Heterostructural Composites of TiO2 Mesh−TiO2 Nanoparticles

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Heterostructural Composites of TiO2 Mesh
TiO2 Nanoparticles Photosensitized with CdS: A New Flexible Photoanode for Solar Cells York R. Smith and Vaidyanathan (Ravi) Subramanian* Chemical and Materials Engineering Department, University of Nevada, Reno, 1664 North Virginia Street, Reno, Nevada 89557, United States

bS Supporting Information ABSTRACT:

A flexible electrorde utilizing TiO2 nanotubes (T_NT) anodically grown on titanium mesh with TiO2 nanoparticles as an overlying layer for solar cell applications has been investigated. Deposition of cadmium sulfide nanocrystals (CdS) for visible light absorption was carried out through successive ionic layer adsorption and reaction (SILAR) method. The addition of a TiO2 nanoparticle layer (T_NT/T_NP) prepared via TiCl4 exposure and subsequent heat treatment on T_NT before CdS deposition was found to increase the deposition and utilization of CdS for the same number of SILAR cycles as compared to without TiO2 nanoparticles. The increased loading of CdS, as a result of T_NP deposition, enhanced visible light absorbance as well as the photoelectrochemical properties. Spectroscopic and photoelectrochemical studies indicate that the T_NP assisted improved dispersion of CdS results in at least 2-times higher photocurrent than did the T_NT/CdS system. Preliminary tests demonstrate the stable performance of the T_NT/T_NP/CdS photoanode in a prototype solar cell configuration.

’ INTRODUCTION Cadmium sulfide nanocrystals have received considerable attention recently due to their unique size-dependent ability to absorb solar radiation. Photoresponses of the CdS and other quantum dots in the visible light spectrum through size quantization effect have been reported.1
4 Researchers have examined ways to augment the performance of large bandgap semiconductors (e.g., TiO2, ZnO, SnO2) by synergistically coupling them with a variety of chalogenides as sensitizers for enhanced visible light absorption.5
8 Another avenue explored has been the use of organic dyes as sensitizers in dye-sensitized solar cells (DSSC) such as a ruthenium metal complex, which has demonstrated promising power conversion.9,10 Despite the high power conversion of DSSCs (11%), the advantage of using quantum dots for harvesting solar energy lies within the quantum confinement effect4,11,12 including the impact ionization, Auger recombination, r 2011 American Chemical Society

and midband effect.13,14 These effects are known to increase the exciton concentration and quantum yield. It is suggested that in a coupled semiconductor system, utilizing multiple excitation generation of charge carriers in quantum dots offers the opportunity for improving solar cell devices’ performance.15
18 A comparison between DSSC and QD solar cells is given by the recent work of Hodes.19 Within the past decade, titania nanotubes (T_NT) have become a popular material of interest due to their simple synthesis and highly improved ability to transport photogenerated charges as compared to their titania nanoparticle (T_NP) counterpart, which is attributed to the 1-D structure.20,21 Furthermore, a Received: October 25, 2010 Revised: February 15, 2011 Published: April 05, 2011 8376

