Disassembly, Reassembly, and Photoelectrochemistry of Etched TiO

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Disassembly, Reassembly, and Photoelectrochemistry of Etched TiO2 Nanotubes David R. Baker and Prashant V. Kamat* Radiation Laboratory and Departments of Chemistry and Biochemistry, and Chemical and Biomolecular Engineering, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: July 10, 2009; ReVised Manuscript ReceiVed: August 14, 2009

Etched TiO2 nanotubes are removed from the titanium foil substrate by sonication and are reassembled onto new electrodes for photovoltaic applications. CdS nanocrystallites were deposited on the restructured electrodes to compare their performance as quantum dot-sensitized solar cells to aligned nanotube electrodes. The sensitized photoresponses of the photoelectrochemical cell created from reassembled TiO2 nanotubes are very similar to aligned TiO2 nanotube arrays. Transient absorption spectroscopy of dispersed tubes indicates that electron transfer from excited CdS nanocrystallites into TiO2 nanotubes occurs at a rate of 2.0 × 1010 s-1. BET surface area analysis is investigated on etched nanotube powder without the need for weight approximation and was found to be 77.0 ( 2.9 m2/g. The importance of nanotube orientation and porosity on the electrode surface in stabilizing accumulated electrons in TiO2 nanotubes is elucidated from the open circuit voltage decay. Nanotube orientation was also seen to affect electron transport in photocurrent experiments. Introduction Nanostructure assemblies have been shown to be useful for designing next generation solar cells.1-6 One-dimensional TiO2 nanoarchitectures have stimulated the advancement of dyesensitized and semiconductor-sensitized solar cells.7-12 Directionality in these systems increases electron mobility and efficiency because the travel time in the TiO2 film is greatly reduced.13,14 The anodically etched TiO2 nanotube has been a key player in developing next generation solar cells. Recent studies from our laboratory9,15 and elsewhere16-19 have shown that etched nanotubes outperform the traditional TiO2 nanoparticulate films. However, there remain several issues that need to be addressed if the technology is to be effectively utilized for commercial application. Several advantages exist if nanotube arrays can be detached and reassembled on other conductive surfaces. The electrochemical etching of titanium films in a fluoride media produces TiO2 nanotube arrays on both sides of an electrode.20-23 Since these electrodes are opaque, only one side will be accessible for illumination if the electrode is directly employed in a solar cell. The electrode’s geometry inherently limits the production yield to half without modification of the incident light. Few efforts have been made to remove TiO2 nanotube arrays as freestanding membranes or bundled arrays.22,24 Removal of nanotubes from the titanium substrate allows one to use the full potential of the material as well as recycle the titanium metal and economize the production. Additionally, the detached tubes can be conveniently transferred onto transparent electrodes for spectroscopic studies or onto flexible conducting substrates for use on nonconventional surfaces. Compared to a film of TiO2 nanoparticles, nanotubes have been shown to provide directionality benefits in oriented arrays through improvements in electron mobility and separation of charges.13,14,25,26 A major issue that one might encounter during the detachment and reassembly of TiO2 nanotubes on another conducting surface is the loss of orientation. The overarching * Address correspondence to this author. E-mail: [email protected]. Web: http://www.nd.edu/∼pkamat.

question is: Can randomly oriented nanotubes still deliver the same beneficial effect of oriented arrays? In order to further assess the behavior of randomly structured nanotubes versus oriented nanotubes, we conducted a photoelectrochemical study by modifying each of these systems with CdS. Sensitization of TiO2 nanostructures with CdS nanocrystallites has been extensively studied.27-36 The deposition of TiO2 nanotubes on transparent substrates and its modification with CdS nanocrystallites allows us to estimate the charge injection rates using transient absorption spectroscopy. Detachment of nanotubes also allows for powder-based studies, such as Brunauer-EmmettTeller (BET) surface area analysis. Experimental Section Electrochemical Etching of Titanium Films. Nanotube films were prepared through the well-established electrochemical etching technique.20-22 First, titanium foil slides (0.8 × 4.0 cm) were sonicated in 2-propanol for 1 h to remove oils and other surface deposits. Foils were then washed with, and stored in, acetone. Electrochemical etching was conducted in a solution of 2 vol% H2O and 0.3 wt % NH4F in ethylene glycol. A potential of 60 V was applied for 1.5 h after an initial 1 V/s ramp. The electrolyte solution was held at room temperature for the duration of the etching process. Etched foils were washed with water, dried in an air stream, and annealed at 450 °C for 3 h in air after a 1 °C/min heating ramp. After the 3 h anneal, they were cooled at 1 °C/min to room temperature. Detachment of TiO2 Nanotubes. The nanotubes were removed from the titanium foil by sonication in water for 50 min using a Fischer Scientific FS30 sonicator. The titanium foils were removed from the solution of suspended nanotubes, and the solution was dried on a hot-plate. The dry nanotubes were then removed from the wall of the container and weighed. BET surface area analysis was conducted on the dried nanotubes with a Quantachrome Instruments Autosorb-1. Nanotubes were resuspended in methanol to a concentration of 1 mg/mL for redeposition onto new electrodes. Redeposition was conducted with electrophoretic deposition (EPD) and dropcasted onto optically transparent indium-doped tin oxide

