Macro-mesoporous TiO2 Microspheres for Highly Efficient Dye

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Macro-mesoporous TiO2 Microspheres for Highly Efficient Dye-Sensitized Solar Cells Pengfei Liu, Yunfeng Li, Yanjie Hu, Xiaoyu Hou, and Chunzhong Li Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 19 Jun 2015 Downloaded from http://pubs.acs.org on June 19, 2015

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Industrial & Engineering Chemistry Research

Macro-mesoporous TiO2 Microspheres for Highly Efficient Dye-Sensitized Solar Cells Pengfei Liu,† Yunfeng Li,†, ‡ Yanjie Hu,*,† Xiaoyu Hou,† Chunzhong Li,*, † † Key

Laboratory for Ultrafine Materials of Ministry of Education, School of Materials

Science and Engineering, East China University of Science & Technology, Shanghai 200237, China ‡

Shanghai Nanotechnology Promotion Center, Shanghai 200237, China

*To whom correspondence should be addressed.

E-mail: [email protected] (Prof. C. Z. Li) and [email protected] (Prof. Y. J. Hu)

1

ACS Paragon Plus Environment


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ABSTRACT In this work, the novel macro-mesoporous TiO2 microspheres (MMTMs) have been synthesized via a spray drying route with fumed silica (FS) as template, followed by calcination and etching. The as-synthesized MMTMs have unique bimodal porous structures of macropores origin from FS template and mesopores accumulated by TiO2 nanoparticles. The macro-mesoporous structure endows the TiO2 microspheres with better surface area and wonderful light scattering property. When MMTMs are employed as the photoelectrodes of dye-sensitized solar cells (DSSCs), short-circuit current (Jsc) and open-circuit voltage (Voc) are both improved and the high power conversion efficiency (PCE) of 8.68% is obtained eventually, which is much higher than P25 photoelectrode. The excellent performance can be attributed to the excellent light-scattering property, better diffusion of electrolyte as well as superior electron transport owing to the unique bimodal pore structure and the dense packed of the internal nanoparticles of MMTMs.

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1. INTRODUCTION With the superiority of stable, low cost and facility manufacturing process, Dye-sensitized solar cells (DSSCs) have been extensively investigated ever since O'Regan and Grätzel reported their breakthrough in 1991.1-3 As frequently reported, the structural design of the photoanode play a key role in determining the performance of DSSCs, which requires strong light scattering, fast electron transport, and high surface area.4, 5 TiO2 nanoparticles which have a high surface area access to the dye adsorption and thus lead to high conversion efficiency, have been widely used in DSSCs. Grätzel had obtained a high power conversion efficiency (PCE) of solar cells by using the TiO2 nanoparticle films as photoanode6. However, TiO2 nanoparticles have size less than 50 nm, which cannot sufficiently scatter or reflect sunlight. Therefore, a light-scattering layer in photoelectrode has been employed to improve the cell efficiency. Submicrometer-sized TiO2 such as rods7, sheets8, 9 and hollow spheres10-12 have been used as the scattering layers to enhance the back-scattered light for improving cell efficiency. However, the submicrometer-sized materials show lower adsorption ability of dye and faster interfacial electron recombination owing to the lower surface area than that of nanoparticles, which extremely limit the improvement of the performance of the DSSCs. As an ideal scattering material for DSSCs, there are three important requirements: large size to enhance the light scattering, high surface area to obtain the more amount of dye loading and dense packed microstructures for fast electron transport.13-15 Much effort has been motivated to 3



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develop hierarchical structured oxides materials to satisfy these conflicting requirements. For instance, porous TiO2 spheres16-18, TiO2-coated multilayered hollow microspheres19, hierarchically structured ZnO20 and three-dimensional hyperbranched TiO221,

22

have been demonstrated to be efficient light scattering

materials for DSSCs. However, complicated steps and high temperatures as well as long time are required in the fabrication of these materials. Therefore, it is still a challenge to find a facile route for the construction of hierarchical structures of TiO2. In this study, we reported the fabrication of macro-mesoporous TiO2 microspheres (MMTMs) by a simple spray drying process with fumed silica (FS) as template, followed by calcination and chemical etching procedures. The as-prepared MMTMs possessed unique macro-mesoporous structure, dense packed internal nanoparticles structures and high specific surface area. When MMTMs were employed as the photoelectrodes for DSSCs, Jsc and Voc were both improved and led to high PCE eventually, which was much higher than that of commercial P25 photoelectrodes. The light scattering ability, electron lifetime and dye loading of MMTMs were investigated in detail. 2. EXPERIMENTAL SECTION 2.1. Preparation of the MMTMs. 4.39 ml titanium (IV) tetrachloride (TiCl4) was added in 100 mL of cold deionized water (DW), and fumed silica (FS) (Degussa A200, Germany) was added into the TiCl4 solution to form precursor solution by different mass ratios of FS/TiO2 from 0 to 0.8 (TiO2: FS=1: 0 named as MTMs and TiO2: FS=1: 0.8 named as MMTMs0.8). The precursor solution can be sprayed into 4


