TiO2-Coated Ultrathin SnO2 Nanosheets Used as Photoanodes for

Article pubs.acs.org/IECR

TiO2-Coated Ultrathin SnO2 Nanosheets Used as Photoanodes for Dye-Sensitized Solar Cells with High Efficiency Jun Xing,†,‡ Wen Qi Fang,†,‡ Zhen Li,§ and Hua Gui Yang*,† †

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science & Technology, Shanghai 200237, People's Republic of China § ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Queensland 4072, Australia S Supporting Information *

ABSTRACT: Ultrathin SnO2 nanosheets were prepared by a hydrothermal method using SnF2 and methenamine as precursor and morphology controlling agent, respectively. Structural characterizations indicate that these ultrathin SnO2 nanosheets having a thickness of approximately 4−6 nm can assemble into a three-dimensional, flowerlike architecture. Due to the higher electron mobility and enhanced light-scattering effect of these hierarchical structures, the dye-sensitized solar cells (DSSCs) based on such SnO2 architectures exhibit much higher cell performance than that of SnO2 nanoparticles. Furthermore, coating a TiO2 layer on these ultrathin SnO2 nanosheets can also significantly improve the short-circuit current, open-circuit voltage, and fill factor. Compared with the plain SnO2 nanosheets, the TiO2 coating on these ultrathin SnO2 nanosheets can lead to more than 7 times improvement in the energy conversion efficiency. With a thin layer of TiO2 coating, the highest overall photoconversion efficiency of DSSCs based on SnO2 nanosheets is approximately 2.82%, which is over 2 times higher than that of DSSCs constructed by conversional SnO2 nanoparticles.

1. INTRODUCTION Over the past decades, two-dimensional (2D) anisotropic nanostructures of semiconductor materials including various metal oxides have attracted great research interest due to their promising properties. The unusual properties are usually determined by exceptionally small thickness of 2D nanostructures because, in such quantum wells, electrons and holes are spatially confined within a finite thickness and can only move in 2D space. Due to these intrinsic geometrical and electronic characteristics, ultrathin 2D nanomaterials have many potential applications in solar cells, catalysis, and electrochemical devices.1−3 Thus shape controlling of semiconductor nanocrystals demonstrated science and technology importance, and ultrathin 2D nanostructures of many functional metal oxides such as TiO2, ZnO, CoO, MnO2, and CuO2 etc. have been achieved.4−7 Dye-sensitized solar cells (DSSCs) have drawn intense attention of both academic and industrial fields for decades owing to their high-efficiency and potential for manufacture at a low-cost.8,9 Although an energy conversion efficiency of 12.3% has been achieved by using TiO2 films as the electrontransporting electrode, researchers are still seeking to find alternative binary or ternary metal oxides such as SnO2, ZnO, Nb2O5, and Zn2SnO4, that may offer additional advantages in terms of device fabrication and cell performance.10−13 SnO2 particularly has been deemed as one of the most promising candidates due to its comparatively larger band gap (3.6 eV), which leads to a low sensitivity of UV degradation and hence improvement of the long-term stability of DSSCs, and faster charge transport dynamics (μe, 100−300 cm2 V−1 S−1) than that of anatase TiO2 (μe < 1.0 cm2 V−1 S−1).14−17 However, due to its higher recombination losses to either dye cations or © 2012 American Chemical Society

oxidized redox species, SnO2-based DSSCs have been developed with less success so far. The main factor limiting the performance of SnO2 photoelectrodes is their positive conduction band potential (∼400 mV higher than that of nanocrystalline TiO2), which leads to a lower attainable opencircuit voltage (Voc) and a higher charge recombination loss of injected electrons with the redox electrolyte. To solve these issues, coating a thin layer of TiO2 is often employed to improve the efficiency of SnO2-based DSSCs, which makes use of the advantages of both the TiO2 and SnO2.10,18−20 To our knowledge, the highest overall photoconversion efficiency of SnO2-DSSCs has been achieved to 6.4% with a TiO2 coated hierarchical SnO2 octahedra as photoelectrodes.20 To achieve large current density and high photon to current conversion efficiency, a photoelectrode for DSSCs requires not only a high surface area for sufficient dye anchoring but also a high light-harvest capability. Several approaches were explored to satisfy these conflicting requirements. As an example, a second layer of larger semiconductor particles with a diameter of 200−400 nm was often used to scatter the transmitted light back for enhancing the light harvest. Moreover, hierarchically structured oxide materials including ZnO aggregates, multilayered SnO2 hollow microspheres, and mesoporous anatase TiO2 beads have been successfully used as light scattering layers without sacrificing the internal surface area needed for effective dye-uptake.10,21−23 Recently, sub-micrometer-sized three-dimensional (3D) self-assembled architectures with sheetlike Received: Revised: Accepted: Published: 4247

