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Phosphor Polymer Nanocomposite: ZnO:Tb3+ Embedded Polystyrene Nanocomposite Thin Films for Solid-State Lighting Applications Jai Prakash, Vinod Kumar, Lucas J B Erasmus, M M Duvenhage, G Sathiyan, Stefano Bellucci, Shuhui Sun, and Hendrik C Swart ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00387 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018
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Phosphor Polymer Nanocomposite: ZnO:Tb3+ Embedded Polystyrene Nanocomposite Thin Films for Solid-State Lighting Applications Jai Prakash1,2 *, Vinod Kumar1,3, L. J. B. Erasmus1, M. M. Duvenhage1, G. Sathiyan2, Stefano Bellucci4, Shuhui Sun5, Hendrik C Swart1* 1
2
Department of Physics, University of the Free State, Bloemfontein, ZA9300, South Africa Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India 3 Center for Energy Studies, Indian Institute of Technology Delhi, New Delhi-110016, India
4
INFN-Laboratori Nazionali di Frascati, Via Enrico Fermi, 40, 00044 Frascati (Roma), Italy 5 Center for Energy, Materials and Telecommunications, Institut National de la Recherche Scientifique, Québec, J3X 1S2, Canada Abstract
Producing novel functional nanomaterials with high reproducibility is extremely desirable but very challenging. Further treatment by reliably combining or embedding nanoparticles into a protecting medium could be a promising approach to retain their properties and realize practical applications. In this work, we report the preparation, characterization and potential applications of flexible and self-standing polymer nanocomposite films embedded with ZnO:Tb3+ nanophosphors suitable for optical devices. The nanocomposite films were prepared from ZnO:Tb3+ phosphor nanoparticles synthesized by a solution combustion method and a solution of polystyrene (PS) in toluene. The structural, morphological, thermal and optical properties of the phosphor polymer nanocomposite (PS/ZnO:Tb3+) films were studied. These studies showed that the ZnO:Tb3+ phosphor nanoparticles were uniformly distributed throughout the polymer films providing higher stability to the thermal and photoluminescence (PL) properties of the films. The PL properties of the nanocomposite films increased with an increase in nanophosphor concentration. The temperature-dependent PL properties of the PS/ZnO:Tb3+ films were also studied. This study reveals that the embedded nanophosphor nanoparticles retain their unique luminescent properties without any significant spectral shift indicating their potential applications in solid-state lighting. Keywords: Nanophosphor, Polymer, Luminescent nanocomposite films, Thermal stability, Solid-state lighting, *
Corresponding author: Email: Jai Prakash (
[email protected],
[email protected]), Hendrik Swart (
[email protected])
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1. Introduction The development of functional nanomaterials with high sensitivity and reproducibility is highly desirable for practical applications but very challenging1-5. Nanostructured materials are very interesting because of their unique properties at the nanoscale level that significantly enhance the physical and chemical properties that are desirable for potential applications6-9. In addition, the reliable and controllable doping of nanomaterials or the embedding of a combination of other functional materials into a suitable medium such as polymer could provide a better platform for practical applications via thin-film or coating nanotechnology 5, 10-15. Zinc oxide (ZnO), a wide-band-gap semiconductor, is a promising material with a high potential for a variety of practical applications including phosphor, sensing, optical, photonic, and light-emitting diodes (LEDs), ultraviolet (UV) laser emitters and solar cells
7, 16-17
. In
addition, ZnO is an environmentally friendly, low-cost material with good thermal and chemical stability. Furthermore, ZnO can be synthesized by simple methods. This semiconductor is also used in the medical field because of its biocompatibility, biodegradability and non-toxicity16, 18. ZnO is a fascinating material in the field of solid-state lighting that has attracted the scientific community, particularly because of its great potential in LED applications. Moreover, ZnO is especially used in phosphor-converted LEDs because of a strong and broad absorption band in the near UV region and several inherent defects7. ZnO-based nanomaterials have been extensively used and studied for several optical devices such as blue, violet, and UV LEDs and laser diodes (LDs), and furthermore there is a growing interest of ZnO as a white LED in recent years
7, 19-20
. These interesting results have been achieved either by various types of defects
induced by different techniques or introducing impurities such as rare earth ions through doping 7, 21
. ZnO has been shown to be a unique host material for doping rare earth ions exhibiting
luminescence at room temperature, and it is known to be an excellent phosphor with high efficiency and low degradation. ZnO doped with rare earth ions has the potential to be a promising multifunctional nanomaterial with a co-existing semiconducting and optical properties. The rare earth ions provide excellent luminescence properties to the host materials because of their sharp and intense emissions due to their 4f intrashell transitions7, 21-22. ZnO doped with Tb3+ (ZnO:Tb3+) is a promising nanomaterial with high luminescence properties that have been applied to white light solid-state LEDs7,
23-24
. In addition, ZnO:Tb3+
nanomaterials exhibit ferromagnetism that varies with temperature24-26. There are only a few 2 ACS Paragon Plus Environment
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reports of ZnO:Tb3+-based white LEDs 7. White LEDs are important as they have several advantages such a higher energy efficiency, a high reliability along with a longer life and a faster response. In addition, these nanomaterials have demonstrated better pollution control compared to traditional lighting