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fine droplet by air-assisted nozzle (pressure: 0.2 MPa). The droplets underwent evaporation, hydrolysis and then were converted into solid spheres containing SiO2 template through a simple spray drying process (the temperature was kept at 250 °C). Subsequently, the obtained solid precursor spheres were transformed into TiO2/SiO2 composites by annealing at 450 °C for 2 h. Then the composites microspheres were treated by diluted HF aqueous solution (20%) for 10 min to remove the FS template, and followed by centrifugation and rinse with DW for three times. Finally, after drying at 80 °C overnight, the hierarchically macro-mesoporous TiO2 microspheres were obtained. 2.2. Preparation of MMTMs Slurry and DSSCs Fabrication. Transparent FTO glasses were cleaned by ultrasonic with acetone, isopropanol and ethanol for 30 min, respectively. The MMTMs slurry was prepared by mixing α-terpineol and ethyl cellulose with the MMTMs. Various TiO2 electrodes were prepared by screen printing method in this work. First, film of P25 was prepared on the FTO glass and then annealed at 450 °C for 30 min, followed by the treatment of 50 mM TiCl4 aqueous solution and the annealing at 450 °C for 30 min. The second and third electrodes, films of MTMs and MMTMs0.6 which deposited on top of the P25, were annealed at 450 °C for 30 min to remove the organic in the slurry, followed by the treatment of 50 mM TiCl4 aqueous solution and the annealing at 450 °C for 30 min. The sintered electrodes were immersed in a 0.5 mM N719 dye solution with a mixture of tert-butyl alcohol/acetonitrile (volume ratio, 1:1) for 24 h. Similarly, Pt counter electrodes were prepared by drop casting a H2PtCl6 solution on FTO glass 5



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substrates, followed by sintering at 380 °C for 30 min. N719 sensitized electrodes were took out, rinsed with DW and ethanol. Sealed with a thermal adhesive film, a sandwich-type cell was assembled with dye-sensitized electrode and Pt-counter electrode.

The

electrolyte,

which

was

composed

of

0.60

M

BMII

(1-butyl-3-methylimidazolium iodide), 0.03 M I2, 0.50 M 4-tert-butylpyridine, 0.10 M guanidinium thiocyanate and acetonitrile solvent, was injected into the sandwiched cells through holes made in the counter electrodes. 2.3. Characterization. The morphologies of the synthesized TiO2 were observed by scanning electron microscopy (SEM: HITACHI S-4800) and transmission electron microscope (TEM: JEOL JEM-2100). X-Ray Diffraction (Rigaku D/max2550) was used to verify crystallinity. The Brunauer-Emmett-Teller (BET) analysis was used to determine the specific surface area and pore size distribution of the microspheres (nitrogen adsorption-desorption, ASAP 2010N). UV-vis diffuse reflectance spectra (Cary-500 spectrometer) were obtained for light scattering analysis. Photocurrent voltage measurements were carried out by a Newport solar simulator (91160A, Newport). The intensity of the lamp was adjusted to 60 mW cm-2. The incident photon to current conversion efficiency (IPCE) was measured using a 300 W Xe light source (Oriel). Electrochemical impedance spectroscopy (EIS) measurements were carried out using a potentiostat (Versastat, Ametek). The dye-loading-amount measurement was performed by desorbing a dye-loaded photoanode in 0.05 M NaOH solution (1:1 H2O/ethanol).

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Figure 1. Schematic illustration of the MMTMs synthesis procedures.

