Template- and surfactant-free synthesis of mesoporous TiO2 spheres with hollow core-shell structure for dye-sensitized solar cells

Advanced Powder Technology 29 (2018) 2161–2167

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Original Research Paper

Template- and surfactant-free synthesis of mesoporous TiO2 spheres with hollow core-shell structure for dye-sensitized solar cells Zheng-Yi Li, Xiao-Dong Qiao, Yi-Yi Liu, Xin-Yu Ye, Yu-Feng Cao, Hong Chen, Bing-Xin Lei ⇑, Wei Sun, Zhen-Fan Sun ⇑ School of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, China

a r t i c l e

i n f o

Article history: Received 21 October 2017 Received in revised form 26 May 2018 Accepted 30 May 2018 Available online 7 June 2018 Keywords: Titania Core-shell structure Microsphere Dye-sensitized solar cell Light-scattering

a b s t r a c t We have demonstrated a simple and effective hydrothermal route to synthesize titania mesoporous spheres with hollow core-shell structure. The synthesis is free of any surfactants or templates. The formation mechanism is investigated on the basis of the results of time-dependent experiments. The as-obtained mesoporous titania spheres with a specific surface area of 21.5 m2 g 1 and diameters of 1.2–2.3 lm are composed of anatase titania nanocrystals. The excellent light scattering property of mesoporous titania spheres with hollow core-shell structure is proved. A higher cell efficiency of 8.27% is achieved with mesoporous titania spheres with hollow core-shell structure as a light scattering layer, compared with a cell efficiency of 6.63% for the P25 film electrode with the similar thickness. The higher cell efficiency is attributed to the hollow core-shell structure scattering layer, resulting in excellent pore fitting for electrolyte diffusion, enhanced light scattering ability, and reduced charge recombination. Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction As a promising third-generation photoelectric conversion device, dye-sensitized solar cell (DSSC) has become a strong competitor against other types of solar cells due to its superiorities such as low manufacturing cost, facile fabrication and relatively high power conversion efficiency (PCE) [1]. Up to now, the efficiency of DSSC based on TiO2 film electrode which was cosensitized by two different dyes in contact with a redox electrolyte (CoII/CoIII) has reached a quite convictive value of 14.3% [2]. As a crucial component of typical DSSCs, the dye-sensitized TiO2 nanocrystalline film electrode plays a key role in capturing incident sunlight. However, the TiO2 nanocrystallite film electrode represents high transparency of visible light, and hence produces low utilization rate of incident sunlight. In order to address this issue, a large particles top layer on a light absorption layer is often utilized to scatter the transmitted light to improve the light utilization efficiency by means of increasing the optical pathways and improving optical confinement in the film electrode, which has been an effective solution to boost PCE of DSSCs [3–8]. The multifarious structures of TiO2 materials can act as a top light scattering layer, such as large solid particles with flat surface ⇑ Corresponding authors. E-mail addresses: [email protected] (B.-X. Lei), [email protected] (Z.-F. Sun).

[9–11], microplates [12], hollow spheres [13–15], and hollow boxes [16,17]. Although the TiO2 solid spheres have a good light scattering ability, their solid structure will lower specific surface area for reducing the adsorption of dyes and simultaneously damage penetration of electrolyte [18]. The hollow TiO2 spheres were also constructed DSSCs because the hollow structure can increase light utilization rate via light scattering effect and also provide accessible pores that benefit permeation of electrolytes [19]. However, the internal cavity may harm surface area of the film electrode. TiO2 spheres with hollow core-shell structure (TiO2-CSSs) are an alternative candidate for the top light scattering layer because they can reflect more light between the inner core and the outside shell, so as to increase the optical path length and enhance light absorption [20,21]. In comparison with hollow TiO2 spheres, the inner core can add specific surface area for dye loading. Therefore, the research on the application of such TiO2-CSSs in DSSCs is very meaningful. To prepare TiO2-CSSs, most of the methods currently used surfactants or polymers that must be removed to form the hollow cavity. The template- and surfactant-free synthesis of mesoporous TiO2-CSSs is a challenging issue. In this work, TiO2-CSSs have been successfully synthesized via a one-step template- and surfactant-free hydrothermal method. TiO2-CSSs were used as the top light scattering layer on the P25-based film electrode for the DSSCs, leading a remarkable

https://doi.org/10.1016/j.apt.2018.05.024 0921-8831/Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