January 3, 2012 February 26, 2012 February 28, 2012 February 28, 2012 dx.doi.org/10.1021/ie2030823 | Ind. Eng. Chem. Res. 2012, 51, 4247−4253

Industrial & Engineering Chemistry Research

Article

mixture of acetonitrile and valeronitrile (volume ratio, 85:15) was injected into the cell. 2.3. Characterization. The crystal structure was determined by X-ray diffraction (XRD, Bruker D8 advanced diffractometer with Cu Kα radiation). Scanning electron microscopy with energy-dispersive X-ray spectroscopy (FESEM, HITACHI S4800) and transmission electron microscopy (TEM, JEM 2100F) were used to examine the structure of the samples. The amount of adsorbed dye was measured by desorbing the dye into 0.1 mM NaOH solution and by absorption measurement of the solution using the adsorption peak intensity of N719 at 530 nm by using a Cary 500 spectrophotometer. The current−voltage tests of DSSCs were performed under one sun condition using a solar light simulator (Oriel, 91160, AM 1.5 globe). The power of the simulated light was calibrated to 100 mW·cm−2 using a Newport Oriel PV reference cell system (model 91150 V). The IPCE test system consisted of a model SR830 DSP lock-in amplifer and model SR540 optical chopper (Stanford Research Systems, Sunnyvale, CA, USA), a 7IL/PX 150 xenon lamp and power supply, and a 7ISW301 spectrometer. The EIS measurements were carried out with an impedance analyzer (Parstat 2273, Princeton) in darkness at a bias potential of −0.65 V and 10 mV of amplitude over the frequency range of 0.1 Hz to 100 kHz. The diffuse-reflectance spectra were measured by using a Cary 500 spectrophotometer.

anatase TiO2 and ZnO have been used as photoanode materials in DSSCs, which show an improvement in efficiency due to their superior light scattering effect and excellent light reflecting ability of the mirrorlike plane facets.24−26 However, to the best of our knowledge, the synthesis and application of hierarchical ultrathin SnO2 nanosheets architecture in DSSCs have been scarcely reported.27−30 In this work, we report a facile method to fabricate hierarchical ultrathin SnO2 nanosheets (SnO2 NSs) with a thickness approximately 4−6 nm and lateral dimension up to sub-micrometers. The ultrathin SnO2 NSs can significantly improve the photoconversion efficiency of SnO2-DSSCs, especially when a TiO2 shell was coated on the ultrathin SnO2 NSs (SnO2 NSs−TiO2). The high crystallinity and 3D hierarchical structure of the SnO2 NSs−TiO2 offer higher electron mobility and enhanced light scattering, resulting in a 231% higher efficiency than that of SnO2 nanoparticle photoanodes. To our knowledge, ultrathin SnO2 NSs, for the first time, were used as photoanodes for DSSCs with high efficiency.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. The ultrathin SnO2 NSs were synthesized by a hydrothermal method. In a typical experiment, 30 mL of as-prepared tin difluoride (SnF2) aqueous solution (0.01 M) was added in 0.035 g of methenamine with stirring continuously. The obtained white turbid suspension was stirred for 30 min before being transferred to a Teflon-lined stainless steel autoclave and then heated in an electric oven at 180 °C for 18 h. A brown product was harvested after centrifugation and dried at 60 °C overnight. For comparison, the SnO 2 nanoparticles (SnO2 NPs) were prepared by adding 0.376 g of SnF2 and 0.08 g of carbon spheres (fabricated by a simple hydrothermal method31) into 30 mL of deionized water under continuous stirring. Then the mixtures were kept in a Teflonlined autoclave under 180 °C for 24 h. Finally, the products were harvested by a centrifugation method and washed with ethanol three times, followed by calcination at 500 °C for 4 h. 2.2. DSSCs Assemble. For the preparation of the SnO2 electrodes with SnO2 NSs and SnO2 NPs, the viscous pastes were prepared by the following procedure. To an ethanol suspension containing 0.5 g of SnO2 NSs or SnO2 NPs, the mixture of ethyl cellulose (0.26 g) and terpineol (2.03 g) was added. The solvent was then evaporated at 45 °C to obtain the viscous paste. The prepared SnO2 paste was spread uniformly on the surface of the FTO glass using the screen-printing technique. Coating of TiO2 on SnO2 electrodes was performed by dipping the SnO2 electrodes into a 40 mM TiCl4 aqueous solution for 30 min at 70 °C until the TiCl4 was hydrolyzed, then washing the electrodes with deionized water to remove residual TiCl4, and then again sintering at 500 °C in air for 30 min. Finally, these SnO2−TiO2 electrodes were soaked into 0.5 mM N719 dye solution in a mixture of acetonitrile and butyl alcohol (volume ratio, 1:1) for 24 h at room temperature and then washed with ethanol and dried in air. The dye-sensitized SnO2 electrode and a thermal decomposition Pt counter electrode were assembled into a sandwich type cell and sealed with a hot-melt gasket of 25 μm thickness made of the ionomer Surlyn 1702 (DuPont). The size of the SnO2 film used was 0.25 cm2 (5 mm × 5 mm). A drop of the redox electrolyte (0.6 M 1butyl-3-methylimidazolium iodide (BMII), 0.03 M I2, 0.10 M guanidinium thiocyanate, and 0.5 M 4-tert-butylpyridine in a

