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Mesoporous SnO2 Spheres Synthesized by Electrochemical Anodization and Their Application in CdSe-Sensitized Solar Cells Md. Anower Hossain, Guangwu Yang, Manoj Parameswaran, James Robert Jennings, and Qing Wang* Department of Materials Science and Engineering, Nanocore, Faculty of Engineering, National UniVersity of Singapore, Singapore 117574 ReceiVed: September 22, 2010; ReVised Manuscript ReceiVed: October 25, 2010
Mesoporous SnO2 spheres of tunable particle size were synthesized for the first time by facile electrochemical anodization of tin foil in alkaline media. As the anodization process involves no sophisticated reactor or toxic chemicals, and proceeds continuously under ambient conditions, it provides an economic way of synthesizing nanostructured SnO2 on a large scale. Structural characterization indicates that these spherical particles consist of an agglomeration of SnO2 nanocrystals, resulting in a high internal surface area. This makes them a promising photoanode material for use in semiconductor-sensitized solar cells (SSCs). By using the successive ioniclayer adsorption and reaction method, a thin layer of CdSe was conformally coated on the surface of SnO2 nanocrystals, which were previously treated with aqueous TiCl4 solution. Efficient charge separation was observed by photoluminescence spectroscopy. After deposition of a ZnS passivation layer onto the CdSe light-harvesting layer, a power conversion efficiency of ∼1.91% was achieved in a regenerative photoelectrochemical cell. Factors dictating interfacial charge recombination and charge separation are discussed and compared to those in its molecular dye-sensitized counterpart. This study represents the first attempt so far of using mesoscopic SnO2 as a photoanode in a SSC device, and characterizing it under simulated AM 1.5, 100 mW cm-2 illumination. The results are a step toward development of highly efficient SSCs employing novel electron transport materials and sensitizers, such as infrared light absorbers PbS, CuInSe2, etc. Introduction Nanostructured tin dioxide (SnO2) has generated great scientific and technological interest in the past two decades because of its diverse properties (optical, electrical, electrochemical, etc.) and potential applications, such as a transparent conducting oxide (TCO)1 for displays, anodic material for lithium ion batteries,2,3 and photoelectrode for dye-sensitized solar cells,4,5 sensors,6,7 etc. SnO2 is an n-type semiconductor with an optical band gap of ∼3.6 eV, which corresponds to an absorption onset of ∼340 nm. Electron mobility in SnO2 is reported to be much higher than in nanocrystalline TiO2,8,9 a common choice of material in the aforementioned applications. In addition, the conduction band minimum (CBM) of SnO2 is ∼0.40 eV more negative compared to anatase TiO2, suggesting more efficient charge injection from light absorbers should be possible. It has been revealed that these intriguing properties are strongly affected by the dimensions and morphology of the SnO2, which has implications for its eventual applications. For instance, nanostructured SnO2 provides a large surface area, which is essential for the photoanode of sensitized solar cells. Furthermore, 1D nanostructured SnO2 has demonstrated much facilitated charge transport in optoelectronic devices.10 Hence controlled growth of nanostructured SnO2 with desired size and shape is of great importance. Various chemical and physical approaches have been developed to synthesize nanostructured SnO2, such as sol-gel,11,12 chemical vapor deposition (CVD),13 hydro/solvothermal methods,14-16 spray pyrolysis,17 magnetron sputtering,18 pulsed laser deposition (PLD),19 thermal evaporation,20 etc. A wide variety of SnO2 nanostructures with interest* To whom correspondence should be addressed. E-mail: qing.wang@ nus.edu.sg.