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improvement in the PCE of 8.27% versus 6.63% for the reference cell made of a single-layer P25 film electrode. 2. Experimental 2.1. Synthesis of TiO2-CSSs All starting chemicals were analytical grade, purchased from commercial sources and used directly without further purification. TiO2-CSSs were prepared by a facile hydrothermal method. In a typical procedure, 0.615 g of NH4F and 1.0 g of Ti(SO4)2 were dissolved in 10 mL distilled water under vigorous stirring for 40 min. Afterwards, 10 mL of HAc was added to the above mixture and stirred for 10 min. Finally, the resulting solution was transferred into a 40 mL Teflon-lined stainless-steel autoclave, heated at 150 °C for 12 h, and followed by cooling to room temperature naturally. The white powder was collected by centrifugation, and then washed for three times with distilled water and ethanol, respectively. Thereafter, the collected powder was dried and annealed at 450 °C for 2 h to convert to anatase TiO2. 2.2. Fabrication of DSSCs Two different kinds of pastes based on commercial P25 nanoparticles and TiO2-CSSs were prepared according to the procedure described in our previous work [22]. The obtained pastes were repeatedly deposited on FTO glass substrates by using the screen-printing technique, kept in air for 3 min and dried for 6 min at 125 °C. Finally, the film electrodes were sintered at different temperatures from 325 °C to 500 °C in air. The thickness of P25 film electrode was controlled to about 16 lm. The TiO2-CSS|P25 film electrode was made up of the TiO2-CSSs scattering layer (4 lm) and the transparent P25 layer (12 lm). The as-prepared film electrodes were made by TiCl4 (40.0 mM) treatment at 70 °C for 30 min, rinsed with distilled water and calcined at 520 °C for 30 min. The TiCl4 treated film electrodes were subsequently sensitized by a 0.5 mM N719 solution (in 1:1 vol ratio of acetonitrile and tert-butyl alcohol) at room temperature for 16 h, and then the dye-sensitized film electrodes were washed with acetonitrile. Pt-counter electrodes were prepared by spincoating 5.0 mM H2PtCl6 isopropanol solution on FTO glass substrates, followed by heating at 400 °C for 15 min. Subsequently, the dye-sensitized TiO2 film electrodes were assembled into a typical sandwich-type cell together with Pt-counter electrodes, separated by a 60-lm-thick hot-melt spacer, after which a I /I3 organic solvent based electrolyte solution was introduced [23]. 2.3. Characterizations and measurements The compositions and crystal structures of the samples were detected on a Rigaku Ultima IV X-ray diffractometer (XRD) equipped with Cu Ka radiation (k = 0.15418 nm) at a scan rate of 10° min 1 in the 2h from 20 to 80°. The morphological structure and size of the samples were characterized by using a field emission scanning electron microscopy (FESEM, JSM-7100F) and a transmission electron microscope (TEM, JEM-2100). To determine the Brunauer-Emmett-Teller (BET) surface area and pore characteristic of the samples, the N2 adsorption-desorption measurements were analyzed by using an Autosorb-iQ surface area analyzer (Quantachrome Instruments US). A UV–visible spectrophotometer (UV-1950, Beijing Purkinje General Instrument Co. Ltd., China) was used to measure the amount of dye adsorbed by film electrodes and diffuse reflectance spectra of the film electrodes. A D-100 profilometer (KLA-Tencor) was measured the thickness of film electrodes.

