TiO2 Hollow Spheres with Synergistic Effect of Plasmonic

Research Article www.acsami.org

Au/TiO2 Hollow Spheres with Synergistic Effect of Plasmonic Enhancement and Light Scattering for Improved Dye-Sensitized Solar Cells Yue-Ying Li,† Jian-Gan Wang,*,† Xing-Rui Liu,† Chao Shen,† Keyu Xie,† and Bingqing Wei*,†,‡ †

State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), Xi’an 710072, China ‡ Department of Mechanical Engineering, University of Delaware, Newark, DE19716, United States ABSTRACT: Au-decorated TiO2 hollow spheres (Au-THS) have been successfully synthesized via a facile one-pot solvothermal method. The Au-THS hybrid features unique hollow structure with a large specific surface area of 120 m2 g−1 and homogeneous decoration of Au nanoparticles, giving rise to enhanced light harvesting and charge generation/separation efficiency. When incorporated into the active layer of dye-sensitized solar cells (DSSCs), an improved power conversion efficiency of 7.3% is obtained, which is increased by 37.7% compared with the controlled P25 DSSC. The underlying mechanism to rationalize the efficiency enhancement can be mainly attributed to the strong synergistic effect of superior light scattering ability of the THS and the plasmonic-enhanced effect rendered by the Au nanoparticles. KEYWORDS: titanium oxide, hollow structure, dye-sensitized solar cell, gold nanoparticle, plasmonic-enhanced effect

■

INTRODUCTION Dye-sensitized solar cells (DSSCs) have received intense scientific and technical interest because of their distinct characteristics of low cost, environmental friendliness, and easy fabrication, as demonstrated in the pioneer report by O’Regan and Grätzel in the early 1990s.1,2 To improve the photovoltaic performance of DSSCs, considerable efforts have been given to new photoanodes,3 electrolytes,4,5 sensitizers,6−8 and counter electrodes.9−12 Of particular note, the photoanode is critical for the DSSC performance, because it plays a crucial role in light harvest and charge transfer promotion. The photoanode materials could be n-type semiconductors with high porosity, nanostructured morphology, and a wide band gap, such as TiO2, ZnO, SnO2, SrTiO3, and so on.13−16 Among these semiconductors, TiO2 has attracted widespread attention owing to its eco-friendliness, low cost, and high chemical/ optical stability.17−21 It should be noted that TiO2 possesses a wide band gap of 3.2 eV, which can only absorb UV light (occupy only ∼5% of the full solar spectrum), leading to a low utilization efficiency of solar energy. To improve the visible light harvesting efficiency, an effective route is to make a rational structure design. Hollow structure is one of the favorable architectures that can render an excellent light-scattering effect.22,23 The most popular strategy to fabricate TiO2 hollow spheres (THS) involves the use of sacrificial hard/soft templates of silica,24 polymer,25,26 carbon particles,27 emulsions,28 and gas bubbles.29 Those methods are advantageous in the versatile preparation of hollow spheres, however, at the expense of high-cost and tedious synthetic © 2017 American Chemical Society

procedures. Therefore, it is highly desirable to explore convenient and cost-effective synthetic strategies to obtain THS with robust hollow structures and high specific surface areas. In addition to the structural design strategy, incorporating metal nanoparticles into DSSCs has emerged as another promising approach to augment light harvesting, carrier generation, and enhanced efficiency.30−35 Upon light illumination, metal nanoparticles contained in the localized plasmonic oscillations transfer energy to the adjacent semiconductor or sensitizer for inducing electron/hole pair generation and separation. Particularly, gold (Au) nanoparticles have been widely studied owing to the high chemical stability and strong visible-light absorption over a wide range. Muduli et al. reported the synthesis of anatase TiO2 nanoparticles (15−20 nm) decorated with Au nanoparticles, and the DSSCs based on the TiO2−Au nanocomposite showed 20% enhancement of photo conversion efficiency (PCE) compared with that based on pure TiO2 nanoparticles.33 Shah et al. introduced Au nanoparticles into the DSSC photoelectrode and achieved 15.23% PCE enhancement.34 Therefore, it is of great interest to combine the light scattering and plasmonic effect by integrating hollow structure and Au nanoparticles into one unit to collectively maximize their light-harvesting effect and carrier generation ability. Received: April 1, 2017 Accepted: August 28, 2017 Published: August 28, 2017 31691

