Electrophoretically Deposited Y2O3:Bi3+,Yb3+ Nanosheet Films as

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Electrophoretically Deposited Y2O3:Bi3+,Yb3+ Nanosheet Films as Spectral Converters for Crystalline Silicon Solar Devices Ryoma Watanabe, Yoshiki Iso,* and Tetsuhiko Isobe* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

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S Supporting Information *

ABSTRACT: Transparent Y2O3:Bi3+,Yb3+ films that exhibit near-infrared (NIR) emission under near-ultraviolet (NUV) excitation are candidates for the spectral converters in crystalline silicon solar devices. In this work, NIR-emitting films were fabricated by the aqueous electrophoretic deposition of Y2O3:Bi3+,Yb3+ nanosheets, and their photoluminescence (PL) properties were enhanced by a postcalcination process. Positively charged Y2O3:Bi3+,Yb3+ nanosheets with adsorbed polyethylenimine (PEI) were deposited as a uniform and dense micrometer thick layer on a transparent conductive substrate by applying an electric field to an aqueous nanosheet dispersion. The resulting nanosheet film showed NIR emission from Yb3+, realized through energy transfer from Bi3+, when excited under NUV light. The NIR PL intensity of the film was improved by calcination at 700−1000 °C under a flow of air. This NIR PL enhancement was explained by the calcination process increasing the crystallinity of the Y2O3:Bi3+,Yb3+ nanosheets and removing adsorbed hydroxyl groups that quenched the NIR emission of Yb3+. Furthermore, the increased energy transfer efficiency of the Bi3+ → Yb3+ transition with increasing calcination temperature also contributed to the observed NIR PL enhancement of the nanosheet film. The NIR PL intensity of the as-deposited nanosheet film did not change under continuous NUV excitation, whereas a gradual increase in the PL intensity was observed for the calcined nanosheet films. This increase in the PL intensity of the calcined films would be caused by the photooxidation of the Bi and Yb ions, which were partially reduced by PEI during the calcination process. KEYWORDS: Y2O3:Bi3+,Yb3+, nanosheet, photoluminescence, near-infrared emission, aqueous electrophoretic deposition, spectral converter, solar device

1. INTRODUCTION The theoretical limit of the photoelectric conversion efficiency of crystalline silicon solar cells, which are the most commonly used type of solar cell, is calculated to be ∼30%, which is wellknown as the Shockley−Queisser limit.1 However, the conversion efficiency of commercial solar cells is only approximately 10%−20%.2 One cause of this efficiency difference is the spectral mismatch between the solar spectrum and the spectral response of solar devices. Crystalline silicon solar cells have low sensitivities in the ultraviolet (UV) and blue regions below 450 nm.3 In particular, the near-ultraviolet (NUV) component of the solar spectrum is inaccessible to commercial silicon solar devices because the encapsulator materials absorb UV light.4 Fluorescent materials that convert NUV light into visible or near-infrared (NIR) light are attractive wavelength convertors to decrease the spectral mismatch between the solar spectrum and solar cells. The band gap of crystalline silicon is 1.1 eV, which means that NIR light with a wavelength of approximately 1100 nm can effectively be used for photoelectric generation because of the suppressed thermalization loss. NIR-emitting materials excited by NUV light are promising as downconverters that © 2019 American Chemical Society

convert one high-energy photon into two low-energy photons.5−7 Downconversion materials should allow the effective utilization of blue and NUV light because of their lower thermalization losses than those of downshifting materials that convert one high-energy photon into one lowenergy photon. Therefore, NIR-emitting phosphors excited by NUV irradiation are promising materials to improve the photoelectric conversion efficiencies of commercial crystalline silicon solar devices. Various phosphors have been suggested as wavelength converters for solar cells, such as organic materials,8 metal complexes,9,10 and inorganic materials.4 Organic compounds readily degrade under NUV irradiation, which is a serious problem that limits their practical use.11,12 In contrast, inorganic phosphors are suitable practical wavelength converters for solar cells because of their excellent stability. Yb3+doped phosphors that emit NIR light at approximately 1000 nm are good inorganic downconverter candidates for solar Received: May 30, 2019 Accepted: June 4, 2019 Published: June 4, 2019 4009