ing properties have been prepared, such as nanoparticles,21 nanowires,22,23 nanotubes,24,25 nanospheres,2 etc. While SnO2 nanocrystals of different morphologies have been prepared, they are limited to either high-temperature hydrothermal or templateassisted approaches. Electrochemical anodization is a templatefree method that can be performed under ambient conditions and used to synthesize various nanostructures, such as honeycomb structured anodized aluminum oxide (AAO),26 oriented TiO2 nanotube arrays,27-30 etc. SnO2 films with disordered pore structures were also prepared via electrochemical anodization of tin foil in acidic media by Liu et al.31 However, no work has been reported thus far on the preparation of monodisperse SnO2 nanocrystals and their assemblies of different morphologies. Here we show a facile two-step method: tin oxide nanocrystals were first synthesized by electrochemical anodization; the obtained nanocrystals were then fabricated into “superstructures”;SnOx (x ) 1, 2) nano/microspheres with mesoscopic pore structures. As the reactions are carried out under ambient conditions, the two-step method could feasibly produce nanostructured SnOx in a continuous fashion, which is particularly important for producing nanostructured materials at a large scale, in contrast with more common hydrothermal and other methods. In this study, the obtained mesoporous SnO2 microspheres were employed as a photoanode material for semiconductorsensitized solar cells (SSCs). It is anticipated that charge injection32 would be greatly enhanced as compared to anatase TiO2, and charge recombination would be significantly inhibited as the semiconductor light absorber could act as a passivation layer because the hole scavenging process is extremely fast.33,34 In addition, the macroscopic morphology ensures efficient light
10.1021/jp109083k 2010 American Chemical Society Published on Web 11/11/2010
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scattering, which in turn enhances light harvesting. By using the successive ionic-layer adsorption and reaction (SILAR) method, a thin layer of CdSe was conformally coated on the surface of the SnO2 nanocrystals to act as a light absorber. We will show here that efficient charge separation has been observed in a regenerative photoelectrochemical cell and promising photovoltaic performance has been achieved. This study represents the first attempt so far of using mesoscopic SnO2 as a photoanode material in a SSC device, and characterizing it under simulated AM 1.5, 100 mW cm-2 illumination. Experimental Section Electrochemical Anodization and Preparation of SnO2 Spherical Particles. In a typical anodization experiment, tin foil (Alfa Aesar, metal basis, 99.8%) of thickness 0.5 mm was used as the anode, and glassy carbon of the same dimension was used as the counter electrode. The basic electrolyte composition was 0.1 M NaOH (Sigma-Aldrich, reagent grade, 98%), 0.05 M NH4F (Sigma-Aldrich, trace metal basis, 99.99%) in ethylene glycol (Sigma-Aldrich, anhydrous, 99.8%). The reaction was carried out with a digital source meter (Keithley 2612A) and tin foil was anodized in a potentiostatic mode at 4 V for 10 h. During anodization, white powder was observed to precipitate out which was then collected from the bottom of electrochemical cell. The collected white powder was dispersed in absolute ethanol and centrifuged several times to remove residual electrolyte, followed by 3 min sonication. The clean and well-dispersed samples were finally aged in ethanol at 55 °C for 1-10 h to obtain the final products. Preparation of CdSe-Sensitized Mesoscopic SnO2 Electrodes. SnO2 spherical particles, ethyl cellulose powder, and R-terpineol were mixed in the same ratio following the same procedure of making TiO2 paste.35 The mixture was then magnetically stirred for 2 days to get homogeneous SnO2 paste. SnO2 electrodes were prepared by screen-printing the above paste onto FTO glass (TEC, 15 Ω/0). The printed films were then sintered in air gradually at 325 °C for 5 min, at 375 °C for 5 min, and at 450 °C for 15 min, and finally at 500 °C for 15 min to obtain semitransparent SnO2 mesoporous electrodes. The thickness of the electrode was determined by an Alpha-Step IQ surface profiler to be ∼2 µm. CdSe was deposited on the electrodes by the SILAR method where the SnO2 spots on FTO were dipped in a solution containing 0.03 M cadmium nitrate tetrahydrate (Cd(NO3)2 · 4H2O, Fluka, >99.0%) in ethanol for 30 s and rinsed with ethanol for 2 min then dried for 2 min in argon atmosphere. Subsequently the dried electrodes were dipped in a solution containing 0.03 M Se2- for 30 s. The Se2solution was prepared by mixing selenium dioxide (SeO2, Sigma-Aldrich, 99.9%) and sodium borohydride (NaBH4, Sigma Aldrich) in ethanol following the reported work by Lee et al.36 After rinsing in ethanol for 2 min and drying again in an argon atmosphere for 2 min, the electrode completes one cycle of deposition. The same procedure was repeated several times to obtain suitable CdSe loading on SnO2 electrodes. A ZnS passivation layer was deposited on a CdSe-coated SnO2 electrode by the SILAR method, using aqueous solutions containing 0.05 M zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, Sigma Aldrich) and 0.2 M sodium sulfide nonahydrate (Na2S · 9H2O, Sigma Aldrich) consecutively.37 Material Characterization. Samples were prepared by washing the as-anodized white powders thoroughly with absolute ethanol several times and then sintering at different temperatures for 3 h with a ramp of 5 deg/min. X-ray diffraction (XRD) measurements were carried out with a Bruker D8, using
Figure 1. X-ray diffraction patterns of an as-prepared sample (a), and a sample sintered in air for 3 h at various temperatures: 300 (b), 500 (c), and 700 °C (d).
