced oxidation processes via heterogeneous photocatalysis in the presence of TiO2 have shown promising results to fully mineralize such contaminates.2-5 Several studies using TiO2 in slurry reactors have demonstrated complete destruction of the dye with reasonably high efficiencies.4,6-9 However, post-treatment recovery of the catalyst can be an inefficient and expensive process.8 Immobilizing TiO2 nanoparticles such as commercially available Degussa P25 or TiO2 particles prepared using sol-gel methods has been carried out on different substrates to counter this problem.10-14 However, when TiO2 particles are cast as films there could be a significant reduction in surface area causing degradation times to be prolonged.15 In an earlier work, we have shown that nanotubes (NTs) of TiO2 demonstrate rapid photocatalytic degradation under UV-vis illumination.16 TiO2 in the form of hollow NTs has recently been synthesized by anodic oxidation of titanium foils in various electrolytes.17-20 Interestingly enough, the synthesis of this material is relatively simple and at the same time, provides a large active surface area without the inherent downfalls of particle immobilization. TiO2 NTs display interesting photoelectrochemical properties compared to TiO2 nanoparticles. For example, there are fewer interfacial grain boundaries in NTs which promotes better charge separation and improved redox activity compared to nanoparticles.21 These innate properties can be exploited to improve photocatalytic degradation of environmental pollutants. * To whom correspondence should be addressed. E-mail: ravisv@ unr.edu. Tel.: +1 775-784-4686. Fax: +1 775-327-5059. Address: MS 388, LMR 474, Rm 310, University of Nevada, Reno, 1664 North Virginia Street, Reno, NV 89431. † Chemical and Metallurgical Engineering Department. ‡ Electrical and Biomedical Engineering Department.
A review by Mehrotra et al.9 on photocatalytic degradation using TiO2 in a slurry system, summarizes various physicochemical parameters that influence semiconductor-assisted photocatalytic oxidation. Some of these parameters include initial dye concentration, light intensity, mass transportation rates/ limitations, initial pH, catalyst size/loading, oxygen saturation, and temperature. Along with these operating conditions, adopting an appropriate kinetic model is critical to understanding the photocatalysis phenomena. To this end, several studies have concentrated on the heterogeneous kinetic model of LangmuirHinshelwood (LH) type, or even applied a more basic power law model. In the case of TiO2 NTs, a few articles16,22-24 report photocatalytic degradation of organic pollutants in water. However, to the best of our knowledge, there is currently no systematic work that has examined the contributions of the physicochemical parameters identified above in the photocatalytic degradation of a model azo compound in the presence of TiO2 NTs. In this work, TiO2 NTs were used as a photocatalyst and the effects of the physicochemical parameters identified above on the photodegradation of a model dye compound, methyl orange, has been presented. The applicability of the LH kinetics to MO photodegradation and the combined effects of light intensity, NT dimensions, and its phase composition have also been investigated. 2. Experimental Section Chemicals. Ammonium fluoride (NH4F, Fischer, 100%), ethylene glycol (C2H4(OH)2, Fischer), deionized water (Millipore Q), acetone (CH3COCH3, Sigma Aldrich, 99.5%, diluted to 50%), methyl orange (Sigma Aldrich), acetic acid (CH3COOH, VWR, 17.4N), sodium acetate (CH3COONa.3H2O, Sigma Aldrich, 99+%), sodium hydroxide (NaOH, Fischer, 99.3%), sodium bicarbonate phosphate (NaHCO3, Sigma Aldrich, g 99.0%), boric acid (B(OH)3, Sigma Aldrich, g 99.5%), and titanium foil of thickness 0.2 mm (ESPI Metals 99.9% purity) were received and used without any further treatment.
