Ag nanoparticle-decorated SiO2@TiO2 hierarchical microspheres improve the efficiency of dye-sensitized solar cells

SiO2@TiO2-Ag (STA) microspheres decorated with Ag nanoparticles (Ag NPs) were prepared and assembled into the photoanode. The photoanode composed of STA microspheres and TiO2 nanoparticles (P25) was prepared by doctor blade method. UV–vis measurement indicates that the introduction of a few STA microspheres observably enhances the light scattering and capturing ability of the photoanode. The photoelectric conversion efficiency of the DSSCs with 2wt% STA photoanode increased to 7.4% from 4.3% comparing with pure P25 TiO2 nanoparticles. The configuration DSSCs have the maximum short circuit current density (Jsc) of 16.0 mA cm−2 and open-circuit voltage (Voc) of 0.780 V, which are significantly higher than the pure TiO2 DSSCs. The significant improvement of the DSSCs performance can be due to the synergistic effect of the superior light scattering of STA and the localized surface plasma resonance (LSPR) effect of Ag NPs modified on the microspheres surface.


Introduction
Dye-sensitized solar cells (DSSCs) have attracted huge attention in recent years due to their relatively low cost, easy production process and moderate photovoltaic performance since designed by Gratzel team in 1991 [1]. Generally, typical DSSCs are composed of a photoanode, a counter electrode (CE), the electrolyte and the photo-sensitizer (dye) [2]. The photoanode plays an important role to achieve high photoelectric conversion efficiency (PCE), which usually is made of porous metal oxide semiconductors such as TiO 2 , ZnO, SnO 2 [3]. Among them,TiO 2 has aroused extensive attention due to its suitable band-edge position, anti-photocorrosion stability and nontoxicity [4]. However, the intrinsic low carrier mobility and the weaker light capture rate of TiO 2 severely limit the efficiency of DSSCs [5]. The low carrier mobility makes electron-hole recombination easier. The nanocrystals in small size have higher transparency, resulting in lower light capture rate. To solve the above problems, design of photoanode materials with strong light capture and efficient electron transfer capacity has become a research hotspot. Nowadays, various methods are used to optimize the photoanode to improve the photovoltaic property of DSSCs by acid surface modification, metallic oxide doping and introduction of scattering layer [6][7][8].
Adding light scattering material into the photoanode is a simple and feasible method to increase the light absorption and increase the performance of the DSSCs. The incident light can be reflected and scattered when passing through the photoanode film, so that improves the absorption and capture ability of light. In general, the scattering material is introduced into the photoanode with double-layer or hybrid structure. Yu group has fabricated layered TiO 2 microspheres as the scattering layer in DSSCs, and the performance of the cell has been significantly improved [9]. Son group has reported that SiO 2 /TiO 2 spheres consisting of a low refractive index core and high refractive index shell enhanced light scattering to improve power conversion efficiency of DSSCs [10]. Tan and Wu group prepared composite solar cells by using anatase nanoparticles and single-crystal anatase TiO 2 nanowires. The DSSCs based on TiO 2 nanowire have light scattering effect and electron fast transmission channel, which make the efficiency of the cell increase from 6.7% to 8.6% [11]. In addition, nanomaterials with different morphologies have been used in the scattering layer of the DSSCs, including nanoplatelets, nanorods, nanofibers, etc.
However, improving the efficiency of dye-sensitized solar cells remains a major challenge. The low absorption in the visible region of TiO 2 obstructs the efficiency of DSSCs. Recently, localized surface plasmon resonance (LSPR) effect of noble metal nanoparticles such as Ag, Au and Pt has been extensively applied in photocatalysis [12] and photovoltaic devices [13]. It is reported that LSPR effect can increase the light capture rate of photoanode, thereby increasing the PCE of DSSCs [14]. The LSPR absorption is located at about 430 nm and 530 nm for spherical Ag and Au nanoparticles, respectively [15]. In addition, the LSPR effect can improve the energy level of TiO 2 to enhance the charge transfer and reduce the electron recombination. Li group synthesized Au-decorated Urchin-like TiO 2 microspheres, which makes the PCE reach to 7.21% [16]. Naphade loaded gold nanoparticles onto TiO 2 nanofibers, thus significantly improving the DSSCs performance [17].
In this work, we synthesized core-shell structure SiO 2 @TiO 2 (ST) microspheres and Ag nanoparticles (Ag NPs)-decorated SiO 2 @TiO 2 -Ag (STA) microspheres via a simple hydrothermal method. The ST or STA microspheres were blended with P25 to manufacture composite photoanode. The improvement in performance can be attributed to the enhancement of light capture rate by strong light scattering and the enhancement of light absorption in LSPR effect due to the addition of Ag NPs. The effects and mechanisms of the STA microspheres on the performance of the DSSCs were explored. (TEOS, C 99.7%), (3-aminopropyl) trimethoxysilane (APTMS, C 98%), 4-tert-Butylpyridine (C 9 H 13 N, C 96%), potassium iodide (KI, C 99.7%), acetone (CH 3 COCH 3 , C 99.7%) and acetylacetone (CH 3 COCH 2 COCH 3 , C 99.5%) were provided by the Sinopharm Chemical Reagent Co. , Ltd. Titanium isopropoxide (TTIP, C 95%) and TiO 2 NPs (P25) were purchased from Aladdin Co., Ltd. N719 ([ 95%) was obtained from Taiwan Yongguang Chemical Industry Co., Ltd. All the water used in the experiment was deionized water. All reagents for analytical purity were used without further purification.
The crystalline structures of the samples were investigated by X-ray diffraction (XRD) carried on a DX-2700 diffractometer using the Cu Ka radiation k = 1.5406 Å ,,(Fangyuan, China). The morphology and microstructure of the samples were observed with a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan) and on a transmission electron microscope (TEM, JEM-2100, JEOL, Japan). The chemical elements of the samples were measured by an energy-dispersive X-ray spectroscopy (EDS, JEM-2100, JEOL, Japan). X-ray photoelectron spectroscopy (XPS) was performed on a PerkinElmer PHI 5000C ESCA instrument with Al Ka X-rays (Thermo Scientific, America). The specific surface area was recorded on a Brunner-Emmet-Teller (BET) apparatus (ASAP2020M ? C, Micromeritics, America). The optical absorption and reflection spectra were measured from the UV-vis spectrophotometer (Lambda750s, PerkinElmer, America). The current-voltage (J-V) curves of DSSCs were collected using an electrochemical Workstation (CHI 660B, China) under simulated sunlight (100 mW/cm 2 ). Each type of DSSCs is measured six times to reduce the error. The incident photon-to-current conversion efficiency (IPCE) was carried out by a SC S100 (Zolix, China). The electrochemical impedance spectroscopy (EIS) was measured at a bias potential of -0.8 V with the frequency range from 0.1 Hz to 100 kHz at an ac amplitude of 10 mV using an electrochemical Workstation (CHI 660B, China) in the dark.