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The Journal of Physical Chemistry C one-step versatile electrochemical anodization technique for the synthesis of T_NT provides an avenue to grow T_NT arrays on virtually any substrate22
25 and different geometries.26,27 These articles focused on the photocatalytic and photoelectrocatalytic activity of T_NTs. Although T_NT grown on Ti foils has shown promising results for photovoltaic applications, the use of metal foils has limitations (such as the opacity, inefficient use of Ti, and reduced flexibility) for device applications. To counter this, metal meshes provide an approach that allows for high flexibility, efficient Ti utilization, and transparency. Applications such as DSSC using T_NT grown on Ti mesh28 and Ti wires29 show promise as flexible photoanodes with power conversion efficiencies of 1.47% and 2.78%, respectively. Anodized T_NT arrays grown on metal foils or conducting glass substrates have been sensitized with CdS through a variety deposition of methods.29
37 The results presented in these works demonstrate improved photoelectrochemical performance of T_NT as well as increased visible light absorbance attributable to CdS. Additional treatments of T_NT sensitized with CdS such as cosensitization with CdSe30 or CdS/ZnO shell36 have also been explored to improve the photoactivity of T_NT
CdS systems. Efforts to increase the surface area of T_NT-based solar cells, by deposition of colloidal T_NP, is often practiced.10,38
42 The arguments underlying the effect of depositing T_NP onto T_NT and the potential for photoactivity improvement include increased surface area, light scattering, dye anchoring in DSSC, and increased charge transport. The combination of T_NT and T_NP as composites has shown to improve photoelectrochemical performance of T_NT alone38 as well in DSSC.39,42
44 However, the use of T_NT coupled with T_NP and CdS has yet to be fully explored. It is hypothesized that the (i) proposed T_NT/T_NP/CdS architecture will enhance available surface for the CdS to deposit, (ii) improved dispersion and deposition of CdS shall favor absorbace of a greater fraction of light that is incident to the same geometrical area, and (iii) such an architecture lends itself to enhanced light utilization and better photoelectrochemical reponses. The work presented here systematically examines the elements of the above hypothesis by using heterostructural T_NT grown on titanium mesh as a flexible underlying backbone, T_NPs as the intermediate overlying layer, and CdS as the visible light harvesting surface layer. The deposition of T_NP onto T_NT from a TiCl4 treatment demonstrates improved dispersion and better loading of CdS deposited via successive ionic layer adsorption and reaction (SILAR). The improved loading and dispersion of CdS results in increased photocurrent due to a greater visible light absorbance. Studies within this area are often limited to reporting the performance of the photoanode in a three-electrode configuration and not the actual performance of such anodes in an integrated device. An additional step further focused on building a preliminary device with the photonanode and its performance tested. The performance data indicate that the device is stable, but further optimization of the system is required.

’ EXPERIMENTAL METHODS Synthesis of T_NT/T_NP Composite. TiO2 nanotubes were synthesized similarly to our previous work.27 Titanium mesh (40 mesh, Alfa Aesar) was cut into strips and sonicated in acetone to remove any grease. The degreased strips were washed with DI water and partially immersed in a fluorinated solution of ethylene

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glycol and water. Nanotube growth was achieved through anodization at 40 V DC with a platinum counter electrode for 4 h with a ramp of 1 V/s. Post anodization, the samples were washed with DI water and sonicated for ∼1
3 s to remove any surface deposits. The nanotubes were then subject to heat treatment for 2 h at 350 °C with a ramp rate of 2 °C/min in air atmosphere. T_NP was deposited onto the T_NTs by immersing the samples in a 0.02 M TiCl4 in DI water for 1 h with constant stirring at room temperature. Nanoparticle growth was achieved by subsequent heat treatment at 450 °C for 2 h at 2 °C/min in air. CdS Nanocrystal Deposition via Successive Ionic Layer Adsorption and Reaction (SILAR) Method. Deposition of CdS nanocrystals onto T_NTs was carried out via the successive ionic layer adsorption and reaction (SILAR) method. The electrodes prepared by the aforementioned procedure were exposed to CdSO4 solution (0.1 M) for 10 min followed by washing with DI water. The samples were then immersed in Na2S solution (0.1 M) for 10 min and washed with DI water again to deposit nanocrystalline CdS. The SILAR process was repeated to an optimal amount of CdS loading. Characterization. Scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) analysis was performed by using a Hitachi S-4700 SEM. X-ray diffraction (XRD) analysis was performed using a Philips 12045 B/3 X-ray diffractometer with a scan rate of 0.6o/min. UV
vis diffuse reflectance (DRUV
vis) was characterized using a Shimadzu UV-2501PC spectrophotometer. Photoelectrochemical Measurements. All photoelectrochemical experiments were conducted in a quartz cell with Pt wire as the counter electrode and Ag/AgCl as the reference electrode using an approach followed in an earlier work.35 Photoelectrochemical measurements were monitored by a potentiostat (Autolab PGSTAT 302). A 500 W Newport Xenon lamp with a 0.5 M CuSO4 far UV cutoff filter illuminated the photocell with a light intensity of ∼90 mW cm
2. Incident photon to current conversion efficiency (IPCE) was measured in a two-electrode system with an Oriel Instruments Optical Power Meter and Newport 77250 monochromator. Device Assembly and Characterization. Device fabrication utilized Pt deposited onto FTO conducting glass via thermal decomposition (0.5 mM chloroplatinic acid in n-propanol) with parafilm as a separator, and a glass slide completed the sandwich construction. The assembly of a prototype device using the anode and testing its preliminary performance were carried out. The setup details are discussed in Results, section C.