10.1021/ie801851p CCC: $40.75 2009 American Chemical Society Published on Web 10/13/2009
Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009
Preparation of TiO2 NTs. TiO2 NTs were prepared over rectangular titanium strips (1 × 4 cm) by anodization in a fluorinated ethylene glycol solution (0.5 wt % NH4F and 10 wt % distilled water). The details of the anodization process can be obtained from a previous work.16 To achieve variation in NT diameter, the applied bias was varied from 20-60 V DC (Agilent E3649A DC power source). The anodization time was fixed at 60 min while the applied voltage was varied.25 Postanodization, all samples were allowed to dry overnight followed by annealing in a tube furnace (Thermo Scientific, Lindberg Blue M BF51866C) with nitrogen atmosphere at 500 °C for 2 h. The furnace was ramped at a rate of 1 °C min-1 with continuous gas flow. Degradation Experiments. An aqueous solution of MO was used as the model pollutant. All photocatalytic experiments were performed with 7 mL of 20 µM MO dye in a custom-made three-electrode cell using TiO2 NTs anodized under the conditions mentioned above. The setup used for the photocatalytic experiments is similar to the one used in earlier work.16 A UV-visible light source (Newport 66902, Oriel Research) was used to illuminate the TiO2 NTs. Far UV radiation was filtered using a 0.5 M copper sulfate solution. The light intensity was varied using neutral density filters (Newport no. 1075910). The optical cell was maintained at 5 cm from the light source with constant agitation of the solution via magnetic stirring to ensure no mass-transport limitations. All the data presented here have been repeated at least twice. Some of the experiments (effects of light intensity and changes in the nanotube diameter) have been performed in triplicate. The error in all these measurements is within (5%. Photocatalytic degradation experiments with no bias were performed with TiO2 NTs and a Pt wire. For the photoelectrochemical experiments, a saturated calomel electrode (SCE) was used as the reference electrode and a platinum wire was used as the cathode. A potentiostat (Autolab PGSTAT 302) was used to control the three-electrode cell. A UV-vis absorption spectrum of the MO dye was recorded periodically using a spectrophotometer (Shimadzu UV2501 PC) by withdrawing small aliquots (∼0.7 mL) of the dye after different time intervals following photoillumination. After each absorption spectrum, the dye was returned back to the photocell. A distinctive peak at 459 nm was observed. This peak was tracked to monitor the change in concentration of MO as a function of time by application of Beer-Lamberts law. A representative series of absorbance spectra obtained as the dye degradation is monitored, is shown in Supporting Information, Figure S1. For exploring the effects of pH, the solution’s initial pH was adjusted by using noninvasive buffers (containing OH groups such as NaOH, B(OH)3, and NaHCO3). The effects of NT diameter and light intensities were performed in triplicates. Control experiments were performed with UV-visible light illumination of the MO in the presence of a titanium sheet in the photocell. We found that the degradation via interaction with the titanium sheet or direct photolysis was negligible compared to when the dye was exposed to the TiO2 NTs. 3. Results The details of photocatalytic oxidation mechanism of various azo dyes have been discussed previously in several articles.26-28 Specifically related to this work, the photodegradation mechanism and intermediates of MO have also been identified.29-31 In general, conduction band electrons (eCB-) and valence band holes (hVB+) are generated when a TiO2 surface is illuminated with light energy greater than the band gap of TiO2. Depending
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upon the type of dye, the photogenerated electrons can either reduce the dye or can react with electron acceptors (O2 absorbed on the surface of Ti3+ or dissolved in water) to create superoxide, O2•-. The photogenerated holes, hVB+, can oxidize the organic molecule or react with OH- or water to form hydroxyl radicals, OH · , which are highly oxidizing species. The heterogeneous photocatalytic degradation of an azo compound can be summarized by the following reactions:32,33 TiO2 + hν f TiO2(eCB- + hVB+)
(1)
TiO2(eCB- + hVB+) f TiO2 + heat
(2)
TiO2(hVB+) + H2O f TiO2 + H+ + OH·
(3)
TiO2(hVB+) + OH· f TiO2 + OH·
(4)
TiO2(eCB-) + O2 f TiO2 + O•2
(5)
+ O•2 + H f HO2·
(6)
dye + OH· f degradation products
(7)
dye + hVB+ f oxidation products
(8)
dye + eCB- f reduction products
(9)
In this study, the change in MO concentration was monitored over time and expressed in terms of fractional conversion, defined as X)
(MO0 - MOt) MO0
(10)
where X is fractional conversion, MO0 is the initial concentration of MO at t ) 0, and MOt denotes the concentration of MO at time t. 3.1. Physicochemical Factors That Influence MO Photodegradation. This section examines the effects of various physicochemical parameters including light intensity, initial dye concentration, external bias, pH, and NTs dimensions (length, diameter, and wall thickness) on the fractional conversion of the dye after UV-vis illumination for different lengths of time. 3.2. Effect of Light Intensity. The effects of changes in the photoillumination intensity on MO degradation were studied by varying the light intensity between 3 and 165 mW · cm-2. The initial MO concentration was maintained at 20 µM. Figure 1 shows the fractional conversion of the dye under no bias conditions. The dye conversion increases from 0.1 to 0.6 as the light intensity increases from 3 and 165 mW · cm-2. It is also observed that the fractional dye conversion remained almost constant at higher light intensities (I > 90 mW · cm-2). A similar observation wherein the degradation of a dye over nanoparticle TiO2 cast as films was noted to initially increase with light intensity, and later becomes independent of light intensity, was reported in an earlier work.11 At lower light intensities, the incident photon flux on the TiO2 surface is not sufficient to excite all the available electron-hole pairs. This implies that too few electron-hole pairs are generated and hence limited holes are available to assist in the photo-oxidation of MO. An increase in light intensity leads to the generation of more electron-hole pairs which contributes to the formation of oxidizing species in greater concentrations.