3. RESULTS AND DISCUSSION 3.1. XRD, SEM, and TEM Studies. The crystallographic structure of the as-synthesized SnO2 NSs was inspected by XRD (see Figure S1 of the Supporting Information). All the diffraction peaks can be indexed to a tetragonal rutile-like SnO2 (JCPDS card No. 41-1445, P42/mnm, a = 4.738 Å, and c = 3.1865 Å) without any detectable impurity. Figure 1a displays a

Figure 1. (a, inset) FESEM images, (b, c) TEM images, and (d) HRTEM image of the as-prepared ultrathin SnO2 NSs.

typical field-emission scanning electron microscope (FESEM) image of an as-synthesized sample. Interestingly, the SnO2 NSs assembled into a three-dimensional, flowerlike architecture. TEM images provide insight into the inherent structure of the ultrathin SnO2 NSs. Parts b and c of Figure 1 show the formation of the nanosheets with lateral dimensions up to hundreds of nanometers. These nanosheets generally are flat 4248

dx.doi.org/10.1021/ie2030823 | Ind. Eng. Chem. Res. 2012, 51, 4247−4253


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Figure 2. Cross-section SEM images of the sintered SnO2 NSs (a) and SnO2 NPs (b) films. Insets are the corresponding enlarged images of these films. (The scale bars in insets of a and b are 1 μm and 500 nm, respectively.) (c) I−V characteristics and (d) IPCE spectra of DSSCs with photoelectrode films composed of SnO2 NSs, SnO2 NSs−TiO2, SnO2 NPs, and SnO2 NPs−TiO2, respectively.

with step edges, and some of them appeared to be self-folded (see Figure 1c), indicating that the nanosheets formed in the solution rather than on the substrate during the process of sample synthesis. This scrolled nature of this sort of structure can be ascribed to the minimization of surface energy. The thickness of the ultrathin nanosheets was measured to be 4−6 nm (see Figure 1c and Figure S2 of the Supporting Information). A thickness of approximately 8.6 nm was also found during the measurements, which corresponds to the thickness of a bilayer nanosheet. A high resolution TEM (HRTEM) image taken from the top face of a single nanosheet shows the lattice fringes were perfectly aligned across the entire surface (see Figure 1d), of which the interfringe distances are 0.264 and 0.335 nm on average, closing to the (101) and (110) lattice spacing of the tetragonal phase of SnO2 crystal. Also, the separation angle between them is measured as 66.8°, which is consistent with theoretical angle (66.7°) of these planes. 3.2. Photovoltaic Performance. To obtain an insight into the photovoltaic performance of SnO2 NSs, we compared four kinds of nanoporous photoelectrodes: a standard SnO2 NPs photoelectrode, SnO2 NSs photoelectrode, and their corresponding TiO2 coated photoelectrodes (denoted as SnO2 NPs−TiO2 and SnO2 NSs−TiO2 (see Figure S3 of the Supporting Information)). The DSSCs are a very complicated system that requires electrodes to be similar in order to achieve a meaningful comparison. Thus, all electrodes for DSSCs tests were prepared by screen printing a thin SnO2 film on fluorinedoped tin oxide (FTO) glass and sintered at the same batch. After the TiO2 coating, both plain and TiO2-coated electrodes were sintered a second time together. The thickness of electrodes was controlled by the screen printing times, showing a similar value of about 4 μm (see Figure 2a,b). The inset in Figure 2a shows that the flowerlike architecture of the SnO2 NSs can be well-maintained after 2 times of high-temperature calcination. The average particle size of the benchmark SnO2

