Submicrometer@nano Bimodal TiO2 Particles as Easily Sintered

Mar 17, 2014 - Submicrometer@nano bimodal TiO2 particles consisting of a submicrometer core and a mesoporous structured shell were synthesized by a ...
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Submicrometer@nano Bimodal TiO2 Particles as Easily Sintered, Crack-Free, and Current-Contributed Scattering Layers for DyeSensitized Solar Cells Mengyu Gao, Yichuan Rui, Hongzhi Wang,* Yaogang Li, and Qinghong Zhang* State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, No. 2999 North Renming Road, Shanghai 201620, P. R. China ABSTRACT: Submicrometer@nano bimodal TiO2 particles consisting of a submicrometer core and a mesoporous structured shell were synthesized by a chemical deposited method. The submicrometer TiO2 particles were first dispersed in the titanium(IV) bis(ammonium lactato) dihydroxide solution in the presence of polyethyleneimine and urea; after reflowing hydrolysis, the suspension was transferred to a Teflon autoclave and subjected to hydrothermal treatment at 150 °C for 24 h. The as-prepared nanocrystal-coated submicrometer TiO2 (NCS-TiO2) showed a core/shell structure where the outer TiO2 nanocrystals with the size of 5 to 7 nm and in anatase phase uniformly coated on the submicrometer TiO2. When the NCS-TiO2 was fabricated as the scattering layer in dye-sensitized solar cells (DSSCs), it adsorbed more dye molecules while still keeping a high light-harvesting efficiency. Moreover, the presence of TiO2 nanocrystals in NCS-TiO2 reduced the sintering stress between the transparent porous layer and the scattering layer, leading to a crack-free and high-quality photoanode film, which also reduced the serial resistance, as verified by electrochemical impedance spectroscopy. Finally, DSSCs with the NCS-TiO2 scattering layer reached an efficiency of 8.36%, which showed a 20% improvement compared with the one without it.

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INTRODUCTION Dye-sensitized solar cells (DSSCs) have been attracting widespread scientific and technological interests due to their low cost, highly theoretical efficiency, and eco-friendly production.1 A typical DSSC is mainly composed of four parts: the photoanode fabricated from mesoporous TiO2 nanoparticles, the dye molecules, the electrolyte usually containing I−/I3− redox couples, and the platinum counterelectrode. The light is mainly absorbed by dye molecules attached to the surface of nanocrystalline TiO2 to generate free excited electrons that can be subsequently injected into the conduction band of TiO2, and thus the TiO2 nanocrystals serve as very crucial part in DSSCs. The conventional TiO2 photoelectrode with nanocrystalline TiO2 (15−30 nm) provides a large surface area for dye adsorption;2−7 however, it is transparent to the solar light and only utilizes some of the incident light. Also, with increasing the thickness of TiO2 film, an opaque film was obtained; however, the straight diffusion lengths of electron were limited to ∼100 μm,8 and some electrons were not able to be collected by the external circuit in a thicker photoanode. Light-scattering materials have been used as an effective and practical approach to achieve better performance DSSC for the reason of elongating the path lengths of incident light within the films, thereby enhancing the light-harvesting capability of the photoanode film.9 © 2014 American Chemical Society

Light scattering is usually achieved via mixing submicrometer particles into a nanocrystalline matrix or placing the submicrometer TiO2 particles at the back of the nanocrystalline film,10−16 and these submicrometer particles are effective and available to enhance light scattering so as to the light harvesting. According to Mie theory, the diameters of particle size near visible-light wavelength and ∼550 nm would have optimal scattering effect for DSSC light harvesting.17 Submicrometer particles are widely used as the scattering centers; however, the bulk particles with very small surface area adsorb few dye molecules and thus contribute little to the photocurrent in a limited thickness.10,11,18,19 Another frequently encountered trouble is that the films easily crack in the subsequent sintering process due to the different shrinkage rate between large scattering particles-based (hundreds of nanometers) scattering film and the tiny nanocrystals-based (15−30 nm) one. To obtain a homogeneous and crack-free photoanode film, researchers mixed the commercial submicrometer particles with the nanoparticles as a composite scattering layers;20−22 for Special Issue: Michael Grätzel Festschrift Received: January 15, 2014 Revised: March 12, 2014 Published: March 17, 2014 16951

dx.doi.org/10.1021/jp500466s | J. Phys. Chem. C 2014, 118, 16951−16958

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Scheme 1. Scheme of Submicrometer@nano Bimodal TiO2 Particles Reducing the Sintering Stress to Form Crack-Free Photoanode Film

