19484
J. Phys. Chem. C 2008, 112, 19484–19492
Enhanced Photoelectrochemical Response of a Composite Titania Thin Film with Single-Crystalline Rutile Nanorods Embedded in Anatase Aggregates Xiao-Mei Song,† Jin-Ming Wu,*,† Ming-Zao Tang,† Bin Qi,† and Mi Yan‡ Department of Materials Science and Engineering and State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed: August 28, 2008; ReVised Manuscript ReceiVed: September 28, 2008
A composite titania thin film consisted of quasi-aligned single-crystalline rutile nanorods embedded in sol-gelderived anatase aggregates was fabricated and its photoelectrochemical behavior was studied in detail. A monolayer of nearly single-crystalline rutile nanorods was first deposited on metallic Ti substrates through a controlled reaction between Ti and hydrogen peroxide. The gaps among the rutile nanorods were then filled with titania nanoparticles through a sol-gel dip-coating approach, which achieved a composite thin film of single-crystalline rutile nanorods embedded in anatase aggregates, after a subsequent thermal treatment at 723 K. The X-ray diffraction measurement revealed that the composite film with thickness of 180 nm contained anatase and rutile in a nearly 50:50 ratio. The UV-visible diffusive reflectance spectra estimated a band gap of 2.99 eV for the rutile monolayer and a slightly increased value of 3.03 eV for the composite film due to the embedding of the anatase nanoparticles. Photocurrent versus potential diagrams, photocurrent transient curves, and open-circuit potential measurement supported the fact that the anatase nanoparticles possessed better inherent photoelectrochemical properties than rutile. However, electrochemical impedance spectra characterization, together with photocurrent transient curves and open-circuit potential measurement, suggests that the single-crystalline rutile nanorods exhibited a higher electron-transfer rate. The combination of the two components in such an appropriate way enhanced significantly the charge separation effect arising from the anatase/rutile couple, which hence combined efficiently the “mixed crystal effect” and the “mixed morphological effect”. A steady-state photocurrent more than double the simple sum of those generated by the two components alone was detected for the current composite film, which can be attributed to the singlecrystalline rutile nanorods that provide channels for rapid electron transfer to the conductive Ti substrates under an applied bias potential. As a result, this study concludes that the present nanostructure magnified significantly the well-established mixed crystal effect, that is, an enhanced charge separation arising from a mixture of anatase and rutile. 1. Introduction Because of its inherent excellent chemical and photochemical stability, as well as ideal photoelectrochemical properties, titania (TiO2) has found various applications in gas sensors,1 photovoltaics,2 photocatalysis,3 and photoelectrocatalysis.4 Among them, photovoltaic electrodes and photocatalysts of titania have received the most attention. For solar energy conversion, titania photoelectrodes, in the form of either porous films5 or ordered nanostructured films,6 were utilized for light-harvesting and light-converting components. When used as photocatalysts, titania particle suspensions, or immobilized titania films, adsorb photons with energy larger than the band gap (ca. 3.2 eV) to generate electron-hole pairs, which migrate to the very surface to degrade the organic pollutants (by holes) or to reduce toxic metallic ions (by electrons) in wastewater.7 Usually, the efficiency of the above-mentioned photon-induced procedure is limited by a high electron-hole recombination rate or a low photogenerated charge transfer rate. The recombination of photogenerated electron-hole pairs could be prevented through either eliminating recombination centers such as structural * Corresponding author: phone +86-571-87953115; fax +86-57187952358; e-mail
[email protected]. † Department of Materials Science and Engineering. ‡ State Key Laboratory of Silicon Materials.
defects8 or introducing electron traps such as noble metal dispersions.9 In recent years, titania has been fabricated in the form of different ordered nanostructures such as ordered macroporous particles,10 nanowires,11 nanorods,12,13 and nanotubes1,6,14-17 by various techniques. In addition to the extremely high specific surface area as a result of the unique ordered nanostructures, such nanostructured titania is also believed to improve the electron-transfer rate, which in turn restrains the recombination of photogenerated electrons and holes. For example, Zhang et al.16 reported that the highly ordered titania nanotube arrays fabricated through the electrochemical anodic oxidation technique, which was originally developed in Grimes’s group,17 offered the potential for improved electron transport as well as higher surface area. Huynh et al.18 argued that nanocrystal shape played an important role in charge separation and transport in a blend of polymer and nanocrystals as photovoltaic device. The rod-shaped CdSe nanocrystals embedded in a polymer led to an order of magnitude increase in power conversion efficiency. On the other hand, titania composited with other semiconductors with narrower band gap, such as CdS/TiO2, has been developed to improve photon-induced properties. 19 Lo´pez-Luke et al.20 reported that the combination of nitrogen-doping and CdSe quantum-dot sensitization of titania thin films is an
10.1021/jp8076886 CCC: $40.75 2008 American Chemical Society Published on Web 11/14/2008