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Figure 1. The effects of varying the light intensity on the photodegradation of methyl orange after continuous UV-vis illumination is plotted. The fractional conversion of methyl orange as a function of time at an intensity of (a) 3, (b) 16, (c) 37, (d) 92, and (e) 165 mW/cm2 was estimated using eq 10. (Initial [MO] ) 20 µM, natural pH).
Figure 2. The photodegradation of MO after continuous UV-vis illumination at various initial dye concentration in the absence of any external bias is plotted. The fractional conversion as a function of time for initial MO concentration of (a) 2.5, (b) 5, (c) 10, (d) 20, (e) 40, (f) 60, (g) 80, and (h) 100 µM was estimated using eq 10. (I ) 165 mW · cm-2, natural pH).
Although an increase in light intensity from 3 to 90 mW · cm-2 increases the fractional conversion of MO, further increases in the light intensity beyond this point result in no significant increase in fractional conversion. Similar results have been observed with different textile dyes.9,34 Such observations can be attributed to either increased electron-hole recombination and/or absence of species to scavenge the photogenerated holes. Primarily, greater electron-hole pair recombination rates at higher light intensities limits fractional conversion.35 Under these conditions fractional conversion becomes independent of light intensity. 3.3. Effects of Initial MO Concentration and External Applied Bias. 3.3.1. Effect of Initial MO Concentration. The influence of initial dye concentration on the degradation of MO was studied by varying the concentration from 2.5-100 µM. On the basis of the observations from section 3.2, the light intensity was maintained at 165 mW · cm-2 and the experiments were performed at the natural pH of the dye. The fractional conversion of the dye over a concentration range from 2.5 to 100 µM under no bias conditions is shown in Figure 2. It is noted that as the dye concentration increases from 2.5-100 µM, the fractional conversion after continuous illumination reduces from 1 to 0.09. Similar observations were also reported with TiO2 nanoparticulate films deposited on different substrates.11,36 At a fixed light intensity, the decrease in conversion of MO with an increase in dye concentration can be attributed to the greater amount of dye competing for degradation and/or the
Figure 3. The photodegradation of MO after continuous UV-vis illumination at various initial dye concentration in the presence of an external bias: VSCE ) +0.0 is plotted. The fractional conversion as a function of time for MO concentration of (a) 20, (b) 40, (c) 60, (d) 80, and (e) 100 µM was estimated using eq 10. (I ) 165 mW · cm-2, natural pH).