DOI: 10.1021/acsanm.9b01021 ACS Appl. Nano Mater. 2019, 2, 4009−4017

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ACS Applied Nano Materials cells. A codopant is required as a sensitizer in Yb3+-doped phosphors to shift the excitation peak to the NUV region. When lanthanide ions, such as Pr3+, Tb3+, and Tm3+, have been used as sensitizers, the NIR photoluminescence (PL) intensity of Yb3+ was very weak because of the narrow excitation peaks and low absorption efficiencies of these sensitizers caused by the forbidden inner-shell 4f−4f transitions.3−19 In contrast, Ce3+ and Bi3+ exhibit broad and strong excitation bands that can facilitate Yb3+ emission. Ce3+ is excited through allowed 4f−5d transitions; however, it is easily oxidized into Ce4+, which is accompanied by the reduction of Yb3+, resulting in a decrease in the PL intensity.20 Bi3+ is more promising as a sensitizer than Ce3+ because of its stable valence state and broad and strong absorption band corresponding to the allowed 6s2 → 6s6p transition. Y2O3 is widely known as a host crystal of inorganic phosphor materials that possesses the favorable attributes of high transparency and good stability against light irradiation and heating. The Y3+ in Y2O3 can be substituted by other trivalent metal ions. For Y2O3:Bi3+,Yb3+, the Bi3+ and Yb3+ ions occupy the Y3+ sites, for which the coordination number is six, in the cubic Y2O3 crystal.21 Herein, the ionic radii of Y3+, Bi3+, and Yb3+ are 90, 103, and 87 pm, respectively.22 Y2O3 doped with Bi3+ exhibits a broad absorption band at 300−400 nm. Y2O3:Bi3+,Yb3+ shows NIR emission from Yb3+ through energy transfer from Bi3+ when excited under NUV irradiation.23−25 The lower toxicity of Y2O3:Bi3+,Yb3+ compared to that of other NIR-emitting materials, such as cadmium chalcogenides, is also advantageous for its practical use.26,27 In previous reports on the preparation of Y2O3 via a liquidphase method, hydroxide precursors synthesized by a hydrothermal method that maintained their morphologies after a postcalcination process.28−34 The precursors were identified as Y2(OH)5.14(NO3)0.86·H2O, Y4O(OH)9(NO3), and Y(OH)3. In those reports, final products with various morphologies, such as microprisms,28 nanorods,29 nanosheets,29 and nanotubes,30 were obtained. To fabricate a transparent fluorescent film for use as a spectral converter in solar cells, the negligibly low light scattering from nanometer-sized particles is attractive. Moreover, the Y2O3:Bi3+,Yb3+ nanoparticles should have a sheet-like morphology to aid their deposition. Nanosheets are twodimensional materials with nanometer-sized thicknesses. Such anisotropic morphology should be suitable for the deposition of dense films. One of the most important technical problems facing spectral converters is realizing sufficiently thick films to harvest the NUV component of the solar spectrum while maintaining high transparency in the visible and NIR regions. Various processes have been used to fabricate nanosheet films, e.g., spin coating,35 dip coating,36 layer-by-layer deposition,37 and electrophoretic deposition (EPD).38 The EPD method involves the deposition of charged particles onto a conductive substrate immersed in a dispersed solution upon application of an electric field.39 EPD products have been fabricated from various inorganic nanomaterials, such as oxides, metals, carbon nanotubes, and semiconductors.38,40−44 Using EPD, homogeneous and dense films can be fabricated on conductive substrates, and the film thickness can be controlled by changing the magnitude and duration of the applied voltage. Furthermore, micrometer-thick films can be deposited from a dilute dispersion of nanoparticles. Aqueous dispersions are preferred for EPD because of their incombustibility, lower environmental load, and lower cost compared with those of