Composite Rutile/Anatase Titania Thin Film effective way to enhance the photon response, which is promising for photovoltaic and photoelectrochemical applications. In addition, the other kind of titania composite, in which the components are all titania but with various crystal structures or shapes, served as well to enhance the photoelectrochemical properties.21,22 Liu et al.21 reported the fabrication of a titania heterogeneous nanostructure by depositing anatase titania nanoparticles on the surface of rutile titania nanorods by a layerby-layer technique, which exhibited synergistic effects between anatase and rutile in decomposition of gaseous acetaldehyde. Yamamoto et al.22a found that cathodically electrosynthesized titania films containing occluded commercial available Degussa P25 nanoparticles resulted in an enhanced performance for the constructed dye-sensitized solar cell device. Enhanced photocatalytic activity was also noticed by Dionysiou and co-workers22b,c for sol-gel-derived titania films with the additive of appropriate amounts of P25. Keshmiri et al.22d,e utilized titania sol and P25 nanoparticles to fabricate titania thick films with ideal photocatalytic activity along with superior mechanical properties. These novel works, together with those studies reporting the enhanced charge separation efficiency induced by ordered nanostructures, gave us hints that single-crystalline rodlike titania embedded in titania nanoparticles would lead very likely to an enhanced photoelectrochemical response. In this paper, we utilized a monolayer of quasi-aligned single-crystalline rutile nanorods on conductive metallic Ti substrates to construct a novel composite titania thin film. A significantly enhanced photoelectrochemical property of the composite film was observed and possible contributions arising from such a novel nanostructure were studied in details through mainly electrochemical approaches. 2. Experimental Procedures 2.1. Fabrication of Thin-Film Electrodes. 2.1.1. Fabrication of Single-Crystalline Rutile Nanorod Monolayer. Ti plates (5 cm × 5 cm × 0.01 cm in size) were pickled at 60 °C for 2 min in a 1:3:6 (v/v/v) mixture of aqueous solutions of HF (55 wt %), HNO3 (63 wt %), and distilled water, followed by cleaning with distilled water in an ultrasonic bath. The precursor for rutile monolayer deposition was prepared by reactions at 80 °C for 48 h of each Ti plate with H2O2 solution (30 w. %, 50 mL, containing 100 mg of hexamethylenetetramine and 1.0 mL of concentrated nitric acid). The resultant solution was subjected to centrifugation at a rotation speed of 3000 rpm for 5 min to remove the suspensions. Several parallel reactions were carried out to collect enough amounts of precursors. The nearly single-crystalline rutile nanorod monolayer (named NR hereafter) was deposited through simply soaking the aspickled Ti substrate in the precursor prepared as described above (50 mL each) at 80 °C for 12 h. After deposition, the substrate was taken out, rinsed with distilled water, and then dried for further use. 2.1.2. Fabrication of Composite Film. Titania sol was prepared as follows: 17.0 mL of titanium(IV) butoxide and 4.8 mL of diethanolamine were dissolved in 67.3 mL of ethanol. After the mixture was stirred vigorously for 2 h at ambient temperature, a solution of 0.9 mL of water and 10 mL of ethanol was added slowly and the solution was further stirred for 2 h. The composite film (designated as NR-1 hereafter) was constructed by dipping the NR substrate in the sol for 1 min to allow thorough penetration of sols into gaps among the rutile nanorod arrays, which was then withdrawn at a speed of 36 cm/min under a relative humidity of ca. 20%. To observe the effect of the sol-gel layer, the dip-coating procedure was
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19485 repeated six times on the NR substrate to achieve the other sample (designated as NR-6). The film was dried at 200 °C for 2 h during the interval of each dip-coating procedure. In addition, sol-gel titania layer was also dip-coated on the aspickled Ti plate (designated as AS-1). All the above-mentioned samples were subjected to a final heat treatment at 450 °C for 1 h in the air atmosphere. 2.2. Thin Film Characterization. Surface morphology was observed with a field emission scanning electron microscopy (FE-SEM, Sirion, FEI). High-resolution transmission electron microscopy (HR-TEM) observation was conducted with a JEM2010 microscope (JEOL, Japan) working at 200 kV. X-ray diffraction (XRD) measurements were conducted on a Rigaku D/max-3B diffractometer with Cu KR radiation, operated at 40 kV, 36 mA (λ ) 0.154 056 nm). The UV-vis diffuse reflection spectra were taken with an Ocean Optics HR4000CG-UV-NIR high-resolution spectrometer. 2.3. Photoelectrochemical Measurement. All the electrochemical experiments were performed in a standard threeelectrode configuration made of Pyrex glass with a platinum plate as counterelectrode and a saturated calomel electrode (SCE) as reference electrode, with an electrochemical working station (CHI-660C, Shanghai Chenhua Instruments). The photoelectrodes were fabricated by attaching a copper wire on the back with silver paste. An epoxy was used to cover the electrode except for the used TiO2 surface. The area of working electrode was set constant at 1 × 1 cm2. Measurements of photocurrent and photopotential signals and current-potential curves were carried out with an active area of 1.0 cm2. A 14-W UV lamp with a central wavelength of 365 nm was used as a light source to measure the photoelectrochemical properties. The average intensity of UV irradiance reaching the sample was measured to be ca. 3.5 mW/cm2, on a UV irradiance meter (model UVA, Beijing Normal University, measured for the wavelength range of 320∼400 nm with a peak wavelength of 365 nm). A 0.5 M Na2SO4 solution (pH ) 6.3) was used as supporting electrolyte, which was exposed to air. Electrochemical impedance spectra (EIS) were obtained at the open-circuit potential of the specimen, with amplitude of 5 mV. The frequency span was from 100 kHz down to 0.01 Hz. The measured EIS spectra were fitted by use of ZSimpWin software (version 3.10, by Echem), which can take over all the complex operations when a job is defined by specifying a data file and selecting a model. It has the capability to autostart with an initial guess of the parameters associated with the selected model and to perform computations a number of times to determine the desired solution on complex nonlinear leastsquares fitting criteria. 