27
. We have recently reported on white light emission from ZnO doped
with Tb3+ phosphor nanomaterials 7. A variation in the tunable color emission from yellow to blue occurred by changing the Tb3+ concentration, whereas white emission could achieved at an optimum Tb3+ doping concentration (5 mol%) with a high color rendering index. This study emphasizes that ZnO:Tb3+ phosphor nanomaterials may potentially be used as a single-phase full-color-emitting phosphor for UV-pumped warm white LEDs for indoor applications. However, these nanomaterials are not suitable for potential applications until they are used in a proper way. As reported in the literature, ZnO nanoparticles (NPs) are not stable and tend to aggregate readily, and a fluorescence quenching effect occurs when these NPs are subjected to water adsorption on their surfaces
28
. Such behavior of the ZnO NPs makes them
ineffective for technological applications. Therefore, as discussed above, research has been focused on the protection of such NPs from aggregation by incorporating them into a suitable matrix
29
. Similarly, for ZnO:Tb3+ nanophosphor NPs, embedding them into a suitable matrix
such as a polymer is a potential solution to avoid their aggregation and provide structural stability for practical applications. This is a promising approach for the development of functional nanosystems that are easy to handle and allow for a wider range of applications in optoelectronics, electrochemistry, thin-film and coating technologies, etc. However, techniques have been developed to synthesize thin films of phosphor materials on solid substrates, but there have always been some challenges of obtaining thick, uniform and high optical quality films 30. The incorporation of nanophosphor materials containing rare earth ions into a polymer matrix has attracted considerable attention of researchers because this approach provides composite materials with improved photoluminescence (PL) properties and a unique combination of different characteristics that allow for multifunctional applications
27, 31
. In addition, these
nanocomposite materials provide good thermal stability, mechanical strength, excellent optical properties along with flexibility and the ability to form thin films or coatings. In recent years, there has been immense activity regarding the synthesis and characterization of nanophosphor materials embedded in polymer matrices to explore thin-film technology for variety of applications because of their lightness, ease of fabrication and variety of other properties27,30,32-35. 3 ACS Paragon Plus Environment
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Several methods have been used to prepare luminescent polymer nanocomposites such as the in situ synthesis of luminescent NPs within a polymer matrix, melt processing, the direct mixing of a nanophosphor powder with a polymer solution or mixing with a monomer solution followed by in situ polymerization.
27, 30, 36
. Chen et al.
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reported Tb3+, Li+ ion co-doped ZnO/
polyethylene glycol (PEG) particles synthesized by a co-precipitation method. It was found that the PEG matrix enhanced the PL properties of Tb3+ providing excellent luminescent nanocomposite materials. Potdevin et al.
30
synthesized self-standing, flexible and thick films of
polyvinylpyrrolidone (PVP)/Y3Ga5O12:Tb3+ by mixing phosphor powder with the PVP solution in isopropanol. The structural, morphological and optical properties of these films were studied and correlated. It was reported that the films exhibited a good dispersion of nanophosphors within the polymer matrix and were suitable for potential applications in optical devices. The mixing of a nanophosphor powder with a polymer solution is the simplest method because it avoids a multistep synthesis process and is therefore not time consuming 30.
Figure 1: Schematic of synthesis process for ZnO:Tb3+ nanophosphor and PS/ZnO:Tb3+ phosphor nanocomposite films.
In the present work, we report on the preparation, characterization and optical properties of flexible and self-standing phosphor nanocomposite films prepared from ZnO:Tb3+ nanophosphor powder (synthesized by a solution combustion method) and a solution of polystyrene (PS) in toluene (Figure 1). The structural, morphological, thermal and optical properties are discussed with special emphasis on the PL properties of the nanocomposite films. Additionally, the PL properties of the nanocomposite films with increasing nanophosphor concentration and temperature-dependent PL were studied for their potential applications in a large variety of optical devices such as solid-state lighting. 4 ACS Paragon Plus Environment
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2. Experimental details 2.1 Materials Terbium nitrate pentahydrate, zinc nitrate tetrahydrate, urea and polystyrene granules were acquired from Sigma Aldrich. No further purification was carried out and these chemicals were used as received. 2.2 Synthesis of ZnO:Tb3+ nanophosphors with varying concentrations of Tb3+ ZnO:Tb3+ nanophosphors with varying Tb3+ concentrations (1-6 mol %) were synthesized using solution combustion, as described in a previous report 7. In brief, urea was mixed with zinc nitrate tetrahydrate, and the mixture was dissolved in distilled water, followed by the addition of terbium nitrate pentahydrate (used as a Tb source with a varying concentration of 0 to 6 mol%) with constant stirring for 20 min. The combustion process was carried out in a muffle furnace maintained at 450 °C as schematically shown in Figure 1. The final product was cooled down and used in the form of a powder for the various characterizations. 