reduction in the light intensity that reaches the TiO2 surface. At very high concentrations, much of the light is screened by the solution and fewer photons are able to reach the TiO2 surface. Thus, the generation of electron-hole pairs is greatly reduced and in turn, the dye degradation is reduced due to absence of oxidizing species. A similar trend was observed for the photocatalytic degradation of Remazol Black 5 and Procion Red MX-5B in the presence of commercial Degussa P 25 TiO2.37 3.3.2. Application of External Bias. Recently we have demonstrated that the application of an external bias assists with increasing the degradation of MO over TiO2 NTs.16 In this work, the application of a bias potential to electrochemically assist in the photodegradation of MO was examined at various MO concentrations (natural pH, I ) 165 mW cm-2). A plot of the fraction dye conversion at a +0.0 V bias vs SCE as a function of MO concentration is shown in Figure 3. The fractional conversion reduces from 1 to 0.5 as the dye concentration increases from 20 to 100 µM (this dye concentration range was chosen since bellow 20 µM, the dye solution becomes completely colorless without external bias). Supporting Information, Figure S2 shows that the application of a minimal bias of 0.0 V versus SCE improves MO degradation by at least 10 times compared to no bias conditions at any concentration of the dye. Thus, the trend noted for conversion of MO under bias conditions is the same as the one noted for no bias conditions. At a bias of +0.0 V and a MO concentration of 20 µM, the fractional conversion is significantly higher compared to the unbiased condition. Generally, higher conversion have been noted when a bias potential is applied and its magnitude is increased.16,22,23,38 The increase in the fractional conversion when a bias is applied can be attributed to the improvement in electron-hole separation as a result of the external driving force that separates the photogenerated charges following illumination. This favors higher concentrations of redox species to be available for photodegradation. 3.4. pH Effect. Industrial dyestuff waste streams can be discharged with wide ranging pH values. Since photocatalysis is a surface phenomenon, the photocatalyst performance can be highly predisposed by the pH of the stream, the nature of the dye, and its ability to absorb onto the photocatalyst surface. In acidic or caustic conditions, the surface of TiO2 can respectively become positively or negatively charged. This property is typically influenced by the zero-point charge (zpc) of the photocatalyst. For TiO2, reported values for the zpc vary, primarily based on the method used for synthesis and range
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Figure 4. The photodegradation of MO after continuous UV-vis illumination at various initial pH in the absence of any external bias is plotted. The fractional conversion of methyl orange as a function of time at initial pH values of (a) 6, (b) 7, (c) 9, and (d) 11 adjusted with noninvasive buffers was estimated using eq 10. ([MO] ) 20 µM, I ) 165 mW · cm-2).
Figure 5. The photodegradation of MO after continuous UV-vis illumination in the presence of TiO2 NTs of different diameters is plotted. The fractional conversion of MO as a function of time using TiO2 NTs of diameters (a) 50, (b) 75, and (c) 120 nm was estimated using eq 10. ([MO] ) 20 µM, natural pH, I )165 mW · cm-2).
from 6-7.39 Thus, for pH values less than pHzpc the surface becomes positively charged (attributed to H+ ions), and for pH values greater than pHzpc the TiO2 surface will be negatively charged (attributed to OH-).40,41 Several studies have examined the effects of pH on photodegradation.36,42-45 These studies have concluded that TiO2 particulate films demonstrate higher photodegradation in acidic media compared to alkaline media. It is worthwhile to mention that the MO molecules are known to be anionic in nature.46 This means that an increase in fractional conversion, for pH < pHzpc, can be ascribed to a positively charged photocatalyst surface being more available for the absorption of the MO molecules. However, under acidic conditions, MO is prone to change to a quinone structure. A visible color change, along with an absorbance peak shift was observed at lower pH values, further supporting the possible presence of a quinone MO structure. The quinone structure is more prone to oxidation over the azo structure due to the sulfonic groups (-SO3-) aiding in capturing hydrogen protons and further enhancing the hydrophobicity of the TiO2 NT surface.44 Transformation of MO to a quinone structure implies that the MO molecules are a different compound, thus the degradation mechanism would be different. Therefore, in this study we limited our analysis of pH effects to just the alkaline region. The pH of the solution was adjusted as discussed in the experimental section and not controlled during the reaction. Figure 4 shows that from natural pH (6.0) to more caustic conditions, the fractional conversion of the dye increases with other conditions (20 µM, I ) 165 mW cm-2) constant. The increase in fractional conversion with an increase in the pH can be explained in that although hydroxyl radicals can be formed through the reaction (OH- + h+ f OH•), the hydroxyl radicals are scavenged more rapidly at a higher pH allowing them to react more readily with the dye. 3.5. Nanotube Diameter Effect. Physical aspects of the NTs such as diameter, wall thickness, and length can influence photoactivity. For example, it has been shown that, the diameter of TiO2 NTs can be easily changed by varying the applied potential during the anodization process.25,47-49 To probe this further, the degradation of MO was examined over TlO2 NTs prepared by varying the anodization potential from 20-60 V DC. This corresponds to TiO2 NTs with an average tube diameter of 50 to 120 nm. Scanning electron micrographs showing the dimensions of the NTs anodized at different voltages are provided in Supporting Information, Figure S3.