The photocurrent-voltage (J-V) characteristics of DSSCs were recorded by using a Keithley model 2400 digital source meter, equipped with a solar light simulator of intensity 100 mW cm 2 (Oriel, Model: 94041A). The intensity of incident light was rectified by a NREL-calibrated Si solar cell to approximate AM 1.5 G one sun before each measurement. The electrochemical impedance spectroscopy (EIS) measurements were made by using a Zennium electrochemical workstation (Zahner, Germany). The frequency range of EIS measurements was 10 mHz to 1000 kHz with an alternative 10 mV signal under forward bias of open voltage in the dark. Incident photon to current conversion efficiency (IPCE) was measured as a function of wavelength from 365 to 800 nm by using a photo current spectra system of CIMPS (CIMPS-PCS) with wideband white LED tunable light source (TLS03). Intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) was obtained by using the same electrochemical workstation.

3. Results and discussion XRD measurement was performed to investigate the crystal structure of the as-prepared TiO2-CSSs (Fig. 1a). All the diffraction peaks are unambiguously assigned to anatase phase TiO2 and are in good agreement with standard PDF card no. 21-1272. The peaks at 25.3°, 37.8°, 48.0°, 53.8°, 55.1°, 62.7°, 70.3° and 75.0° correspond to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (2 2 0) and (2 1 5) planes, respectively. According to the Scherrer equation, the crystal size estimated from the full width at half maximum of the (1 0 1) peak is about 15 nm. The XRD result illustrates that the asprepared TiO2-CSSs are very pure without any impurity phase and highly crystalline. The BET surface area and the pore size distribution for the as-prepared TiO2-CSSs were investigated by N2 adsorption-desorption isotherm measurement. As shown from Fig. 1b, the curve of adsorption isotherm of the as-prepared TiO2-CSSs displays a type III hysteresis loop, which indicates the presence of mesopores. It is found that the as-prepared TiO2-CSSs have a BET surface area of 21.5 m2 g 1 with an average pore diameter of 4 nm and a pore volume of 0.108 cm3 g 1. The mesopores are propitious to the electrolyte penetration. In the case of P25 sample (Fig. S1), the specific surface area is 54 m2 g 1, which is larger than that of TiO2-CSSs sample. Fig. 2 shows the typical FESEM images of the as-prepared TiO2-CSSs. A low magnification FESEM image (Fig. 2a) of the asprepared TiO2-CSSs reveals that the TiO2 sample has a spherical shaped structure with diameters of 1.2–2.3 lm. The magnified image (Fig. 2b) of the broken sphere remarkably displays that a core is inside the shell, a so-called ‘‘core-shell’’ structure. As can be seen from Fig. 2b, the thickness of the outer shell is roughly 100 nm. A FESEM image (Fig. 2c) under high magnification clearly shows that the surface of the out shell is made up of nanoparticles and there are many mesopores. The detailed TiO2-CSS was further investigated by TEM (Fig. 2d). The gradual contrast of the TEM image from the edge to the center of the sphere indicates that TiO2-CSSs are indeed a core-shell structure. As shown in Fig. 2e, the lattice spacing of 0.352 nm is discriminated and can be ascribed to the (1 0 1) plane of the anatase phase of TiO2. Time-dependent experiments were investigated the growth mechanism of this hollow core-shell structure. The products from different time intervals were characterized by FESEM, as shown in Fig. 3. After the reaction is maintained for 2 h (Fig. 3a), it can be observed that the solid microspherical product is formed. The surface of the sphere is composed of tiny nanoparticles and has an excellent porosity. When the hydrothermal process is prolonged to 4 h (Fig. 3b), the solid sphere is transformed to core-shell structure sphere. The distance between the core and the shell is small. When the reaction is carried out for 6 h (Fig. 3c) and 12 h

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Fig. 1. (a) XRD pattern and (b) Nitrogen adsorption-desorption isotherms (the inset shows the pore size distribution) of the as-prepared TiO2-CSSs obtained after 12 h.