Cu KR1 radiation (λ ) 0.154059 nm). Absorption spectra were collected with use of a Shimadzu UV-vis-NIR spectrophotometer (Solidspec-3700). For the reflectance measurement, BaSO4 was used as the reference. Morphology of the particles was characterized by scanning electron microscopy (SEM, Philips XL 30 FEG) and transmission electron microscopy (JEOL JEM 2010F). Photoluminescence spectra were collected with a spectrofluorophotometer (Shimadzu-RF5301), and surface area was determined by using the Brunauer-Emmet-Teller (BET) method (ASAP 2020, Micromeritics). Fabrication and Characterization of CdSe-Sensitized SnO2 Solar Cells. Platinized counter electrodes were fabricated on FTO with small holes drilled into one corner. After cleaning, a thin layer of Pt was deposited onto the FTO by thermal decomposition of hexachloroplatinic acid. Photoanodes and counter electrodes were sealed together in a sandwich configuration with a hot-melt polymer (Surlyn, DuPont). The interelectrode space was filled with an electrolyte by vacuum backfilling. Holes were sealed by using a small piece of hotmelt polymer and a microscope coverslip. The electrolyte was composed of 0.125 M sulfur, 0.5 M Na2S · 9H2O, and 0.2 M KCl in 7 mL of deionized water and 3 mL of methanol. IPCE spectra were measured with a spectral resolution of 5 nm, using a 300 W xenon lamp and a grating monochromator equipped with order sorting filters (Newport/Oriel). The incident photon flux was determined by using a calibrated silicon photodiode (Newport/Oriel). Photocurrents were measured with an autoranging current amplifier (Newport/Oriel). Control of the monochromator and recording of the photocurrent spectra were performed with a PC running the TRACQ Basic software (Newport). Current-voltage characteristics under simulated AM 1.5 illumination were measured with a Keithley Source Meter and the PVIV software package (Newport). Simulated AM 1.5 illumination was provided by a Newport class A solar simulator and light intensity was measured with a calibrated Si solar cell. The active area of the cells was defined by a mask to be 0.1199 cm2. Results and Discussion Phase Identification and Structural Examination. Crystal structures of samples were characterized by powder X-ray diffraction (XRD). As shown in Figure 1a, the XRD pattern of the as-prepared sample indicates the presence of tetragonal tin(II) oxide hydroxide (Sn6O4(OH)4, JCPDS No. 84-2157), which
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Figure 2. (a) SEM image of colloidal particles obtained from electrochemical anodization. (b) High-resolution TEM image of the colloidal particles consisting of small crystal grains.