’ RESULTS A. Surface and Optical Characterization. 1. Surface Characterization

1.1. Scanning Electron Microscope Analysis. Scanning electron micrographs of the photoanodes at various stages of synthesis are shown in Figure 1 and Supporting Information Figure S1. The figures contain images at identical magnification for T_NT, before and after TiCl4 treatment, and before and after CdS deposition. The T_NT arrays grow radialy outward around the Ti wire uniformly with tight compaction as shown in Figure 1a. Such a growth process leads to the formation of T_NT bundles that are interspaced with several fissures at irregular intervals, unlike T_NT grown on Ti foils where such fissures are absent. This can be attributed to the curvatureinduced stress generated as a result of the radial outward growth 8377

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Figure 1. SEM images of TiO2 nanotubes (T_NT) prepared by anodization of Ti mesh followed by heat treatment at 350 °C for 2 h in air. Parts (a)
(c) show representative images of T_NT under low and high magnification, indicating the overall growth details and the nanotube morphology.

Figure 2. XRD of the (a) T_NT, (b) T_NT/T_NP, (c) T_NT/CdS, and (d) T_NT/T_NP/CdS performed at a scan rate of 0.6 deg/min.

of the nanotubes on the underlying Ti backbone. A closer magnification (Figure 1b and c) reveals well-defined T_NT arrays with similar lengths of ∼12 μm, diameter of ∼120 nm, and wall thickness of ∼20 nm. Supporting Information Figure S1 shows a step-by-step comparison of the T_NT surface (S1 a,a0 ), changes to the T_NT surface after TiCl4 treatment and calcination (S1 b,b0 ), before CdS deposition (S1 c,c0 ), and after CdS deposition (S1 d,d0 ). It is noted that the untreated T_NT has smooth walls (S1 a,a0 ). The TiCl4 treatment with subsequent calcination results in transformation of the surface to what appears as small T_NPs homogeneously decorated on the T_NT surface with some tubes partially or totally filled with nanoparticles. Similar observations on the decorating of T_NT with T_NP via TiCl4 have been reported elsewhere.39 The deposition of CdS on T_NT by the

SILAR process results in the formation of distinct particles on the T_NT. Further treatment of T_NT/T_NP using cadmium and sulfide salt solutions using the SILAR approach leads to the formation of a much denser coating on the surface. On the basis of these images, it can be concluded that the T_NP and CdS nanoparticles are deposited within the tube as well as along the rims of the tube. 1.2. X-ray Diffraction Measurements. The XRD spectra of the samples are shown in Figure 2. The annealed samples demonstrate a distinct XRD pattern that confirms the formation of crystalline TiO2 with anatase phase indexed to a standard JCPDS card (PDF no. 21-1272, labeled “A”). One can also observe the signals from the underlying Ti (PDF no. 44-1294, labeled “T”). For the most part, the XRD reflections in T_NT and T_NT-T_NP are similar. The overlapping in the XRD of the 8378

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Figure 3. Energy dispersion spectroscopy (EDS) of (a) TiO2 nanotubes with CdS (T_NT/CdS) and (b) T_NT with TiCl4 treatment followed by deposition of CdS (T_NT/T_NP/CdS). Both samples were subject to 10 SILAR cycles for CdS deposition. Note: The spectrum is provided to show the elements that have been identified.