Fig. 3. FESEM images of the as-prepared TiO2 spheres at different reaction time. (a) 2 h; (b) 4 h; (c) 6 h; (d) 12 h.

Fig. 2. FESEM (a, b, c) and TEM (d, e) images of the as-prepared TiO2-CSSs obtained after 12 h.

(Fig. 3d), the spheres are still core-shell structure. The distance between the core and the shell merely becomes larger. Anatase TiO2-CSSs (Fig. 2) were obtained by thermal treatment of the product (Fig. 3d) at 450 °C for 2 h. In order to study photovoltaic performances of the synthetic products prepared at different hydrothermal reaction time (2, 4, 6, 12 h), the typical J-V characteristics curves were measured based on the synthetic products as the light scattering layer. Fig. S2 and Table S1 show the photovoltaic performances of the DSSCs based on the TiO2-2h|P25, TiO2-4h|P25, TiO2-6h|P25 and TiO2-12h|P25 films. It is concluded that the cell based on TiO2-12h|P25 film presents an optimal PCE. Fig. 4 shows the J-V characteristics for DSSCs with TiO2 -CSS|P25 film electrode, including a reference cell based on P25 film

Fig. 4. J-V curves of DSSCs based on P25 and TiO2-CSS|P25 film electrodes under AM 1.5 G simulated sunlight irradiation (100 mW cm 2).

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Table 1 Photovoltaic parameters of DSSCs based on P25 and TiO2-CSS|P25 film electrodes. Cell

Voc (V)

Jsc (mA cm

P25 TiO2-CSS|P25

0.81 0.82

11.42 14.11

2

)

FF (%)

PCE (%)

72.12 71.08

6.63 8.27

electrode. Table 1 summarizes the photoelectric characteristics of the two DSSCs. The reference cell (Cell-P25), assembled with the P25 film electrode, exhibits an overall PCE of 6.63% accompanying with an open circuit voltage (VOC) of 0.81 V, a short-circuit current density (JSC) of 11.42 mA cm 2 and a fill factor (FF) of 72.12%. As expected, the object cell (Cell-TiO2-CSS|P25) based on TiO2-CSS| P25 film electrode shows significantly improved VOC and JSC when compared to the Cell-P25. In virtue of the superior JSC of up to 14.11 mA cm 2 and VOC of up to 0.82 V, the scattering layer of TiO2-CSSs can indeed succeed in enhancing the overall PCE to 8.27%. Compared to Cell-P25, the overall PCE of Cell-TiO2-CSS|P25 is enhanced by 24.7%. The outstanding PCE of Cell-TiO2-CSS|P25 roots in the improved light utilizing efficiency, the multiple reflectivity and the reduced charge recombination rate due to the superior light scattering effect, the high crystallinity and the relatively few trapped states of the TiO2-CSSs. To determine the amount of dye loading in the two different film electrodes, the dye-sensitized film electrodes were soaked in a 1.0 mM NaOH solution to detach the dyes. Fig. 5a shows the UV–vis absorption spectrum of the dyes desorbed from the two different film electrodes. It can be seen that, unfortunately, an obvious decrease in the dye loading amount can be found in TiO2-CSS|P25 film (1.32  10 7 mol cm 2) compared to P25 film (1.64  10 7 mol cm 2), which is due to the low specific surface area of the top scattering layer (TiO2-CSSs). Crucially, its JSC for TiO2-CSS|P25 film electrode is still much better. Such a significant