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example, Ito and coworkers made the scattering layers composed of 400 nm TiO2 and 10 nm TiO2 with mass ratio of about 10:1,20 and an efficiency as high as of 10% was achieved by crack-free DSSCs. Huang and coworkers used commercial mixture (80 wt % 400-nm CCIC TiO2 particles and 20 wt % 18 nm CCIC TiO2 particles) as one of their scattering layers in DSSCs.21 Agarwala et al.23 also used P25 nanoparticles in their process, which acted as active binders to prevent formation of microcracks. However, it is time-consuming to well blend the bulk and tiny particles in the paste-fabricating process, and whether a homogeneous paste is obtained is still a question. It would be better if the tiny nanocrystalline TiO2 can be coated on the master particles in situ growth. Core/shell-structured TiO2 is an approach to reduce the sintering stress between the scattering layer and the porous layer; meanwhile, the integrated structure showed multifunctions in photocatalysis.24−26 We present a facile process to coat the submicrometer TiO2 particles with tiny nanocrystals, and the bimodal TiO2 particles were used as the scattering layer of DSSCs (Scheme 1). By using the negative charged titania precursor,27 titanium(IV) bis(ammonium lactato) dihydroxide (TALH), via a process of controlled hydrolysis and condensation, 5 to 7 nm TiO2 nanocrystals were successfully coated on 400 nm sized submicrometer TiO2 particles. The modification enhanced the specific surface area, and the presence of TiO2 nanocrystals in NCS-TiO2 reduced the sintering stress between transparent porous layer and scattering layer. The high efficiency of 8.36% was achieved in the presence of the crack-free scattering layer, which was much higher than that of the one without it.

PREPARATION OF DOCTOR-BLADING PASTES

Two kinds of TiO2 pastes were prepared: one consisting of hydrothermal prepared nanocrystalline TiO2 (20 nm, Paste A) was used to form the transparent porous layers, and the other consisting of S-TiO2 or NCS-TiO2 (Paste B) was used to print light-scattering layers of the DSSCs. The methods of preparing nanocrystalline TiO2 and paste A were mainly according to the literature.20 Paste B was mainly prepared according to the literature.28 In brief, 3.0 g of NCS-TiO2 (S-TiO2) was ground for 60 min in the mixture of 20 mL of ethanol, 0.5 mL of acetic acid, 10.0 g of terpineol, and 1.5 g of ethyl cellulose to form a slurry; then, the slurry was dispersed by ultrasonic and magnet tip. Paste B was then prepared by removing the ethanol and water from the mixture solution with a rotary-evaporator. Preparation of Photoanodes. The FTO glass (2.2 mm thick, 14 Ω/□, Nippon Sheet Glass, Japan) was cleaned in a detergent solution using ultrasonic bath for 20 min and then rinsed with water and ethanol. To form a TiO2 compact layer,29 the FTO glass was immersed in a 40 mM aqueous TiCl4 solution at 70 °C for 30 min and washed with water and ethanol. Paste A and Paste B were coated on the FTO glass plates with doctor-blade method. We use 1T to stand for onefold doctor blade printing transparent porous layer and 2T to stand for two-fold doctor blade printing transparent porous layers. The films dried at 125 °C for 7 min after doctor blade printing. Surface profiler (Wyko NT9100, Veeco, USA) was used to verify the thickness of the transparent layers within 10− 13 μm and the scattering layers within 5−8 μm. The electrodes coated with the TiO2 pastes were gradually sintered under an airflow at 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min.20 The TiO2 films were treated again with 40 mM TiCl4 solution and sintered at 450 °C for 30 min for a better contact between the individual nanocrystals.30 The active areas of the working electrodes were 0.25 cm2 by scraping off the excess area. Fabrication of DSSCs. The sintered photoanodes were immersed in ethanol solution containing 0.3 mM cis-di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4-carboxylic acid-4′-tetrabutylammonium carboxylate ruthenium(II) (N719, Solaronix) for 24 h. Then, the photoanodes were rinsed with ethanol to remove excess amounts of dye and dried in oven. The counter electrode was platinized by applying a drop of 5 mM H2PtCl6 in 2-propanol onto a FTO glass substrate and annealing it in air at 380 °C for 10 min. The counter electrode was then placed directly on the top of the dye-sensitized TiO2 film sealed with polymer foil (Surlyn 1702, DuPont) as a spacer frame. The