2.3 ZnO:Tb3+/PS nanocomposite thin film preparation First, a transparent solution of PS was obtained by dissolving PS granules in a toluene solvent at 60-80 °C. The solution was magnetically stirred for 1 h, followed by ultrasonication for 30 min. The PS and ZnO:Tb3+ nanocomposite was prepared by mixing the ZnO:Tb3+ (5 mol% of Tb) nanophosphor powder (0, 1, 2, 3, and 5 wt %) within a transparent solution of PS in toluene, followed by constant stirring. Thin films of the nanocomposites were obtained by casting the solution into a petri dish, and the films were dried overnight at 25-30 °C for outgassing (Figure 1). Free-standing, transparent and semitransparent films were obtained with increasing ZnO:Tb3+ wt%. 2.4 Characterization techniques X-ray diffraction (XRD): The synthesized ZnO:Tb3+ nanocomposite powders and ZnO:Tb3+/PS nanocomposite thin films were characterized using a Bruker D8 X-ray diffractometer operating at 40 kV and 40 mA using Cu Kα X-rays at a wavelength of 0.15406 nm to study the crystalline nature of the powders and the nanocomposite films. Field emission scanning electron microscopy (FE-SEM) and Transmission electron microscopy (TEM): A model JSM-7800F instrument from JEOL was used for imaging the ZnO:Tb3+ nanophosphor powder and ZnO:Tb3+/PS nanocomposite thin films. An electron beam of energy 5.0 kV was used with a magnification ranging from ×5,000 to ×30,000 to acquire the 5 ACS Paragon Plus Environment
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SEM micrographs in the present experiments. The film thicknesses were also measured by using cross sectional (X)-SEM. The sizes of the ZnO:Tb3+ NPs were measured from the TEM micrographs obtained by using TEM (Philips CM100 Analytical). Energy-dispersive spectroscopy (EDS) mapping was also performed to confirm the presence of elements in the ZnO:Tb3+ phosphor NPs. X-ray Photoelectron Spectroscopy (XPS): The ZnO:Tb3+ nanophosphor and ZnO:Tb3+/PS nanocomposites films were investigated by XPS to investigate the chemical changes that occurred in the systems and chemical states of the elements. In a ultra-high vacuum system with a base pressure of 3×10−9 torr, a 100 µm diameter monochromatic Al Kα X-ray beam (hν = 1486.6 eV) generated by a 25 W, 15 kV electron beam was used for the XPS measurements. High-resolution scans were recorded at a pass energy of 11 eV, a resolution of 0.5 eV and a step size of 1 eV. The C 1s peak at 284.4 eV was used as the reference energy for charge correction. Multipack version 9 software was used to analyze the spectra. Secondary ion mass spectrometry (SIMS): TOF-SIMS measurements were performed on an ion TOF SIMS 5 system. A pulsed 30 keV Bi+ primary ion beam operating at a DC current of 30 nA with a pulse repetition frequency of 10 kHz (100/s) was used to acquire chemical images of the phosphor in the positive secondary ion polarity mode. Moreover, an oxygen sputter gun at an energy of 1 kV and a DC current of 250 nA was used to raster an area of 300 × 300 µm on the sample. The analytical field-of-view was 100 × 100 µm with a 256 × 256 pixel digital raster. UV-vis spectroscopy: UV-vis diffuse reflectance measurements were carried out in air for the ZnO:Tb3+ nanophosphor and ZnO:Tb3+/PS nanocomposites films in the wavelength range of 250–800 nm using a Lambda 950 UV-Vis-NIR spectrophotometer with an integrating sphere from PerkinElmer. Photoluminescence (PL) spectroscopy: PL measurements were carried out at room temperature using a He–Cd laser emitting at 325 nm to excite the samples. An iHR320 spectrometer from Horiba was used to analyze the luminescence, which was measured using a thermoelectrically cooled R928 photomultiplier tube from Hamamatsu. Temperature-dependent PL measurements were also conducted on the PS and PS/ZnO:Tb3+ nanocomposites films in a temperature range of 30-100 °C using the same laser source. Infrared spectroscopy: A Nicolet 6700 FT-IR spectrometer from Thermo Scientific was used to measure the FTIR spectra operating in transmittance mode. The FTIR spectra of the polymer and 6 ACS Paragon Plus Environment
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nanocomposites films were compared to study the changes that occurred due to incorporation of the ZnO:Tb3+ nanophosphor within the polymer matrix. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) analyses: TGA was performed using a Perkin-Elmer TGA7 thermogravimetric analyzer. The thermal stability of the thin pristine polymer film and nanophosphor-embedded nanocomposite film (ZnO:Tb3+/PS) with 5 wt% of ZnO:5 mol% Tb3+ was studied. Furthermore, a DSC analysis was also performed on the polymer and nanocomposite films to study their thermal stability. 3. Results and discussion 3.1 ZnO:Tb3+ nanophosphor The ZnO:Tb3+ nanophosphors, as synthesized by a solution combustion method with varying Tb3+ concentrations (1-6 mol%), were examined by XRD measurements to determine their structure and crystalline behavior. The XRD patterns showed highly crystalline nanophosphors with a hexagonal wurtzite structure with a preferred (101) orientation, as shown in Figure 2 (a). An additional peak at 29.3° also appeared that corresponds to Tb2O337, and its intensity increased with increasing Tb concentration due to an excess of Tb, which formed Tb2O3at a specific concentration of Tb3+, that occupied the Zn2+ (or interstitial) sites 7.
Figure 2. (a) XRD spectra of the ZnO:Tb3+ nanophosphors with different doping concentrations of Tb where "*" indicates the presence of Tb2O3 (standard XRD data of Tb2O3 and ZnO are shown on top). (b) PL emission of the ZnO:Tb3+ nanophosphors with different Tb3+ concentrations. The emitting color was easily tuned from yellow to blue by varying the Tb3+ concentration. Near white emission was achieved for the 5 mol% doping concentration of Tb3+ in ZnO:Tb3+. [Reproduced with permission from ref. 7. Copyright (2014) Elsevier].