3. RESLULTS AND DISCUSSION 3.1. Material Characterization. The MMTMs were fabricated via a facile spray drying route with fumed silica as template, subsequently followed by calcination and etching procedures. Figure 1 showed the schematic illustration of the formation of MMTMs. Firstly, the precursor solution including TiCl4 and silica template were sprayed into fine droplets and then these droplets were transferred into solid precursor spheres by a spray drying process at 250 °C. These spheres had relative smooth surface morphology and were composed of the intermediated Ti compounds and silica template (Supporting information Figure S1). After calcination, anatase TiO2 was formed from the intermediated Ti compounds. Finally, with the removal of silica by HF etching, MMTMs were obtained and the bimodal macro-meso pores were formed owing to the unique chain-like structures of fumed silica (Supporting information, Figure S2). Various MMTMs with different pores structures could be designed and fabricated by varying the titanium precursor/FS weight ratio.

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Figure 2. SEM images of (a, b) MMTMs0.6 before etching; (c, d) MMTMs0.6 after the removal of silica template; (e) Element mapping images of MMTMs0.6 before etching; (f) Element mapping images of MMTMs0.6 after etching.

Figure 2 showed the SEM images and element mapping images of MMTMs with 60 wt% fumed silica template. After calcination, the as-prepared composites exhibited a perfect spherical shape (Figure 2a and b) and the rough surface morphologies were observed due to the formation of TiO2 crystal particles and the existence of silica, which was different from the pure MTMs (Supporting information Figure S3). As shown in Figure 2c and d, pores were formed after the removal of fumed silica by HF etching. The multi-modal pore structures could be attributed to the unique chain-like, assembled structures of primary SiO2 particles. All the SEM images showed that the obtained microspheres had a diameter ranging from 2 - 5 µm and the macroscopic morphology remained well after the HF etching. Furthermore, 8


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the element mapping analysis in Figure 2e and f indicated that the MMTMs were composed of Ti and O elements and fumed silica template could be removed clearly by HF etching. However, when increasing the content of fumed silica to 80 wt%, the crushed sample were obtained (Supporting information Figure S4), indicating that more silica template could not facilitate the retaining of pores structures.

Figure 3. (a) Low-magnification, (b) High-magnification, (c) High-resolution TEM images of MMTMs0.6, (d) XRD patterns of MMTMs0.6 before and after calcination.

The TEM were performed to further confirm the morphology structures and macro-mesoporous characteristics of as-synthesized MMTMs. Figure 3a and b showed that the obtained microspheres had an average diameter of 3 µm, which was agreement with the observation of SEM images. In addition, it was clearly observed that the existence of the foam-like and highly porous structure with macro- and meso-pores interconnected in a single sphere. These pores had mesopore diameter ranging from 5-70 nm. A HRTEM image in Figure 3c demonstrates porous spheres 9



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were composed of TiO2 nanocrystallites with diameter of 5-10 nm. It was clear that the TiO2 nanoparticles had good crystallization and the interplanar spacing of the lattice fringes of 0.35 nm further depicted the (101) plane of anatase TiO2. Figure 3d showed the XRD patterns of MMTMs0.6 before and after calcination. No sharp peaks were found before calcination, suggesting that the sample was amorphous phase after spray drying. However, after calcination, the sharp peaks indicated that the sample had good crystalline nature. The diffraction peaks could be indexed to the anatase TiO2 (JCPDS NO. 21-1272). To further investigate internal pore structures of the MMTMs, the BET surface area and the corresponding pore size distribution were characterized. After the removal of fumed silica template, a high specific area of 116.2 m2/g was obtained for the MMTMs0.6 (Supporting information, Figure S5). Clearly, as shown in Table 1, with the increase of the content of fumed silica, the corresponding specific surface area was also increased. The highest specific surface area of 129.7 m2/g was obtained by 40 wt% silica template. There was a balance of specific surface area and pore size at 40 wt% silica. When the FS concentration reached to 60 wt%, pore size increased and the specific surface area decreased. The microsphere was crushed when the FS concentration reached to 80 wt%. The corresponding pore size distribution was also depicted for studying the pore information (Supporting information Figure S6). Pure TiO2 microspheres had only tiny meso-pores with a diameter of 3.5 nm. Bimodal pore structures were observed for average pore diameter of 10 and 40 nm with fumed silica as template. For 10


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MMTMs0.6, a broad pore size distribution of 20-100 nm was obtained. When the concentration of silica increased to 80 wt%, the large pores disappeared because of the crush of microspheres structures. The broad pore size distribution might play a critical role in the scattering of the incident light. Table 1. The performance of DSSCs with various TiO2 photoelectrodes Samples

Jsc

Voc 2

FF

η

Adsorbed dye (10 mol/cm )