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EXPERIMENTAL SECTION Preparation of NCS-TiO2. Polyethyleneimine (PEI Mw 1800) (Aldrich) 2.0 g was diluted in 300 mL of deionized water, which was further purified by filtration (Mill-Q). Then, 8.5 g of submicrometer anatase TiO2 particles (S-TiO2) (300− 400 nm, Sinopharm Chemical Reagent, Shanghai, China), 7.0 g of urea, and 6.0 mL of TALH (50 wt % in aqueous solution, Alfa Aesar) were added to PEI solution in sequence. After stirring and sonicating, the suspension was transferred into a flask reflowing at 95 °C for 24 h under stirring. Finally, the suspension was placed in an 80 mL Teflon autoclave and subjected to hydrothermal treatment at 150 °C for 24 h to form white precipitate. The precipitate was washed with deionized water and ethanol and collected by centrifuging. The NCSTiO2 particles were dried in a vacuum oven at 80 °C for 24 h. 16952

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electrolyte (600 mM of 1,2-dimethyl-3-propylimidazolium iodide, 50 mM iodine, 100 mM of lithium iodide, and 500 mM of tert-butylpyridine that were dissolved in acetonitrile) was injected from a predrilled hole on the counter electrode into the space between the sandwiched cells. Characterization. The morphology of the samples was evaluated by field-emission scanning electron microscopy (FESEM, Model S-4800, Hitachi, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). The Brunauer−Emmett−Teller (BET) specific surface area was measured by nitrogen adsorption apparatus (model ASAP 2020, Micromeritics Instrument, USA). All as-samples were degassed at 150 °C for 6 h prior to BET measurements. A desorption isotherm was used to determine the pore size distribution using Barrett−Joyner−Halender (BJH) method. Diffuse reflectance spectra of TiO2 films and the transmittance spectra of the dye-sensitized photoanodes were measured with a spectrophotometer (Lambda 950, Perkin−Elmer, USA). J−V curves of the DSSCs were measured by using a Keithley 2400 source meter under illumination of simulated sunlight (100 mW/cm2) provided by a solar simulator (Model 96160 Newport, USA) with an AM 1.5G filter. The incident-photonto-current conversion efficiency (IPCE) spectra were measured as a function of wavelength from 350 to 800 nm using a specially designed IPCE system (Newport, USA) for DSSCs. The electrochemical impedance spectroscopy (EIS) measurements were performed by using an electrochemical analyzer (Zahner−Elecktrik, Germany) and carried out by applying bias of the open-circuit voltage (Voc) without electric current under 100 mW/cm2 illumination and recorded over a frequency range of 100 mHz to 100 kHz with AC amplitude of 10 mV.

Figure 1. TEM (a,b) and HRTEM (c) image of NCS−TiO2 hydrothermal treated at 150 °C for 24 h.

RESULTS AND DISCUSSION Morphological Characteristic of NCS-TiO2 and TiO2 Films. Figure 1 shows the TEM images of NCS-TiO2; it can be found that the TiO2 nanocrystals in a crystallite size of 5 to 7 nm were coated on the surface of submicrometer TiO2 successfully (Figure 1a). The HRTEM image (Figure 1c) shows that TiO2 nanocrystals were well crystallized, and the lattice fringes corresponded to the (101) plane of anatase. We tentatively suggested the formation mechanism as that, when the S-TiO2 was dispersed into the PEI solution, the positive group NH2+ was adsorbed by S-TiO2.31,32 During slow heating process, complexation of the negatively charged TALH on STiO2 occurred in the solution. With further heating of the solution to 95 °C, the ammonia released by urea hydrolysis slowed down the hydrolysis and condensation of TALH, and a dense tiny anatase TiO2 film covered on the surface of STiO2.33 The microscope image and optical photo of TiO2 films are shown in Figure 2. To study the crack of TiO2 films, the double transparent porous layers (2T) were fabricated with a thickness of ∼12 μm, which were a little thicker than a conventional one, and it cracked slightly in view of microscope with 40 times magnification from Figure 2a. The TiO2 films contained double-transparent porous layers, and S-TiO2 scattering layer (2T-S) was seriously cracked, as shown in Figure 2b. However, the TiO2 films with double-transparent porous layers and NCS-TiO2 scattering layer (2T-NCS) were completely integrity in the view of microscope with 40 times magnification (Figure 2c). The TiO2 films were all fabricated by the same process, and the thickness of porous layers was a little different. We suggested the reason why the 2T-NCS films were integrity while the 2T-S showed a serious crack is that the