3. Results and Discussion 3.1. Composite Film Characterization. Chemical analysis revealed that the precursor derived from the Ti-H2O2 reaction contained ca. 4.8 mM hydrated Ti(IV) ions and 0.66 M nitrate ions, with a pH value of ca. 0.6. The concentration of nitrate ions was higher than that of the original value (0.2 M) because of solvent evaporation during preservation at 80 °C for 48 h to prepare the precursor. The hydrated Ti(IV) ions in the precursor were supposed to exist in the form Ti(OH)22+ in such an acidic environment,23 which was a result of the balance between the corrosion of the Ti substrate by H2O2 and the precipitation of the amorphous titania.24 Such a precursor was found to be appropriate for deposition of a rutile monolayer on the as-pickled Ti substrate at a low temperature of 80 °C for 12 h. Figure 1 shows the FE-SEM surface morphology of the rodlike titania monolayer, the sol-gel anatase film, and the
19486 J. Phys. Chem. C, Vol. 112, No. 49, 2008
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Figure 1. FE-SEM surface morphology of (a) rutile nanorod film NR, (b) sol-gel anatase film AS-1, and (c, d) composite film NR-1.
constructed composite film. Most of the rodlike titania with sizes of ca. 60 nm in diameter and 180 nm in length grew upside from the substrate; however, parts of the nanorod lay parallel to the substrate (Figure 1a). The sol-gel anatase film was smooth and crack-free, even after the subsequent thermal treatment. Observations on artificially introduced fragments estimated a thickness of ca. 90 nm for the sol-gel film (Figure 1b). For the composite film, the interspace among the nanorods was filled thoroughly with spherical titania nanoparticles with sizes of ca. 30 nm. The top of some nanorods, which was pyramidlike, can be seen clearly exposed on the very surface of the composite film (Figure 1c). Cross-sectional observation of the film by FE-SEM revealed that the thickness was ca. 180 nm (Figure 1d). The constructed composite film as a whole was dense and homogeneous. It is noted that such a film thickness is appropriate to utilize UV light with a central wavelength of 365 nm, because the penetration length of such UV light in titania is estimated as 100∼250 nm.25 The increase in weight for the sol-gel film after the subsequent thermal treatment was estimated to be 0.038 mg/ cm2, which was in good accordance with the film thickness estimated from SEM observation. The weight for anatase nanoparticles embedded in the composite film was ca. 0.041 mg/cm2; therefore, it is concluded that the amount of titania in the composite film (NR-1) is roughly the sum of those for the nanorod monolayer (NR) and the sol-gel film (AS-1). In the present investigation, fabrication parameters have been carefully optimized to construct the composite film, with special attention paid to avoid a simple cover of the sol-gel titania layer, which has been verified to contribute negatively to the photocatalytic activity of a titania nanorod film that separated from the Ti substrate with an intermediate condensed anatase layer.3 The abundant hydroxyl groups as well as some superoxide groups26 on the surface of the as-deposited titania nanorods were supposed to favor the penetration of titania sol into the
gaps among the nanorod arrays. However, not all the rodlike titania grew vertical to the Ti substrate because of the various facets of Ti grains, which leaves a space for further optimizing the microstructure and hence the photochemical properties of the current composite film. Figure 2 shows a typical HR-TEM image taken from a part of the rodlike titania. Parallel fringes as demonstrated partly in the figure were observed throughout the whole rod, suggesting that the titania nanorod was nearly single-crystalline. The distance between the two adjacent fringes was measured to be ca. 0.33 nm, which corresponds well to the interplanar distance of rutile (110). Figure 3 shows XRD patterns of the nanorod monolayer and the achieved composite film. The rodlike titania film was identified to be well-crystallized phase pure rutile, which agrees well with the HR-TEM observations. The rutile monolayer grew preferably along the [001] direction to give an abnormal weak (101) peak, according to JCPDS card 21-1276. The sol-gelderived titania crystallized to phase pure anatase (XRD data not shown here). As a result, the composite film exhibited additional XRD peaks corresponding to anatase, except those attributable to the nanorod monolayer. For the composite film, the fraction of rutile can be estimated according to the formula26
WR ) 1/(1 + 0.8IA/IR)
(1)
where IA and IR are X-ray integrated intensities of the (101) diffraction of anatase and the (110) diffraction of rutile, respectively. Because of the abnormal growth of rutile in the current investigation, the intensity of rutile (101) was taken as IR. The calculation revealed that the NR-1 composite film contained 52.4% anatase and 47.6% rutile by weight. The NR-6 thin film contained 88.5% anatase and 11.5% rutile by weight, due to more sol-gel anatase deposited on rutile nanorods.