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The PL properties of the ZnO:Tb3+ nanophosphors at all doping concentrations of Tb3+ ions were studied at room temperature using a 325 nm laser excitation source, and the recorded PL spectra are shown in Figure 2b. An excitation wavelength of 325 nm (~3.8 eV) was used because it has an energy greater than that of the band gap of ZnO (393 nm, ~3.16 eV)7 (Figure S1) and can excite an electron from the valence band directly into the conduction band as well as deep levels within the band gap38. Even though, other wavelengths less than 393 nm have also been used as an excitation wavelength such as 36039, 353 and 340 nm40, but 325 nm wavelength has been used extensively for the excitation of ZnO based nanomaterials7, 38, 41-42. In undoped ZnO, an orange-red band was observed and attributed to intrinsic defects (Oi and Vo) in ZnO 7, whereas in Tb3+-doped ZnO, a green emission peak at 543 nm and several other small weak peaks at 489, 586, 622, 444 and 420 nm were observed that correspond to the 5D4-7F5, 5D4-7F6, 5
D4-7F4,
5
D4-7F3, 5D3-7F4 and
5
D3-7F5 transitions of Tb3+, respectively. These transitions
correspond to various emission bands such as blue, green, and red that are attributed to both induced defects and Tb3+ transitions7, 23. The PL intensity of the Tb3+ emission increased at a 5 mol% Tb concentration, and at 6 mol% Tb concentration, the PL intensity decreased. The observed decrease in the intensity may be due to the formation of Tb2O3 at the higher Tb concentration. It was concluded that undoped ZnO exhibited a broadband orange-red emission ranging from 500 to 850 nm, whereas the higher intensity of white light emission was observed in doped ZnO with 5 mol% Tb, which was the optimized Tb concentration. The nanophosphor ZnO:Tb3+ doped with 5 mol% Tb3+ powder was further used to synthesize thin nanocomposite films by embedding the NPs into the PS polymer matrix, as will be discussed in detail in the next section. The nanophosphor powder was further investigated using SEM and TEM. The SEM and TEM micrographs of ZnO:Tb3+ (doped with 5 mol% Tb+3) along with the corresponding EDS spectrum are shown in Figure 3. The SEM micrograph of the ZnO:Tb3+ nanophosphor powder exhibited a nanoflake-like structure, as shown in Figure 3 (a). Interestingly, these ZnO-based nanoflake structures are promising nanomaterials for sensing applications7,43. Moreover, the TEM image of the ZnO:Tb3+ nanophosphor exhibited spherically shaped NPs, as shown in Figure 3 (b). Using ImageJ software, the sizes of these NPs were found to be 11±3 nm, as estimated from the TEM images (Figure S2 and Figure 3 (c) inset). The EDS spectrum showed the presence of Tb in the ZnO:Tb3+, which agrees with the XRD result confirming the doping of Tb3+ in ZnO (Figure 3c). 8 ACS Paragon Plus Environment
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Figure 3. (a) SEM and (b) TEM micrographs and (c) EDS of the ZnO:Tb3+ nanophosphor powder at a 5 mol% Tb3+ concentration. The EDS spectrum shows all the elements present in the nanophosphor. The Cu signals are from the Cu TEM grid. Inset shows the size distribution of the ZnO:Tb3+ nanophosphor particles extracted from the TEM images.
3.2 ZnO:Tb3+/PS nanocomposite thin films 3.2.1 Structural and morphological properties The polymer nanocomposite films embedded with ZnO:Tb3+ nanophosphor as obtained from the petri dish were free-standing and remarkably flexible. The as-obtained films were transparent at the lower doping wt% values of the nanophosphor, whereas films became semitransparent with increasing ZnO:Tb3+ wt% values, as shown in Figure 4 (a). It has been reported that the luminescent films are not completely transparent but become semitransparent due to the dispersion of the nanophosphor particles within the polymer matrix27, 30. The PS and ZnO:Tb3+/PS nanocomposite films were evaluated using SEM to investigate the surface morphologies and thicknesses of the films. Figures 4 (b-f) show the SEM micrographs of the PS film and ZnO:Tb3+/PS nanocomposite films at different ZnO:Tb3+ concentrations. It is evident from the SEM micrographs that the surface of the pure PS film was rather smooth, whereas the embedding of ZnO:Tb3+ nanophosphor NPs modified the surface of 9 ACS Paragon Plus Environment
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the ZnO:Tb3+/PS nanocomposite films. The morphologies of the nanocomposite films were coarse and heterogeneous with increasing ZnO:Tb3+ concentration in the nanocomposite films. This finding may be due to the agglomeration of NPs near the surface as a result of the interconnection with the primary NPs30. The similar morphologies have been reported in phosphor/ZnO-embedded polymer nanocomposite films30, 44-45. Potdevin et al. reported that the phosphor NPs were well dispersed within the polymer film along with some agglomerated NPs composed of the primary NPs overflowing onto the surface of the nanocomposite film. The films were approximately 40 µm thick as characterized by X-SEM, as shown in the inset of Figure 4(b).
Figure 4: (a) Photographs of the thin PS and ZnO:Tb3+/PS nanocomposite films (from left to right-PS film and nanocomposite film with 1, 2, 3 and 5 wt% ZnO:Tb3+ nanophosphor. SEM micrographs of the (b) PS film and ZnO:Tb3+/PS nanocomposite films with ZnO:Tb3+ concentrations of (c) 1 wt%, (d) 2 wt%, (e) 3 wt% and (f) 5 wt%. Inset in (b) shows a cross-sectional SEM of the film.