Figure 5 shows the fractional conversion of the dye with TiO2 NTs of different diameters. The experiments were conducted using 20 µM MO and a light intensity of 165 mW · cm-2. The 50 nm diameter tubes show a conversion of 0.5. As the diameter increases, the conversion increases. The 75 and 120 nm diameter tubes show almost identical dye conversion after 120 min of continuous illumination. It can thus be inferred that the increase in diameter does improve the conversion. However, increasing the diameter from 75 to 120 nm does not proportionally improve the dye conversion. In a related experiment reported by Zhuang and co-workers, there is a slight initial increase followed by a decrease in the photocatalytic degradation of MO over TiO2 NTs as the NT diameter increases.50 Therefore, no trend was concluded from these results. 3.6. Kinetics of MO Degradation over TiO2 Nanotubes. Many factors play a vital role in determining the reaction kinetics of photocatalytic reactions. To better understand the reaction kinetics of such a system, the relationships between light intensity, concentration, and surface area are critical. However, the combined effects of these variables is rather complex and require much in-depth analysis. The analysis presented here is only a first step in this direction and is expected to assist further comprehensive studies of these parameters for TiO2 NTs in photocatalytic reactions. 3.6.1. Determination of Overall Reaction Rate. The initial overall reaction rate can be determined from the changes in the concentration of the dye using the following general equation:51 rMO )
-d[MO] dt
(11)
where -d[MO]/dt is the observed initial degradation rate of the organic pollutant, MO. The experiments conducted for section 3.2.1 were used to estimate the initial overall reaction rate. Figure 6a shows the rate versus concentration plot thus developed. Three regions are noted as shown in the figure. A region where the rate increases with increasing dye concentration, a region where the rate remains independent of the dye concentration, and a region where the rate decreases with increasing MO concentration. Notice that as the dye concentration increases, the initial rate increases although the fractional conversion decreases (Figure 2). Similar observations are also reported for the photodegradation of other dyes.3,33,52,53 3.6.2. Langmuir-Hinshelwood Mechanism. The purpose of sections 3.5.2, 3.5.3, and 4 is to (i) identify the LH region, (ii) determine what happens in this region (i.e., perform a
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kr(I, s) )
Figure 6. (a) The overall initial rates of reaction at different MO concentrations are plotted. The dye concentration was varied between 2.5-100 µM (I ) 165 mW · cm-2, natural pH). (b) The linearized LangmuirHinshelwood plot for various initial dye concentrations of 2.5-100 µM (I ) 165 mW · cm-2, natural pH). The continuous line indicates the dye concentration range where the LH kinetics is valid. Deviation from linearity at higher MO concentration shows that LH kinetics is not valid in the [MO] > 40 µM.