Composite Rutile/Anatase Titania Thin Film
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19487
Figure 2. Typical HR-TEM image of a nearly single-crystalline rutile nanorod.
TABLE 1: Band Gap of Rutile Nanorod Thin Film Alone and Composited with Sol-Gel Anatase for One and Six Layersa band gap, eV
NR
NR-1
NR-6
direct indirect
3.26 2.99
3.03 2.82
3.13 3.02
a The band gap was estimated from the corresponding UV-vis diffuse reflectance spectra shown in Figure 4, assuming an indirect and direct transition between bands for titania, respectively. Values shown in boldface type were assumed to be more reasonable.
Figure 3. XRD patterns of the rutile nanorod thin film alone (NR) and composited with sol-gel anatase for one layer (NR-1) and six layers (NR-6).
Figure 4. UV-vis diffuse reflectance spectra of the rutile nanorod thin film alone (NR) and composited with sol-gel anatase for one layer (NR-1) and six layers (NR-6).
Figure 4 illustrates the UV-vis diffuse reflectance spectra of NR, NR-1, and NR-6 thin films. The band gap (Eg) of the titania thin films can be estimated from27-30
Rhν ) A(hν - Eg)n
(2)
where R is the absorption coefficient, hν is the photon energy, and A is a constant. In the case that the fundamental absorption
of the titania crystal possesses an indirect transition between bands, n ) 1/2; for a direct transition between bands, n ) 2. By plotting R1/2 and Rhν2 against hν, respectively, the intercepts defining the band gap for the various titania thin film can be obtained, which are listed in Table 1. The band gap for anatase and rutile is 3.2 and 3.0 eV, respectively;28-30 therefore, the indirect transition model gave a relatively reasonable Eg of ca. 2.99 eV for the rutile monolayer. On the contrary, the direct band gap of 3.13 eV was relatively reasonable for NR-6 because it consisted mainly of anatase. Similarly, it is very likely that the composite film of NR-1 possessed a direct band gap of 3.03 eV, which was between that of rutile and anatase. However, considering that the composite film NR-1 contained nearly 50% anatase by weight, the band gap of 3.03 eV seems unreasonably lower, the reason for which remains unclear at present. The reason why the band gap of rutile nanorod film was fitted better by the indirect transition model while that composited with sol-gel anatase layer was more correctly estimated by the direct one is also not clear. However, Reddy et al.27 reported that the indirect transition model applied well to a commercial anatase powder, while the direct transition model gave more realistic band gaps for their laboratory-fabricated anatase nanoparticles. 3.2. Electrochemical Photovoltaic Characterization. Cyclic voltammograms (CV) of the NR-1 thin-film electrode in 0.5 M Na2SO4 solution, either in the dark or under UV illumination, are shown in Figure 5. There appeared almost no photocurrent in the dark. Under UV illumination, significant anode photocurrent was detected at potential beyond -0.15 V, in accordance
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Figure 5. Cyclic voltammograms for rutile nanorod film composited with one layer of sol-gel anatase (NR-1), with and without UV illumination. Sweep rate 10 mV/s; working solution 0.5 M Na2SO4 electrolyte.
Song et al.
Figure 7. Photocurrent transient curves of various electrodes under UV pulsed illumination in 0.5 M Na2SO4 electrolyte. Working electrodes were constructed from rutile nanorod film (NR), rutile nanorod film composited with one layer of sol-gel anatase (NR-1), and one-layer sol-gel anatase layer on as-pickled Ti substrate (AS-1). The UV light intensity reaching the sample was 3.5 mW/cm2. The working electrode potential was kept constant at 0.5 V vs SCE.
(0.32 mA/cm2). In addition, the incident photon-to-current conversion efficiency (IPCE, percent) of the three kinds of titania thin film can be calculated by20,32
IPCE )
Figure 6. Typical photocurrent vs potential diagrams of various electrodes under UV illumination in 0.5 M Na2SO4 electrolyte. The working electrodes were constructed from rutile nanorod film (NR), rutile nanorod film composited with one layer of sol-gel anatase (NR1), and one-layer sol-gel anatase layer on as-pickled Ti substrate (AS1). The scan rate was 10 mV/s. The UV light intensity reaching the sample was 3.5 mW/cm2.