two samples with minor accentuations in the S/N ratio corresponding to anatase peaks is noted. This suggests that the heat treatment after TiCl4 deposition leads to the formation of a predominantly anatase phase of crystalline TiO2. The XRD analysis of the T_NT/T_NP/CdS was also performed and compared to T_NT/CdS as well as T_NT/T_NP. All relevant peaks are labeled in Figure 2. Alternative approaches reported in the literature point to the tracking CdS formation on ITO.45 We choose to examine the product of SILAR deposition on ITO to (i) ensure that XRD analysis can be performed with all of the available and deposited CdS (because the deposits from SILAR process may coat the interior of the T_NT or T_NT/ T_NP composite), (ii) visually confirm formation of a product after the SILAR process, and (iii) perform spectroscopy to determine the optical features of the deposited CdS without interference from the T_NT or T_NT/T_NP. Supporting Information Figure S2 shows the XRD of ITO and the deposits over the ITO. The reflections in Figure S2 characteristic to ITO have been subtracted from the CdS-ITO sample. The normalized XRD spectra for CdS deposited onto ITO show a characteristic peak at 2θ = 26.5°. It is well-known that CdS can exist in wurtzite as well as cubic phases with very similar d-spacing.46 The wurtzite phase has three prominent peaks at 2θ = 24.8°, 26.5°, and 28.2° [JCPDS file no. Hex 060314], while the cubic phase has one prominent peak at 2θ = 24.8° [JCPD file no. Hex 10-0454]. As a result, it is difficult to analyze the recorded XRD spectra and assign a particular unique phase, and the probability of observing reflections for both wurtzite and cubic phase of CdS is quite possible. Similar XRD reflections for CdS deposited onto ITO via SILAR method using CdCl2 and thiourea have been reported.47 Moreover, the characteristic reflection for the anatase phase of titania is 2θ = 25° (PDF no. 21-1272). Because many of these peaks almost overlap, it is difficult to determine the phase and size of CdS, and thus other spectroscopic characterization methods are required.

1.3. Energy Dispersion X-ray Analysis (EDX). EDX is used to identify the elements that are deposited on a surface and track the presence of possible impurities.48
51 EDX analysis was performed on the T_NT and T_NT/T_NP films after SILAR treatment. A large area as shown in Figure 3 was monitored in the analysis. Larger dispersion and greater deposition is noted when the SILAR process is used with T_NT/T_NP. This inference complements the results obtained from SEM analysis. Further, EDX analysis identifies the elements Ti, O, Cd, and S in the samples. This shows that the samples contain only the elements that were planned for deposition. Of particular interest is the clear identification of Cd and S. For both T_NT/CdS and T_NT/T_NP/CdS, the Cd:S ratio was found to be 1:1, indicating the formation of stoichiometric CdS. It is interesting to note a 17.4% (EDX analysis) increase in CdS loading with the addition of T_NP on the T_NT under otherwise similar conditions (same number of SILAR cycles). Thus, the elemental analysis confirms the formation of CdS deposits after the SILAR treatment and an increase in CdS deposition due to the addition of T_NP on the T_NT. While EDX is useful to identify deposited elements, it is to be noted that further rigorous analysis could be carried out to obtain compositional ratios. 2. Optical Characterization 2.1. Absorbance Measurements. Absorbance spectroscopy can be used to further confirm CdS formation as CdS has a characteristic optical signature. A slight change in coloration from colorless to yellowish orange is noted after 10 cycles of the SILAR process using ITO. Specifically, one can note the formation of a dense yellow layer due to successive deposition as shown in the picture in Figure 4a. To probe this further, absorbance measurements were conducted, and a representative series of spectra is shown in Figure 4b. Two characteristic features noted in the absorbance are (i) visible light absorbance starting at 550 nm and (ii) a peak at 415 nm (Figure 4b). Similar absorbance spectra of films prepared in this manner and featuring these 8379

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Figure 4. (a) Photograph comparing an anodized T_NT and T_NT after deposition of T_NP and CdS nanoparticles using the SILAR method. (b) The absorbance spectra of the CdS prepared by the SILAR process on indium tinoxide (ITO)-coated glass plate. (c) UV
vis absorbance spectra of (a) T_NT, (b) T_NT/CdS, (c) T_NT/T_NP, and (d) T_NT/T_NP/CdS obtained from the diffuse reflectance measurements. (d) Difference absorbance spectra of (a) T_NT/T_NP, (b) T_NT/CdS, and (c) T_NT/T_NP/CdS obtained from the data in (c).