DOI: 10.1021/acsami.7b04624 ACS Appl. Mater. Interfaces 2017, 9, 31691−31698

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) XRD patterns and (b) Raman spectra of THS and Au-THS, respectively. method. The thickness of the printing film was ∼3 μm, and the active areas were fixed at 0.25 cm2 (0.5 cm × 0.5 cm). The organics were removed by sintering at 450 °C for 30 min. Subsequently, the asprepared film was immersed into 0.5 mM N719 ruthenium dye ethanol solution for 18 h at room temperature in the dark. The dyesensitized films were rinsed with ethanol to remove the physically absorbed dye molecules and dried at 60 °C for 15 min. The sensitized photoanodes were assembled with the Pt-modified counter electrode and I−/I3− based liquid electrolyte. The device was characterized without any encapsulation. Characterization and Measurement. The crystalline structure was obtained by using X-ray diffraction (XRD, X’Pert PRO MPD, Philips) with a Cu Kα radiation (λ = 1.5418 Å). Raman spectrum was recorded on Renishaw Invia Raman microscope (laser wavelength: 532 nm) at room temperature in the spectral range of 75−1100 cm−1. The morphology and structure of samples were characterized with a field emission scanning electron microscopy (FE-SEM, FEI Nano SEM 450) and transmission electron microscopy (TEM, FEI Tecnai F30G2). The pore structures were characterized by the nitrogen absorption/desorption measurement at 77K (Belsorp, Japan). Brunauer−Emmett−Teller (BET) method and nonlocal density functional theory (NLDFT) calculation were used to determine the specific surface area and the corresponding pore size distribution. The UV−vis spectra were measured by ultraviolet−visible (UV−vis) spectrophotometer (PerkinElmer Lambda 35 UV−vis-NIR). To measure the amounts of dye absorption, the N719-sensitized TiO2based composite photoanodes were immersed in a 10 mL of 0.1 M NaOH water/ethanol (v/v = 1:1) solution to remove the dye molecules. The solution was analyzed by UV−vis spectroscopy to determine the dye absorption amounts by using the adsorption peak intensity (A) of N719 at 515 nm based on the Beer−Lambert law, i.e., A = k × c × l; here k is a constant (1.41 × 104 mol L−1 cm−1), c is the concentration of the dye, and l is the thickness of the quartz cuvette (i.e., 1 cm). Current density/voltage (J−V) measurements were carried out on a Keithley model 2420 digital source meter controlled by Test point software under a standard light intensity of 100 mW· cm−2 from Air Mass (AM) 1.5G solar simulator. The light intensity was calibrated using a standard reference silicon solar cell equipped with a KG-5 filter in order to reduce the mismatch between simulated light and standard AM1.5G solar light in the region of 350−700 nm to less than 2%. A black mask (0.25 cm2) was attached on top of the device to avoid stray light completely. Incident photon-to electron conversion efficiency (IPCE) were measured with monochromatic incident light produced by a 300 W xenon lamp in DC mode (Newport, 2936-R). Electrochemical impedance spectrum (EIS) were measured by the electrochemical workstation (CHI660C, Shanghai, China) under light irradiation at open-circuit potential, with the frequency ranging from 0.1 Hz to 105 Hz and the perturbation amplitude of 10 mV.

In this work, Au-decorated TiO2 hollow spheres (Au-THS) are synthesized in order to achieve their dual functions of efficient light-scattering and plasmonic-enhanced effect. The experimental results indicated that the DSSC performance is greatly improved by incorporating Au-THS into the active layer of the photoanode. The optimized device can give rise to a high PCE of 7.3%, which is about 37.7% enhancement compared with the controlled P25 DSSC (5.3%). The performance improvement can be attributed to the unique Au-THS structure that holds synergistic effects of light-scattering and plasmonic enhancement to promote light harvest and carrier generation/ separation efficiency.

■

EXPERIMENTAL SECTION

Materials Synthesis. All chemicals were analytical grade reagents and used without further purification. THS were prepared via a facile template-free solvothermal method. Briefly, tetrabutyl titanate (1 mL) was added to a mixture of acetylacetone (5 mL) and isopropanol (40 mL) to obtain an orange-yellow precursor under magnetic stirring for 30 min at room temperature. The precursor solution was then transferred into a Teflon-lined autoclave (100 mL) and kept at 200 °C for 10 h. After the yellow precipitates were cooled to room temperature, they were washed with ethanol several times via centrifugation and dried at 90 °C in air. Au-THS were prepared by in situ deposition of Au nanoparticles on the surface of THS. The loading amount of Au nanoparticles is controlled by the ratio of Au-precursor (HAuCl4) and TiO2, which is targeted to be about 1 wt % with particle sizes in the 5−15 nm range. Typically, the as-synthesized THS (0.25 g) was dispersed in 25 mL of ethanol, and then 0.25 mL of (3-aminopropyl) trimethoxysilane (APTMS) was added. The mixture was stirred vigorously for 6 h to ensure uniform modification of APTMS on the surface of THS, followed by filtration and washing. The as-collected APTMS-modified THS were redispersed into a mixture of HAuCl4·3H2O and ethanol under stirring for 24 h. The precipitates were washed with ethanol and dried at 90 °C overnight. Finally, the as-prepared THS and Au-THS were calcined at 500 °C for 4 h. DSSC Assembly. As the current collector, fluorine-doped tin oxide (FTO) glass was cut into rectangular pieces with 1.5 × 2 cm2 and then ultrasonically cleaned with cleanser essence, acetone, deionized water, and ethanol, respectively, and finally dried under N2 flow to ensure a clean surface. The photoanodes were prepared according to our previous work.16 The TiO2 blocking layer was prepared through a sol− gel and spin coating method. The absorbing layers were screen-printed onto the blocking layer. Typically, 0.35 g of mixture powder with different Au-THS/P25 mass ratio (0, 0.3, 0.5, 1, 2, and 3 wt %) were added into 1 mL of DI water, 0.1 mL of terpineol, and 0.3 mL of glacial acetic acid in a agate mortar to form paste, which was then printed onto the blocking layer of FTO substrate by screen-printing 31692