with the fact that titania is an n-type semiconductor.25 Anode photocurrent density increased with increasing applied potential, which gradually reached a saturated value of ca. 1.36 mA/cm2 at potentials beyond 0.5 V. No current peak due to electrochemical oxidation can be discerned in the present system because only supporting electrolyte existed in the solution. The cathodic current observed at potentials more negative than -0.3 V under both dark and illumination conditions can be attributed to the electrochemical reduction of dissolved oxygen on the electrode,16 which was enhanced remarkably under UV illumination. Figure 6 demonstrates the typical photocurrent versus potential curves of AS-1, NR, and NR-1 thin-film electrodes under UV illumination in 0.5 M Na2SO4 supporting electrolyte. In the dark, when the electrode potential was linearly swept from -0.8 to 1.0 V, there was no obvious current response for all the film electrodes (data not shown). Under UV illumination, the photoelectrochemical response appeared for all three thin films. The saturated current density increased in the order of NR < AS-1 < NR-1. The rutile nanorod film exhibited the least photocurrent response, which is not surprising because anatase possesses more surface-active sites (electron traps such as hydroxyl groups or adsorbed oxygen) than rutile, favoring the photocurrent response.29,31 Such trends persist even when positive bias potential is applied, wherein electric-field-assisted separation of electron-hole pairs contributes additionally to the photocurrent.31 Interestingly, the saturated current density, as determined at the potential of 1.0 V versus SCE, for the composite film (NR-1) was ca. 0.95 mA/cm2, which was more than double the sum of those for NR (0.08 mA/cm2) and AS-1
1240jSC × 100 λIi
(3)
where Ii is the incident light power (in milliwatts per square centimeter), λ is the illumination wavelength (in nanometers), and jSC is the saturated photocurrent density (in milliamps per square centimeter). It can be calculated that the IPCE values for NR, AS-1, and NR-1 are 7.8%, 31.1%, and 92.2%, respectively. Therefore, it is clear that, for the present composite thin film, there exist some synergistic effects between the anatase nanoparticles and the embedded single-crystalline rutile nanorods. To provide a rough comparison, it is noted that a CdSe quantum dot-sensitized and nitrogen-doped titania film exhibited a highest IPCE of ca. 95% at λ ) 300 nm.20 The photoelectrochemical properties of the three thin films were further examined by photocurrent transient measurement and the results are shown in Figure 7. Once again, the steadystate photocurrent density of NR-1 (1.12 mA/cm2) was more than doubled the sum of those for NR (0.12 mA/cm2) and AS-1 (0.35 mA/cm2). For the two titania films NR and AS-1, a common behavior for titania electrode was observed when the UV light was on: a fast increase followed by a gradual decrease in the photocurrent as a function of illumination time. The photocurrent response can be explained in terms of a classical onset of recombination, and the curve can be fitted according to a simple kinetic approach based on exponential law:32-34
It ) I∞ + (I0 - I∞) exp(-kt)
(4)
where It is the photocurrent value after switching the UV light on for a duration of t, I0 is the photocurrent value when the UV light was on, I∞ is the steady-state photocurrent, and k is a constant reflecting the recombination rate of photogenerated charges.32,34 The fitting results for the data in the interval of 20∼40 s in Figure 7 gave a much lower rate constant for the NR film (0.164 s-1) than that for the AS-1 film (0.434 s-1), which means the NR film possessed a significantly lower recombination rate of photogenerated charges.32 Therefore, the photocurrent transients suggested that although the anodic photocurrent of the NR film was the smallest among the three film electrodes, due mainly to the inferior photoelectrical response of rutile to anatase (considering the similar amounts of titania in the two thin films),29,31 the single-crystalline rutile
Composite Rutile/Anatase Titania Thin Film
Figure 8. Variation in the open circuit potential for rutile nanorod film (NR), rutile film composited with one layer of sol-gel anatase (NR-1), and one-layer sol-gel anatase layer on as-pickled Ti substrate (AS-1) thin film photoanodes in 0.5 M Na2SO4 electrolyte under chopped UV light. The UV light intensity reaching the sample was 3.5 mW/cm2.
nanorod reduced remarkably the recombination rate of photogenerated carriers. The enhanced electron-transfer rate of singlecrystalline rutile nanorods can be attributed mainly to the elimination of grain boundaries, which act as electron scattering centers. The subsequent thermal treatment to crystallize sol-gel titania nanoparticles contributes additionally to electron transfer because of the further decreased crystal defects within the nanorods. As will be discussed later, such an enhanced electron transfer of the nearly single-crystalline rutile nanorods magnified significantly the so-called “mixed crystal effect” between anatase and rutile arising from the band match,13 which hence combined effectively the mixed crystal effect and the mixed morphological effect and resulted in a significantly enhanced photocurrent response under UV illumination. On the other hand, for the composite thin film NR-1, the photocurrent transient, which generated promptly when the UV light was on, increased gradually with further UV illumination time. Such transient behavior was also observed by Zhang et al.,16 who conducted the photocurrent transient measurement for annealed titania nanotube arrays in mixed solution of 0.5 M Na2SO4 and 0.1∼1 M methanol, and also by Tsuchiya et al.34 for as-anodized amorphous titania nanotube arrays in 0.1 M Na2SO4 electrolyte. The latter case was explained by the gradual filling of traps acting as recombination centers in the amorphous titania nanotube. 