10.1021/jp9065357 CCC: $40.75  2009 American Chemical Society Published on Web 09/17/2009

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electrodes (OTE). EPD films were deposited in a 4:1 solvent mixture of acetonitrile:methanol at 125 V/cm. Dropcast films were deposited to a concentration of 0.5 mg/cm2 so as to be similar to the aligned TiO2 films. Modification with CdS. Electrodes were sensitized with the successive ion layer absorption and reaction (SILAR) technique. The films were immersed in a 0.1 M CdSO4 solution for 5 min and then washed with water. The electrode was then immersed in a 0.1 M Na2S solution for 5 min and washed with water. This constituted one cycle. Electrodes were cycled 8 times and used as formed without annealing of the CdS. Characterization of Films. Photoelectrochemical tests were conducted using a Keithley 2601 source-meter unit. Incident light was provided at 100 mW/cm2 by a 300 W xenon lamp and aqueous CuSO4 310 nm cutoff filter. Working electrodes were placed in a three-armed electrochemical cell with a platinum gauze counter electrode in a 0.1 M Na2S redox couple solution degassed with nitrogen. Because a two-electrode configuration was used in all the experiments, the third arm of the cell was sealed from the atmosphere to prevent oxygen diffusion. Transient absorption spectroscopy (TA) was conducted using the instrumentation and arrangement described earlier.37 Excitation of the sample was carried out using a Clark MXR-2010 Ti:Sapphire laser system capable of delivering 775 nm laser pulses of 1 mJ/pulse (width of 150 fs) at a rate of 1 kHz. A fraction (5%) of the beam was used to generate a probe pulse. The second harmonic (387 nm) was used for excitation of the sample.

Baker and Kamat

Figure 1. Illustrations of (A) aligned and (B) random TiO2 nanotubes. SEM images of (C) aligned and (D) random nanotubes. Random nanotubes were dropcasted onto a carbon paper.

Results and Discussion Detachment of TiO2 Nanotubes from Titanium Surface. Sonication of electrochemically etched titanium electrodes was carried out in water. Extended sonication (>10 min) in the same environment results in nanotubes detaching from the titanium foil. The amount of time the foils are exposed to sonication determines their dispersibility in the medium. Initially, the nanotube bundles are removed from the surface. Further sonication causes dispersion as individual nanotubes. These suspended nanotubes can be redeposited onto any desired electrodes (carbon paper, OTE, or Ti foil). The primary difference between the two electrodes in Figures 1A and 1B is the orientation of the TiO2 nanotubes. Figures 1C and 1D show SEM images of TiO2 nanotubes as prepared by anodization of titanium film and after their reassembly on a carbon paper electrode from solution. The parent TiO2 nanotubes anchored on titanium show an ordered array of tubes of diameter of ∼80 nm. The electrodes prepared from the dismantled TiO2 nanotubes show a random assembly of similarly sized nanotubes. This process of removal and restructuring of TiO2 nanotubes eliminates the fragility of detached oriented membranes and allows for application on more flexible electrodes such as polymers or aluminum foil. Also, solution-based nanotubes allow a wide range of spectroscopic experiments to be conducted on transparent electrode surfaces. After sonication, the titanium foils were imaged to determine what remained on the surface. It was seen that two distinct regions occupied the surface, schematically shown in Figure 2A and physically visible in the SEM image (Figure 2C). One area is a smooth film of TiO2 covering the titanium metal while the other region consists of small deposits of TiO2 in the shape of the original nanotubes, but these deposits are only slightly protruding above the surface. These “footprints” are assumed to be the base of the nanotubes which have been