Research Article


Figure 2. FE-SEM images of THS (a,b) and Au-THS (c,d). (e−g) EDS mapping images of Ti, O, and Au elements, respectively.

■

RESULTS AND DISCUSSION

density of TiO2 is enhanced by the electron transfer from Au nanoparticles to the TiO2 conduction band.41 The morphology of the as−prepared THS and Au-THS samples is examined by FE-SEM. As shown in Figure 2a, the THS are composed of uniform submicron spheres with an average diameter of around 900 nm. The high-resolution SEM in Figure 2b displays rough surface of the spheres, which is constructed by small TiO2 nanoparticles. In addition, the hollow interior is clearly observed from some broken spheres, which is favorable for enhancing the light scattering ability.28,29 The sphere shape is well-preserved when Au nanoparticles are decorated onto THS (Figure 2c,d). The homogeneous distribution of Ti, O, and Au elements in the EDS mapping (Figure 2e−g) manifests the successful incorporation of Au. Figure 3a shows the typical TEM image of the Au-THS hybrid spheres. The sharp contrast between the edge and center demonstrates the clear hollow structure. The average diameter of the hollow interior is ∼700 nm, and the shell thickness is estimated to be ∼90 nm, which are in good agreement with the SEM observation. Figure 3b exhibits the corresponding high-resolution TEM (HRTEM) image of AuTHS, in which the lattice spacings of 0.35 and 0.24 nm can be attributed to the (101) crystal planes of anatase TiO2 phase and the (111) crystal planes of Au NPs, respectively. The HRTEM image also indicates that both TiO2 and Au NPs are of highly crystalline nature, which is consistent with the characteristic peaks of the XRD patterns.

The phase structure of the as-prepared samples were characterized by XRD. As shown in Figure 1a, the diffraction peaks of THS at around 25.6°, 37.9°, 48.2°, 54.2°, 55.3°, 62.9°, 69.0°, 70.7°, and 75.3° can be readily indexed to (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of anatase TiO2 (JCPDS no. 21-1272). Subsequent treatment of THS with APTMS and HAuCl4 was carried out to decorate Au nanoparticles on the surface of THS, and the corresponding XRD pattern was displayed in Figure 1a. In addition to the diffraction peaks of the anatase TiO2, another four peaks appeared at 38°, 44°, 64°, and 77° can be wellassigned to (111), (200), (220), and (311) planes of Au nanoparticles, respectively.30,32 Raman spectra were employed to confirm the crystallographic structure of Au-THS. As shown in Figure 1b, both spectra of the THS and Au-THS samples exhibit identical Raman peaks centered at 142, 197, 396, 515, and 636 cm−1, which can be ascribed to the Eg, Eg, B1g, B2g, and Eg modes of the anatase TiO2, respectively.33,36,37 It is worth noting that the Raman peak at ∼142 cm−1 is red-shift with higher intensity for the case of Au-THS sample. The intensity enhancement phenomenon is presumably resulting from the plasmonic electromagnetic effect induced by Au nanoparticles,38−40 while the red-shift can be ascribed to the fact that the electronic 31693