The presence of embedded ZnO:Tb3+ nanophosphors in the composite films was also confirmed by the XRD patterns of the ZnO:Tb3+/PS nanocomposite films (Figure S3). The XRD 10 ACS Paragon Plus Environment
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patterns of the nanocomposite films revealed that the embedded ZnO:Tb3+ nanophosphors were highly crystalline with a preferred (101) orientation appearing as a diffraction peak at 36.2°
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with an increasing ZnO:Tb3+ nanophosphor concentration. Furthermore, the distribution of the ZnO:Tb3+ nanophosphor in the composite film was studied using SIMS analysis. Figure 5 shows the ion maps, overlays and depth profiles of C+ and various other hydrocarbon components that correspond to PS. It can be seen that all the ions are uniformly distributed throughout the film. Similarly, the ZnO:Tb3+/PS nanocomposite embedded with 5 wt% of the ZnO:Tb3+ nanophosphor was analyzed by TOF SIMS to determine the distribution of the nanophosphor throughout the film. Figure 6 shows the ion map overlays for C+, Zn+ and Tb+ at 2, 40 and 200 overlay scans. The results also demonstrate that the intensities of the Zn+ and Tb+ ions increased with an increasing number of scans. This is an indication that the ZnO:Tb3+ nanophosphor NPs were encapsulated within the PS, and more carbon compounds were found on the surface of the film.
Figure 5: TOF-SIMS ion maps (spectroscopy mode) of the PS film and depth profiles of C and other hydrocarbon components.
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The depth profile in Figure 6 also indicates that the intensity of Zn+ increased with the number of scans. An SIMS study of the ZnO:Tb3+ nanophosphor was also performed and showed a uniform distribution of doped Tb3+ in the ZnO host, as reported earlier 7.
Figure 6: TOF-SIMS image (spectroscopy mode) of the ZnO:Tb3+/PS nanocomposite embedded with 5 wt% of ZnO:Tb3+ and depth profiles of several components of the polymer including C+ and Zn+.
Similarly, the 3D ion maps and overlays of C+, Zn+ and Tb+ in the ZnO:Tb3+/PS nanocomposite embedded with 5 wt% of the ZnO:Tb3+ nanophosphor is shown in Figure S4 and confirms the presence of additional carbon on the surface. The results also show higher intensities of Zn+ and Tb+ ions while sputtering deeper into the film, which became constant after
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a certain time period. Furthermore, no clustering of Zn+ and Tb+ was observed, which confirms that the ZnO:Tb3+ nanophosphor was distributed evenly throughout the PS film. 3.2.2 Chemical properties: FTIR and XPS studies To investigate the chemical behavior of the embedded ZnO:Tb3+ nanophosphor within the PS matrix, FT-IR spectra were recorded for the PS and ZnO:Tb3+/PS nanocomposite films. Figure 7 (a) shows the FTIR spectra of PS and ZnO:Tb3+/PS nanocomposite films with varying ZnO:Tb3+ concentrations. The FTIR spectra exhibited the characteristic absorption peaks of PS at approximately 1600, 1500 and 1450 cm-1 that correspond to the aromatic C-C vibration of the styrene unit. Other major characteristic peaks at approximately 760 and 700 cm-1 were assigned to the C-H out-of-plane bending vibration of the benzene ring in PS. Most of the absorption bands as observed in the FTIR spectra of the PS and nanocomposite films were well matched with those observed in the literature46-48.
Figure 7: (a) FTIR spectra of the PS and ZnO:Tb3+/PS nanocomposite films. (b) XPS wide survey spectra of ZnO:Tb3+/PS at different embedded wt% values of ZnO:Tb3+ NPs.
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Interestingly, no appreciable shift in the peak position and deformation related to the broadening or sharpening of the peaks were found in the FTIR spectra, which ruled out chemical bond formation between the embedded nanophosphor particles and the PS components within the ZnO:Tb3+/PS nanocomposite films. However, a decrease in the intensity of some of the peaks at 460, 2345, and 3650 cm-1 with an increase in the ZnO:Tb3+ nanophosphor concentration was observed. In addition, the intensity of the peak at 3450 cm-1 increased with an increase in the ZnO:Tb3+ concentration. These observed changes in the peak intensity showed a blending of the ZnO:Tb3+ nanophosphor within the PS matrix with increasing concentration. Similar observations have been reported by Jangir et al.46 and Chae et al.
48
in ZnO and PS
nanocomposites. The films were further examined using XPS for elemental compositions. Figure 7 (b) shows an XPS survey scan of the PS and ZnO:Tb3+/PS nanocomposites films. Carbon (C) was detected in PS (0 wt%), whereas elemental Zn, O, Tb and C were detected in the ZnO:Tb3+/PS nanocomposite films (1, 3, and 5 wt%). All binding energies were corrected for the charge shift using the C 1s peak of graphitic carbon (binding energy = 284.6 eV) as in reference8, 13, 49-50. The peak intensities of the corresponding elements in the ZnO:Tb3+/PS nanocomposite films increased with an increase in the ZnO:Tb3+ concentration. These nanocomposite films were also investigated after surface sputtering, and similar results were obtained, which showed that the ZnO:Tb3+ NPs were properly embedded within the PS matrix. 3.2.3 Thermal properties: TGA and DSC studies The thermal stability of the PS and ZnO:Tb3+/PS nanocomposite (embedded with 5 wt% ZnO:Tb3+) films was investigated by TGA and DSC analyses, as shown in Figure 8. The PS film showed a two-step degradation process: the first step degradation Tonset was just above 100 °C (with a final temperature at 180 °C), and the second step degradation Tonset was at 340 °C (with a final temperature at 440 °C, as shown by the TGA curves in Figure 8 (a). Similarly, a two-step degradation process occurred after the incorporation of 5 % ZnO:Tb3+ into the PS matrix. In the ZnO:Tb3+/PS nanocomposite film, the degradation temperatures due to water and polymer decomposition were recorded at 130-165 °C and 340-440 °C, respectively. The PS film showed a 5 % weight loss at 350 °C, whereas the degradation temperature increased to 356 °C for the ZnO:Tb3+/PS nanocomposite film after the addition of 5 wt% ZnO:Tb3+ NPs. The final weight loss in the ZnO:Tb3+/PS nanocomposite film (9.9 %) showed greater stability compared to that of 14 ACS Paragon Plus Environment
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the pristine PS film (2.4 %), indicating that the addition of phosphor NPs improved the thermal stability of the phosphor nanocomposite film 51-52 (Figure 8a).
