Electrophoretically Deposited Y2O3:Bi3+,Eu3+ Nanosheet Films with

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Electrophoretically deposited YO:Bi ,Eu nanosheet films with high transparency for near-ultraviolet to red light conversion Yuta Kosuge, Yoshiki Iso, and Tetsuhiko Isobe Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04334 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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Langmuir

Electrophoretically deposited Y2O3:Bi3+,Eu3+ nanosheet films with high transparency for near-ultraviolet to red light conversion

Yuta Kosuge, 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

1 ACS Paragon Plus Environment

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ABSTRACT Fluorescent films were fabricated by depositing Y2O3:Bi3+,Eu3+ nanosheets, which emit red light under near-UV irradiation. The Y2O3:Bi3+,Eu3+ nanosheets were obtained by calcining hydroxide precursor nanosheets synthesized through a hydrothermal method. An aqueous dispersion of positively charged Y2O3:Bi3+,Eu3+ nanosheets with polyethyleneimine adsorbed to the surface was prepared for their deposition. Fluorescent nanosheets were electrophoretically deposited on a transparent conductive substrate under a constant voltage. The obtained nanosheet films were dense and uniform, and showed excellent photostability against the excitation light. Growth of the nanosheet film caused a decrease in transmittance and an increase in photoluminescence intensity. The former effect was attributed to light scattering from inner voids and the rough surface of the film. A polyvinylpyrrolidone (PVP) coating on the film improved the transmittance to be greater than 70% over the visible region. These effects were attributed to anti-reflection effects at the film surface owing to the low refractive index of PVP. Furthermore, suppression of light scattering by coating the rough surface with a smooth PVP film and filling of voids in the nanosheet film with PVP also improved the transmittance.


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Langmuir

1. INTRODUCTION Phosphors, which convert near-UV light to visible light, have been considered for use as wavelength converters for applications in white light-emitting diodes (LEDs) for solid-state lightning and liquid crystal display backlights,1–5 invisible security inks,6 and bio-imaging.7 For example, white LEDs with a remote phosphor structure are composed of a near-UV or blue LED and a spectral converting film.4,5 Transparent spectral converting films are applicable to security printings.6 These applications require spectral converting films with the following characteristics: (i) high photostability under continuous excitation, (ii) strong broadband absorption in the near-UV region, (iii) transparency to visible light, and (iv) a high photoluminescence (PL) quantum yield (QY).8,9 Visible light-emitting phosphors includes organic dyes, metal complexes, and inorganic phosphors. Inorganic phosphors can be categorized into micrometer-sized materials and nanometer-sized ones. Although organic dyes and metal complexes have good properties that meet the requirements (ii)–(iv), their low photostability is a serious problem for practical use.8,10 Micrometer-sized inorganic phosphors meet the requirements (i), (ii), and (iv); however, light scattering decreases their transparency considerably.11 Conversely, nanometer-sized inorganic phosphors can realize (iii)


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because light scattering intensity is proportional to the sixth power of the particle diameter, on the basis of Rayleigh scattering.12 There have been many reports on nanometer-sized inorganic phosphors such as quantum dots, which are single-crystalline semiconductor nanoparticles emitting light through an interband transition, and nanophosphors doped with metal ions in a host crystal as emission centers and sensitizers for excitation. Transparent fluorescent films using the nanophosphors have also been fabricated by many researchers.11,13–16 In a previous study, we reported on the synthesis and PL properties of YVO4:Bi3+,Eu3+ nanophosphors, which converted near-UV light to red light.17–19 Compared with quantum dots, YVO4:Bi3+,Eu3+ nanophosphors have the following advantages: (i) self-absorption does not occur owing to no overlap between the absorption and emission spectra; and (ii) they can be synthesized by an aqueous route with a low environmental load. However, YVO4:Bi3+,Eu3+ nanoparticles react with organic molecules adsorbed on their surface under continuous excitation light irradiation through photoreduction of V5+ to V4+, resulting in a decrease of PL intensity.20 We focused on nanometer-sized Y2O3:Bi3+,Eu3+ with sheet-like morphology as a new spectral conversion material to address the photodegradation problem.21 The Y2O3:Bi3+,Eu3+ nanosheet exhibited red emission corresponding to the 4f–4f transition of Eu3+ through an energy transfer from Bi3+ to