Research Article


IPCE measurement was employed to investigate the effect of the Au-THS incorporation amount on the light-harvesting capability. As shown in Figure 5b, the intensity of IPCE increases with the increasing incorporation amount of Au-THS from 0.3 wt % to 1.0 wt % and then decreases with more incorporation, the trend of which is identical to the Jsc, indicating the presence of 1.0 wt % Au-THS maximizes the light-harvesting capability. The enhanced light-harvesting capability is further confirmed by UV−vis absorption. As shown in Figure 5c, all the photoanodes possess strong absorption intensity within the wavelength range. The absorption intensity increases with the increasing Au-THS incorporation amount, indicating enhanced optical absorption properties in the presence of Au-THS. It is noted that the 3.0 wt % Au-THS-DSSC device exhibits poor photovoltaic performance albeit with the highest absorption intensity, which may be attributed to the excess Au-TiO2 incorporation that results in increased trapping of photogenerated electron and increased light absorption by Au nanoparticles, which transforms/part of the incident solar power into heat.42 This hypothesis is validated by the dark J−V curves, as shown in Figure 5d. The dark current represents the recombination of electrons with I3− ions, which would result in the loss of photocurrent.43 Compared with the 1.0 wt % Au-THS-device, the dark current onset of the 3.0 wt % Au-THS-device shifts to a lower potential, and a larger dark current is produced at the same potential. These observations indicate a higher electron− hole recombination rate between transferred electrons and I3− ions in the 3.0 wt % Au-THS-device. Therefore, a particular incorporation amount of Au-TiO2 composite could give rise to strong light-harvesting capability and high charge generation/ separation efficiency, and thus substantially enhance the DSSC performance. To investigate the role of hollow structure and Au nanoparticles in the light-harvesting capability, DSSCs with 1.0 wt % THS and Au-THS incorporation were fabricated. Figure 6a shows the J−V curves of the DSSC devices. It is observed that the Jsc of the DSSC is significantly increased after THS and Au-THS incorporation. Specifically, the Au-THSDSSC and the THS-DSSC exhibit higher Jsc of 18.71 and 16.40 mA cm−2 than the controlled P25-DSSC, thereby rendering enhanced PCE of 7.30% and 6.90%, respectively. The higher PCE can be attributed to the enhanced light-harvesting capability, which is confirmed by the IPCE. As shown in

Figure 3. (a) TEM and (b) HRTEM images of Au-THS.

The specific surface area and pore size distribution of THS and Au-THS were investigated through nitrogen adsorption/ desorption measurement. The resulting N2 adsorption/ desorption isotherms in Figure 4a shows type-IV isotherm plots with an H2 hysteresis loop. The BET specific surface area of THS and Au-THS are ∼126 and ∼120 m2 g−1, respectively, indicating the introduction of Au nanoparticles exerts a negligible effect on the textural structure. Figure 4b displays the corresponding pore size distribution of the samples. Both THS and Au-THS samples are of mesoporous structure with a narrow pore size distribution centered at 4−5 nm. Figure 5a shows the J−V curves of the DSSCs with different Au-THS incorporation amounts. It is observed that the incorporation of Au-THS does not change the open circuit voltage (Voc) of the DSSC (∼0.700 V), because Voc is primarily determined by the energy difference between the Fermi level of the illuminated TiO2 semiconductor and the Nernst potential of I−/I3− redox couple electrolyte.1 The short-circuit current density (Jsc) value increases to 15.70, 17.30, and 18.71 mA cm−2 when 0.3, 0.5, and 1.0 wt % of the Au-THS composite is incorporated into the photoanode, respectively. Higher incorporation content of 2.0 and 3.0 wt % would reduce the Jsc to 16.80 and 16.01 mA cm−2, respectively. Table 1 summarizes the photovoltaic performance of DSSCs in our work. Encouragingly, the Au-THS-DSSC with an optimal incorporation amount of 1.0 wt % manifests a maximum PCE of 7.30%, which is 37.7% higher than that of the controlled P25-based DSSCs (5.30%). It should be worth noting that the PCEs of DSSCs herein are based on a thin photoanode film of 3 μm, which can be further improved by optimizing the film thickness. As shown in Table 1, the PCE of the 1.0 wt % AuTHS-DSSCs could reach 8.31% when the film thickness is increased to ∼7.5 μm.

Figure 4. (a) Nitrogen adsorption/desorption isotherms and (b) pore size distribution of THS and Au-THS. 31694


Research Article


Figure 5. J−V curves (a), IPCE (b), UV−vis absorption spectra (c), and dark J−V curves (d) of the DSSCs with different amounts of Au-THS.

Table 1. Photovoltaic Performance of the DSSCs sample 1.0 wt % Au-THS 0.3 wt % Au-THS 0.5 wt % Au-THS 2.0 wt % Au-THS 3.0 wt % Au-THS 1.0 wt % THS P25 P25 1.0 wt % Au-THS

Voc (V) 0.700 0.700 0.700 0.700 0.690 0.700 0.690 0.700 0.700

± 0.009 ± 0.001 ± 0.007 ± 0.005 ± 0.003 ± 0.002 ± 0.002 ± 0.002 ± 0.005

Jsc (mA cm−2) 18.71 15.70 17.30 16.80 16.01 16.40 12.91 16.60 20.80

FF (%)