34 Zhang et al.16 did not provide an explanation; but the addition of methanol in the solution could possibly be the cause, as diffusion of photogenerated holes to the very surface to be trapped by the additionally adsorbed methanol is time-consuming. In the present investigation, no additional organics were added to the electrolyte and the titania film was well-crystallized through a subsequent thermal treatment. Therefore, we speculated that the inhibition of the recombination process due to enhanced electron transfer along the embedded single-crystalline rutile nanorods could be the reason for the transient observed in Figure 6. The recombination process was surpassed by other factors contributing to an enhanced photocurrent response; as a result, the photocurrent increased gradually with prolonged UV illumination time. Such positive factors arising from the so-called mixed crystal effect will be schematically shown in Figure 11 and described later. Figure 8 shows the open-circuit potential changes of AS-1, NR, and NR-1 thin-film electrodes under UV illumination. In the dark, AS-1 and NR-1 films exhibited an initial open-circuit potential of ca. 0.12 V versus SCE, while the rutile nanorod film (NR) possessed a negative initial open-circuit potential of ca. -0.08 V versus SCE, which suggests an accumulation of
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19489 electrons on the NR film electrode even without UV illumination. When the UV light was on, the potential dropped by ca. 0.50, 0.54 and 0.10 mV for NR-1, AS-1, and NR thin-film electrodes. The decreased photovoltage as a result of the sudden generation of electrons and holes in the thin-film electrodes is directly linked to electron accumulation in the titania film following band-gap excitations. When the illumination is cut off, the accumulated electrons transfer to solution, to the backcontact, or to the surface states, and as a result the open circuit potential slowly relaxes in all cases.35 Compared to NR, the significant drop in the open-circuit potential for NR-1 and AS-1 indicates that the electron accumulation in these two film electrodes was more significant than that in the NR film. This can be attributed to the sol-gel anatase nanoparticles in the two films, which, in comparison with rutile, tend to increase the surface redox potentials and to prolong the carrier lifetime.29 Interestingly, when the UV light was off, recovery of the opencircuit potential for NR-1 was remarkably slower than that for AS-1, which implies more efficient charge separation or less charge recombination for the composite thin film. Electrochemical impedance spectra (EIS) are a common approach to characterize the interfacial properties of nanostructure electrodes. Figure 9 shows the measured and calibrated Nyquist plots for AS-1, NR, and NR-1 thin-film electrodes in the dark and under UV illumination. For calibration, the same circuit model is used for all the three thin-film electrodes due to their similar electrode components with titania films adhering on Ti substrates. Figure 10 shows the equivalent circuit proposed for NR, AS-1, and NR-1 thin-film electrodes in the dark and under UV illumination. In the equivalent circuit, the constant phase element Q1 represents the double-layer capacitance on the electrode. Rct is the apparent resistance and W represents the Warburg impedance due to mass transfer from the bulk of the electrolyte to the reaction site. Rn and Q2 in parallel combination correspond to cations/anions extruded into the titania film to maintain electroneutrality. Rs includes both solution resistance and uncompensated potential drop. For the equivalent circuit under UV illumination, Q3 is added to compensate capacitance on the electrode. As can be seen from Figure 9, the fitting results using such equivalent circuit models are in good agreement with the experimental data. Figure 9 demonstrates that, in the dark, all three thin-film electrodes exhibited a large semicircle at the range of both high and low frequencies, suggesting that the electrode process is electron-transfer-limited over the whole frequency range from 0.01 Hz to 100 kHz.36 Under UV illumination, the semicircle radius at high frequency decreased significantly. Meanwhile, a linear part appeared at low frequency, which arises from the mass-transfer-limited electrode process, for all three titania thin films.37 The extracted parameters for the circuit elements evaluated by fitting the impedance data are summarized in Table 2. Rs are almost constant for all cases except NR in the dark, which may be ascribed to a deviation in the position of the reference electrode.37 Under UV illumination, Q1 increases more than 10 times larger than those in the dark for all three electrodes. The AS-1 electrode exhibited relatively smaller double-layer capacitance (143.0 µF · cm-2) than NR (462.6 µF · cm-2), because the gaps among the rutile nanorods contributed to the capacity. After the gaps were filled with anatase nanoparticles, additional interfaces (those between rutile nanorods and anatase nanoparticles) were introduced, which led to an even enhanced capacity for NR-1 (559.2 µF · cm-2). For all three thin-film electrodes,
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Figure 9. Measured and calibrated Nyquist diagrams of rutile nanorod film (NR), rutile film composited with one layer of sol-gel anatase (NR-1), and one-layer sol-gel anatase layer on as-pickled Ti substrate (AS-1) thin films: (a) in the dark and (b) under UV illumination.
Figure 10. Typical equivalent circuit for the composite thin-film electrode with rutile nanorods embedded in sol-gel anatase aggregates (NR-1): (a) in the dark and (b) under UV illumination.
Figure 11. Schematic image showing the transfer of hole-electron pairs photogenerated in (A) anatase nanoparticles, (B) rutile nanorods, and (C) anatase nanoparticles neighboring the rutile nanorods of the composite thin-film electrode with rutile nanorods embedded in sol-gel anatase aggregates (NR-1).