increase in the JSC can be attributed to light reflecting and scattering effects of TiO2-CSSs. Fig. 5b shows the diffuse reflection spectra of the TiO2-CSS|P25 and P25 films without dye loading. The TiO2CSS|P25 film exhibits higher diffuse reflectance in the whole wavelength range from 400 to 800 nm compared to P25 film. which can be ascribed to multiple-reflection of TiO2-CSSs. The possible reflecting and scattering of sunlight in P25 and TiO2-CSS|P25 films is illustrated in Fig. 5c and d. The multiple-reflection occurring between the core and the shell of the spheres could capture the incident light in the film electrode for a longer duration, which is more favourable for the absorption of more light by the dyes. IPCE measurement (Fig. 6a) was then performed to further understand the scattering effect. The IPCE value obtained from TiO2-CSS|P25 film electrode is 80.5%, approximately 1.3-fold higher than that of the P25 film electrode (60.3%). Because the diameters of TiO2-CSSs were comparable to the wavelength of the visible light, the higher IPCE performance of TiO2-CSS|P25 film electrode at longer wavelengths is probably the result of better light scattering efficiency. Considering the same film electrode thickness and the lower dye loading, such an improved IPCE performance of TiO2-CSS|P25 film electrode at shorter wavelengths should primarily be the result of higher charge collection efficiency due to slow electron recombination in TiO2-CSSs scattering layer. The lifetimes of electrons (se) in the Cell-TiO2-CSS|P25 and CellP25 were measured. Fig. 7a depicts se for Cell-TiO2-CSS|P25 and Cell-P25, which is obtained by measuring the decay of VOC as a function of time (the inset of Fig. 7a) [24]. As shown in Fig. 7a, the se of Cell-TiO2-CSS|P25 is longer than that of Cell-P25, indicating that the higher chance for the photogenerated carriers of CellTiO2-CSS|P25 can be extracted to an external load and thus obtains higher charge collection efficiency for Cell-TiO2-CSS|P25. The longer se of the Cell-TiO2-CSS|P25 confirms the suppressed recombination of photogenerated electrons, stemmed from that the surface trapping and detrapping in highly crystalline TiO2-CSSs

Fig. 5. (a) UV–visible absorption spectra of dye detached from the two films. (b) Diffuse reflectance spectra of the two films. Schematic of the utilization of incident light of P25 film electrode (c) and TiO2-CSS|P25 film electrode (d).

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Fig. 6. IPCE spectra of DSSCs based on P25 and TiO2-CSS|P25 film electrodes. Fig. 8. EIS spectra of DSSCs based on P25 and TiO2-CSS|P25 film electrodes.

scattering layer is much less. The TiO2-CSSs scattering layer contributes to the increase in the VOC for Cell-TiO2-CSS|P25. This outcome is consistence with the dark current results (Fig. 7b). In order to investigate charge transport and recombination properties in the DSSCs, EIS measurements were used. Fig. 8 shows the Nyquist curves of the two different DSSCs. Each curve contains two semicircles. The smaller one at high frequency is assigned to charge-transfer resistance (Rct) and capacitance (C1) at the PtFTO|electrolyte and the FTO|TiO2 interfaces, and the larger one at low frequency is associated with electron transfer and recombination resistance (Rre) and capacitance (C2) at the dye-sensitized TiO2|electrolyte interface [25–27]. The values fitted by Z-view software using the equivalent circuit (the inset of Fig. 8) are given in more detail in Table 2. The series resistance (Rs) is caused by the sheet resistance of FTO, current collector contacts, etc. The value of Rs changes very slightly, which indicates that the morphology of film electrode might have very little relationship with Rs. The value of Rct for two cells is not significantly different because the counter electrode (Pt-FTO), electrolyte(I /I3 ), and FTO|P25 interface were identical in the two DSSCs. However, the Rre value of Cell-TiO2-CSS|P25 (77.34 X) is obviously bigger than that of CellP25 (70.57 X). The lifetime of the electrons (se = Rre  C2) in the film electrode is calculated to be 68.3 ms, 35.6 ms for Cell-CSTiO2|P25, Cell-P25, respectively. Generally, the larger Rre and longer