organic systems. In our previous study, transparent redemitting films with micrometer-scale thicknesses were prepared by aqueous EPD of Y2O3:Bi3+,Eu3+ nanosheets.40 However, in the case of NIR-emitting phosphors doped with Yb3+, the adsorption of hydroxyl (OH) groups quenches the emission; the energy of excited Yb3+ is lost through a nonradiative relaxation process involving OH vibration.23,45 In particular, when using fluorescent nanomaterials that have a large specific surface area, the effect of quenching on the PL properties is large. In this study, we fabricated Y2O3:Bi3+,Yb3+ nanosheet films, which are candidates for the spectral converters of crystalline silicon solar devices, via an aqueous EPD method, and improved their NIR PL intensity by calcining at 700−1000 °C to remove the adsorbed OH groups that quench the PL emission of Yb3+. The Y2O3:Bi3+,Yb3+ nanosheets were prepared by calcining hydroxide precursor nanosheets synthesized via a hydrothermal method, as we reported previously.46 The EPD films were deposited onto transparent conductive substrates from an aqueous dispersion of positively charged Y2O3:Bi3+,Yb3+ nanosheets with adsorbed polyethylenimine (PEI).

2. EXPERIMENTAL SECTION 2.1. Materials. Y(NO3)3·6H2O (99.99%) and Bi(NO3)3·5H2O (99.95%) were purchased from Kanto Chemical. Yb(NO3)3·3H2O (99.9%) was purchased from Mitsuwa Pure Chemical. Ethylene glycol (EG; 99.5%) and polyvinylpyrrolidone (PVP; K25) were purchased from Wako Pure Chemical Industries. Ethanol (99.5%) was purchased from Kanto Chemical. Triethylamine (99.0%) was purchased from Tokyo Chemical Industry. PEI (Mw = 25000 g mol−1, Mn = 10000 g mol−1) was purchased from Sigma-Aldrich. All reagents were used without further purification. 2.2. Preparation of Precursor Nanosheets and Their Calcination To Obtain Y2O3:Bi3+,Yb3+ Nanosheets. The synthesis of Y2O3:Bi3+,Yb3+ nanosheets was performed via the same method as in our previous work.46 The procedure is summarized in Scheme 1 (A). Bi(NO3)3·5H2O (0.01 mmol) was dissolved in EG (2.6 mL). Y(NO3)3·6H2O (0.94 mmol) and Yb(NO3)3·3H2O (0.05 mmol) were dissolved in ultrapure water (30 mL). The aqueous solution was added to the EG solution, and then the mixture was added to triethylamine (3.0 mL). The total volume of the resulting suspension was adjusted to 40 mL by adding ultrapure water. The suspension was placed in a polytetrafluoroethylene vessel with a volume of 50 mL and then heated in a stainless-steel autoclave (DAB-2, Berghof) at 160 °C for 4 h. After cooling the autoclave to room temperature in a water bath, the precipitate was isolated by 5 min of centrifugation at a centrifugal force of ∼11,000g (10,000 rpm using a rotor with a diameter of 10 cm). The precipitate was redispersed in ultrapure water (20.0 mL) by ultrasonication and then centrifuged under the same conditions. This cycle of washing and centrifugation was performed three times. The resulting precipitate was freeze-dried overnight to obtain the dried precursor. The precursor was placed in an alumina combustion boat, heated to 700 °C at a heating rate of 10 °C min−1 under an air flow of 300 mL min−1, and then kept at temperature for 2 h to obtain the calcined sample. 2.3. Electrophoretic Deposition of Y2O3:Bi3+,Yb3+ Nanosheets and Spin Coating of PVP. The EPD conditions are summarized in Scheme 1(B). An aqueous PEI solution (10 mL, 0.3 wt %) was adjusted to pH 5.5 by adding 1 M nitric acid. The calcined sample (0.48 g) was added to the PEI solution and dispersed by ultrasonication for 10 min. After letting the dispersion stand for 1 h, the supernatant (pH 7) was collected to give a stable nanosheet dispersion. EPD was carried out using the prepared dispersion (0.81 wt %). An indium tin oxide (ITO)-coated quartz glass substrate (25 mm × 50 mm × 1 mm thick, 50 Ω □−1) was used as the cathode. A stainless-steel plate (25 mm × 50 mm × 1 mm thick, SUS-304) was 4010