Preparation of SiO 2 @TiO 2 (ST)
The method in the literature [18] was modified to synthesize ST. 0.12 g SiO 2 microspheres were added into the 30 mL acetonitrile and 90 mL anhydrous ethanol with ultrasonic dispersion for 30 min. The mixture was put in a 20°C water bath, and added 1 mL of ammonia. After reacted for 30 min, 1 mL TTIP was slowly added to the mixture and the reaction continued for 12 h. SiO 2 @TiO 2 can be obtained by centrifuging, washing and drying.

Preparation of SiO 2 @TiO 2 -Ag (STA)
0.35 g SiO 2 @TiO 2 microspheres were dispersed in 100 mL ethanol solvent. 0.35 mL (3-aminopropyl) trimethoxysilane (APTMS) was added into solution at 80°C for 4 h to obtain the APTMS modified SiO 2 @TiO 2 . Subsequently, the obtained APTMS modified SiO 2 @TiO 2 sample was added into 30 mL deionized water and ultrasonic for 30 min to disperse evenly. Then, 8 mg silver nitrate was added to solution stirred for 5 min at 90°C water bath and 40 mg sodium citrate was added at 90°C reacting for 30 min. Finally, Ag nanoparticles-decorated SiO 2 @ TiO 2 microspheres (SiO 2 @TiO 2 -Ag) were obtained by centrifuging and washing with deionized water.