sintering stress between scattering layer and porous has been reduced. The thermal expansion coefficients, α, for anatase TiO2 are known to be strongly dependent on the crystallographic orientation; the one that is perpendicular to [001] has a higher coefficient (αa = −2.9 × 10−6, αc = +6.6 × 10−6, both at room temperature).34 Thus during the high-temperature sintering and the subsequent cooling down, the maximum crack width is calculated to be 1.25 nm for the submicrometer TiO2 particles in a size of 400 nm. The contraction during the cooling of the nanocrystals in a crystallite size of 10 nm was negligible; the porous nanocrystal coating might also efficiently diminish the tiny cracks between the submicrometer TiO2 particles. We also observed the top and cross-sectional view of the 2TNCS films in Figure 3. Figure 3a,b reveals that the films are crack-free structure. The cross profile of 2T-NCS electrode is shown in Figure 3c,d; there the FTO layer in a thickness of 300 nm, porous layer, and scattering layer are clearly observed in the sequence of bottom to up. The porous layer is in a thickness of ∼11.5 μm, while the scattering layer is ∼7.5 μm. BET Analysis. The N2 adsorption−desorption curves are presented in Figure 4. The isotherms of S-TiO2 powders have the types of IV, and the curves exhibit a hysteresis loop at high relative pressure of 0.8 to 1.0, indicating the presence of mesopores (type IV).35 The shape of the hysteresis loop is type H3, associated with plate-like particles giving rise to slit-shaped pores.35 NCS-TiO2 shows isotherms of type IV and two hysteresis loops, and the N2 adsorption volume of NCS-TiO2 was higher than that of S-TiO2, suggesting an increasing specific surface area. At relative pressure of 0.5 to 0.9, the hysteresis loop is type H2, which can be assigned to the ink-bottle pores

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Figure 4. N2 adsorption−desorption isotherms and pore-size distribution curves (inset) of S-TiO2 and NCS-TiO2.

the NCS-TiO2 shows the similar shape and type of the hysteresis loop as S-TiO2. The inset of Figure 4 shows the porediameter distribution as estimated by the BJH method, and one can see that the pore size of the products after coating the tiny nanocrystals became smaller. The presence of TiO2 nanocrystals enhanced the specific surface area from 5.5 to 16.7 m2/g and could provide a well-interconnected porous crystalline network after sintering. According to the specific surface area of 168 m2/ g for the TiO2 nanocrystals obtained via the TALH hydrolysis and subsequently hydrothermal treatment,36 one can calculate that 12.0 m2 surface area resulted from the nanocrystal coating in 1.0 g of NCS-TiO2, which corresponded to 0.071 g TiO2 nanocrystals in the coating (7.1 wt %). The calculated value 7.1 wt % was lower than that of TiO2 (10.4 wt %) derived from the prescribed amount of TALH, which showed that some colloidal nanocrystals were not recovered during the centrifugation. Although the coating mass is still not so high, it provides a coating in a thickness of more than 50 nm on the surface of the

Figure 2. (a) Microscope image of 2T photoanode after dye-uptaking process with 40 times magnification. (b) Optical image of 2T−S photoanode and (c) microscope image of 2T-NCS photoanode after dye-uptaking process with 40 times magnification.

originated from the packing voids among the nanocrystals coated on the S-TiO2. At higher relative pressure (0.9 to 1.0),