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ACS Applied Nano Materials Scheme 1. Summary of the (A) Y2O3:Bi3+,Yb3+ Nanosheet Synthesis and (B) EPD Conditions

The zeta-potential profiles were measured with a zeta-potential analyzer (Zetasizer Nano Z, Malvern Instruments). Fourier transform infrared (FTIR) absorption spectra of the samples in KBr disks were measured on an FTIR spectrometer (FT/IR-4200, JASCO). Transmission spectra were measured on a UV−vis−NIR optical absorption spectrometer (V-570, JASCO) coupled to an integrating sphere (ISN470, JASCO). PL and photoluminescence excitation (PLE) spectra were recorded on a fluorometer (FP-6600, JASCO) equipped with a NIR photomultiplier tube (R5509−43, Hamamatsu Photonics). Changes in the PL intensity during continuous NUV excitation were measured on the same apparatus. Absolute PL quantum yield (PLQY) of a powder sample using a quantum efficiency measurement system (QE-2000-311C, Otsuka Electronics).

3. RESULTS AND DISCUSSION 3.1. Characterization of the As-Prepared Y2O3:Bi3+,Yb3+ Nanosheets. The XRD pattern of the hydrothermally prepared precursor (Figure S1) was not assigned to an International Center for Diffraction Data card, but the observed profile was similar to that of the reported hydroxide precursor of Y2O3.47,48 The XRD pattern of the calcined sample was assigned to the cubic phase of Y2O3. XRF analysis revealed that the actual elemental composition of the precursor was Y:Bi:Yb = 91:1:8, which was close to the nominal ratio of 94:1:5. The calcined sample had a Y:Bi:Yb ratio of 90:1:9, which reasonably corresponded to the composition before calcination within experimental error. Figure 1 shows the TEM images and corresponding size distributions of the hydroxide precursor and Y2O3:Bi3+,Yb3+. Low-magnification TEM images are provided in Figure S2. The nanosheets of both the precursor and calcined samples were square in shape with a side length of ∼200 nm. The average lateral sizes calculated from the size distributions were 200 ± 36 and 175 ± 25 nm for the precursor and Y2O3:Bi3+,Yb3+, respectively, indicating that calcination caused the nanosheets to shrink. Judging from the AFM images in Figure 2, the thicknesses of the precursor and Y2O3:Bi3+,Yb3+ nanosheets were ∼20 and ∼15 nm, respectively, revealing that the nanosheet thickness also decreased during calcination. Figure 3 depicts the PLE and PL spectra of the NIR emission of the Y2O3:Bi3+,Yb3+ nanosheets. An excitation peak assigned to the 1S0 → 3P1 transition of Bi3+ was observed at 329.2 nm.49 Multiple emission peaks assigned to the 2F5/2 → 2 F7/2 transition of Yb3+ were observed in the wavelength region of 900−1100 nm, arising from Stark splitting of the 2F5/2 and 2 F7/2 levels of Yb3+.49,50 The strongest emission peak was observed at 976.8 nm. 3.2. Characterization of the Electrophoretically Deposited Y2O3:Bi3+,Yb3+ Nanosheet Films. 3.2.1. Properties of the Y2O3:Bi3+,Yb3+ Nanosheet Dispersion Used for EPD. The zeta potential of the Y2O3:Bi3+,Yb3+ nanosheet dispersion was dependent on the pH and was tens of millivolts under neutral and acidic conditions (Figure S3). The positively charged particle surface is derived from the protonated PEI adsorbed onto the nanosheets. EPD is preferably performed under neutral conditions to improve safety and avoid the dissolution of Y2O3.51 The mean zeta potential was 49.7 ± 7.7 mV at pH = 7. The average hydrodynamic size of the nanosheets as determined by DLS was 193 ± 98 nm (Figure S4). This value was close to the mean particle size obtained from the TEM image, implying that the nanosheets were not extensively aggregated in the dispersion used for the EPD process.