characteristics have been reported in earlier works.3 This suggests that the material formed after the SILAR process on ITO is indeed CdS. In the SILAR process, the deposition of CdS on T_NT and T_NT/T_NP electrodes is achieved through the exposure to Cd2þ and S2
ions by successive immersion in CdSO4, water, Na2S, and water. After each cycle, a small, thin layer of CdS is deposited onto the TiO2 structure. Increasing the deposition cycles leads to the formation of new CdS as well as the growth of smaller CdS into larger CdS crystal aggregates. To track the deposition/growth of CdS on the TiO2 surface, diffuse reflectance UV
vis spectroscopy was used. It was found that successive SILAR cycles lead to increased visible light absorbance of the TiO2 electrode (results not presented), which is consistent with previous reports.35 From the absorbance (Figure 4c) and difference absorbance (Figure 4d), increased absorbance in the visible spectrum is noted for T_NT/T_NP/CdS over T_NT/CdS electrode. The increased absorbance is attributed to increased loading of CdS and complements the EDX analysis. It is also important to mention that the absorbance profile of CdS on

T_NT/CdS and T_NT/T_NP/CdS shown in Figure 4c mimics the profile shown in Figure 4b. This suggests that the interaction with ITO, T_NT, or T_NT/T_NP surfaces does not alter the CdS absorbance profile significantly. B. Photoelectrochemical Properties of the Films. Figure 5a shows the current voltage properties of the electrodes under a constant illumination of ∼90 mW/cm2 intensity. T_NT shows a photocurrent of 0.5 mA/cm2. The point of zero current or apparent flat-band potential is noted at
0.92 V (vs Ag/AgCl reference). The addition of an overlying T_NP layer leaves the photocurrent at magnitudes similar to T_NT with only a marginal increase in the range from
0.8 to
0.4 V and in an increase in fill factor. However, the presence of CdS directly on the T_NT has a significant benefit. An increase in the photocurrent from 0.5 to 1.5 mA/cm2 can be observed. This 3-fold increase can be attributed to the additional photocurrent generated by the visible light absorbing CdS overlying on the T_NT. A further increase from 1.5 mA/cm2 in T_NT/CdS to ∼2 mA/cm2 in T_NT/ T_NP/CdS can be noted, making this system the best result in this series. This increase can be attributed to improved 8380

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Figure 5. (a) i
V characteristics of (a) T_NT, (b) T_NT/T_NP, (c) T_NT/CdS, and (d) T_NT/T_NP/CdS obtained under photoillumination of the different electrodes using Ag/AgCl as a reference electrode and a Pt wire as a counter electrode. (b) XYY plot comparing photocurrent density (ISC) and photovoltage (VOC) responses of T_NT/ T_NP with varying CdS deposits (CdS deposits were varied by changing the number of SILAR cycles on the T_NT/T_NP samples).

distribution of the CdS on the T_NP and the effective separation of the photogenerated charges. It is also noteworthy to mention that the addition of CdS causes the apparent flat band potential to shift more negative. This effect is more prominent in the T_NT/T_NP/ CdS photoanode as shown in Figure 5a. The variations in the photocurrent and photovoltage responses with the number of SILAR cycles are given in the XYY plot shown in Figure 5b. In general, as the number of cycles increases, the photocurrent increases and peaks at ∼2.0 mA/cm2 at ∼10 cycles. The responses of the different anodes under discontinuous illumination are shown in Figure 6a. Such measurements are indicative of the reproducibility of photoresponses. All of the films show an instantaneous change in current upon illumination. The current retracts to original values almost instantaneously as well, once the illumination is switched off. This trend is repeated with every on
off cycle (labeled in Figure 6a). There is a difference in the magnitude of the change in photocurrent based on the type of anode used. The variations in the magnitudes of the photocurrent as noted from the ordinates of Figure 5a follow the trend T_NT/T_NP/CdS > T_NT/CdS > T_NT/T_NP > T_NT. This trend is similar to the one noted in Figure 5a. The reproducible responses to continuous on
off cycles also indicate that the anodes are stable under illumination and photoresponse occurs only upon illumination. The Voc or open circuit potential is a measure of the magnitude of charge built-up after photoillumination.52,53 This information is

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Figure 6. (a) Photocurrent density versus time of (a) T_NT, (b) T_NT/T_NP, (c) T_NT/CdS, and (d) T_NT/T_NP/CdS measured using a two-electrode system with Pt as the counter electrode. (b) Open circuit potential (VOC) measurements of (a) T_NT, (b) T_NT/T_NP, (c) T_NT/CdS, and (d) T_NT/T_NP/CdS.