Figure 3. Top and cross-sectional FE−SEM images of 2T-NCS electrode: (a,b) top view and (c,d) cross-sectional view. 16954

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N719 dye usually shows inferior absorption in the long wavelength region from 700 to 800 nm, and thus it is meaningful to add a scattering layer.37,38 Apparently, 1T-NCS showed the best light utilization ability due to the bifunctions of both light scattering and dye adsorption. Photocurrent−Voltage Property Measurements. The performance of photocurrent density (J) versus photovoltage (V) for cells is plotted in Figure 6, along with detailed

bulk particles (Figure 1b), and it is meaningful to adsorb more dye molecules. Light-Scattering Ability of NCS-TiO2. The light-scattering effect was evaluated by measuring the diffuse reflectance of the films. Owing to the fact that the films composed of S-TiO2 scattering layer and double-transparent porous layer had serious cracks, we evaluated the light-scattering ability by singletransparent porous layer with S-TiO2 (1T-S) and NCS-TiO2 (1T-NCS) scattering layers. Figure 5a shows the UV−vis

Figure 6. I−V curves of DSSC devices made with 1T, 1T-S, 1T-NCS, and 2T-NCS electrodes.

parameters listed in Table 1. It can be seen that the Jsc values of 1T-S and 1T-NCS were about 1.2 and 1.3 times higher than that of 1T, respectively, while the Voc was not appreciably changed. The higher Jsc can be attributed to the better lightharvesting of the photoanodes, which is consistent with the results from UV−vis measurement. The amount of absorbing dye molecules was determined by the following method: the sensitized photoelectrodes were separately immersed into a 20 mL volumetric flask that contained 5 mL of 0.1 M NaOH water/ethanol (1:1 v/v) solution. The amount of dye loaded on the photoanodes was measured by the UV−vis absorption spectrum of the resultant solution using ε = 1.41 × 104 M−1 cm−1 as the molar extinction coefficient for N719 at 515 nm.18 It can be observed that the absorbing amount of dye molecules of 1T-NCS is larger than 1T-S in the same thickness, indicating that coating of the TiO2 nanocrystals on the submicrometer TiO2 increased the amount of dye absorbing. This is coinciding with the analysis of BET. However, the fill factor of 1T-S was much lower than that of 1T-NCS. Presumably, the fact that sintering stress of 1T-S made the photoanodes have many defects. The higher fill factor made 1T-NCS reach an efficiency enhancement of 16% compared with 1T-S. It is attributed to the fact that TiO2 nanocrystals reduced the sintering stress, and the crack-free 2T-NCS in a thickness of 18.17 μm was effortlessly assembled, while the 2T-S showed a serious crack. Finally, 2T-NCS showed a cell efficiency of 8.36%, which was 20% higher than double-transparent porous layers (2T) with the thickness of 11.47 μm. Incident Photon-to-Current Conversion Efficiency Performance. The IPCE spectra for the DSSCs can provide further evidence about the scattering effect and the generating of photoelectrons. In Figure 7, 1T-S and 1T-NCS show better IPCE than 1T in the entire wavelength, whereas in the region of 600−800 nm they are almost the same. Such difference in scattering efficiency might be related to light-scattering of

Figure 5. (a) UV−vis reflectance spectra of different electrodes. (b) UV−vis transmittance spectra of different dye-sensitized electrodes.

diffuse reflectance spectra of different films in the visible-light region. The films with the scattering layers had stronger reflectance than the single-transparent porous layer (1T). The reflectance of 1T-S was ∼85%, while 1T-NCS was decreased to of ∼80%, which suggested that 10 wt % nanocrystal coating resulted in the noticeable reduce of reflectance. So, the enhanced uptaking of dye by the more nanocrystal coating was at the cost of reducing of reflectance. Figure 5b shows the transmittance spectra of the three films after the dye-uptaking process. 1T had the transmittance of ∼30% at the short wavelength region (from 400 to 550 nm) and showed a much higher transmittance at the long wavelength region (from 550 to 800 nm). When the scattering layers of the S-TiO2 and NCS-TiO2 were introduced (1T-S and 1T-NCS), the transmittance of short-wavelength light was almost completely suppressed, and long-wavelength transmittance was