used as the anode. The substrate was cleaned in acetone under ultrasonication for 10 min and then dried under ambient conditions. The distance between the electrodes was kept at 10 mm using an acrylic spacer. The nanosheets were deposited on a region (25 mm × 10 mm) of the transparent conductive substrate (cathode) by application of a constant voltage of 2.5 V for 1−30 min and then dried under ambient conditions overnight. The nanosheet films formed on the substrate by EPD for 5 min were heated to 700−1000 °C at a heating rate of 10 °C min−1 under an air flow of 300 mL min−1 and then kept at the designated temperature for 2 h. An ethanol solution of PVP (100 μL, 24 wt %) was added dropwise onto a nanosheet film and then spin coated at 3000 rpm for 60 s to obtain a PVP-coated nanosheet film. 2.4. Characterization. X-ray diffraction (XRD) patterns were measured on an X-ray diffractometer (Rint-2200, Rigaku) with a Cu Kα radiation source and a monochromator. Crystallite sizes were calculated from the XRD peak widths by the Scherrer equation. X-ray fluorescence (XRF) spectra were measured using an X-ray fluorescence spectrometer (ZSXmini II, Rigaku). Elemental compositions were determined by the fundamental parameter method. Particle images were captured with a transmission electron microscope (TEM; Tecnai 12, FEI). Samples for TEM were prepared by drying a drop of an ethanol dispersion of each sample on a carbonreinforced collodion film on a copper grid (COL-C10, Oken Shoji). The nanosheets were also observed using an atomic force microscope (AFM; Hitachi, AFM5200S) in dynamic force mode. The samples for AFM were prepared by drying a drop of an ethanol dispersion of each sample on a negative-type silicon wafer (Nilaco, < 0.02 Ω cm at 0 °C). The film structures were observed with a scanning electron microscope (SEM; Inspect F50, FEI). The hydrodynamic size distributions were measured by dynamic light scattering (DLS) using a high-performance particle sizer (HPPS, Malvern Instruments). 4011

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Figure 1. TEM images (top) and corresponding size distributions (bottom) of the (a) hydroxide precursor and (b) Y2O3:Bi3+,Yb3+ nanosheets.

uniform thickness of 2.5 μm. Figure 5 shows SEM images of the calcined nanosheet films. When the nanosheet films were calcined at a higher temperature, morphological changes and partial fusion of the nanosheets were observed (see Figure S6 for a magnified SEM image). Figure 6 shows the XRD profiles of a bare ITO-coated quartz glass substrate, an as-deposited nanosheet film before calcination, and nanosheet films after calcination at 700 and 1000 °C. All of the observed XRD peaks, except for the sharp peaks attributed to ITO, were assigned to cubic Y2O3. As the calcination temperature increased, the intensity of the Y2O3 peaks increased, and the peaks became sharper. The crystallite sizes calculated from the (222) peak by the Scherrer equation were 59, 104, and 142 Å for the as-deposited film and nanosheet films calcined at 700 and 1000 °C, respectively, indicating that calcination improved the crystallinity of the films. The net XRD profiles of the nanosheet films were obtained by subtracting the data for the ITO-coated quartz glass substrate from the XRD profiles of the film samples (Figure S7). The net peak intensities relative to the strongest peak were calculated for each sample. Table 1 summarizes the net peak intensities and corresponding peak positions for the film samples. The net relative intensities of the (222) and (400) peaks changed upon calcination. As the calcination temperature increased, the change became more pronounced. These results suggested that the nanosheet orientation in the deposited films might change during calcination. The NIR emission energy from Yb3+ is a harmonic overtone of the vibration energy of the OH groups, thus the emissive energy in the NIR region is readily lost through the nonradiative relaxation process of OH vibration.52 FTIR spectra of the nanosheet films, displayed in Figure 7(a), were

Figure 2. AFM images of the (a) precursor hydroxide and (b) Y2O3:Bi3+,Yb3+ nanosheets.