shown in Figure 6b. Just as in the case of photocurrent responses, an instantaneous change in photovoltage is noted immediately on photoillumination. The peak voltage is reached almost immediately on illumination and remains at a steady value upon continuous illumination. However, once the illumination is turned off, a gradual decay of Voc is noted. This observation is noted with all of the samples. The Voc change follows the trend: T_NT/T_NP/CdS > T_NT/CdS > T_NT/T_NP > T_NT, which is similar to the photoanode performances noted with photocurrent. The similarities in the trends observed for photocurrent and Voc show that these responses are related. In this instance, it suggests that the higher the Voc, the higher is the photocurrent. Similar observations have also been noted in other related systems.3,54 After the stable responses to photoillumination were confirmed, the effectiveness of the material as photoanode by estimating the IPCE was examined (Figure 7).52,55 IPCE (incident photon conversion efficiency) is a measure of the number of electrons generated for every 100 photons absorbed by the photoactive material and is estimated using the equation:52 IPCE ð%Þ ¼

1240  isc ðA=cm2 Þ  100 Iinc ðW=cm2 Þ  λ ðnmÞ

ð1Þ

where isc is the short circuit current, Iinc is the incident light power, 8381

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Figure 7. Incident photon to current conversion efficiency (IPCE %) of (a) T_NT, (b) T_NT/T_NP, (c) T_NT/CdS, and (d) T_NT/ T_NP/CdS.

Scheme 1. A Prototype Solar Cell Designed To Test the Performance of the Flexible Photoanode: T_NT/T_NP/CdS

and λ is the wavelength of light. T_NT and T_NT/T_NP show an IPCE of 4% at 360 nm. The presence of CdS with T_NT and T_NT/T_NP increases the IPCE to 20% and 32% at 360 nm, respectively. These increases in the IPCE can be attributed to the improved light harvesting by the CdS, and the results complement the observations noted in the photoelectrochemical responses. Further, it is noteworthy to mention that the IPCE profile follows the absorbance spectra of CdS, where the maximum IPCE values of 27% for T_NT/CdS and 43% for T_NT/T_NP/ CdS are noted. These observations confirm that the CdS nanocrystals are instrumental in the enhancement of the solar-to-electric conversion. C. Preliminary Device Assembly and Characterization. The results pertaining to the performance of the photoanode under the conditions encountered by an actual photovoltaic device can be very useful to the designing of better PV systems. Therefore, in this segment, preliminary results indicate the activity of the photoanode in an integrated device environment is presented. It should be noted that the purpose of this section is to show that the materials fabricated earlier can be used as a photoanode in a solar cell, and further optimization of the assembly should be carried out to improve the device performance. There are several approaches to designing and fabricating an integrated device. One of the common classification is devices with two56,57 and three58,59 electrodes. The three-electrode architecture typically involves obtaining the photoelectrochemical

Figure 8. (a) i
V characteristics of a preliminary prototype solar cell under photoillumination at different orientations. The cell was held at an orientation normal to the incident light (0°) as well as at an angle (45°) to the incident light while performing i
V measurements. (b) The photoresponse of the prototype solar cell is stable as noted through several on
off cycles.

properties with respect to a reference (such as silver- or mercurybased electrodes). This work focuses on elucidating the performance of the photoanode in a two-electrode configuration. Scheme 1 shows the structure of the assembled device used to test the photoanode. The device is a simple prototype of a real solar cell and consists of an anode and a cathode with Na2S (0.1 M) as the electrolyte. The in-house fabricated T_NT-based materials (T_NT/T_NP/CdS) were tested by using the anode in the device. The cathode was prepared using thermal decomposition of a Pt source on FTO. Commercially available parafilm (two layers, each of 127 μm thickness, total 254 μm) was cut with a cavity of the size of 1 cm  1 cm and placed as a spacer between the anode and the cathode. The assembly was compacted using stationary clips as shown in the picture (Supporting Information Figure S3). The electrolyte was injected in the spacer cavity using a microliter syringe. The cell was allowed to stabilize for ∼10 min to let the electrolyte permeate uniformly and to check that there was no leak. Illumination of the device was performed using UV
vis light, and the photoelectrochemical responses were recorded by connecting the cell to a potentiostat. 8382

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The Journal of Physical Chemistry C Scheme 2. Deposition of CdS on T_NT (A) Improves Visible Light Absorbancea

a

The use of T_NP on T_NT (B) allows for increased dispersion of CdS, thus favoring improved visible light absorbance by the CdS and resulting in higher charge generation.