Figure 2. Illustrations of (A) postsonicated and (B) cutaway of reetched titanium electrodes. (C) SEM image of postsonicated titanium foil shows the two distinct regions of “footprints” and smooth TiO2 barrier layer. (D) The re-etched foil shows how the footprint region can etch to form more nanotubes while the barrier layer etches evenly.

removed. The smooth region is thought to be the barrier layer between the nanotube array and the titanium metal. These foils were re-etched following the same procedure described above, and the two areas etched differently, shown schematically in Figure 2B and in an SEM image in Figure 2D. One area formed nanotubes while the other remained smooth. The footprint, therefore, acts as a template to generate the next set of nanotube arrays by electrochemical etching. The same piece of titanium metal can thereby be reused to harvest a large quantity of TiO2 nanotubes. It is postulated that the footprints change the applied electric field at the surface to be similar to the original etching process, allowing for further etching into the foil. The thickness of the barrier layer prevents new pits from forming in the smooth regions, preventing new tubes from forming, resulting in the region etching at the same rate, and retaining the original smoothness.

Etched TiO2 Nanotubes Removing the nanotubes allows for a detailed characterization independent of the titanium substrate. For example, one can obtain accurate measurement of BET surface area of pristine TiO2 nanotubes. Previously, Chen and co-workers reported the surface area for etched TiO2 nanotubes to be 285 m2/g.38 Because accurate determination of TiO2 nanotube weight is difficult when bound to the titanium substrate, one would expect significant error in the estimation of surface area. Removing nanotubes from the foil allows BET analysis to be done on a powder with exact measurements for the TiO2 weight. In the present study we obtained ∼0.3 mg/cm2 of TiO2 nanotubes from the initial etching of titanium foil (dimension 0.8 cm × 3 cm). Analysis showed that the surface area of the nanotube powder is 77.0 ( 2.9 m2/g. A back-of-the-envelope calculation for the surface area of TiO2 nanotubes, assuming smooth walls, 8 µm length, 100 nm outer diameter, and 70 nm inner diameter, yields 34 m2/g. This value matches the powder experiments within 1 order of magnitude, implying nanotubes are not as rough as previously thought. Redeposition of Nanotubes on Electrode Surface and Photoelectrochemical Evaluation. Two commonly employed methods, electrophoretic deposition (EPD) and dropcasting, were considered to reassemble the TiO2 nanotubes on titanium or optically transparent electrodes (OTE). EPD was rather cumbersome to perform in one step because of the settling of nanotubes during the application of electric field. In order to deposit the entire contents of the solution on the electrode surface, the electric field had to be turned on for a brief time (30-60 s) followed by stirring of the solution and reapplication of the electric field. This on-off cycle and stirring procedure needed to be followed several times until all the nanotubes from suspension were deposited on the electrode surface. EPD of TiO2 was possible because of surface hydroxyl groups, which provide a negative surface charge and render the movement of nanotubes to the positive electrode. The dropcast method worked well for redeposition as long as the amount that dropped at a time remained small (∼20 µL/ cm2). The film was air-dried between intervals and finally annealed in air at 400 °C. The electrodes prepared by EPD and dropcasting methods were evaluated for their photoelectrochemical performance. No noticeable difference in their overall performance could be seen, suggesting there was no preferential alignment with EPD. Also, neither technique yielded a more even nanotube distribution than the other. Films deposited on titanium electrodes by the dropcast method were chosen for modification with CdS nanocrystallites using the SILAR method described above. Surface Modification with CdS and Charge Injection Process. Films of TiO2 nanotubes on OTE and other electrodes were modified with CdS nanocrystallites using the SILAR (sequential ionic layer adsorption and reaction) process using the method described earlier.15 Because the SILAR method was not quite effective at depositing CdS on OTE we employed a colloidal CdS drop cast method to obtain a reference. This reference electrode was useful to compare the absorption and spectroscopic properties of pristine CdS nanocrystalline film with that of TiO2-CdS electrode. The absorption spectra of CdS nanoparticles prepared in the presence and absence of TiO2 nanotubes are shown in Figure 3. The onset of absorption of these nanoparticles occurs at 520 nm, confirming the formation of CdS with a bandgap of approximately 2.4 eV. Figure 4 shows an SEM image of restructured TiO2 nanotubes before and after modification with CdS nanoparticles. Clusters of average diameter ∼10 nm,

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Figure 3. Absorbance spectra of films made from colloidal (a) CdS and (b) redeposited TiO2 nanotubes coated with CdS.

