In situ synthesis, enhanced luminescence and application in dye sensitized solar cells of Y2O3/Y2O2S:Eu3+ nanocomposites by reduction of Y2O3:Eu3+

Y2O3/Y2O2S:Eu3+ nanocomposites were successfully prepared by reducing Y2O3:Eu3+ nanocrystals. The obtained Y2O3/Y2O2S:Eu3+ nanocomposites not only can emit enhanced red luminescence excited at 338 nm, but also can be used to improve the efficiency of the dye sensitized solar cells, resulting an efficiency of 8.38%, which is a noticeable enhancement of 12% compared to the cell without Y2O3/Y2O2S:Eu3+ nanocomposites. The results of the incident photon to current, dynamic light scattering, and diffuse reflectance spectra indicated that the enhancement of the cell efficiency was mainly related to the light scattering effect of Y2O3/Y2O2S:Eu3+ nanocomposites. As a phosphor powder, the emission at ~615 nm of Y2O3/Y2O2S:Eu3+ was split into two sub-bands. Compared with Y2O3:Eu3+, the 5D0 → 7F0 and 5D0 → 7F1 emissions of Y2O3/Y2O2S:Eu3+ showed a little red-shift.

Rare earth (RE) compounds were extensively applied in the fields of high-performance magnets, luminescence devices, catalysts, and other functional materials. Most of these functions depend strongly on the composition and structure of materials [1][2][3][4][5] . In particular, nano-sized luminescent materials have attracted considerable attention since Bhargava et al. reported that doped nanocrystalline phosphors yielded high luminescence efficiencies [6][7][8][9] . With rapidly shrinking size, nanomaterials usually exhibit novel physical and chemical properties due to their extremely small size and relatively large specific surface areas [10][11][12][13] .
It is well known that host material is an important factor to obtain high efficient luminescent properties. Among various host materials, Y 2 O 3 not only has good chemical and photochemical stabilities and high melting points, but also can be easily doped with RE ions. Especially, Y 2 O 3 :Eu 3+ phosphor is used for high efficiency cathode-ray tubes and field emission displays because of its excellent luminescence efficiency under ultraviolet excitation [14][15][16][17] . Y 2 O 2 S:Eu 3+ has been used as a red "no mill" phosphor for decades. Its high brightness, excellent color definition, and linear response in the wide range of current density make it promising for the future generation of display equipment 18-22 . Composite materials formed by combining two or more materials could present complementary properties that have shown important technological applications 23,24  In the past decade, the dye-sensitized solar cell (DSSC) has become one of the most promising solar cells in the renewable energy research and development field for its potentially low fabrication cost and relatively good efficiency [25][26][27] . The concept of integrating a down-conversion layer into a solar cell has attracted significant attention because it not only can remove the load of spectral matching from the semiconductor itself, minimize the thermalization losses in cells, and move this task to a separation component, but also can offer the opportunity to improve light harvesting and thereby the efficiency of the solar cells [28][29][30]

Discussion
Sample numbers and corresponding experimental conditions are given in Table 1 Figure 4 shows the XPS spectrum of Y 2 O 3 /Y 2 O 2 S:Eu 3+ nanocomposites. Obviously, Y 3+ was identified by its Y 3 s, Y 3p, Y 3d, and Y 4 P speaks, O 2− was identified by the O 1 s and O KLL peaks, Eu 3+ was identified by the Eu 4d peak, and S 2+ was identified by the S 2p peak. The Y 3d 3/2 spectral peaks were at 156.7, 158.3, and 158.8 eV, and the S 2p 2/3 spectral peaks were at 167.7 and 170.1 eV. In addition, the O 2 s spectrum can be fitted by three peaks located at 628.8, 531.1, and 532.0 eV.
For comparison, the luminescence properties of the Y 2 O 3 :Eu 3+ (without sulfuration) nanocrystals were investigated first, as shown in Fig. 5. For the excitation spectra of Y 2 O 3 , the broad band extending from 200 to 300 nm is assigned to the charge transfer transition from the 2p orbital of O 2− to the 4 f orbital of Eu 3+ , which is related closely to the covalency between O 2− and Eu 3+ and the coordination environment around Eu 3+ . The sharp lines in Fig. 5(a) correspond to the f-f transitions of the Eu 3+ ions. Figure 5(b) shows the emission spectra of Y 2 O 3 :Eu 3+ excited at different wavelengths. It is found that the peak at ~615 nm of Y 2 O 3 :Eu 3+ was much stronger than that at ~630 nm. When the excitation wavelength was 259 nm, the emission intensities were the strongest. Figure 6( 3+ , the 5 D 0 → 7 F 0 (~583 nm), 5 D 0 → 7 F 1 (509-602 nm), and 5 D 0 → 7 F 2 (614-633 nm) transitions of the Eu 3+ ions were observed. The luminescence was dominated by the emission at ~615 nm. The 5 D 0 → 7 F 1 emission was split into three sub-bands due to local fields around Eu 3+ and their separations depend on the energy for the  direct excitation from the 7 F 0 ground level to the 5 D 0 excited level. For the Y 2 O 3 /Y 2 O 2 S:Eu 3+ , the 5 D 0 → 7 F 0 and 5 D 0 → 7 F 1 showed a little red-shift. The luminescence was dominated by the emission at ~630 nm. In addition, the emission at ~615 nm was split into two sub-bands. Figure 7( Figure 8(b) shows the emission spectra of the Y 2 O 3 /Y 2 O 2 S:Eu 3+ nanocomposites excited at different wavelengths. When the excitation wavelength was 338 nm, the emission intensities were the strongest. Figure 9 shows the luminescence decay curve for the Y 2 O 3/ Y 2 O 2 S:Eu 3+ nanocomposites excited at 280 nm and monitored at 620 nm. It is noted that the decay curve cannot be fitted with the single exponential function, while a        the performance improvement, as shown in Fig. 10(b). (b) The enhancement of the efficiencies of the TiO 2 -Y 2 O 3 / Y 2 O 2 S:Eu 3+ composite cells were related to the light scattering of Y 2 O 3 /Y 2 O 2 S:Eu 3+ , as shown in Fig. 10(c,d). (c) It is noted that the sintering process was necessary during preparation of the photoelectrode, which has been described in the Experimental section. And thus, some Ti 4+ ions will be substituted by S 6+ during the sintering process, which was beneficial for enhancing photoelectric properties 31 . In addition, the decrease of the efficiency of TiO 2 -1%Y 2 O 3 /Y 2 O 2 S:Eu 3+ was related to the decrease of the amount of dye adsorption and lower interfacial electron transfer 32