NPs are around 30 nm (shown in Figure 2b). All photoelectrodes were tested under an illumination of one sun condition (AM 1.5 globe, 100 mW·cm−2). The characteristic current−voltage (I−V) curves and corresponding incident photon to current conversion efficiency (IPCE) spectra of these DSSCs are given in Figure 2c,d, and their detail device photovoltaic parameters derived from the I−V curves are summarized in Table 1. Table 1. Comparison of Short-Circuit Photocurrent Density (Jsc), Open-Circuit Photovoltage (Voc), Fill Factor (FF), and Overall Photoconversion Efficiency (η) along with the Amount of Adsorbed Dye N719 for the Films Composed of SnO2 NSs, SnO2 NSs−TiO2, SnO2 NPs, and SnO2 NPs− TiO2, Respectivelya film SnO2 NSs SnO2 NSs− TiO2 SnO2 NPs SnO2 NPs− TiO2

adsorbed dye (×10−8 mol·cm−2]

Jsc (mA·cm−2)]

Voc (mV)

FF (%)

η (%)

1.50 1.60

2.6 5.7

300 649

0.30 0.48

0.23 1.79

2.32 2.43

1.5 5.0

258 481

0.30 0.31

0.12 0.74

The active areas of the photoelectrodes were about 0.25 cm−2, and the thickness of the photoelectrodes was about 4 μm.

a

The data in Figure 2c suggest that the open-circuit photovoltage (Voc) of SnO2 NSs DSSCs (Voc = 300 mV) is 40 mV higher than that of plain SnO2 NPs DSSCs (Voc = 258 mV). As indicated in this work and reports of other researchers, the open-circuit potential of SnO2-based DSSCs changes with altering morphology of SnO2 in the photoelectrode, which can be attributed to the different surface structures and exposed facets of these nanostructured SnO2.19,32 The surface of all nanosheets in this work can be deemed as a typical low-index 4249



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Figure 3. (a) Diffuse-reflectance spectra of SnO2 NSs and SnO2 NPs films with a thickness of about 4 μm. (b) Dark current density−voltage characteristics of DSSCs with photoelectrode films composed of SnO2 NSs, SnO2 NSs−TiO2, SnO2 NPs and SnO2 NPs−TiO2, respectively.

SnO2 NSs−TiO2 shifts to a much higher potential which confirms the reduced electron recombination in electrode. However, the upward shift in the dark-current onset of the SnO2 NPs−TiO2 electrode is not as obvious as that of the SnO2 NSs−TiO2. One possible reason for this phenomenon may be due to the comparatively higher concentration of surface states on the SnO2 NPs electrode which can countervail the passivation and recombination suppression effects of TiO2 coating. Minimizing the charge recombination in DSSCs also gives rise to an increase in shunt resistance and thereby improves the fill factor (FF).34 This may be one of the major reasons for the observed lower FF for the SnO2 NPs−TiO2 than the SnO2 NPs−TiO2 DSSCs. 3.4. Electrical Impedance Spectra Analysis. To better understand the dynamics of electron transport and recombination in the SnO2 electrodes, electron transfer between the counter electrode and electrolyte, and I3− diffusion in the electrolyte, further studies were carried out with electrical impedance spectra (EIS) in the dark at a bias direct current (DC) voltage of 0.65 V and 10 mV of amplitude over the frequency range of 0.1 Hz to 100 kHz. Figure 4 shows the Nyquist plots under the open-circuit condition for the DSSCs using SnO2 NSs, SnO2 NSs−TiO2, SnO2 NPs, and SnO2 NPs− TiO2, respectively. An equivalent circuit model (see the inset in Figure 4) was given to recognize the total series resistance (RS),