Figure 4. SEM images of redeposited nanotubes before (A) and after (B) CdS coverage. CdS aggregates can be seen in (B) to be ∼10 nm in diameter and are evenly distributed over the surface of the nanotubes.

previously determined to be aggregates of CdS particles, covered the TiO2 nanotube evenly. Figures 5A and 5B show time-resolved transient absorption spectra recorded following 387 nm laser pulse excitation of CdS in the absence and presence of TiO2 nanotubes deposited as a thin film on an OTE, respectively. It has been shown earlier that metal chalcogenide nanoparticles undergo degradation on the TiO2 surface when exposed to light in the presence of air.39 In order to minimize the photodegradation during laser excitation, the transient absorption measurements were conducted under vacuum. When excited at 387 nm, the TiO2-CdS and CdS films showed a bleaching in the 480-540 nm region. The transient bleaching is broad because of polydispersity of CdS particles formed in solution. The presence of a TiO2 surface provides better control of particle size, as seen in the narrow band of bleaching of Figure 5A. With increasing time the bleaching recovers as the separated charges disappear either via recombination or by electron transfer to TiO2 (reactions 1 and 2).

CdS + hν f CdS(h+ + e-) f CdS

(1)

CdS(h+ + e-) + TiO2 f CdS(h+) + TiO2(e-)

(2)

Figure 5C compares the bleaching recovery recorded following 387 nm laser pulse excitation of TiO2-CdS and CdS films at bleaching maximum. The analysis of transient bleaching recovery in elucidating the kinetics of the charge injection process is described in our previous studies.9,40 The recovery is multiexponential and was analyzed with biexponential kinetics (eq 3). Fitted values are tabulated in Table 1.

y ) y0 + A1e-t/τ1 + A2e-t/τ2

(3)

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Figure 5. Time-resolved transient absorption spectra of (A) randomly oriented TiO2 nanotube film on OTE coated with CdS, and (B) a film of colloidal CdS dropcast onto OTE. (C) The bleaching recovery normalized to peak response.

Figure 6. Photocurrent efficiency (IPCE) of (A) TiO2-CdS electrodes with (a) aligned and (b) random geometries. (B) White light response (λ > 310 nm) of aligned versus random nanotubes, (a, b) with and (c, d) without CdS.

The bleaching recovers faster in the TiO2-CdS sample (τ1 ) 31.0 ps and τ2) 2.9 ps) than CdS alone (τ1 ) 81.0 ps and τ2) 7.3 ps). The faster recovery in the TiO2-CdS system arises as the electron transfer to TiO2 dominates the deactivation of excited CdS. If electron transfer is assumed to be the dominant pathway responsible for the faster bleaching recovery of CdS on TiO2 surface, we can estimate the electron transfer rate using eq 4.

kET ) 1/τTiO2-CdS - 1/τCdS

with CdS using the SILAR deposition method and employed as anodes in a photoelectrochemical cell with a platinum counter electrode and 0.1 M Na2S as redox electrolyte. The photocurrent action spectra of the two electrodes are shown in Figure 6A. The incident photon conversion efficiency (IPCE) was recorded by monitoring the short circuit photocurrent (ISC) at different excitation wavelengths (λ), using eq 5.

IPCE (%) )

(4)

If we substitute slow and fast recovery lifetimes in eq 4, we obtain upper and lower limits for the electron injection rate constants. The electron transfer rate constants obtained from these results were 2.0 × 1011 and 2.1 × 1010 s-1, thus confirming the ultrafast nature of electron transfer. As shown previously, the heterogeneity of the surface and the varying degree of interaction between the two particles introduce different time constants with which electron transfer proceeds. Photoelectrochemistry. Photoelectrochemical behavior of films constructed of redeposited TiO2 nanotubes was compared with as-formed arrays on titanium films. Because the aligned nanotube arrays are anchored on titanium metal substrate, the TiO2 nanotubes in solution were redeposited onto a clean titanium foil to account for any reflection and other substrate properties of the titanium foil. These electrodes were sensitized

1240 × ISC(A) × 100% λ(nm) × Pinc(W)