3.2.2. Properties of the Y2O3:Bi3+,Yb3+ Nanosheet Films and Their Changes Induced by Calcination. The film weight increased monotonically with increasing deposition time, whereas the growth rate gradually decreased (Figure S5(a)). This can be explained by the increase in film resistance resulting in a decrease in the actual electric field applied to the dispersion.39 Because the film weight increased in proportion to the film thickness (Figure S5(b)), the density of the deposited film was constant along the film growth direction. Figure 4 shows SEM images of the surface and cross section of a nanosheet film deposited for 5 min. The nanosheets were densely deposited onto the substrate, and the film had a 4012

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Figure 3. (a) PLE and (b) PL spectra of the as-prepared Y2O3:Bi3+,Yb3+ nanosheets.

Figure 4. SEM images of the (a) surface and (b) cross section of the nanosheet film fabricated by EPD for 5 min.

Figure 6. XRD profiles of the ITO-coated quartz glass substrate and nanosheet films fabricated by EPD for 5 min before and after calcination at 700 and 1000 °C.

calcination temperature increased, indicating the loss of adsorbed OH groups during calcination. 3.2.3. Optical Properties of the Nanosheet Films. The transmission spectrum of the nanosheet film remained unchanged after calcination at 700−1000 °C, as shown in Figure 8. The transparency of the film was almost maintained, even after calcination. In the NUV region between 300−400 nm, the transmittance of the calcined film samples was lower than that of the as-prepared sample. This may be explained by enhanced surface light scattering, possibly due to an increase in the surface roughness during calcination. The transmittances of the nanosheet films were lower than that of the bare substrate in the visible and NIR regions, where Y2O3:Bi3+,Yb3+ has no absorption. Furthermore, a gradual decrease in transmittance was observed as the wavelength decreased. Light scattering at the rough surface of the nanosheet films might be the origin of their decreased transparency compared with that of the bare substrate. High transparency in the visible and NIR regions is required for the spectral down converters of solar cells to sufficiently harvest solar light; therefore, improving the transparency of the nanosheet films is required. This problem can be solved by coating the nanosheet films with a transparent polymer.40 A nanosheet film was coated with a smooth and uniform PVP layer, which exhibited no absorption in the NUV, visible, and NIR regions (see Figure S8 for a photograph and SEM image of the PVP-coated nanosheet film). As shown in Figure 8, the PVP coating enhanced the transmittance of the Y2O3:Bi3+,Yb3+ nanosheet film over the whole wavelength region. This may be

Figure 5. SEM images of nanosheet films calcined at different temperatures of (a) 700 °C, (b) 800 °C, (c) 900 °C, and (d) 1000 °C.

measured to evaluate their OH contents. The absorption band at ∼3400 cm−1 was assigned to the O−H stretching vibration {ν(OH)}.53 The absorption peaks at 1395 and 1506 cm−1 were attributed to the symmetric {νs(CO32−)} and asymmetric {νas(CO32−)} stretching vibration of carbonate (CO32−) derived from the absorption of carbon dioxide in the atmosphere.54 The absorption peak originating from the Y− O stretching vibration {ν(Y−O)} appeared at ∼660 cm−1.55 Figure 7(b) shows the ratio of the absorbance of the O−H stretching vibration {Aν(OH)} to that of the Y−O stretching vibration {Aν(Y−O)}, Aν(OH)/Aν(Y−O), for the nanosheet films before and after calcination. This ratio decreased as the 4013

DOI: 10.1021/acsanm.9b01021 ACS Appl. Nano Mater. 2019, 2, 4009−4017

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ACS Applied Nano Materials Table 1. Relative Intensities of the Net XRD Peaks for the As-Deposited and Calcined Nanosheet Films Calcined at 700 °C

As-deposited Miller index

Peak position (degree)

(222) (400) (440) (622)