± 0.02 ± 0.03 ± 0.04 ± 0.05 ± 0.03 ± 0.04 ± 0.03 ± 0.04 ± 0.06

56.10 57.01 56.21 57.10 58.03 60.12 59.23 56.11 57.32

± 0.41 ± 0.12 ± 0.74 ± 0.58 ± 0.46 ± 0.31 ± 0.22 ± 0.20 ± 0.38

PCE (%) 7.30 6.26 6.80 6.71 6.41 6.90 5.30 6.51 8.31

± 0.07 ± 0.06 ± 0.01 ± 0.02 ± 0.03 ± 0.02 ± 0.01 ± 0.04 ± 0.03

film thickness (μm) 3.0 3.0 3.0 3.0 3.0 3.0 3.0 7.5 7.5

is calculated from the peak intensity at 515 nm based on Beer− Lambert’s law. It is estimated to be about 4.88 × 10−7, 4.23 × 10−7, and 3.56 × 10−7 mol cm−2 for the P25-, THS-, and AuTHS-anodes, respectively. These results suggest that the incorporation of HTS or Au-HTS would result in a lower dye absorption amount due to the existence of large hollow interior. A lower dye absorption value typically shows a lower PCE of DSSC. However, the THS- and Au-THS-DSSCs possess much higher PCEs even with a small dye absorption amount. The better photovoltaic performance reveals the collectively enhancement of light scattering and plasmonic effect caused by the unique hollow structure and Au nanoparticles in the Au-THS composite, which could ultimately increase the light harvesting capability and the charge generation/separation efficiency. EIS is used to deep study the carrier transportation and recombination at the electrolyte/electrode interfaces of DSSC.

Figure 6b, the IPCE values of the THS- and Au-THS- DSSCs are much higher than that of the controlled P25-DSSC in the light wavelength range of 400−800 nm. Figure 6c further presents much higher UV−vis absorption intensity of the DSSCs with THS and Au-THS incorporation, which is consistent with the IPCE results. The enhanced optical absorption property of the THS-DSSC can be ascribed to the strong light scattering rendered by the hollow structure of THS. In addition, when a same amount of the Au-THS composite is incorporated into the P25 photoanode, the absorption intensity is higher than that of the THS photoanode. The better optical absorption can be resulted from the plasmonic effect caused by the Au nanoparticles. The significant role of hollow structure and Au nanoparticles is validated by analyzing the absorption amount of the N719 dye in the photoanodes.40 Figure 6d exhibits the corresponding UV−vis absorption curves, in which the dye absorption amount 31695


Research Article


Figure 6. (a) J−V curves, (b) IPCE curves, and (c) UV−vis absorption spectra of the controlled P25, THS, and Au-THS-DSSCs. (d) Absorption spectra of the N719 dye desorbed from photoanode.

Figure 7. Nyquist plots (a) and Bode phase plot of the EIS spectra (b) of the controlled P25, THS, and Au-THS-DSSCs.

Figure 7a shows the resulting Nyquist plots. Typically, there are three semicircles in the Nyquist diagram as the frequency increase, which are corresponding to the Warburg diffusion Ws of the electrolyte (Ws), the charge transfer resistance within the photoanode-dye/electrolyte interface (R2) and Pt/electrolyte interface (R1), respectively.44 The Ws and R1 values are almost identical because of the similar conducting substrates, counter electrodes, and I−/I3− electrolytes. The only difference is R2,

which is determined by the diameter of the semicircle component. Clearly, the Au-HTS-DSSC possesses the smallest R2 value, thus facilitating the electron transport rate that may minimize the electron/hole recombination. The electron lifetime (τe) during the photovoltaic process can be obtained from the bode phase plots of EIS according to τe = 1/ (2πf max),45 here f max is the frequency at the maximum phase angle. As shown in Figure 7b, the f max of Au-THS-DSSC (55 31696


Research Article


Figure 8. Schematic illustration of the DSSC performance enhancement caused by Au-TiO2 architecture.

■

ACKNOWLEDGMENTS The authors acknowledge the financial supports of this work by the National Natural Science Foundation of China (51772249, 51402236, 51472204), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.: 123-QZ-2015), the Key Laboratory of New Ceramic and Fine Processing and State Key Laboratory of Control and Simulation of Power System and Generation Equipment (Tsinghua University, KF201607, SKLD17KM02), the Fundamental Research Funds for the Central Universities (G2017KY0308), and the Program of Introducing Talents of Discipline to Universities (B08040).

Hz) is much lower than that of the P25- (271 Hz) and HTSDSSCs (141 Hz), implying a longer electron lifetime that enables a smaller charge recombination probability and thus a substantial improvement of the Jsc. Figure 8 illustrates the schematic of the Au-TiO2 architecture for the DSSC performance enhancement. First, the uniform hollow structure could extend the light transfer distance in the photoanode by scattering light, enabling more light absorbed by the N719 dye molecules to generate hole−electron pairs effectively. Second, the Au nanoparticles on the shell could induce localized plasmonic oscillation to transfer hot electrons to the adjacent TiO2 semiconductors upon light illumination, and finally, accelerate the electron−hole pair generation/ separation efficiency.