represents the major crystalline phase in the as-prepared product, suggesting tin metal was oxidized to Sn(II) during the anodization process. Tin(II) oxide hydroxide dehydrated upon treatment at 300 °C and formed tin(II) monoxide (SnO, JCPDS No. 06-0395, Figure 1b). The diffraction peaks of SnO vanish after thermal annealing in air at 500 °C for 3 h and it exhibits an XRD pattern being indicative of the (110), (101), (200), (210), (310), (301), and (201) planes of the tetragonal rutile structure of SnO2 (JCPDS No. 41-1445). The diffraction peaks of SnO2 become much sharper after annealing at 700 °C for 3 h, indicating a larger grain size and improved crystallinity. The grain size estimated from the Scherrer equation is ∼6 nm, suggesting the nanocrystalline nature of SnO2. Morphology Evolution. The morphology of the as-prepared colloidal particles was characterized with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The sample was collected from the clear colloidal solution after washing with ethanol and a subsequent dispersion in an ultrasonic bath. As shown in Figure 2a, these colloidal particles have a uniform size of ∼60 nm. HRTEM measurements (Figure 2b) suggest that these nanoparticles consist of crystal grains of 5-6 nm, presumably being tin(II) oxide hydroxide as indicated by XRD measurements. It will be shown later that these agglomerated crystal grains serve as building blocks which can form various SnO2 superstructures. The clear colloidal solution in ethanol was then aged in an oven at 55 °C for different durations. White sediment was collected and annealed in air at 500 °C for 3 h. As the SEM images in Figure 3 show, after calcination the sedimented particles are almost monodisperse and have a spherical shape,
Figure 3. SEM images of SnO2 spheres obtained by aging the colloidal solution for different durations: (a) 1, (b) 3, and (c) 10 h. All these samples were sintered in air at 500 °C for 3 h prior to SEM measurement. The scale bar is 1 µm. The insets show enlarged images of SnO2 spherical particles.
which is distinct from the primary colloidal particles (Figure 2a). Magnified images (insets) reveal that the spheres have a rough and granular surface, implying they are built up by agglomeration of small colloidal particles. The size of the spheres is found to be in the range ∼100 nm to ∼1 µm, dependent on aging time (Figure 3). This time-dependent particle size suggests that the spherical particles are grown by continuous collection of tin(II) oxide hydroxide nanocrystals. As shown in Figure 4, selected area electron diffraction (SAED) measurements indicate that all the annealed spherical particles are polycrystalline in nature, consistent with the XRD measurements. Closer investigation by HRTEM (insets in Figure 4) reveals that regardless of aging time, the size of small particulates on the surface is ∼5-6 nm, which is the same as the as-anodized nanocrystals shown in Figure 2b, indicating that
Synthesis of Mesoporous SnO2 Spheres
Figure 4. TEM images of SnO2 spheres obtained by aging the colloidal solution for different durations: (a) 1, (b) 3, and (c) 10 h. All these samples were sintered in air at 500 °C for 3 h prior to TEM measurement. The insets show the SAED patterns of respective samples and their HRTEM images.
the spherical particles are formed by agglomeration of these nanocrystals. The mechanism of crystal growth and morphological evolution has been extensively studied with various materials, where aggregation and coarsening are treated as the two major growth mechanisms. Driven by surface energy minimization, crystal growth by aggregation can occur by interactions between randomly oriented particles (coalescence), or between highly oriented particles (oriented attachment).38 On the basis of the size and shape of particles obtained at different aging stages (Figure 4), it seems unlikely that the growth of spherical particles is via coalescence of two or more identical particles. In addition, while preferential interactions between crystallographically oriented nanocrystal grains in the homogeneous
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21881 colloidal solution may take place at the initial aging stage and form larger colloidal particles (as shown in Figure 2), it is very unlikely that the subsequent growth occurs by the same mechanism because of restricted rotational and translational motion in the sediment. Coarsening (Ostwald ripening) requires transport of solute from small particles to large ones as a result of a chemical potential gradient between solid-liquid interfaces as well as nonuniformity of crystallities under experimental conditions. As tin(II) oxide hydroxide nanocrystals are not soluble in ethanol under ambient conditions, presumably small crystal grains from smaller particles could detach to homogenize the concentration gradients, which leads to complete dissolution of smaller crystallities which become mobilized.39,40 These small crystal grains migrate and redeposit