Methods
Preparation of samples. All of the chemicals used in this paper were analytical grade and used as received without further purification. In the synthesis of Y 2 O 3 /Y 2 O 2 S:Eu 3+ , 3 mL of Ln(NO 3 ) 3 (Ln = Y and Eu) aqueous solution (0.5 mol/L) was added to 3 mL deionized water, and the solution was thoroughly stirred, then an aqueous solution of NaOH (0.25 M) was added into the above solution. Subsequently, the milky colloidal solution was transferred to a 50 mL Teflon-lined autoclave, and heated at 100 °C for 5 h. The systems were then allowed to fast cool to room temperature. The final products were collected by means of centrifugation, washed with deionized water and ethanol, dried at 80 °C for 4 h in air. And then the Y 2 O 3 :Eu 3+ precursor was obtained. 0.1 g of the Y 2 O 3 :Eu 3+ precursor and some (1.0, 1.5, and 2.0 g) sulfur powder were put into a porcelain boat with different proportions of sulfur powder, and then sintered at 600 °C for 1 h in N 2 atmosphere.

Fabrication of photoelectrodes.
Fabrication of photoelectrode and the assembly of DSSCs: several pastes, from homogeneously mixing Y 2 O 3 /Y 2 O 2 S:Eu 3+ and TiO 2 (Degussa P25) into 1.5 mL of TiO 2 colloid. The TiO 2 colloid was prepared following the previously published synthesis procedure 33 2 [Ru(4-carboxy-4′ -carboxylate-2,2′ bipyridine) 2 (NCS) 2 ] dye (N719, Solaronix SA, Switzerland) in acetonitrile and tert-butanol (volume ratio, 1:1) for 48 h at room temperature. The Pt counter electrodes were prepared following the previous literature 34 . The dye-sensitized photoanode was assembled with a Pt counter electrode into a sandwichtype cell. The sandwich-type cell was further fixed together with epoxy resin. The space between the electrodes was filled with the electrolyte, which comprised 0.6 M 1-propyl-2,3-dimethyl-imidazolium iodide, 0.05 M I 2 , 0.1 M LiI, and 0.5 M tert-butylpyridine (TBP) in 3-methoxypropionitrile (3-MPN), by capillary action. Materials Characterization. The crystal structure was analyzed by a Rigaku (Japan) D/MAX-rA X-ray diffraction meter equipped with graphite monochromatized Cu Kα radiation (λ = 1.541874 Å), keeping the operating voltage and current at 40 kV and 40 mA, respectively. The sizes and morphologies of the final products were determined by using a JEOL JEM-2010F transmission electron microscope (TEM) operated at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed using a VG ESCALABMK II with a Mg KR (1253.6 eV) achromatic X-ray source. The photoluminescence spectra were recorded using a Hitachi F-4600 fluorescence spectrophotometer at room temperature. For comparison of the luminescence properties of different samples, the luminescence spectra were measured with the same instrument parameters (2.5 nm for slit width and 700 V for PMT voltage). The luminescence decay curve was recorded by a Spex 1403 spectrometer under the excitation of a third harmonic (355 nm) of a Nd:YAG pulsed laser.
Photovoltaic properties. Photovoltaic measurements were carried out with a solar simulator (Oriel, USA) equipped with an AM 1.5 G radiation (1 sun conditions, 100 mW cm −2 ) filter was used as the light source. The irradiation area of DSSCs is 0.09 cm 2 . The electron transport and recombination properties were measured by intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) (Zahner Elektrik, Germany).