■

■

(1) O’Regan, B.; Grätzel, M. A Low-cost, High-efficiency Solar Cell Based on Dye-Sensitized. Nature 1991, 353, 737−740. (2) Bach, U.; Lupo, D.; Comte, P.; Moser, J.; Weissortel, F.; Salbeck, J.; Spreitzer, H.; Grätzel, M. Solid-state Dye-sensitized Mesoporous TiO 2 Solar Cells with High Photon-to-Electron Conversion Efficiencies. Nature 1998, 395, 583−585. (3) Bella, F.; Griffini, G.; Gerosa, M.; Turri, S.; Bongiovanni, R. Performance and Stability Improvements for Dye-sensitized Solar Cells In the Presence of Luminescent Coatings. J. Power Sources 2015, 283, 195−203. (4) Kim, J.; Lee, C.; Lee, S.; Cho, H.; Kim, J. Bimodal Porous TiO2 Structures Templated by Graft Copolymer/Homopolymer Blend for Dye-Sensitized Solar Cells with Polymer Electrolyte. J. Power Sources 2016, 336, 286−297. (5) Yella, A.; Lee, H.; Tsao, H.; Yi, C.; Chandiran, A.; Nazeeruddin, M.; Diau, E.; Yeh, C.; Zakeeruddin, S.; Grätzel, M. Porphyrinsensitized Solar Cells with Cobalt(II/III)-based Redox Electrolyte exceed 12% Efficiency. Science 2011, 334, 629−633. (6) Hao, Y.; Saygili, Y.; Cong, J.; Eriksson, A.; Yang, W.; Zhang, J.; Polanski, E.; Nonomura, K.; Zakeeruddin, S.; Gratzel, M.; Hagfeldt, A.; Boschloo, G. A Novel Blue Organic Dye for Dye-Sensitized Solar Cells Achieving High Efficiency in Cobalt-based Electrolytes and By CoSensitization. ACS Appl. Mater. Interfaces 2016, 8, 32797−32804. (7) Lu, F.; Wang, X.; Zhao, Y.; Yang, G.; Zhang, J.; Zhang, B.; Feng, Y. Studies on D-A-p-A Structured Porphyrin Sensitizers with Different Additional Electron-withdrawing Unit. J. Power Sources 2016, 333, 1− 9. (8) Eom, Y.; Choi, I.; Kang, S.; Lee, J.; Kim, J.; Ju, M.; Kim, H. Thieno [3,2-b][1] benzothiophene Derivative as A New π-bridge Unit in D−π−A Structural Organic Sensitizers with Over 10.47% Efficiency for Dye-Sensitized Solar Cells. Adv. Energy. Mater. 2015, 5, 1500300. (9) Dong, J.; Wu, J.; Jia, J.; Fan, L.; Lan, Z.; Lin, J.; Wei, Y. Cobalt Selenite Dihydrate as An Effective and Stable Pt-free Counter

CONCLUSIONS Au-THS submicron hollow spheres have been successfully synthesized via a facile template-free and one-spot solvothermal method. The distinct hollow structure and the presence of Au nanoparticles could improve the light harvesting capability and charge generation/separation efficiency by synergistic effect of light scattering capability and plasmonic enhancement. The incorporation amount of Au-THS is found to show significant influence on the photovoltaic performance of DSSCs. A maximum power conversion efficiency of 7.30% is achieved when 1 wt % Au-THS is incorporated into the photoanode, which is increased by 37.7% relative to the controlled P25DSSC. The enhanced DSSC performance demonstrates that hollow spheres combined with noble nanoparticles are applicable to optimize device performance, such as solar cell and photocatalyst.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.-G.W.: [email protected]. *E-mail for B.W.: [email protected]. ORCID

Jian-Gan Wang: 0000-0001-5582-0573 Keyu Xie: 0000-0001-7719-9095 Bingqing Wei: 0000-0002-9416-1731 Notes