Fabrication of DSSCs
FTO glass (2.2 mm thickness, 14 X/sq) was ultrasonically cleaned with acetone, ethanol and deionized water for 15 min, respectively. Briefly, the TiO 2 slurry was prepared using 0.1 g P25 powders dispersed in 0.5 mL acetylacetone. The TiO 2 slurry with three kinds of STA contents was obtained by replacing the TiO 2 slurry with the mass percentage of 1wt%, 2wt% and 3wt%. Then, the obtained slurry was coated on a FTO glass by doctor blade method with a square area of 0.25 cm 2 homogeneous films. The FTO glass was dried in an oven at 80°C and annealed at 450°C for 30 min. These photoanodes were labeled as 1%STA, 2%STA and 3%STA. The annealed photoanode was soaked in a 3.  The morphology and microstructure of the assynthesized SiO 2 , ST and STA samples were investigated by SEM and TEM. It is found that SiO 2 nanospheres have good monodispersity and uniform particle size with average diameter of 350 nm (Fig. S1). It can be seen that the ST spheres with an average diameter of 400 nm are tightly coated by nano-TiO 2 shell (Fig. 2a). The STA sample shows raspberry-like rough surface, because Ag nanoparticles were deposited on the surface of ST spheres with the reduction of silver ions by sodium citrate. The core-shell structure of STA can be discerned in Fig. 2c. Due to the effect of surface silane-amination, Ag nanoparticles were evenly dispersed on the surface of the sphere with the uniform size [19]. The HRTEM image shows that the size of Ag nanoparticle is about 10 nm. The lattice fringes were measured to be about 0.22 nm, which is corresponding to the d 111 spacing of Ag. In addition, the elemental mapping results indicate the presence and uniform distribution of Ti, O, Si, Ag, C in STA microsphere (Fig. 3). The Si and Ti elements distribution cases further identify that the STA sphere is the core-shell structure with SiO 2 core and TiO 2 shell, while the Ag element signal is mostly scattered on the surface of STA sphere. It also confirms that core-shell structure ST modified by Ag nanoparticle dispersed uniformly on surface to form the hierarchical structure STA.
The bonding environment and the chemical state of the SAT sample were analyzed by XPS. The elements of Ti, Si, O, Ag and C presented in the survey curve of the STA samples from the binding energies of electrons (Fig. 4a). The carbon element can be attributed to the remnant carbon form reaction. Figure 4b demonstrates that STA samples display two separated peaks for Ag 3d electron. The binding energies of 368.2 eV and 374.2 eV are corresponded to Ag 3d 5/2 and Ag 3d 3/2 , respectively. In addition, the 6.0 eV difference of the doublet state 3d electron confirmed the formation of metallic Ag [20].
The specific surface areas of the ST and STA microspheres samples were characterized by nitrogen adsorption/desorption isotherms (Fig. S2). The BET-specific surface areas were 63.25, 58.16 and 50.50 m 2 /g for ST, STA and P25, respectively. The BET surface area of ST and STA microspheres was larger than that of commercial P25. The large specific  surface area of ST and STA can provide more adsorption sites to increase the amount of dye adsorption, which is beneficial to the photovoltaic performance of DSSCs.
UV-vis results shown in Fig. 5 illustrate the light response of P25, ST and STA samples. It can be seen that P25 mainly absorbs UV light below the wavelength of 400 nm because of the wide band gap of TiO 2 [21]. The absorption of P25 and ST is similar in UV region because SiO 2 has no absorption in the UV and visible regions [22]. However, ST sample increases the absorption in the visible region obviously. The results show that SAT samples can significantly enhance the light harvest in the visible region. It is well-known that the strong absorption in the visible region is attributed to the LSPR effect of Ag NPs [23]. In addition, the LSPR effect of Ag NPs can improve the capture ability of photogenerated electrons. The reflectance of STA is distinct lower than that of P25 and ST due to light capture caused by strong LSPR of Ag NPs. The reflectance of ST is slightly higher than that of P25 after the wavelength of 500 nm can be ascribed to size-dependent scattering effect [24].  Table 1. The different amounts of STA added into P25 have different photovoltaic properties. The current density and conversion efficiency increase with the doping amount of STA and then decrease with the more of STA. The results show that the optimal STA to P25 ratio is 2 wt%, and the corresponding J sc and PCE are 16.0 mA cm -2 and 7.4%, respectively. The results indicate that the PCE of the DSSCs with 2 wt% STA increased about 70% more than that of the device with pure P25 photoanode. The increase of J sc and PCE may be mainly due to the light capture enhancement of LSPR effect of Ag NPs and light scattering. Moreover, the surficial Ag NPs could promote the photogenerated electron-hole separation and reduce recombination [14]. All these factors will be beneficial on photovoltaic performance of DSSCs. However, the current density and photoelectric conversion efficiency will be reduced by the excessive addition of STA. Excessive Ag may lead to electron trapped on the surface of Ag, thus reducing the current density. Meanwhile, excess Ag will increase the electron back reaction with the electrolyte when Ag is in contact with the electrolyte. The V oc of the four DSSCs does not change significantly because V oc is related to the energy difference between the Fermi level of TiO 2 and the redox potential in the electrolyte [25]. IPCE is an important character to evaluate the rate of light capture and quantum efficiency in DSSCs. It indicates the utilization of sunlight by DSSCs [26]. The IPCE chart of DSSCs is illustrated in Fig. 6b. Addition STA to P25 could increase the IPCE in visible region. The IPCE of DSSCs with 2 wt% STA photoanode is highest, which is consistent with the results of J-V plot. The 2 wt% STA photoanode-based The electrochemical impedance spectroscopy (EIS) was utilized to study the electron transport and    recombination processes in DSSCs [27]. Figure 7a and b shows the Nyquist and Bode plots of DSSCs. The fitting data of equivalent circuit are presented in Table 2. It can be seen there are two semicircles of P25 photoanode in Fig. 7a. The larger semicircle represents the interface transfer impedance R2 and stands for the photoanode/dye/electrolyte interface in DSSCs. The small semicircle stands for the Pt electrode-electrolyte interface R3. The intercept on the real axis represents the series resistance R1 in the equivalent circuit. Since the Pt electrode and external circuit are used in same testing condition, different DSSCs should show similar R3 and R1. The R2 is inversely related to the J sc of DSSCs. R2 of 2 wt% STA photoanodes decreased significantly which indicated the modification of Ag will improve the electron transport rate in DSSCs and lessen the recombination of electrons and holes in DSSCs. Furthermore, when the amount of STA doping continues to increase, excessive Ag may become a recombination center that is not conducive to the charge transfer process, leading to an increase in R2. The FF value reflects the parameter related to the intrinsic resistance of DSSCs, so the decrease in R2 can explain the increase in the FF value of DSSCs [28]. The electron lifetime (s) in DSSCs can be evaluated from the Bode phase plots of EIS on the basis of the equation s = 1/(2pf max ) [29], where f max is the maximum peak frequency. As shown in Fig. 7b, the peak of doped STA photoanode moved to the low frequency direction compared to pure P25, indicating that addition of STA could prolong the electron lifetime. The s of 2wt% STA of DSSCs (15.91 ms) is greater than that pure P25 of DSSCs (7.39 ms). These further confirmed that the addition of an appropriate amount of STA will benefit the electron transfer inside the photoanode, reduce charge recombination and promote the conversion efficiency of DSSCs.