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Eu3+, following near-UV excitation corresponding to the 6s2–6s6p transition of Bi3+. The Y2O3:Bi3+,Eu3+ nanosheet was composed of more stable ions than YVO4:Bi3+,Eu3+. Y2O3 as a host crystal is photostable to near-UV irradiation, because its band gap energy is higher than the energy of near-UV photons. The excellent stability of the PL intensity from

Y2O3:Bi3+,Eu3+

nanosheets

under

continuous

near-UV

irradiation

was

experimentally confirmed in our previous report.21 Y2O3 nanosheets were prepared by calcining hydrothermally-synthesized yttrium hydroxide precursor nanosheets, while maintaining their morphology. The PLQY was improved by removing surface hydroxyl groups, which causing emission quenching of Eu3+, during the calcination. The sheet-like particles have large contact area each other, therefore well-ordered multilayer films would be piled up densely. Hence, nanosheet films were deposited from dispersion of the nanosheets by the following methods. There have been reports on various processes to fabricate films from nanometer-sized

particles,

e.g.,

Layer-by-layer

methods,22

Langmuir–Blodgett

methods,23 drop-casting,24 and electrophoretic deposition (EPD).16,25–28 To convert the excitation light effectively, a sufficient film thickness is necessary for the spectral conversion film. We focused on the EPD method to fabricate micrometer-thick films from the nanometer-sized materials. In the EPD method, surface-charged particles are


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directly deposited on an electrode from a dispersion under the application of a voltage.29 The advantage of the EPD method is that homogeneous and dense films can be fabricated on conductive substrates with various shapes.30,31 Moreover, the film thickness is controlled by the magnitude and duration of the voltage application. Both organic and aqueous dispersion media can be used for EPD. For example, a film was deposited at an applied voltage of 600 V from negatively charged GdOCl nanoplates dispersed in cyclohexane.26 A composite film was also fabricated by applying a voltage of 3.0 V from an aqueous dispersion of negatively charged YVO4:Bi3+,Eu3+ nanoparticles and resin nanoparticles.16 The use of an aqueous medium for EPD has the following advantages: (i) the surface potential of the dispersed particles is readily controlled by adjusting the pH; (ii) a film can be fabricated under the application of a lower voltage compared with that required for organic media; and (iii) the environmental load is low.32 In a previous report, transparent films were fabricated through a dense deposition of positively charged Al(OH)3 nanoplates by EPD followed by polymer coating of the film surface.27 The transparency of the EPD films was improved by the surface treatment. In this study, transparent fluorescent films of Y2O3:Bi3+,Eu3+ nanosheets were fabricated and characterized. The Y2O3:Bi3+,Eu3+ nanosheets was prepared by calcining


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hydroxide precursor nanosheets synthesized via a hydrothermal method. An aqueous dispersion of positively charged Y2O3:Bi3+,Eu3+ nanosheets was prepared by the addition of polyethyleneimine (PEI), which adsorbed to the as-prepared nanosheets. We fabricated nanosheet films on an indium tin oxide (ITO) transparent conductive glass substrate by EPD. Furthermore, the improvements of transparency by coating with a polyvinylpyrrolidone (PVP) layer on the deposited film were investigated.

2. EXPERIMENTAL SECTION 2.1. Reagents Y(NO3)3·6H2O (99.99%), Bi(NO3)3·5H2O (99.5%), Eu(NO3)3·6H2O (99.95%), and ethanol (99.5%) were purchased from Kanto Chemical. Ethylene glycol (99.5%) and polyvinylpyrrolidone (PVP) were purchased from Wako Pure Chemical Industries. Triethylamine

(99.0%)

was

purchased

from

Tokyo

Chemical

Industry.