The authors declare no competing financial interest. 31697


Research Article

ACS Applied Materials & Interfaces Electrode in Dye-sensitized Solar Cells. J. Power Sources 2016, 336, 83−90. (10) Gao, J.; Yang, Y.; Zhang, Z.; Yan, J.; Lin, Z.; Guo, X. Bifacial Quasi-solid-state Dye-sensitized Solar Cells with Poly (vinyl pyrrolidone)/Polyaniline Transparent Counter Electrode. Nano Energy 2016, 26, 123−130. (11) Bao, C.; Li, F.; Wang, J.; Sun, P.; Huang, N.; Sun, Y.; Fang, L.; Wang, L.; Sun, X. One-Pot Solvothermal in Situ Growth of 1D SingleCrystalline NiSe on Ni Foil as Efficient and Stable Transparent Conductive Oxide Free Counter Electrodes for Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 32788−32796. (12) Lei, B.; Luo, Q.; Yu, X.; Wu, W.; Su, C.; Kuang, D. Hierarchical TiO2 Flowers Built from TiO2 Nanotubes for Efficient Pt-free Based Flexible Dye-sensitized Solar Cells. Phys. Chem. Chem. Phys. 2012, 14, 13175−13179. (13) Yang, Y.; Jin, Q.; Mao, D.; Qi, J.; Wei, Y.; Yu, R.; Li, A.; Li, S.; Zhao, H.; Ma, Y.; Wang, L.; Hu, W.; Wang, D. Dually Ordered Porous TiO2-rGO Composites with Controllable Light Absorption Properties for Efficient Solar Energy Conversion. Adv. Mater. 2017, 29, 1604795. (14) Dong, Z.; Lai, X.; Halpert, J. E.; Yang, N.; Yi, L.; Zhai, J.; Wang, D.; Tang, Z.; Jiang, L. Accurate Control of Multishelled ZnO Hollow Microspheres for Dye-Sensitized Solar Cells with High Efficiency. Adv. Mater. 2012, 24, 1046−1049. (15) Qian, J.; Liu, P.; Xiao, Y.; Jiang, Y.; Cao, Y.; Ai, X.; Yang, H. TiO2-coated Multilayered SnO2 Hollow Microspheres for DyeSensitized Solar Cells. Adv. Mater. 2009, 21, 3663−3667. (16) Li, Y.; Hao, H.; Qin, L.; Wang, H.; Nie, M.; Hu, Z.; Gao, W. Y.; Liu, G.S. Synthesis and Characterization of Ho3+-doped Strontium Titanate Down-Conversion Nanocrystals and Its Application in DyeSensitized Solar Cells. J. Alloys Compd. 2015, 622, 1−7. (17) Crossland, E.; Noel, N.; Sivaram, V.; Leijtens, T.; AlexanderWebber, J. A.; Snaith, H. Mesoporous TiO2 Single Crystals Delivering Enhanced Mobility and Optoelectronic Device Performance. Nature 2013, 495, 215−219. (18) Li, Z.; Que, Y.; Mo, L.; Chen, W.; Ding, Y.; Ma, Y.; Jiang, L.; Hu, L.; Dai, S. One-pot Synthesis of Mesoporous TiO2 Microspheres and Its Application for High-efficiency Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 10928−10934. (19) Yang, H.; Sun, C.; Qiao, S.; Zou, J.; Liu, G.; Smith, S.; Cheng, H.; Lu, C. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638−641. (20) Liu, G.; Sun, C.; Yang, H.; Smith, S.; Wang, L.; Lu, G. Q.; Cheng, H.-M. Nanosized Anatase TiO2 Single Crystals for Enhanced Photocatalytic Activity. Chem. Commun. 2010, 46, 755−757. (21) Yang, H.; Liu, G.; Qiao, S.; Sun, C.; Jin, Y.; Smith, S.; Zou, J.; Cheng, H.; Lu, G. Solvothermal Synthesis and Photoreactivity of Anatase TiO2 Nanosheets with Dominant {001} Facets. J. Am. Chem. Soc. 2009, 131, 4078−4083. (22) Xie, F.; Li, Y.; Dou, J.; Wu, J.; Wei, M. Facile Synthesis of SnO2 Coated Urchin-like TiO2 Hollow Microspheres as Efficient Scattering Layer for Dye-sensitized Solar Cells. J. Power Sources 2016, 336, 143− 149. (23) He, X.; Li, X.; Zhu, M. The application of Hollow Box TiO2 as Scattering Centers in Dye-sensitized Solar Cells. J. Power Sources 2016, 333, 10−16. (24) Hwang, S.; Yun, J.; Jang, J. Multi-shell Porous TiO2 Hollow Nanoparticles for Enhanced Light Harvesting in Dye-sensitized Solar Cells. Adv. Funct. Mater. 2014, 24, 7619−7626. (25) Wang, L.; Sasaki, T.; Ebina, Y.; Kurashima, K.; Watanabe, M. Fabrication of Controllable Ultrathin Hollow Shells by Layer-by-layer Assembly of Exfoliated Titania Nanosheets on Polymer Templates. Chem. Mater. 2002, 14, 4827−4832. (26) Djojoputro, H.; Zhou, X.; Qiao, S.; Wang, L.; Yu, C.; Lu, G. Periodic Mesoporous Organosilica Hollow Spheres with Tunable Wall Thickness. J. Am. Chem. Soc. 2006, 128, 6320−6321. (27) Xia, Y.; Mokaya, R. Hollow Spheres of Crystalline Porous Metal Oxides: A Generalized Synthesis Route via Nanocasting with Mesoporous Carbon Hollow Shells. J. Mater. Chem. 2005, 15, 3126−3131.