on the surface of larger particles, giving rise to a rough surface composed of a large number of loosely packed building block nanocrystals. As the deposition of small crystal grains may proceed in all directions, spherical particles are eventually formed which was confirmed from rough surface of spheres synthesized from an aggregation of smaller nanocrystallities. The above growth mode leads to a mesoporous structure of the final spherical particles, as a result of randomly packed nanocrystal grains. BET surface area of the annealed irregular particles at 500 °C was determined to be 21.45 m2/g, with average pore diameter ∼12 nm. This high internal surface area affords a high loading of light harvesting materials, and the pore diameter ought to be sufficient to allow infiltration of electrolytes or hole transport materials. In addition, these films are found to have an opaque white appearance, implying strong light scattering. These properties render the spherical SnO2 particles interesting as a photoanode material for SSCs. Optical Properties of CdSe-Sensitized SnO2. Nanocrystalline CdSe is a widely used semiconductor sensitizer for nanocrystalline TiO2 because of its wide absorption spectrum covering most of the visible light range. However, sizedependent electron injection from excited CdSe quantum dots to TiO2 has been observed by femtosecond transient absorption measurements, because of the relatively high CBM of anatase TiO2.41 Efficient charge separation at the CdSe/TiO2 interface can only be achieved with small CdSe quantum dots. In contrast, the CBM of SnO2 is ∼0.40 eV more negative than TiO2, so much more efficient charge injection is anticipated. While the monochromic photoresponse42 and ultrafast carrier dynamics43 of CdSe-sensitized SnO2 have been investigated, no currentvoltage (J-V) characteristics of complete solar cells have been reported so far. Here CdSe was deposited on mesoporous SnO2 spheres by the SILAR method.44 As shown by the UV-vis spectrum in Figure 5, the absorption onset of CdSe-coated SnO2 occurs at ∼700 nm, being slightly blue-shifted compared to its bulk counterpart. For comparison, the same deposition procedure was applied to a mesoporous Al2O3 electrode. In this case an absorption onset of ∼750 nm was observed, indicating slightly larger CdSe particles, which is consistent with the larger pore size (∼20 nm) of the Al2O3 electrode. Steady-state photoluminescence (PL) measurements were carried out to study the charge injection from CdSe to the two metal oxides upon illumination. As indicated in Figure 5, CdSe-coated Al2O3 gave emission at ∼640 nm in the visible light region when it was excited at 450 nm. In comparison, no appreciable emission was observed from the CdSe-coated SnO2 electrode. It is not surprising that the CdSe-sensitized Al2O3 electrode presents a clear radiative signal upon illumination, since Al2O3 has a very wide band gap and charge injection from the excited state of CdSe to the conduction band of Al2O3 is not favorable. As a
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Figure 5. UV-vis spectra (line) and PL spectra (dot) of CdSesensitized SnO2 (blue) and Al2O3 (red) electrodes prepared by 3 cycles of SILAR deposition. The excitation wavelength is 450 nm for PL measurement. Insets show the photographs of the two sensitized electrodes.
consequence, excited electrons recombine radiatively with holes in the HOMO levels of the CdSe particle. It is known that SnO2 has a much lower CBM than TiO2 in the same media. Charge injection from excited CdSe to the conduction band of SnO2 is deemed to be much more efficient. Leventis et al. recently reported comparative studies of charge separation between PbSsensitized mesoscopic TiO2 and SnO2.45 Transient optical measurements showed much faster charge injection for the PbSsensitized SnO2 electrode, where the quantum yield was close to unity when Li+ was added to the electrolyte. The complete quenching of PL from the CdSe-sensitized SnO2 electrode upon excitation also suggests efficient charge injection, which is critical for charge separation and consequently the photovoltaic performance of the device. Photoelectrochemical Properties of CdSe-Sensitized SnO2. To study the photovoltaic performance of CdSe-sensitized mesoscopic SnO2 electrodes, sandwich-type thin layer cells were fabricated with FTO/SnO2/CdSe as the photoanode, platinized FTO as the counter electrode, and a polysulfide electrolyte as the hole transporter. To prevent recombination at the SnO2/CdSe/ electrolyte interface, SnO2 electrodes were treated with TiCl4 aqueous solution prior to CdSe deposition, and a thin passivation layer of ZnS was deposited onto the above-sensitized electrode as well by two SILAR deposition cycles. Figure 6a shows the incident photon-to-collected-electron conversion efficiency (IPCE) spectra of CdSe-sensitized SnO2 solar cells. Electrodes without TiCl4 pre-treatment or ZnS post-treatment exhibit very poor IPCE, with the peak quantum efficiency