Polyethyleneimine (PEI; Mw ≈ 25000, Mn ≈ 10000) was purchased from Sigma-Aldrich. HNO3 solution (69%) was purchased from Nacalai Tesque. All reagents were used without further purification.


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2.2 Preparation of Y2O3:Bi3+,Eu3+ nanosheets The Y2O3:Bi3+,Eu3+ nanosheets were prepared by calcination of hydroxide precursors synthesized via the hydrothermal method as reported in our previous work.21 Bi(NO3)3·5H2O (0.0040 mmol) was dissolved in ethylene glycol (2.6 mL). Y(NO3)3·6H2O (0.956 mmol) and Eu(NO3)3·6H2O (0.040 mmol) were dissolved in ultrapure water (30 mL). The aqueous solution was added to the ethylene glycol solution, and then the mixture was added to triethylamine (3 mL). The total volume of the solution was adjusted to 40 mL by the addition of ultrapure water, and the resulting suspension

was

stirred

for

10

min.

The

suspension

was

placed

in

a

polytetrafluoroethylene vessel with a volume of 50 mL and heated in a stainless-steel autoclave (Berghof, DAB-2) at 160 °C for 4 h. After cooling the autoclave to room temperature by placing it in water, the product was isolated by centrifugation at ~11,000 ×g (10,000 rpm using a rotor with a diameter of 10 cm) for 5 min and washed with ultrapure water. This process was repeated three times. The product dispersed in ultrapure water (10 mL) was freeze-dried overnight and heated to 600 °C at a heating rate of 10 °C min−1 in an air flow of 300 mL min−1, and maintained at that temperature for 2 h to obtain calcined samples.


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2.3 Fabrication of nanosheet films by electrophoretic deposition The calcined powder sample (0.48 g) was added to 10 mL of an aqueous solution of PEI (0.3 wt%) under vigorous stirring and dispersed by ultrasonication for 10 min. The pH value of the dispersion was adjusted to 7 by addition of 1 M HNO3 solution, and the nanosheets became positively charged attributed to adsorption of PEI.33 The dispersion was allowed to stand for 1 h, then the supernatant was used for electrophoretic deposition. Its concentration was 1.3 wt%. An ITO-coated glass substrate (25 mm × 50 mm × 1 mmt, 50 Ω □−1) and a stainless-steel plate (25 mm × 50 mm × 1 mm, SUS-304) were used as the cathode and anode, respectively. The distance between both electrodes was maintained at 10 mm, and they were dipped vertically into the dispersion to a depth of 10 mm. A nanosheet film was deposited on the dipped area (25 mm × 10 mm) of the ITO-coated glass substrate during the application of a constant voltage of 2.5 V for 1–30 min, followed by drying under ambient conditions for several tens of minutes. After cutting the undeposited part of substrate, an ethanol solution of PVP (24 wt%) was spin-coated (3,000 rpm for 1 min) on the deposited nanosheet film (25 mm × 10 mm) and dried under ambient conditions to fabricate a PVP-coated nanosheet film.


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2.4 Characterization X-ray diffraction (XRD) profiles were measured on an X-ray diffractometer (Rigaku, Rint-2200), with a Cu Kα radiation source and monochromator. Elemental compositions were determined by an X-ray fluorescence (XRF) analyzer (Rigaku, ZSX mini II). Electron microscope images were captured by a transmission electron microscope (TEM; FEI, Tecnai G2 F20) and a scanning electron microscope (SEM; FEI, Inspect F50). Samples for TEM observations were prepared by drying a drop of ethanol dispersions of each sample on a carbon-reinforced collodion film of copper grid (Okenshoji, COL-C10). Nanosheets were also observed with a scanning probe microscope (SPM; Hitachi, AFM5200S) in dynamic force mode. The samples for SPM were prepared by drying a drop of an ethanol dispersion of each sample on an n-type silicon wafer (Nilaco,

Electrophoretically Deposited Y2O3:Bi3+,Eu3+ Nanosheet Films with

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