Rct decreased significantly when UV light was on, especially for NR (from 457 700 Ω · cm2 in the dark to 1470 Ω · cm2 under UV illumination) and NR-1 (from 295 700 Ω · cm2 in the dark to only 818 Ω · cm2 under UV illumination). The fitted parameter Keff, which is derived from eq 538
Keff ) 1/(RctQ1)
(5)
defines the recombination rate constant of the overall photoelectrochemical reaction. Table 2 also shows that, for the three electrodes, Keff increased remarkably after UV illumination, which is not surprising because of the increasing amounts of photogenerated electrons and holes. Also of interest is the order of the Keff values for the three films under UV illumination, NR < NR-1 < AS-1, suggesting the smallest overall recombination rate constant for single-crystalline rutile. During EIS measurement, under UV illumination, the open potential increased due to the separation of photogenerated electrons and holes, resulting in higher Faraday current under
high driving force of the electric field. Therefore, UV illumination decreased electron-transfer resistance and increased electron transfer through the electrolyte/electrode interface. Figure 9b and Table 2 indicate that, under UV illumination, the composite film exhibited the smallest semicircle radius among the three thin-film electrodes, suggesting a smaller electron-transfer resistance Rct for photogenerated electrons to transfer through the titania thin-film electrode to the Ti substrate and then to the out circuit (818 Ω · cm2 for NR-1), while the rutile nanorod film exhibited a large resistance (1470 Ω · cm2 for NR), only slightly less than that of the sol-gel anatase film (1697 Ω · cm2 for AS1). Zhang et al.16 argued that, with the same chemical composition, a compact film exhibits much less resistance for electron transfer than a porous one. Therefore, the relatively large resistance for the NR thin film did not mean necessarily a poor electron-transfer rate. The large area of interspaces among the rutile nanorods resulted in the large electron-transfer resistance. Filling the interspace with sol-gel anatase nanoparticles led to a reduced Rct. The resistance for AS-1 was still slightly higher than that for NR, in spite of the fact that the former was much more compact. This fact indicated clearly that single-crystalline rutile nanorods possessed a significantly enhanced electrontransfer rate compared with the thin film consisting of nanoparticles, as supported also by the recombination rate derived from eq 4. Thus, the combined effect of filled interspaces among the nanorods and the excellent electron-transfer ability of the single-crystalline rutile nanorods achieved remarkably reduced resistance along the electrolyte/electrode interface. On the basis of the experimental results, we propose here a qualitative model to explain the combined mixed crystal effect and mixed morphological effect arising from the present nanostructured composite thin film, as illustrated schematically in Figure 11. Most regions of the composite thin film consisted of nearly single-crystalline rutile nanorods, which distributed separately from each other and embedded in sol-gel anatase nanoparticles. The nanorods grown directly from the Ti substrate without any compact intermediate layer thus achieved good adhesion to the conductive Ti substrate and hence ideal electron transfer through the rutile/Ti interface. The single-crystalline nature of the nanorods contributed additionally to the electrontransfer rate in the film. However, because of the great difference between the photocurrent response of anatase and rutile, the rutile nanorod film (NR) itself exhibited poor photoelectrochemical properties. On the other hand, the photocurrent response of sol-gel anatase films was also not satisfactory, because of the relatively poor electron-transfer rate arising from the grain boundary as well as interfaces along the nanoparticles. The inherent shortcomings of the two films were well overcome
Composite Rutile/Anatase Titania Thin Film
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19491
TABLE 2: Parameters for Circuit Elementsa parameters by fittingb c
electrodes
equivalent circuits
NR AS-1 NR-1
Rs[Q1(RctW)Q2R] Rs[Q1(RctW)Q2R] Rs[Q1(RctW)Q2R]
NR AS-1 NR-1
(RsQ3)[Q1(RctW)Q2R] (RsQ3)[Q1(RctW)Q2R] (RsQ3)[Q1(RctW)Q2R]
Rs (Ω · cm ) 2
In the Dark 128.6 (3.1%) 4.7 (1.1%) 8.7 (2.1%)
Q1 (µF · cm-2)
Rct (Ω · cm2)
Keff (s-1)
31.1 (1.5%) 13.8 (7.4%) 93.1 (1.4%)
457700 (11.5%) 26280 (3.2%) 295700 (9.4%)
0.0703 2.76 0.0363
UV Illumination 3.8 (2.4%) 462.6 (1.5%) 4.5 (1.9%) 143.0 (1.8%) 4.9 (3.7%) 559.2 (1.3%)
1470 (4.5%) 1697 (1.3%) 818 (1.2%)
1.47 4.12 2.19
a Parameters were evaluated by fitting the impedance data to the equivalent circuits for the three thin-film electrodes in 0.5 M Na2SO4 with and without UV illumination. b Numbers in parentheses are estimated errors. c The geometric area of the thin-film electrode is 1 × 1 cm2. Refer to Figure 10 for equivalent circuit symbols.