Pinc is the incident light power at wavelength λ. Both electrodes show similar response with a photocurrent onset just below 550 nm. This photoresponse matches well with the absorption spectra in Figure 3, confirming the origin of the photocurrent generation to be the excitation of CdS. The reproducibility of the photocurrent generation of the two TiO2-CdS electrodes to the illumination with visible light is shown in on/off cycles of Figure 6B. The initial photocurrent response of the reassembled (random) TiO2-CdS nanotube cell is greater than the aligned TiO2-CdS nanotube electrode. However, a quick drop in the photocurrent within the first few seconds shows the redox couple reacting at the surface of the nanotubes after which mass transfer limitations within the TiO2-CdS film sharply reduce the current. A separate trend of

TABLE 1: Fitted Biexponential Values for CdS and TiO2-CdS Films on OTE of Time-Resolved Transient Absorption Bleaching Recovery CdS TiO2-CdS

(5)

y0

A1

τ1 (ps)

A2

τ2 (ps)

-0.15 -0.062

-0.89 ( 0.03 -0.88 ( 0.02

81.47 ( 17.9 31.45 ( 2.84

-0.11 ( 0.03 -0.12 ( 0.02

7.30 ( 0.81 2.93 ( 0.24

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Figure 7. Open circuit potential response to illumination (ON-OFF) (A) comparing TiO2-CdS electrodes made with (a) aligned and (b) random nanotubes. White light was turned off at 60 s. (B) Electron lifetimes versus open circuit potential were calculated using eq 6 for the two electrodes and plotted.

slow photocurrent growth was seen for aligned TiO2-CdS nanotubes with a significantly smaller initial current spike. Both these electrodes maintain a steady state photocurrent in about 15 s. The stabilized photocurrent observed for the aligned TiO2-CdS was ∼1 mA/cm2 as compared to the ∼0.8 mA/cm2 from random TiO2 nanotubes redeposited on a titanium electrode. The photocurrents of the same cells without CdS are shown as traces c and d in Figure 6B. The bare aligned nanotubes provide ∼4 times more photocurrent than random nanotubes. After CdS deposition the rise in photocurrent is nearly the same for both electrodes implying random nanotubes transport electrons from the sensitizer as effectively as aligned nanotubes. The photoelectrochemical behavior was further assessed by monitoring the cell potential response to on-off cycles of visible light illumination (Figure 7A). While the rise in potential following the illumination is similar in both cases, the decay of the potential upon turning the light off differed. Because the photopotential directly represents electron accumulation within the electrode system, the decay of the potential upon stopping the illumination represents the removal of electrons in charge recombination and/or charge transfer across the semiconductor interface from redox couple back-reaction or oxygen scavenging. We analyzed the open circuit potential (VOC) decay using eq 6:41-43

( )

kBT dVOC τe ) e dt

to remain high implies that this technique would be greatly beneficial for commercial applications of etched nanotubes. Conclusions Sonication of TiO2 nanotube arrays prepared by anodization of titanium films results in the disassembly of the nanotubes, providing a means to continuously synthesize TiO2 nanotubes from the same titanium sheet. In powder form, detached nanotubes provide more accurate measurements for BET surface area. These TiO2 nanotubes, when reassembled on conducting glass or titanium electrodes, provide a means to anchor CdS nanocrystallites by the SILAR method. The photoelectrochemical behavior of these reassembled TiO2-CdS electrodes show a slight decrease in photocurrent response as compared to oriented TiO2-CdS electrodes, while photopotential increases slightly under similar conditions. The benefits of the onedimensional TiO2 nanostructure can still be retained even in a randomly oriented system, creating opportunities for a multitude of new substrates to be used. Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is contribution number NDRL 4816 from the Notre Dame Radiation Laboratory. References and Notes

-1

(6)

The dependence of electron lifetime (τe) of TiO2-CdS electrodes on the VOC is shown in Figure 7B. The randomly distributed TiO2-CdS system shows shorter lifetimes as compared to oriented TiO2-CdS system. Faster decay kinetics observed with reassembled tubes is attributed to two factors: first, the barrier layer of TiO2, which lies between the titanium foil and the oriented arrays, inhibits electron transfer. The random nanotube structures do not have a barrier layer and, therefore, transfer electrons directly to titanium substrate. The second kinetic factor is that randomly oriented nanotubes have a higher intertube porosity, compared to close-packed aligned nanotubes, providing additional space the electrolyte solution can occupy. Increased porosity allows for faster mass transfer of the redox couple, which explains the initial photocurrent response and VOC scavenging lifetimes. Restructuring the nanotubes onto new substrates does not significantly reduce the solar cell’s performance. In fact, the ability of the photocurrent

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