(28) Wu, M.; Wang, G.; Xu, H.; Long, J.; Shek, F. L.; Lo, S. M. F.; Williams, I. D.; Feng, S.; Xu, R. Hollow Spheres based on Mesostructured Lead Titanate with Amorphous Framework. Langmuir 2003, 19, 1362−1367. (29) Cheng, S.; Yan, D.; Chen, J. T.; Zhuo, R. F.; Feng, J. J.; Li, H. J.; Feng, H. T.; Yan, P. X. Soft-template Synthesis and Characterization of ZnO2 and ZnO Hollow Spheres. J. Phys. Chem. C 2009, 113, 13630− 13635. (30) Du, J.; Qi, J.; Wang, D.; Tang, Z. Facile Synthesis of Au@ TiO2 Core−shell Hollow Spheres for Dye-sensitized Solar Cells with Remarkably Improved Efficiency. Energy Environ. Sci. 2012, 5, 6914− 6918. (31) Elbohy, H.; Kim, M. R.; Dubey, A.; Reza, K. M.; Ma, D.; Zai, J.; Qian, X.; Qiao, Q. Incorporation of Plasmonic Au Nanostars into Photoanodes for High Efficiency Dye-Sensitized Solar Cells. J. Mater. Chem. A 2016, 4, 545−551. (32) Luoshan, M.; Bai, L.; Bu, C.; Liu, X.; Zhu, Y.; Guo, K.; Jiang, R.; Li, M.; Zhao, X. Surface Plasmon Resonance Enhanced Multi-shellmodified Upconversion NaYF 4 : Yb 3+ , Er 3+ @ SiO 2 @Au@TiO 2 Crystallites for Dye-sensitized Solar Cells. J. Power Sources 2016, 307, 468−473. (33) Muduli, S.; Game, O.; Dhas, V.; Vijayamohanan, K.; Bogle, K. A.; Valanoor, N.; Ogale, S. B. TiO2-Au Plasmonic Nanocomposite for Enhanced Dye-sensitized Solar Cell (DSSC) Performance. Sol. Energy 2012, 86, 1428−1434. (34) Shah, A. A.; Umar, A. A.; Salleh, M. M. Efficient Quantum Capacitance Enhancement in DSSC by Gold nanoparticlesPlasmonic Effect. Electrochim. Acta 2016, 195, 134−142. (35) Yang, N.; Yuan, Q.; Zhai, J.; Wei, T.; Wang, D.; Jiang, L. Enhanced Light Harvesting In Plasmonic Dye-Sensitized Solar Cells by Using A Topologically Ordered Gold Light-Trapping Layer. ChemSusChem 2012, 5, 572−576. (36) Ohsaka, T.; Izumi, F.; Fujiki, Y. Raman Spectrum of Anatase, TiO2. J. Raman Spectrosc. 1978, 7, 321−324. (37) Wang, H.; Shao, W.; Gu, F.; Zhang, L.; Lu, M.; Li, C. Synthesis of Anatase TiO2 Nanoshuttles by Self-sacrificing of Titanate Nanowires. Inorg. Chem. 2009, 48, 9732−9736. (38) Lu, J.; Lu, J.; Liu, H.; Liu, B.; Gong, L.; Tok, E. S.; Loh, K. P.; Sow, C. H. Microlandscaping of Au Nanoparticles on Few-Layer MoS2 Films for Chemical Sensing. Small 2015, 11, 1792−1800. (39) Zhao, J.; Zhang, Z.; Yang, S.; Zheng, H.; Li, Y. Facile Synthesis of MoS2 Nanosheet-silver Nanoparticles Composite for Surface Enhanced Raman Scattering and Electrochemical Activity. J. Alloys Compd. 2013, 559, 87−91. (40) Bai, Y.; Yu, H.; Li, Z.; Amal, R.; Lu, G.; Wang, L. In Situ Growth of a ZnO Nanowire Network Film for Enhanced Dye-sensitized Solar Cell Performance. Adv. Mater. 2012, 24, 5850−5856. (41) Xiang, Q.; Yu, J.; Cheng, B.; Ong, H. Microwave−hydrothermal Preparation and Visible-light Photoactivity of Plasmonic Photocatalyst Ag−TiO2 Nanocomposite Hollow Spheres. Chem. - Asian J. 2010, 5, 1466−1474. (42) Naphade, R.; Tathavadekar, M.; Jog, J. P.; Agarkar, S.; Ogale, S. Plasmonic Light Harvesting of Dye Sensitized Solar Cells by Aunanoparticle Loaded TiO2 Nanofibers. J. Mater. Chem. A 2014, 2, 975−984. (43) Zarick, H. F.; Hurd, O.; Webb, J. A.; Hungerford, C.; Erwin, W. R.; Bardhan, R. Enhanced Efficiency in Dye-sensitized Solar Cells with Shape-controlled Plasmonic Nanostructures. ACS Photonics 2014, 1, 806−811. (44) Han, L.; Koide, N.; Chiba, Y.; Mitate, T. Modeling of an Equivalent Circuit for Dye Sensitized Solar Cells. Appl. Phys. Lett. 2004, 84, 2433. (45) Kern, R.; Sastrawan, R.; Ferber, J.; Stangl, R.; Luther, J. Modeling and Interpretation of Electrical Impedance Spectra of Dye Solar Cells Operated under Open-circuit Conditions. Electrochim. Acta 2002, 47, 4213−4225.

31698


TiO2 Hollow Spheres with Synergistic Effect of Plasmonic

Recommend Documents