In an earlier study, one of the key benefits of using Ti wires for growing T_NT was identified as directional independence of photoactivity. Alternately, the T_NT grown on Ti wires can absorb light from all directions.27 To test if this were true using T_NT prepared over a mesh, the device with the T_NT-based photoanode was rotated over an axis by 45° from the path of illumination. The i
V profile of the assembled with responses at different orientations is shown in the Figure 8a. There was no significant change in the photoresponse as shown. This indicates the orientation-free aspect of the device. Such an architecture can provide immense flexibility in commercial applications in the form of a significantly reduced sensitivity to the solar movement and hence reduced or perhaps eliminating the need for sensitive tracking equipment. The reproducible nature of the preliminary device is evident from the multiple on
off cycles shown in Figure 8b. It is to be noted that the power conversion efficiency and fill factors are low as compared to other systems. The fill factor is a measure of how much power can be extracted from an assembled solar cell device. A low fill factor could mean a significant recombination at the electrode
electrolyte interface. Employing different electrolytes could improve the fill factor. Resistive losses in the device may also bring down the solar cell performance. These factors have to be studied further to improve the performance of the solar cell.

’ DISCUSSION The optical, surface, and photoelectrochemical properties of the photoanode discussed earlier indicate three important roles of the T_NP as a promoter of the solar-to-electric conversion capabilities of the T_NT and CdS. A detailed discussion of these aspects is presented below. 1. Enhancing CdS Deposition on T_NTs. The distribution of CdS nanocrystals on TiO2 nanoparticle-based films and the largely resulting improvements in photoelectrochemical performance through better charge separation have been studied, and

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the benefits are well documented.54,60,61 Here, we present a complete picture of the stage-wise improvement in the T_NT/ T_NP/CdS photoanode performance, and the roles of T_NPs are evident from the comparative analysis of the SEM, EDX, and absorbance measurements. These measurements highlight that T_NPs, as an intermediate layer between T_NT and the CdS, promote the dispersion of the CdS on the T_NT surface. The absorbance measurements provide evidence complementing the SEM images that T_NT/T_NP has more CdS deposits as compared to T_NT alone. In fact, a linear increase in absorbance after successive layered deposition is noted to be effective only in the presence of the T_NPs. Under conditions of illumination over a constant geometrical cross-section, greater dispersion of CdS is the basis for the improved photoelectrochemical properties as noted from Figures 5
7. The i
t, V
t, and IPCE measurements clearly indicate that the T_NP assisted in improved CdS dispersion translated effectively to enhancing the photoelectrochemical properties of the anode. Scheme 2 shows the role of the T_NP as a booster of the photoelectrochemical responses of T_NT and CdS. 2. Supporting CdS with Minimal Alteration of the CdS Optical Properties. Cations of Cd deposited initially on T_NT and/or T_NT/T_NP interact with sulfide anions to form nanoparticles of CdS through electrostatic charge neutralization during each SILAR cycle. A repetition of the SILAR cycles leads to more CdS formation and eventual densification of the CdS layer.62 A close comparison of the IPCE response and absorbance spectra of CdS shows an interesting feature. The absorbance profile of the CdS does not show any significant red-shift with the SILAR cycles. In related studies, it has often been observed that there is a red-shift in the absorbance with increasing deposition of CdS.63,64 Such red-shifts have been attributed to the size increase that accompanies multiple layers of deposits.65 In some cases, employing TiO2 nanoparticles have resulted in a red shift of 50
100 nm in the onset absorbance as well.66 The absence of such shifts in our samples suggests that the T_NT or the T_NT/T_NP does not cause significant changes to the optical property of the CdS. Further, it is well-known that CdS nanocrystals demonstrate size-dependent absorbance. Exploiting this property of CdS by preparing them in different sizes and employing an underlying layer consisting of T_NT/T_NP may be quite effective. For example, maintaining the optical integrity can play a critical role in enhancing absorbance by the process of coupling CdS of different sizes to further improve the broader absorbance of the solar spectrum. Subsequently, this can promote more efficient solar-to-electric conversion. A conceptual diagram of such a photoanode for a “rainbow solar cell” architecture was proposed by Kamat and co-workers in a recent article.67 3. Boosting Open Circuit Potential through Improved Charge Separation. The point of zero current in the i
V profile of a photoanode indicates the location of an apparent flat band potential.68,69 A negative shift in the zero current potential suggests better electron accumulation in the photoanode.3 It is noted from Figure 5 that the deposition of the T_NP causes no increase in photocurrent or shift in the flat band potential. Alternately, the deposition of CdS causes significant increases in the photocurrent but only a marginal negative shift in the zero current potential (Figure 5). This suggests that visible light assisted photogenerated electrons in CdS are rapidly transferred to the T_NT and do not accumulate in the photoanode but move to the base of the photoanode constituting anodic 8383