29.15 33.80 48.65 57.70

Relative intensity (−) Peak position (degree) 100.0 70.8 42.1 34.3

29.20 33.95 48.65 57.85

Calcined at 1000 °C

Relative intensity (−) Peak position (degree) 86.8 100.0 39.0 32.1

29.15 33.80 48.75 57.80

Relative intensity (−) 84.3 100.0 35.8 30.7

Figure 9 shows the PLE and PL spectra of the nanosheet films before and after calcination at 700−1000 °C. For the

Figure 7. (a) FTIR spectra of nanosheet films fabricated by EPD for 5 min before and after calcination at 700 and 1000 °C. (b) Changes in the absorbance ratio of Aν(OH)/Aν(Y−O) with calcination temperature. The absorbance ratios for the nanosheet films before calcination are plotted at 25 °C.

Figure 9. PLE and PL spectra of the (a) Bi3+ and (b,c) Yb3+ emission of the nanosheet films fabricated by EPD for 5 min before and after calcination at 700−1000 °C. The excitation and emission wavelengths for each spectrum are summarized in Table S1.

visible emission of Bi3+ (Figure 9(a)), an excitation band assigned to the 1S0 → 3P1 transition of Bi3+ and an emission band assigned to the 3P1 → 1S0 transition of Bi3+ were observed at 331 and 500 nm, respectively.23 The excitation and emission bands of Bi3+ are composed of two peaks because the doped Bi3+ occupied two kinds of Y3+ sites, which have C2 and S6 symmetries in cubic Y2O3.23 The emission peak at ∼500 nm is attributed to the Bi3+ occupying the C2 symmetry sites, and the shoulder peak at ∼420 nm is attributed to the Bi3+ occupying the sites with S6 symmetry. Notably, a PL peak corresponding to the 3P0 → 1S0 transition did not appear because it is completely spin forbidden.23 The maximum PL intensity of Bi3+ was achieved for the nanosheet film calcined at 800 °C; a decrease in the PL intensity was observed for the films calcined at 900 °C and above (Figure S9). This implies that calcination at 800 °C enhanced the probability of Bi3+ → Yb3+ energy transfer because of the more uniform distribution of Bi3+ and Yb3+ in the Y2O3 host crystal, which resulted in the suppression of the concentration quenching. Therefore, the decrease in the PL intensity of Bi3+ by calcination at 900 °C and above is attributed to the increased energy transfer probability, which does not contradict the monotonical

Figure 8. Transmission spectra of the ITO-coated quartz glass substrate and nanosheet films fabricated by EPD for 5 min before and after calcination at 700−1000 °C. The transmission of the PVPcoated nanosheet film is also shown.

attributed to the suppression of the surface scattering and reflection by applying the smooth resin layer. 4014

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nanosheet film. The NIR PL intensities of the Y2O3:Bi3+,Yb3+ nanosheet films improved by calcination at 700−1000 °C under an air flow. XRD and FTIR analyses indicated that an increase in the crystallinity and the removal of adsorbed OH groups enhanced the NIR PL intensities of the nanosheet films after calcination. Furthermore, judging from the change in the visible PL intensity of Bi3+, an improvement in the Bi3+ → Yb3+ energy transfer efficiency with increasing calcination temperature also contributed to the observed NIR PL enhancement. The NIR PL intensity of the as-deposited nanosheet film did not change under continuous NUV excitation, whereas a gradual increase in the NIR PL intensities of the calcined films was observed. This increase would be caused by photooxidation of Bi and Yb ions that had been partially reduced by PEI during calcination. These techniques to improve the transparency and NIR PL intensity of nanomaterials prepared through an aqueous route provide a large contribution to aid in the development of spectral converters for solar cells. However, PLQY of the Y2O3:Bi3+,Yb3+ nanosheet powder calcined at 1000 °C showed 0.50% only. Unfortunately, we conclude that nanometer-sized Y2O3:Bi3+,Yb3+ is not promising for spectral converters in solar devices; therefore, further searches for efficient NIR PL-emitting nanomaterials are necessary.