Table 2 Electrochemical parameters of DSSCs based on P25 and TiO2-CSS|P25 film electrodes. Cell

Rs (X)

Rct (X)

Rre (X)

C2 (mF)

se (ms)

P25 TiO2-CSS|P25

19.93 19.81

1.072 1.283

70.57 77.34

504.59 883.2

35.60 68.30

se are, the slighter the electron recombination at the dye sensitized TiO2|electrolyte interface is. The longer se for Cell-TiO2-CSS|P25 reflects the effective decrease of electron recombination rate during the electrons transfer across film electrode, which is ascribed to rapid charge transport and the fast diffusion of I3 in lightscattering layer (TiO2-CSSs). In the case of TiO2-CSSs overlayer, a macroporous channel is formed between adjacent spheres, which results in a very high porosity and therefore offers a pathway for expedited electrolyte diffusion and simultaneously accelerates the dye regeneration [28,29]. Apart from that, the electron transport is rapid due to a reduced number of grain boundaries and recombination sites for TiO2-CSSs scattering layer. For these reasons, Cell-TiO2-CSS|P25 shows less recombination occurring at the dye-sensitized-TiO2|electrolyte interface, hence brings about a higher Voc. This result agrees well with the J-V data.

Fig. 7. (a) se for DSSCs based on P25 and TiO2-CSS|P25 film electrodes as a function of VOC. The inset shows VOC decays as a function of time. (b) The dark current densityvoltage curves of DSSCs based on P25 and TiO2-CSS|P25 film electrodes.

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Fig. 9. (a) IMPS and (b) IMVS of DSSCs based on P25 and TiO2-CSS|P25 film electrodes.

IMPS and IMVS measurements were further performed to examine the electron transport and recombination times of the DSSCs derived from the P25 and TiO2-CSS|P25 film (Fig. 9). In IMPS measurements, the time delay in the photoanode film corresponds to the electron transfer time of the excited electrons. In IMVS measurements, the time delay in the photoanode film is assigned to the electron recombination time (electron lifetime) for excited electrons to be recombined to electrolytes and dye [30,31]. In Fig. 9a, the transport time of the TiO2-CSS|P25 photoanode is shorter than that of the P25 photoanode. In Fig. 9b, the recombination time of the TiO2 -CSS|P25 photoanode is larger than that of the P25 photoanode. These results demonstrate that the TiO2-CSSs scattering layer has lower trapping sites and thus, leads to a fast electron transport rate and a lower charge recombination. This results agree well with the decay of VOC and EIS data.

4. Conclusions In summary, we have demonstrated that TiO2-CSSs can be fabricated via a facile and efficient aqueous route. The key process does not use templates or surfactants. TiO2-CSSs were constructed by a great many of anatase nanoparticles. TiO2-CSSs were 1.2–2.3 lm in diameter and with a specific surface area of 21.5 m2 g 1 and thickness of the outer shell of 100 nm. TiO2-CSSs were converted from solid spheres and the distance between the core and the shell was easily controlled by adjusting the reaction time. TiO2-CSSs were positive in causing light scattering in a broad wavelength region, therefore, enhancing the light harvesting capability of the film electrode. The DSSC assembled using the TiO2-CSSs as scattering layer demonstrated an overall light conversion efficiency of 8.27% which was 1.24 times of the overall efficiency (6.63%) obtained from the P25 film electrode. The improvement of DSSC performance was due to the excellent pore fitting for electrolyte diffusion, the enhanced light scattering, and the reduced charge recombination.

Acknowledgement This work is financially supported by the National Natural Science Foundation of China (21561010), the Natural Science Foundation of Hainan Province (20162023, 2017CXTD007), the Key Science and Technology Program of Haikou City (2017042), the Haikou key laboratory of electrochemical energy storage and energy conversion materials (2016-032), and the Undergraduate Training Programs for Innovation and Entrepreneurship (30182040406).

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