Photovoltaic performances of DSSCs
The sandwiched DSSCs cell structure and reaction mechanism are presented in Fig. 8. The dye (N719) molecule becomes excited state after absorbing photon energy under light irradiation. It produces an electronic transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The photo-excited electrons jump from dye (N719) into the CB of TiO 2 and then inject to the adjacent Ag NPs. At the same time, P25 nanoparticles also absorb short-wavelength photons energy to generate electrons and holes in conduction band and valence band, respectively. Due to the continuous band structure of Ag NPs, the electron mobility from TiO 2 to Ag NPs will remain at the continuous energy level near the Fermi level. Meanwhile, the injected electrons in the CB of TiO 2 transferred to the conductive substrate (FTO). Finally, the electrons move from the external circuit to the Pt electrode. The photogenerated electrons collected by the Ag NPs will promote the charge separation process. Therefore, Ag NPs can decrease the electron recombination reaction from the CB of TiO 2 to the dye and a back reaction with the electrolyte. SiO 2 / TiO 2 core-shell sphere is used as scattering medium center of Ag surface plasma and incident light. Altogether, the proposed STA will improve the efficiency of electron collection and provide beneficial influence on the property of DSSCs.

Conclusion
In short, we successfully synthesized SiO 2 @TiO 2 -Ag core-shell spheres modified by Ag nanoparticles on the surface. The as-prepared STA spheres are used as efficient multifunctional scatter introduced into P25 powder to construct composite photoanode. The DSSCs of 2 wt% STA in photoanode possess the highest current density of 16.0 mA cm -2 and conversion efficiency of 7.4%, which is obviously high than that of pure P25 DSSCs. The LSPR effect of surface Ag nanoparticles broadens the visible light absorption of DSSCs and promotes the photogenerated electron-hole separation.