crystal facet. On the other hand, SnO2 nanoparticles have mixed crystal facets and many crystal edges, steps, or defects also exist on the surface. Such shifts in the flat band potential along crystallographic orientation have also been observed before in TiO2 nanosheets with a high percentage of {001} facets.33 The comparative lower open circuit photovoltages of the plain SnO2 DSSCs may be due to the ∼400 mV more positive conduction-band edge of SnO2 than that of TiO2. Because the modification of TiO2 can effectively suppress the interfacial electron combination on electrodes and shifts the electronic band to a more negative value, the open-circuit photovoltages of TiO2-coated SnO2 photoelectrodes are therefore observed to be significantly improved. The SnO2 NSs−TiO2 DSSCs have an efficiency of 1.79% and a maximum IPCE of 35% at 530 nm, with a short-circuit current (Jsc) of 5.7 mA·cm−2 and an open-circuit voltage (Voc) of 649 mV, which was >2-fold improvement in the device performance than the SnO2 NPs−TiO2 DSSCs. However, as given in Table 1, the amount of dye absorption on the SnO2 NSs is 35% lower than that of SnO2 NPs, and the TiCl4 treatment seems to have no obvious improvements on dye absorption. The higher dye loading of SnO2 NPs electrode is attributed to its comparative surface area; the SnO2 NPs with 20−30 nm in diameter are much smaller than these SnO2 NSs (shown in Figure 2). Further, the SnO2 NSs film had a much higher diffuse reflection in the visible range of 300−800 nm than that of SnO2 NPs film (shown in Figure 3a), indicating that the incident light was significantly scattered within the SnO2 NSs film. Therefore, the higher efficiency value of SnO2 NSs−TiO2 DSSCs is most likely given rise by an enhanced light harvesting due to multiple light reflecting and scattering between the hierarchical mirrorlike structure of the SnO2 NSs, rather than the high surface area. In addition, such architecture is compatible for light absorption by the dye molecules and for the diffusion of I−/I3− through the semiconductor matrix. 3.3. Dark Current−Voltage Analysis. To examine the recombination of the electrons with the electrolyte on different SnO2 photoelectrodes, the dark current−voltage characteristics of the plain and TiO2 modified SnO2 NSs and SnO2 NPs electrodes were measured. The dark current is not a direct measurement of the recombination rate. However, it is meaningful when similar cells need to be compared. As illustrated in Figure 3b, compared to the SnO2 NPs electrode, the onset of recombination current occurs at higher potential for SnO2 NSs electrode. After TiCl4 treatment, the onset of

Figure 4. EIS for DSSCs with photoelectrode films composed of SnO2 NSs, SnO2 NSs−TiO2, SnO2 NPs, and SnO2 NPs−TiO2, respectively. The inset displays the equivalent circuit model. 4250



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charge-transfer resistance (R 1 , R 2 , and W s ), and the corresponding constant phase angle element (CPE) in DSSCs. In general, the first semicircle in the high-frequency region (105−102 Hz) represents the impedance corresponding to the redox reaction of I−/I3− at the Pt/electrolyte interface (R1), while the semicircles and arc in intermediate frequencies (102 to 10 Hz) and low−frequency region (10 to 0.1 Hz) give information on the impedance at the oxide/dye/electrolyte interface (R2) related to the charge transport/recombination and Warburg diffusion process of the electrolyte (Ws), respectively.35−37 The fitted data are all listed in Table 2, which reveals several features. First, the RS and R1 values for all of the DSSCs are all

electrons in the oxide film (τr) can be estimated from the maximum angular frequency (ωmax) of the impedance semicircle arc at middle frequencies in the Bode spectrum. The reaction can be described as τr = 1/ωmax = 1/2πf max, where f max is the maximum frequency of the midfrequency peak.10,38 From Table 2, it can be found that the τr for DSSCs with SnO2 NSs, SnO2 NSs−TiO2, SnO2 NPs, and SnO2 NPs−TiO2 photoelectrodes were 7.17, 14.73, 5.78, and 14.70 ms, respectively. Among these photoelectrodes, the SnO2 NSs−TiO2 photoelectrode shows the highest τr value (14.73 ms), implying the longest electron lifetime. This phenomenon indicates that modifying SnO2 NSs by TiO2 coating has the effect of increasing the lifetime of electrons for recombination, and further enhancing the cell performance. 3.5. Film Thickness Studies. The effect of the film thickness of these SnO2 NSs−TiO2 and SnO2 NPs−TiO2 electrodes were also investigated to obtain the optimized film thickness. The thickness of the SnO2 films was adjusted between 3 and 17 μm. As indicated in Figure 5, the thicker layer gave higher photocurrent and lower photovoltage. This is a typical phenomenon in DSSCs regardless of the kind of particles and dyes used.39 The highest overall photoconversion efficiency of the SnO2 NSs−TiO2 DSSCs is about 2.82% with a film thickness of about 14.5 μm, remarkably higher than that of the SnO2 NPs−TiO2 DSSCs (∼0.85%, ∼14.6 μm).