Figure 8: (a) TGA and (b) DSC curves of the PS and PS/ZnO:Tb3+ (5 wt%) nanocomposite films.
Similarly, DSC measurements were carried out for the PS and ZnO:Tb3+/PS nanocomposite (5 wt% ZnO:Tb3+ NPs) films, as shown in Figure 8 (b). The glass transition temperature (Tg) for all the PS and ZnO:Tb3+/PS nanocomposite films was 80 °C, suggesting that the addition of phosphor NPs into the PS matrix did not affect the Tg of the nanocomposite film. Chen et al.53 reported similar results and attributed these findings to intermolecular hydrogen bonding interactions that could not be reduced by the incorporation of phosphor NPs into the PS matrix to such an extent that more energy would be required for any transformation in the nanocomposite film. The DSC curve showed a deep endothermic peak at 418 °C, which is also supported by the TGA curves for both the PS and nanocomposite films. 3.2.3 Optical properties: UV–Vis and PL properties Figure 9 shows the UV−vis reflectance spectra of ZnO:Tb3+ nanophosphor NPs, PS and ZnO:Tb3+/PS nanocomposite films. As shown in the UV-vis spectra, no appreciable absorption bands were observed for the PS film, whereas the ZnO:Tb3+/PS nanocomposite films showed a sharp band edge at approximately 400 nm that is attributed to the intrinsic band-gap absorption of ZnO on account of electron transitions from the valence band to the conduction band (O2p → Zn3d)
7, 54
. Interestingly, the band edge for the nanocomposite films was found to be shifted
towards higher wavelengths with increasing ZnO:Tb3+ concentration, indicating that the band gap of the nanocomposite films decreased with increasing nanophosphor concentration. 15 ACS Paragon Plus Environment
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Figure 9: UV-Vis spectra of the PS, ZnO:Tb3+ nanophosphor NPs and ZnO:Tb3+/PS nanocomposite films (with increasing ZnO:Tb3+ nanophosphor concentration, i.e., 1, 2, 3 and 5 wt%).
The luminescent properties of the polymer-based nanocomposite films mainly depend on the phosphor materials embedded within the polymer matrices30, 55. The potential application of ZnO:Tb3+ nanophosphor powder in solid-state lighting have been studied and reported 7. As discussed in Figure 2 (b), the PL spectra of the ZnO:Tb3+ nanophosphor NPs (at 5 mol% Tb doping) showed white light emission with various emission bands at 543 nm (higher intensity), 489, 586, 622, 444 and 420 nm corresponding to the 5D4-7F5, 5D4-7F6, 5D4-7F4, 5D4-7F3, 5D3-7F4 and 5D3-7F5 transitions of Tb3+, respectively
7, 56-57
. However, to explore the potential and
practical applications of the polymer-based phosphor nanocomposite thin films, it is important to confirm that the nanophosphor retains its unique optical properties. Therefore, the PL spectra of ZnO:Tb3+/PS nanocomposite films with different concentrations of ZnO:Tb3+ phosphor NPs were recorded and compared with the PL spectrum of the PS film, as shown in Figure 10 (a). As shown in the PL spectra, the PS film did not show significant emission bands, except a broad hump between 350-500 nm, which is similar to that reported in the literature58-59. The PL properties of the ZnO:Tb3+/PS nanocomposite films showed combination PL spectra, which include contributions from the PS film and the ZnO:Tb3+ nanophosphor, as observed in Figure 10 (a) and Figure 2 (b). 16 ACS Paragon Plus Environment
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Figure 10: (a) PL spectra of the PS and PS/ZnO:Tb3+ nanocomposite films embedded with optimized nanophosphor powder (shown in Figure 1) with increasing concentration (1-5 wt%). (b) Temperature-dependent PL spectra of the PS/ZnO:Tb3+ (5 wt%) nanocomposite films at different temperatures ranging from 30-100 °C. As mentioned above, the Tb3+ emission peaks clearly observed at 489, 543 (higher intensity), 586, and 622 nm correspond to the 5D4-7F6, 5D4-7F5, 5D4-7F4, and 5D4-7F3 transition levels of Tb3+, respectively, in the nanocomposite films. The other peaks at 444 and 420 nm correspond to the 5D3-7F4 and 5D3-7F5 transition levels of Tb3+ that were merged with the polymer peak. As shown in Figure 10 (a), the PL intensity from the polymer matrix decreased with increasing ZnO:Tb3+ concentration. Whereas, the PL intensities that correspond to the f-f transition level of Tb in the phosphor nanocomposite films increased with an increase in the nanophosphor concentration. Similarly, Chen et al. 36 reported co-doping of Tb3+ and Li+ ions in ZnO/PEG and observed green luminescence spectra corresponding to the f–f transitions of Tb3+. The strong emissions at 484 and 545 nm were due to Tb3+ in the doped ZnO/PEG nanocomposites. In addition, an increased PL intensity of Tb3+ was observed in the ZnO:Tb3+/PEG nanocomposite compared to that of the pure ZnO:Tb3+ nanophosphor. In the present case, an increase in the PL intensity of Tb3+ from the ZnO:Tb3+/PS nanocomposite films was due to an increase in the amount of the nanophosphor ZnO:Tb3+ NPs. Although, the PL 17 ACS Paragon Plus Environment