through formation of a composite film of anatase nanoparticles embedded in single-crystalline rutile arrays. When the composite film was subjected to UV illumination, the two components of anatase nanoparticles and rutile nanorods alone contribute to the photocurrent in a common way, that is, charge separation shown as paths A and B, respectively, in Figure 11. The photocurrent response of the composite film should have not exceeded the sum of the two components’ response. However, a mixed crystal effect could be expected along the anatase/rutile interface: because of the band match between the two crystals, the photogenerated electrons on the conduction band (CB) of anatase could be injected to the CB of rutile nearby and the photogenerated holes on the valence band (VB) of rutile injected to the VB of anatase, which thus improved the charge separation and hence an improved photocurrent response.21,39 The electrons injected to rutile preferred to migrate along the single-crystalline nanorods to the conductive Ti substrate, as illustrated as path C in Figure 11, because of the high electron-transfer rate, which in turn enhanced significantly the mixed crystal effect. As a result, the combined mixed crystal and mixed morphological effects of the present composite film resulted in a synergistic effect that led to a photocurrent response more than double the simple sum of the contributions arising from the two components alone. The composite film fabricated at present is not optimized, because the nearly single-crystalline rutile nanorods were not well-aligned. Further study on low-temperature growth of singlecrystalline rutile could possibly lead to a monolayer with nanorods growing vertically to the Ti substrate, with controllable sizes as well as interspaces among the rods. In addition, the two components that construct the novel composite film could be modified separately, on the basis of numerous reports that have appeared recently concerning the relationship between structure and properties of titania and other semiconductors concerned, especially those focused on extension of the responsible light wavelength.40,41 Thus, we believe such a composite film provides a novel approach to titania with ideal photoelectrochemical properties, which contribute to its applications in photovoltaics and photoelectrocatalysis. 4. Conclusions Quasi-aligned rutile nanorods with nearly single-crystalline structures were deposited on conductive Ti substrates at a low temperature of 80 °C for 12 h via a precursor derived by Ti-H2O2 interactions. Interspaces among the nanorods were then filled with sol-gel anatase nanoparticles to construct a composite film with thickness of ca. 180 nm. Under UV illumination, the composite film exhibited a steady-state pho-
tocurrent density more than double the simple sum of those generated by the two components alone. The anatase nanoparticles possessed better inherent photoelectrochemical properties than rutile, while the single-crystalline rutile nanorods exhibited a higher electron-transfer rate. Combination of the two components in such an appropriate way enhanced significantly the charge separation effect arising from the anatase/rutile couple, because the single-crystalline rutile nanorods embedded in the composite film provided channels for rapid transfer of photogenerated electrons to the conductive substrates under an applied bias potential, which hence combined efficiently the mixed crystal and mixed morphological effects. Acknowledgment. This work is supported by the National Natural Science Foundation of China (NSFC) under Project 50502029. References and Notes (1) Varghese, O. K.; Gong, D. W.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV. Mater. 2003, 15, 624. (2) Zubavichus, Y. V.; Slovokhotov, Y. L.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Gratzel, M.; Shklover, V. Chem. Mater. 2002, 14, 3556. (3) Wu, J. M.; Zhang, T. W.; Zeng, Y. W.; Hayakawa, S.; Tsuru, K.; Osaka, A. Langmuir 2005, 21, 6995. (4) Zanoni, M. V. B.; Sene, J. J.; Anderson, M. A. J. Photochem. Photobiol. A: Chem. 2003, 157, 55. (5) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weisso¨rtel, F.; Salbeck, J.; Spreitzer, H.; Gra¨tzel, M. Nature 1998, 395, 583. (6) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (7) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (8) Wu, J. M.; Huang, B.; Zeng, Y. H. Thin Solid Films 2006, 497, 292. (9) Wang, X. C.; Yu, J. C.; Yip, H. Y.; Wu, L.; Wong, P. K.; Lai, S. Y. Chem.sEur. J. 2005, 11, 2997. (10) Yi, G. R.; Moon, J. H.; Yang, S. M. Chem. Mater. 2001, 13, 2613. (11) Wen, B. M.; Liu, C. Y.; Liu, Y. J. Phys. Chem. B 2005, 109, 12372. (12) Wu, J. M.; Qi, B. J. Phys. Chem. C 2007, 111, 666. (13) Liu, Z. F.; Yamazaki, T.; Shen, Y. B.; Meng, D.; Kikuta, T.; Nakatani, N. J. Phys. Chem. C 2008, 112, 4545. (14) Shankar, K.; Tep, K. C.; Mor, G. K.; Grimes, C. A. J. Phys. D: Appl. Phys. 2006, 39, 2361. (15) Park, J. H.; Kim, S.; Bard, A. Nano Lett. 2006, 6, 24. (16) Zhang, Z. H.; Yuan, Y.; Fang, Y. J.; Liang, L. H.; Ding, H. C.; Shi, G. Y.; Jin, L. T. J. Electroanal. Chem. 2007, 610, 179. (17) Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A. Sol. Energy Mater. Sol. Cell 2006, 90, 2011. (18) Huynh, W. U.; Peng, X. G.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923. (19) Xiao, M. W.; Wang, L. S.; Wu, Y. D.; Huang, X. J.; Dang, Z. Nanotechnology 2008, 19, 015706. (20) Lo´pez-Luke, T. L.; Wolcott, A.; Xu, L. P.; Chen, S. W.; Wen, Z. H.; Li, J. H.; Rosa, E. D. L.; Zhang, J. Z. J. Phys. Chem. C 2008, 112, 1282. (21) Liu, Z. Y.; Zhang, X. T.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami, T.; Fujishima, A. Langmuir 2007, 23, 10916.
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