TiO2 Hollow Spheres with Synergistic Effect of Plasmonic

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04624 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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ACS Applied Materials & Interfaces

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, USA

KEYWORDS：titanium oxide, hollow structure, dye-sensitized solar cell, gold nanoparticle; plasmonic-enhanced effect

ACS Paragon Plus Environment

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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.


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INTRODUCTION Dye-sensitized solar cells (DSSCs) have received intense scientific and technical interest due to its distinct characteristics of low cost, environmental friendliness, and easy fabrication since the pioneer report by O’Regan and Grätzel in the early 1990s.1,

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To improve the photovoltaic

performance of DSSCs, considerable efforts have been paid on 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, etc.13-16 Among these semiconductors, TiO2 has attracted widespread attention owing to its eco-friendliness, low cost, high chemical, and 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 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 high-cost and tedious synthetic 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.


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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 light scattering and plasmonic effect by integrating hollow structure and Au nanoparticles into one unit to collectively maximize their light harvest effect and carrier generation ability. 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


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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) were 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 cooled down to room temperature, the yellow precipitates 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 5-15 nm range. Typically, the as-synthesized THS (0.25 g) was dispersed in 25 ml ethanol and then added 0.25 ml (3aminopropyl) trimethoxysilane (APTMS). 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 re-dispersed 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 ultrasonic 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


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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 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 30min. Subsequently, the as-prepared film was immersed into 0.5 mM N719 ruthenium dye ethanol solution for 18 h at room temperature in the dark. The dye-sensitized 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 751100 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 (Perkin-Elmer Lambda 35 UV-VISNIR). To measure the amounts of dye absorption, the N719 sensitized TiO2-based composite


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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., 1cm). 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. RESULTS AND DISCUSSION 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


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the anatase TiO2, another four peaks appeared at 38°, 44°, 64°, and 77° can be well-assigned 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 ~142cm-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,

39, 40

while the red-shift can be ascribed to the fact that the

electronic density of TiO2 is enhanced by the electron transfer from Au nanoparticles to the TiO2 conduction band.41

※： TiO2

※

(b)

Eg

Au-THS THS

☆： Au

☆ ※ ☆ ※

Au-THS ※※

※☆ ※※ ※ ☆

THS

Intensity (a.u.)

(a) Intensity (a.u.)

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B1g

Eg

Anatase TiO2

Eg

B2g

JCPDS 21-1272

20

30

40

50

60

70

80

150

2 theta (deg.)

300

450

600

-1

750

Raman shift (cm )

Figure 1. (a) XRD patterns and (b) Raman spectra of THS and Au-THS, respectively. 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


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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 and d). The homogeneous distribution of Ti, O and Au elements in the EDS mapping (Figure 2e-g) manifests the successful incorporation of Au.

(a)

(b)

(c)

(d)

(e)

(f)


(g)

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Figure 2. FE-SEM images of THS (a and b) and Au-THS (c and d). (e-g) EDS mapping images of Ti, O, and Au elements, respectively. 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 Au-THS, in which the lattice spacings of 0.35 nm 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.

(a)

(b)

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 m2 g-1 and ~120 m2 g-1, respectively, indicating the introduction of Au nanoparticles exerts a negligible effect on the textural structure.


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Figure 4b displays the corresponding pore size distribution of the samples. Both THS and AuTHS samples are of mesoporous structure with a narrow pore size distribution centered at 4-5nm.

(a)

(b) 0.16

120

Au-THS THS

dV/dlogD (cm3 g-1 nm-1)

Quantity adsorbed (cm3 g-1)

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Au-THS THS 0.12

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

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 wt.%, 0.5 wt.% and 1.0 wt.% of the Au-THS composite is incorporated into the photoanode, respectively. Higher incorporation content of 2.0 wt.% 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


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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.% Au-THS-DSSCs could reach 8.31% when the film thickness is increased to ~7.5µm. Table 1. Photovoltaic performance of the DSSCs. Sample

Voc

Jsc

FF

PCE

Film thickness

(V)

(mA cm-2)

(%)

(%)

(µm)