increase in the NIR PL intensity of Yb3+, as shown in Figure 9(b) and (c). Multiple NIR emission peaks from Yb3+ were detected at 900−1100 nm for all of the nanosheet film samples, as shown in Figure 9(c). As the calcination temperature increased, the NIR PL intensity increased monotonically. Considering the above results from the XRD, FTIR, and PL measurements, this trend can be explained by the following reasons: (i) increased crystallinity, (ii) removal of adsorbed OH groups, which quench Yb3+ emission, and (iii) increased probability of energy transfer from Bi3+ to Yb3+ with increasing calcination temperature. However, PLQY of Y2O3:Bi3+,Yb3+ nanosheet powder calcined at 1000 °C, which showed the highest NIR PL intensity, was only 0.50% (see also PL spectra for the PLQY measurement in Figure S10). From this result, efficient NIR emission attributed to a quantum cutting effect would be unrealistic. Indeed, a quantum cutting effect of Gd2O3:Bi3+,Yb3+ was not observed in a recent work.56 Finally, the photostability of the NIR PL of the nanosheet films was evaluated. Figure 10 shows the changes in the PL

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b01021. XRD profiles of the precursor and calcined samples (Figure S1); TEM images of the precursor and Y2O3:Bi3+,Yb3+ nanosheets (Figure S2); zeta potential and DLS analyses of the Y2O3:Bi3+,Yb3+ nanosheet dispersion (Figure S3 and S4); plots of the film growth (Figure S5); magnified SEM image of a nanosheet film calcined at 1000 °C (Figure S6); net XRD profiles of the nanosheet films before and after calcination at 700 and 1000 °C (Figure S7); photograph and SEM image of the PVP-coated nanosheet film (Figure S8); excitation and emission wavelengths used for the PL and PLE spectra measurements presented in Figure 9 (Table S1); changes in the PL intensity of the Bi3+ emission with calcination temperature (Figure S9); PL spectra of Y2O3:Bi3+,Yb3+ nanosheet powder calcined at 1000 °C for the PLQY measurement (Figure S10). (PDF)

Figure 10. Changes in the normalized NIR PL intensities of the nanosheet films fabricated by EPD for 5 min before and after calcination at 700−1000 °C under continuous excitation for 4 h.

intensity at 976 nm of the calcined films under continuous 332 nm excitation for 4 h. The initial PL intensities were normalized to 100%. The PL intensity of the as-deposited nanosheet film did not change over the 4 h, whereas gradual increases in the PL intensities of the calcined films were observed. There was no correlation between the increase in PL intensity and the calcination temperature. The films fabricated by EPD were composed of Y2O3:Bi3+,Yb3+ nanosheets with adsorbed PEI. The increase in PL intensity during irradiation of the calcined films would therefore be related to photooxidation of Bi and Yb ions that had been partially reduced by PEI during calcination. Unfortunately, evaluating the changes in the valence states of the ions was difficult because of their low doping concentrations.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Tel: +81 45 566 1558; Fax: +81 45 566 1551 (Y.I.). *E-mail: [email protected]; Tel: +81 45 566 1554; Fax: +81 45 566 1551 (T.I.). ORCID

4. CONCLUSIONS Using an aqueous EPD method, we fabricated Y2O3:Bi3+,Yb3+ nanosheet films that converted NUV light into NIR light. Y2O3:Bi3+,Yb3+ nanosheets with adsorbed PEI were deposited in a uniform and dense micrometer-thick layer on a transparent conductive substrate under an applied electric field. The nanosheet films were translucent because of light scattering at their rough surfaces. However, the transparency was drastically improved by apply a smooth PVP layer on top of the

Yoshiki Iso: 0000-0001-7483-2828 Tetsuhiko Isobe: 0000-0002-0868-5425 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support by the Futaba Research Grant Program of the Futaba Foundation and a 4015

DOI: 10.1021/acsanm.9b01021 ACS Appl. Nano Mater. 2019, 2, 4009−4017

Article

ACS Applied Nano Materials

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research grant of the Iketani Science and Technology Foundation.

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DOI: 10.1021/acsanm.9b01021 ACS Appl. Nano Mater. 2019, 2, 4009−4017