(m2/g)

65.87

7.50

0.97

49.5

833

68.06

5.58

0.47

46.1

13.41

819

65.86

7.23

0.51

78.3

MMTMs0.4

15.82

819

64.98

8.42

0.78

129.7

MMTMs0.6

16.01

827

65.54

8.68

0.60

116.2

MMTMs0.8

14.55

824

66.71

8.00

0.64

117.9

(mV)

P25

14.82

768

MTMs

9.84

MMTMs0.2

2

SBET

(%)

(mA/cm )

-7

3.2. Photovoltaic Performance. To investigate the photovoltaic performance of MMTMs, various photoelectrodes of ~7.5 µm MMTMs deposited on top of ~5 µm P25 were fabricated (Supporting information, Figure S7). For comparison, photoelectrode of P25 was also fabricated. The thicknesses of all films were controlled to be ~12.5 µm and the active area was 0.25 cm2. The corresponding performance parameters of various photoelectrodes were shown in Table1. The BET surface area and dye loading was improved with the increase concentration of FS from 0 to 40 wt%, leading to the increase of Jsc (from 9.84 to 15.82 mA/cm2). When the concentration of the FS exceeded to 40 wt%, the larger macropores were formed owing to the aggregation of the FS, thus the surface area and dye loading were decreased. Interestingly, as the larger pore size could also facilitate the diffusion of electrolyte, the Jsc was improved from 15.82 to 16.01 11



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mA/cm2 even though MMTMs0.4 possessed the better light-scattering property (Supporting information, Figure S8d) and the more dye loading (Table 1). When the concentration of the FS exceeded to 60 wt%, the Jsc was decreased. This might be attributed to the declining of scattering ability resulting from the crush of microspheres. As a result, MMTMs0.6 showed the best photovoltaic performance (η =8.68%).

Figure 4. (a) I-V curves, (b) IPCE spectra, (c) Nyquist diagrams of the EIS in the dark, (d) Diffuse-reflectance spectra.

The photocurrent (I)-voltage (V) of P25, MTMs and MMTMs0.6 were shown in Figure 4a. The MMTMs0.6 showed the best PCE of 8.68%, corresponding to a 15.7% and 55.6% increment compared to P25 (7.50%) and MTMs (5.58%). It was worth noting that, compared to P25, Jsc and Voc of MMTMs0.6 were improved by 8.0% 12


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(14.82 to 16.01 mA/cm2) and 7.7% (768 to 827 mV), respectively. This could be attributed to the enhanced scattering effect and the suppressing of photogenerated electrons recombination by dense nanoparticles in MMTMs0.6. Compared with MTMs, the Jsc of MMTMs0.6 was improved from 9.88 to 16.01 mA/cm2 as a result of the higher surface area and the more dye loading. However, the Voc of MTMs was higher than that of MMTMs, this mainly attributed to the integrity of the macrosphere was damaged and the path of charge transfer was frustrated after etching. On account of the excellent Jsc and Voc, MMTMs0.6 revealed the greatest DSSCs performance of the three electrodes. To elucidate the advantageous properties of MMTMs0.6 over the P25 and MTMs, the corresponding IPCE curves were measured. As shown in Figure 4b, the largest IPCE value in three spectra was obtained with MMTMs0.6 as the scattering layer. The increase in short wavelength (300-600 nm) could be contributed to the enhanced light scattering intensity and the high dye loading amount, while that the increase in wavelength of 600-800 nm was only result from the enhanced light scattering effect. As known, suppressing the back electron transfer could improve the Voc and FF of DSSCs23. Electron transfer properties were investigated by electrochemical impedance spectroscopy (EIS) without lights. Figure 4c showed the Nyquist plots of the DSSCs which assembled with three different electrodes. The equivalent circuit was in the inset and the corresponding resistance and electron lifetime values were shown in Supporting Information, Table S1. Two semicircles were observed from the Nyquist diagrams. The semicircles in the high-frequency region represented the 13