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intensities corresponding to the Tb3+ emissions from nanocomposite films were less compared to that of the ZnO:Tb3+ nanophosphor powder (as shown in Figure 2) because of the lower concentration (1-5 wt %) in the PS matrix, these nanocomposite films showed the same trend of emission bands, as shown in Figure 2. Similar results have been reported by Saladino et al. 27 for a YAG:Ce nanophosphor embedded in a poly(methyl methacrylate) (PMMA) matrix. It is also important to note that no defect related emission was observed from ZnO in either of the samples. The temperature-dependent PL measurements were further carried out at higher temperatures for the nanocomposite films embedded with ZnO:Tb3+ (5 wt%) phosphor NPs. The PL spectra of the PS/ZnO:Tb3+ (5 wt%) nanocomposite films as a function of temperature ranging from 30 to 100 °C are shown in Figure 10 (b). It was found that the PL intensity of the Tb3+ peaks decreased continuously with increasing temperature. A similar trend has been reported in rare earth ion-based phosphor polymer nanocomposites60-62. For example, Balamurugan et al.61 studied the temperature-dependent PL properties of π-conjugated polymer– Eu3+ complexes and reported that the PL properties decreased with increasing temperature from 0 to 100 °C. Temperature-dependent PL measurements were performed for a Tb3+ and Eu3+ codoped laponite matrix and demonstrated that the PL emission of Tb3+ decreased with increasing temperature without changes in the PL properties of Eu3+ 63. This decrease was attributed to the partial energy transfer from Tb3+ to Eu3+. Vallerini et al.64 also demonstrated a decrease in the PL intensity of colloidal CdSe/ZnS core/shell quantum dots embedded in a PS matrix with increasing temperature. Similarly, Basu et al.60 studied the temperature-dependent PL properties of a Eu-complex embedded in polymer matrices and found that the Eu3+ PL intensity decreased when temperature increased from 30 to 50 °C due to thermal quenching. The decrease in the PL intensity of the Tb3+ peaks with increasing temperature in the PS/ZnO:Tb3+ (5 wt%) nanocomposite films could be attributed to thermal quenching, as reported by Basu et al.60 Similarly, a decrease in the PL property of the polymer matrix was also observed with increasing temperature65, as shown in Figure S5. The Commission Internationale de l’ (CIE) parameters were calculated to better understand the change in the photometric characteristics of the ZnO:Tb3+/PS nanocomposite films with varying embedded wt% of the nanophosphor (more detail is provided in the supporting information). The color coordinate was very close to white light emission (Figure 18 ACS Paragon Plus Environment
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S6), as reported for the ZnO:Tb3+ nanophosphor7. This finding also reveals that by increasing the wt% of the ZnO:Tb3+ nanophosphor in the nanocomposite film, white light emission can be obtained. The PL spectra of the ZnO:Tb3+/PS nanocomposite films (at different wt% of ZnO:Tb3+ nanophosphor, as shown in Figure 10) did not show any significant shift in the PL peak positions (emission wavelength) compared to that of the ZnO:Tb3+ nanophosphor, as shown in Figure 2. Similarly, no shift in the peak positions was observed in the PL spectra of the nanocomposite films at different temperatures, as shown in Figure 10 (b). However, a redshift of less than 3 nm was observed due to a change in the refractive index in the surroundings of the embedded ZnO:Tb3+ NPs within the polymer matrix, as also reported in the some studies 27, 66. Rao and coworkers 66 studied the effect of various media with different refractive indices such as oxide gels and polymers on the PL properties of phosphor materials. A small blueshift in the maximum emission wavelength was observed with an increase in the refractive index of the medium 66. The quantum yields (QYs) of the ZnO:Tb3+/PS nanocomposite films and the ZnO:Tb3+ nanophosphor were also determined (see Figure S7 and other experimental details), and a range of 0.21-0.33 % was obtained, as shown in Table S1. The PL properties of the ZnO:Tb3+/PS nanocomposite films do not show a significant spectral shift compared to the nanophosphor NPs, which is particularly important for advanced practical applications in solid-state lighting in home appliances. In addition, the polymer matrix protects the nanophosphor materials providing better long-term stability; therefore, such luminescent nanocomposites can potentially to be used in devices for indoor or outdoor lighting. 4. Conclusion We demonstrated a simple technique of synthesizing luminescent and flexible nanocomposite PS films embedded with ZnO:Tb3+ nanophosphors by mixing nanophosphor powder in a polymer solution. The structural, morphological and optical properties of these nanocomposite films were studied using a multitechnique approach including SEM, XRD and SIMS, XPS, FTIR, TGA, DSC, UV-visible, and PL techniques. Investigations revealed that nanophosphor particles were uniformly distributed throughout the nanocomposite films, and the PL properties increased with increasing nanophosphor concentration. Furthermore, the temperature-dependent PL study of the nanocomposite films showed that the PL intensity decreased with increasing temperature due to thermal quenching. These studies exhibited that 19 ACS Paragon Plus Environment
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embedded ZnO:Tb3+ nanophosphor NPs retained their unique luminescent properties in the ZnO:Tb3+/PS nanocomposite film without a significant spectral shift indicating their potential applications in solid-state lighting. Acknowledgment The authors would like to thank Dr. Coetsee for carrying out the XPS measurements, Prof. R. E. Kroon for conducting the quantum yield measurements and Dr. Anurag Pandey for assisting in other characterization methods. The authors would also like to acknowledge the South African Research Chairs Initiative of the Department of Science and Technology (Grant No. 84415), National Research Foundation (Grant No. 93214, R.E. Kroon) and the National Laser Center of South Africa for providing research support. The authors (JP and VK) also acknowledge the Department of Science and Technology, New Delhi (India) for providing an INSPIRE Faculty award and research grant. J P also acknowledges FRQNT, Quebec, Canada for providing a Merit Scholarship Award (2017) and INRS, Quebec, Canada for the postdoctoral research grant.