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photocurrent. However, with the T_NP/CdS deposits, a further increase in photocurrent as well as a large negative shift of ∼120 mV is noted as compared to the T_NT/CdS system. This shows that T_NP boosts the open circuit potential by retaining the CdS generated electrons, which also favors a higher photocurrent generation. This aspect may be important in applications such as a capacitor or in startup/shutdown sequences of portable systems wherein a high Voc is often required for activating the device. Role of Redox Couple as a Hole Scavenger To Promote Photoelectrochemical Properties. The mechanism of visible light assisted photoexcitation in T_NT/T_NP/CdS photoanode followed by a charge transport can be summarized by the following equations: vis light

CdS sf CdSðe
hÞ electron
hole separation

ð2Þ

CdSðeÞ þ TiO2 f CdS þ TiO2 ðeÞ electron transf er to TiO2 ð3Þ CdSðhÞ þ red f CdS þ ox hole transf er to redox couple ð4Þ The electrons transferred to the TiO2 are collected and generate anodic current. An essential part to maintain the stability of the CdS sensitizer is to effectively scavenge holes accumulated in the valence band of CdS. If hole accumulation becomes too great, CdS will begin to oxidize over a period of time. Thus, sacrificial electrolytes, such as sulfides, are used. Often used are polysulfide electrolytes (Na2S
S
NaOH), which have shown to have better performance than in other redox couples in a TiO2
CdS sandwich solar cell device.70 Changing the surface tension of the polysulfide electrolyte by the addition of methanol in an otherwise similar system has also been examined.71 The role of the electrolyte will be one of the deciding factors in determining how sustainable the photocurrent shall be when further improvements such as photocurrent stability and device scale-up are considered. A combination of large and small bandgap (CdS) semiconductors to form nanocomposites such as TiO2
CdS has been used in several applications. Photocatalytic production of hydrogen,72 photovoltaics,25 and environmental remediation73 are applications wherein TiO2-nanotube quantum dots have been beneficially used. While the nanocomposite developed here holds promise as a photoanode in PV applications, it may also be useful to improve the outcomes in any or all of the abovementioned areas. The possible multifunctional aspect of the T_NT/T_NP/CdS presented here thus cannot be understated. Further work is required to tune the trilayered composite based on the end application to maximize the performance of this architecture.

’ CONCLUSIONS A wet chemical approach to deposit TiO2 nanoparticles and CdS on a polycrystalline titanium dioxide film prepared by anodization of a Ti mesh is presented. The optical, electronic, and photoelectrochemical properties of the nanostructured trilayer composite have been examined. The role of T_NP as a promoter for (i) enhancing CdS deposition on T_NT, (ii) supporting CdS deposition with minimal alteration of its optical (absorbance) properties, and (iii) boosting open circuit potential

through improved charge separation has been highlighted. T_NPs help with improving absorbance by 42%, photocurrent by 33%, and photovoltage by 11%. A proof-of-application of the composite in the form of a photoanode in a two-electrode solar cell has been demonstrated.

’ ASSOCIATED CONTENT

bS

Supporting Information. SEM images of T_NT with T_NP and T_NP/CdS deposits, XRD of the CdS deposits on ITO, and a photograph of the assembled solar cell. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (775) 784-4686. Fax: (775) 327-5059. E-mail: ravisv@ unr.edu.

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