preliminary evaluation of the effects of multiple operating parametersslight intensity, dye concentration, and geometry), and (iii) help identify the parameters that assist in explaining the effects in (i) and (ii) using auxiliary characterization methods such as photoelectrochemistry to provide an insight into the activity of the nanotubes. Several researchers have reported that heterogeneous photocatalytic degradation of organic dyes at low concentrations often follows Langmuir-Hinshelwood (LH) kinetics.29,54,55 At low concentration, the rate of photocatalytic degradation of MO can be expressed as a function of rMO ) f(I, s, [MO], K)
(12)
where I is the light intensity, s is the catalyst surface area, [MO] is the concentration of MO, and K is a lumped adsorptiondesorption equilibrium parameter. Figures 1, 2, and 5 confirm this dependence of overall rate on these parameters. Ray et al. have developed empirical correlations that demonstrate the relationship between rate constant, light intensity, and surface area.56,57 The rate expression given in eq 11 and the Ray and Beenackers approach was used to form a modified equation that determines the photocatalytic degradation rate: rMO )
kIasnIβ krK[MO] K[MO] -d[MO] ) ) n β dt 1 + K[MO] 1 + kIas I 1 + K[MO] (13)
[
]
kIasnIβ
(14)
1 + kIasnIβ
where a, n, and β are constants. It is to be noted that kl and a are not to be combined since a is a pre-exponential constant for the term sn and all constants, but kl, are dimensionless. Before examining the effects of these parameters, as mentioned earlier the range where LH kinetics is valid has to be determined. Therefore, degradation data obtained at fixed light intensity (I) and surface area (s) were used to determine the region where LH mechanism is valid. Assuming that the adsorbed dye on the catalyst is in equilibrium with the bulk, the region where LH kinetics is expected to be followed can be reasonably described by a linear plot of eq 13. Figure 6b shows the plot of 1/[MO] and 1/rMO for a concentration range of 2.5-100 µM. One can note from Figure 6b that between 0.025-0.4 µM-1 the relation between the concentration and rate constant is linear with a positive slope. This confirms that the photocatalytic degradation of MO over TiO2 NTs indeed follows LH kinetics only in the range of low dye concentrations (2.5-40 µM). The approach used in Figure 6b is similar to the criteria used in related works to determine the range where LH mechanism is valid.58,59 The purpose of the figure is to only show the region where this mechanism is applicable. At [MO] > 40 µM, rate constant still changes with concentration. However, the deviation from linearity at [MO] > 40 µM indicates that the LH mechanism is not applicable. It is not possible to identify all reaction products using simple UV spectra analysis. Hence it can only be concluded that LH kinetics was observed for a low concentration domain and shows consistency for a single-site adsorption pathway.11 To fully conclude that a single-site adsorption pathway is indeed valid, further spectroscopic studies would be required to confirm adsorbed species onto the NTs as well as analysis of intermediate products. The LH kinetics is thus identified to be followed in the dye concentration range between 2.5 and 40 µM. At constant light intensity, surface area, and low dye concentrations it was found that K[MO] , 1 which results in eq 13 taking the form rMO ) krK[MO] ) ko[MO]
(15)
where ko is a lumped observed reaction rate constant. The values of ko for different dye concentrations and pH are shown in Table 1. It is noteworthy to mention here that, when one examines Table 1, the value of ko decreases with increasing concentration. The fact that kr and ko are f(I), suggests that any change in light intensity will also influence the values of kr and ko. Though the light intensity is maintained constant, the light reaching the photocatalyst surface reduces due to the increasing concentration of the dye.60 As the dye concentration increases from 2.5 µM, Table 1. A Comparative Summary of the Observed Rate Constants (ko) Obtained under Various Conditions Discussed in the Text experimental group
value of controlled parameter
overall rate constant ko [min-1]
concentration [µM]a
2.5 5 20 40
0.022 0.0147 0.0049 0.0029
pHb
7.23 9.03 11.03
0.0047 0.0066 0.0115
a
I ) 165 mW/cm2, natural pH. b [MO] ) 20 µM; I ) 165 mW/cm2.
Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009 Table 2. List of Parameter Values Obtained Using eq 14 for Investigating the Effect of Reaction Rate Constant on NT Diameter and Light Intensity parameter
50 nm NT
75 nm NT
120 nm NT
kI [µM min-1] a β n
0.02 0.021 0.52 0.021
0.032 0.004 0.75 0.003
0.086 0.0007 0.919 0.002
Table 3. The Results of the Photoelectrochemical Measurements Performed with the TiO2 NTs as the Photoanode, Pt Wire As the Cathode, and Calomel Electrode as the Reference in a 1 M KOH Electrolytea
a
higher fractions of incident light is screened from reaching the TiO2 surface causing a decrease in the value of the rate constants (as shown in Table 1). Similar results are reported in other related papers.23,37,61 For example, the overall rate constant for the photocatalytic degradation of Remazol Black 5 and Procion Red MX-5B in the presence of commercial Degussa P 25 TiO2 film reduces as the dye concentration increases.37 3.6.3. Estimating Constants in Rate Equation. The earlier section has identified the MO concentration range where LH kinetics is applicable as 2.5-40 µM. In this section, the effects of varying light intensity (I) and surface area (s) in the concentration range of 2.5-40 µM has been estimated. Equation 14 was solved using a nonlinear routine at a concentration within the LH regime. The values of the constants kI, a, β, and n thus estimated for the NTs with diameters varying from 50 to 120 nm is given in Table 2. The correlation coefficient for the reported values indicates a good fit for all the obtained parameters (R2 g 0.9). The changes in the values of these constants and its relation to the physical parameter they help understand, are of primary interest. For example, the increase in the value of kI with nanotube diameter is indicative of better photocatalytic activity with larger diameter nanotubes. In a related work, Ollis has examined the relationship between reaction rate parameters, light intensity, and charge recombination dynamics for dyes that follow LH kinetics.53 For a reaction dominated process, the reaction rate constant follows a linear relationship, or rather kr ∝ I
(16)
In a recombination dominated process, the reaction rate constant is proportional to the square root of the light intensity, or rather kr ∝ I0.5
(17)
These power terms can be taken as a leading indicator of the nature of interaction between photon flux and catalyst activity. The values of β shown in Table 2 vary from 0.5 to 0.9 indicating that as the NT diameter increases, the recombination rate decreases. Thus the analysis presented here provides a preliminary but valuable insight into the contributions of the NT dimensions toward photocatalytic activity suggesting that the increase in diameter favors a reduction in recombination of photogenerated charges. This hypothesis can be validated using simple experimental procedures as will be discussed in section 4.1. The development of correlations between the various parameters is rather complex and should be treated as a separate topic in itself. This work shows that, in principle, the application of the empirical correlations to the nanotubes-dye system appears promising. However, a further step (future work) in this direction will be to perform a comprehensive and critical analysis that involves (i) measuring the light intensity at the TiO2 surface, (ii) employing quantum yield estimates, and (iii) estimating equilibrium adsorption constant values to further draw insight into the relations between these parameters.
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NT diameter [nm]
photocurrent [mA]
VOC [mV]
50 75 120
0.738 0.769 0.898
305 382 399
Photocurrent data is measured with respect to +0.0 VSCE.
4. Discussions The last section examines the factors that contribute to improving the photoactivity of the NTs. Three main aspects are considered here, first, the physical features of the TiO2 (dimensions of the NTs), second, the preparation method (annealing conditions), and finally the antase-to-rutile ratio in the crystalline NTs. 4.1. Photovoltage and Photocurrent Effects. Among the physical features, anodization voltage, anodization time, bath temperature,62 and bath composition63 are main factors that influence the dimensions of the TiO2 NTs and consequently the photoactivity. As the anodization voltage increases the NT diameter, its length, and its thickness generally increase.63-66 After anodization, a heat treatment step converts amorphous NTs to a crystalline material. The transformation into a crystalline phase is known to improve the photoactivity of the NTs. Measurement of photoelectrochemical properties is a simple method to examine the effects of dimension changes on photoactivity of crystalline TiO2. Therefore, the photocurrent and voltage responses of the crystalline NTs were examined. Table 3 shows the photoelectrochemical data for NTs of different dimensions. The results indicate that the photocurrent and photovoltage increase as the NT dimension increase. The magnitude of the photocurrent correlates to the efficiency of electron transport through the nanotubes to the base (conducting Ti substrate) and hole transfer at the TiO2-electrolyte interface.67 It is also reported that thicker NT walls demonstrate greater band bending and lower the recombination losses of photogenerated charges at the surface. This implies that photoillumination of thicker walled NTs can result in a higher photocurrent.62 The fact that in this work the photoresponses are higher with larger diameter tubes clearly indicates improved charge separation and greater transport of electrons to the base of the NTs array. Besides, the wall thickness of the NTs prepared in this work shows an increase from 16 to 25 nm as the anodization voltage increases from 20 to 60 V. This also confirms the previously noted observation of improved charge separation with increased diameter. Furthermore, the larger diameter tubes demonstrate greater surface area65 and lengths which allows easier diffusion of redox species (i.e., electrolyte and dye molecules). MO molecules are very small (