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charge transfer resistance at the Pt/electrolyte interface, and the semicircles in the medium-frequency region represented the charge transfer resistance at the photoelectrode/dye/electrolyte interface23. It was clear that the semicircles for MTMs and MMTMs0.6 were remarkably larger, indicating that the recombination resistance were much larger. According to the equation τr =R2×CPE2, lifetime (τr) values of photoelectrons in photoanode could be calculated. R2 represented the recombination resistance and CPE2 represented chemical capacitance24. As shown in Table S1, the τr of P25, MTMs and MMTMs0.6 was 32 ms, 103 ms and 89 ms, respectively. The fact confirmed the excellent property of slower charge recombination rate of MTMs and MMTMs which possessed dense internal nanopaticals with respect to reference P25 which had numerous defects, grain boundaries and surface states. With the longer electron lifetime and slower charge recombination rate, DSSCs which based on MTMs and MMTMs showed a higher Voc. Light-scattering performance of the three films was revealed by diffuse-reflectance spectra (Figure 4d). The higher reflectivity meant more excellent light-scattering property, and indicating better light harvesting efficiency of the photoelectrodes. As the macro-mesoporous and the individual spheres could both act as light-scattering center, MMTMs0.6 revealed an excellent light scattering property (Figure 4d) compared to P25 in 400-800 nm. Due to the presence of macroporous, MMTMs0.6 shows a better light-scattering in 400-600 nm compared to MTMs. The light-scattering property of MMTMs0.6 and MTMs was similar in 600-800 nm.

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Figure 5. (a) Light scattering effect in films and (b) internal microsphere.

There were three primary factors contributing to the excellent DSSCs performance of the MMTMs electrode. Firstly, when MMTMs were employed in electrode, as the individual spheres and the macro-mesoporous could both act as light-scattering center (Figure 5), the electrode possessed the superior light harvesting efficiency, which could be observed from the diffuse-reflectance spectra. The enhanced light scattering could obviously increase the quantity of photogenerated electrons and enhance the DSSCs performance. Secondly, the macro-mesoporous structure could also facilitate the diffusion of electrolyte in the electrode which contribute to the higher Jsc. Thirdly, the

dense

internal

nanoparticals

suppressed

the

photogenerated

electrons

recombination at TiO2/dye/electrolyte interface, Voc and FF were improved which could be observed from the Nyquist diagrams. With the excellent light-scattering and lower charge recombination, MMTMs0.6 leads to considerably improving of power conversion efficiency.

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4. CONCLUSION In summary, we have fabricated the bimodal macro-mesoporous TiO2 microspheres via spray drying process. The as-synthesized MMTMs0.6 has large diameter (2-5 µm), macro-mesoporous structure, high surface area (116.2 m2/g) and dense packed of internal nanoparticles. Therefore, the bimodal macro-mesoporous TiO2 microspheres can enhance light scattering, facilitate the diffusion of electrolyte as well as electron transport in photoelectrode. The DSSCs assembled with MMTMs0.6 shows a highest power conversion efficiency of 8.68%. These advantages make MMTMs0.6 an ideal light scattering material in DSSCs. These results have demonstrated that aerosol spray drying with FS as template is an effective route for design and fabrication of porous materials.

AUTHOR INFORMATION

Corresponding Author *Fax: +86 21 64250624. Tel: 86 21 6425 0949. E-mail: [email protected] (Prof. C. Z. Li); [email protected] (Prof. Y. J. Hu). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21322607, 21371057), the Basic Research Program of Shanghai (14JC1490700), the Special Research Fund for the Docoral Program of Higher Education of China (20120074120004), the Shanghai Rising-Star Program (13QA1401100), the Research 16


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Project of Chinese Ministry of Education (113026A), Project funded by China Postdoctoral Science Foundation (2014M561497) and the Fundamental Research Funds for the Central Universities.

Supporting Information Available TEM images of fumed silica template. SEM images of precursor spheres, MTMs, MMTMs0.8. Nitrogen adsorption desorption isotherms of MMTMs0.6. BET surface area of different fumed silica concentration and Pore size distribution of different fumed silica concentration. Cross-sectional SEM image of the MMTMs electrode. I-V curves, IPCE spectra, Nyquist diagrams and Diffuse-reflectance spectra of various DSSCs. Detailed simulative value of recombination resistance (R2) and electron lifetime value (τr) from EIS spectra calculated by equivalent circuit. This information is available free of charge via the Internet at http://pubs.acs.org/.

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For Table of Contents Only

The unique macro-mesoporous structure endows the TiO2 spheres with excellent light scattering property and high specific surface area. Moreover, as the dense packed of the internal nanoparticals, electron transport was also facilitated. In a result, DSSCs of MMTMs electrode exhibits high power conversion efficiency of 8.68%.

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Macro-mesoporous TiO2 Microspheres for Highly Efficient Dye

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