Table 2. EIS Parameters of the Four DSSCs Determined by Fitting the Experimental Data to the Equivalent Circuit Model (See Figure 4) DSSCs SnO2 SnO2 SnO2 SnO2

NSs NSs-TiO2 NPs NPs-TiO2

RS (Ω)

R1 (Ω)

R2 (Ω)

Ws (Ω)

ωmax (Hz)

τr (ms)

21.56 24.17 24.84 23.93

10.49 18.54 9.11 9.07

5.01 6.08 5.71 40.63

0.17 0.15 0.71 0.64

137.7 67.89 173.27 68.01

7.17 14.73 5.78 14.70

similar, simply because all of the SnO2 electrodes and counter electrodes used in DSSCs assembly were printed and heated in the same condition. Second, the R2 values of the surfacemodified SnO2 NSs−TiO2 and SnO2 NPs−TiO2 electrodes are higher than those of plain SnO2 electrodes. Here, R2 is usually thought to be mainly determined by the charge recombination resistance, with partial contribution from transport resistance. Moreover, the electron mobility in SnO2 is over 2 orders of magnitude higher than that in TiO2, which means that the charge transport resistance is lower in SnO2 than in TiO2 and the TiO2 coating would, to some extent, increase the charge transport resistance in SnO2 electrodes further. However, the TiO2 layer can prohibit the electron−hole recombination on the SnO2 surface and reduce the charge recombination resistance in SnO2 electrodes. Thus, a trade-off relation exists between the TiO2 coating and the R2 value. As mentioned before, the TiCl4 treatment did not effectively slow electron recombination in SnO2 NPs electrode, which can be invoked to explain the much higher R2 value of the SnO2 NPs−TiO2 electrode. Finally, from the low values of Ws, we can conclude that diffusion coefficients of I3− ions in these electrodes are pretty high. According to the EIS model, the lifetime of

4. CONCLUSIONS In summary, we have demonstrated a facile and nontoxic approach for one-pot synthesis of a hierarchical ultrathin SnO2 NSs with a thickness approximately 4−6 nm and lateral dimension up to sub-micrometers. Used as photoanode material of DSSCs, the ultrathin SnO2 NSs can greatly improve the photoconversion efficiency in DSSCs because of its higher electron mobility and enhanced light scattering. It was found that the SnO2 films composed of 3D hierarchical nanosheet structures show about 50 mV higher open-circuit voltages than that of SnO2 nanoparticle films. After surface modification by TiCl4, the optimal SnO2 NSs−TiO2 DSSCs exhibited Jsc = 10.4 mA·cm−2, Voc = 603 mV, and η = 2.82%, indicating a 19.5, 98, and 231% increase in Jsc, Voc, and η, respectively, compared to the SnO2 NPs−TiO2 photoelectrodes with similar thickness. These results in this work demonstrate that the SnO2 NSs− TiO2 can be used as promising photoelectrode material for lowcost and high-efficiency DSSCs. In addition, the synthetic strategy developed in this work to prepare ultrathin SnO2 NSs might be applied to prepare other 2D sheetlike oxides.

Figure 5. DSSCs device performance characterization for SnO2 NSs−TiO2 and SnO2 NPs−TiO2 photoelectrodes with different film thickness. Voc and efficiency (black line); Jsc and fill factor (blue line). 4251



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ASSOCIATED CONTENT

S Supporting Information *

Figures showing X-ray diffraction pattern of the as-prepared SnO2 nanosheets, TEM image of ultrathin SnO2 nanosheets, and HRTEM image of TiO2-coated SnO2 nanosheets. This information is available free of charge via the Internet at http:// pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: (+86) 21 64252127. E-mail: [email protected]. Author Contributions ‡

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Scientific Research Foundation of East China University of Science and Technology (Grant YD0142125), Pujiang Talents Programme and Major Basic Research Programme of Science and Technology Commission of Shanghai Municipality (Grants 09PJ1402800 and 10JC1403200), Shuguang Talents Programme of Education Commission of Shanghai Municipality (Grant 09SG27), National Natural Science Foundation of China (Grants 20973059, 91022023, and 21076076), Fundamental Research Funds for the Central Universities (Grant WJ0913001), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning and Program for New Century Excellent Talents in University (Grant NCET-09-0347).

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REFERENCES

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TiO2-Coated Ultrathin SnO2 Nanosheets Used as Photoanodes for

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