1.0 wt.% Au-THS

0.700±0.009

18.71±0.02

56.10±0.41

7.30±0.07

3.0

0.3 wt.% Au-THS

0.700±0.001

15.70±0.03

57.01±0.12

6.26±0.06

3.0

0.5 wt.% Au-THS

0.700±0.007

17.30±0.04

56.21±0.74

6.80±0.01

3.0

2.0 wt.% Au-THS

0.700±0.005

16.80±0.05

57.10±0.58

6.71±0.02

3.0

3.0 wt.% Au-THS

0.690±0.003

16.01±0.03

58.03±0.46

6.41±0.03

3.0

1.0 wt.% THS

0.700±0.002

16.40±0.04

60.12±0.31

6.90±0.02

3.0

P25

0.690±0.002

12.91±0.03

59.23±0.22

5.30±0.01

3.0

P25

0.700±0.002

16.60±0.04

56.11±0.20

6.51±0.04

7.5

1.0 wt% Au-THS

0.700±0.005

20.80±0.06

57.32±0.38

8.31±0.03

7.5

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


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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 lightharvesting 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-THSDSSC 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.


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(b)

Au-THS amount 3.0wt.% 2.0wt.% 1.0 wt.% 0.5wt.% 0.3wt.% P25

80

IPCE (%)

Current density (mA cm-2)

(a)18 12 Au-THS amount 3.0 wt.% 2.0 wt.% 1.0 wt.% 0.5 wt.% 0.3 wt.% P25

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800

Dark current density (mA cm-2)

(c) Absorbance (a.u.)

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-0.5 -1.0

Au-THS amount 3.0 wt.% 2.0 wt.% 1.0 wt.% 0.5 wt.% 0.3 wt.% P25

-1.5 -2.0 -2.5 -3.0 0.0

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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. 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-THS-DSSC and the THSDSSC 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 6b, the IPCE values of the THS- and Au-THS- DSSCs are much higher than that of the


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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 in good 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 is calculated from the peak intensity at 515 nm based on Lambert-Beer’s law. It is estimated to be about 4.88×10-7, 4.23×107

, and 3.56×10-7 mol cm-2 for the P25-, THS- and Au-THS-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.


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12

60

IPCE (%)

(b) 90

Current density (mA cm-2)

(a)18

1.0 wt.% Au-THS 6

1.0 wt.% THS

30

1.0 wt.% Au-THS 1.0 wt.% THS P25

P25

0 0.0

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700

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800

420

480

540

600

660

Wavelength (nm)

Wavelength (nm)

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. EIS is used to deep study the carrier transportation and recombination at the electrolyte/electrode interfaces of DSSC. 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


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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/(2πfmax),45 here fmax is the frequency at the maximum phase angle. As shown in Figure 7b, the fmax of Au-THS-DSSC (55 Hz) is much lower than that of the P25- (271 Hz) and HTS- DSSCs (141 Hz), implying a longer electron lifetime that enables a smaller charge recombination probability and thus a substantial improvement of the Jsc. 50

(a) 240

(b)


40


Phase (degree)

200 160

-Z'' (ohm)

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30

120

20

80

10

40 0

0

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200

300

Z' (ohm)

400

500

0 0.1

1

10

100

Frequency (Hz)

1000

10000

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


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upon light illumination, and finally, accelerate the electron-hole pair generation/separation efficiency.

Figure 8. A schematic illustration of the DSSC performance enhancement caused by Au-TiO2 architecture. 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 P25-DSSC. The enhanced DSSC performance demonstrates that hollow spheres


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combined with noble nanoparticles are applicable to optimize device performance, such as solar cell and photocatalyst. Corresponding Authors *E-mail: [email protected] (J.-G. Wang); [email protected] (B. Wei) ACKNOLEDGMENTS 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 (Tsinghua University, KF201607), the Fundamental Research Funds for the Central Universities, and the Program of Introducing Talents of Discipline to Universities (B08040). REFERENCES (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 TiO2 Solar Cells with High Photon-to-Electron Conversion Efficiencies. Nature 1998, 395, 583-585. (3) Bella, F.; Griffini, G.; Gerosa, M.; Turri, S. Performance and Stability Improvements for Dyesensitized 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


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TOC


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TiO2 Hollow Spheres with Synergistic Effect of Plasmonic

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