Supporting Information Available: Absorption spectrum, XRD data, TEM images, temperature dependent PL, Commission Internationale de l’ (CIE) parameters, SIMS results and quantum yield calculation.
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of ZnO Nanophosphors for Lighting Applications. Ultrasonics Sonochemistry 2014, 21, 1549-1556. (55) Mishra, S. B.; Mishra, A. K.; Revaprasadu, N.; Hillie, K. T.; Steyn, W. J. V.; Coetsee, E.; Swart, H. C. Strontium Aluminate/Polymer Composites: Morphology, Luminescent Properties, and Durability. J. Appl. Poly. Sci. 2009, 112, 3347-3354. (56) Kumar, V.; Ntwaeaborwa, O. M.; Coetsee, E.; Swart, H. C. Role of Deposition Time on the Properties of ZnO:Tb3+ Thin Films Prepared by Pulsed Laser Deposition. J. Coll. Interface Sci. 2016, 474, 129-136. (57) Kumar, V.; Ntwaeaborwa, O. M.; Holsa, J.; Motaung, D. E.; Swart, H. C. The Role of Oxygen and Titanium Related Defects on the Emission of TiO2:Tb3+ Nano-Phosphor for Blue Lighting Applications. Optical Materials 2015, 46, 510-516. (58) Jeeju, P. P.; Sajimol, A. M.; Sreevalsa, V. G.; Varma, S. J.; Jayalekshmi, S. Size-Dependent Optical Properties of Transparent, Spin-Coated Polystyrene/ZnO Nanocomposite Films. Polymer International 2011, 60, 1263-1268. (59) Durães, J. A.; Drummond, A. L.; Pimentel, T. A. P. F.; Murta, M. M.; Bicalho, F. d. S.; Moreira, S. G. C.; Sales, M. J. A. Absorption and Photoluminescence of Buriti oil/Polystyrene and Buriti oil/poly(methyl methacrylate) Blends. Eur. Pol. Journal 2006, 42, 3324-3332. (60) Basu, B. B. J.; Vasantharajan, N. Temperature Dependence of the Lluminescence Lifetime of A Europium Complex Immobilized in Different Polymer Matrices. J. Luminescence 2008, 128, 1701-1708. (61) Balamurugan, A.; Reddy, M. L. P.; Jayakannan, M. [small pi]-Conjugated Polymer-Eu3+ Complexes: Versatile Luminescent Molecular Probes for Temperature Sensing. J. Mater. Chem. A 2013, 1, 2256-2266. (62) Zhang, H.; Song, H.; Yu, H.; Bai, X.; Li, S.; Pan, G.; Dai, Q.; Wang, T.; Li, W.; Lu, S.; Ren, X.; Zhao, H. Electrospinning Preparation and Photoluminescence Properties of Rare-Earth Complex/Polymer Composite Fibers. J. Phys. Chem. C 2007, 111, 6524-6527. (63) Xu, Q.; Li, Z.; Wang, Y.; Li, H. Temperature-Dependent Luminescence Properties of Lanthanide(III) β-Diketonate Complex-Doped LAPONITE®. Photochem. & Photobiol. Sci. 2016, 15, 405-411. (64) Valerini, D.; Cretí, A.; Lomascolo, M.; Manna, L.; Cingolani, R.; Anni, M. Temperature Dependence of the Photoluminescence Properties of Colloidal CdSe/ZnS Core/Shell Quantum Dots Embedded in a Polystyrene Matrix. Phys. Rev. B 2005, 71, 235409. (65) Kong, F.; Wu, X. L.; Huang, G. S.; Yuan, R. K.; Yang, C. Z.; Chu, P. K.; Siu, G. G. Temperature-Dependent Photoluminescence from MEH-PPV and MEH-OPPV Containing Oxadiazole in the Main Chain. Appl. Phys. A 2006, 84, 203-206. (66) Bhat, S. V.; Govindaraj, A.; Rao, C. N. R. Tuning the Emission Bands of Nanophosphors Through the Refractive Index of the Medium. Chem. Phys. Lett. 2006, 422, 323-327.
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ACS Applied Nano Materials
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