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A color-tunable fluorescence composite bulk based on carbon dots and Y2WO6: Eu3+ as blue and red emitting materials, respectively, was fabricated. Due to the different excitation regions of the two materials, the emission color of the composite can be tuned from red, across magenta into the blue via varying excitation wavelengths. At the same time, the material is still considerably optically transparent, which demonstrates a clear advantage for integration into real applications.
© 2016 Optical Society of America
1. Introduction
Carbon nanomaterials, such as nanodiamonds, fullerene, carbon nanotubes, graphene sheets and fluorescent carbon nanoparticles or carbon dots have attracted huge attention since their unique [1] properties and potential applications, including catalysis, electrode, adsorption, luminescence materials etc [2]. For a long time, a stream of researches embracing semiconductor quantum dots have been carried out due to their wide applications [3–7]. Unfortunately, some limitations such as high toxicity and cost are also included in semiconductor quantum dots [8]. Carbon dots (CDs), as a good candidate to replace semiconductor quantum dots, have triggered considerable research interests owing to the superiority in low toxicity, excellent biocompatibility, and abundance of raw material in nature [9]. As a definition of ultra-small fragments of carbon materials, consisting of numerous functional groups on their surface, carbon dots can be readily prepared on a large scale by many routes [10]. As a consequence, these novel carbon dots emitters have been applied in chemical sensing, biosensing, bioimaging, nanomedicine, photocatalysis and electrocatalysis. Despite recent amazing developments of carbon dots, the total solid-state luminescence have been less reported compared to its solution state, leading to its limited application.
Rare-earth (RE3+) doped materials, of significance to lighting and display, are extensively deployed in light-emitting diodes (LEDs), field emission displays (FEDs), plasma display panels (PDPs), and biomarkers [11–15]. Due to the attractive luminescence properties, metal tungstates, such as RE3+-Y2WO6 [16] have been studied as a candidate for White-LED phosphor. In 2011, Huang [17] studied the structure of tetragonal Y2WO6 and the site occupation of Eu3+ dopant, and in 2013, Qian [18] investigated the luminescence properties of Sm3+-doped phosphor with excellent color rendering. Although the studies associated with Y2WO6 has been immensely developed, it is significant to make further exploration due to its excellent luminescence properties, such as “color-tunable application”.
Aiming at obtaining controllable and desired performance for various applications, color-tunable emission is widely pursued recently. In general, some common methods are implemented: i) composition adjustment of mixed phosphors, such as the white-light emitting can be obtained by optimizing the ratio of trichromatic phosphor; ii) concentration adjustment with steady excitation, such as Ca9Gd(PO4)7: Eu/Mn [19], NaYF4: Yb/Tm/Er/Gd [20]; iii) Change in density and pulsed width with single excitation wavelength, such as LiNbO3: Yb/Er/Tm [21,22], NaNbO3: Er/Yb/Al [23]. Furthermore, there are also several investigations on modifying the excitation wavelength, such as CaWO4: Bi [24], whose disparate emission properties under different excitation wavelengths were also analyzed.
However, the color-tunable emission depending on excitation wavelength is extremely rare compared to the above mentioned methods. Herein, we present the fabrication and characterization of an interesting novel material composite based on CDs and Y2WO6: Eu3+ with promising, tunable emission properties. The as-fabricated material exhibits an interesting color-tunable emission depending on the excitation wavelength.
2. Experimental
Materials
Methl Methacrylate (MMA), benzoyl peroxide (BPO) and Y2O3 (99.99%), Eu2O3 (99.99%), (NH4)10H2(W2O7)6 (99.95%), HNO3 and citric acid (CA) monohydrate (>99.5%) were purchased from Aladdin-reagent Ltd (Shanghai, China). Reagent-grade ammonium bicarbonate, trisodium citrate dehydrate, ethanol, hexamethylene, and toluene were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the materials are used as received without further treatment.
Synthesis
- 1. Synthesis of hydrophilic CDs: The prepared route is similar to the previous report [25] with a modification. Typically, 0.4 g ammonium bicarbonate, 3.0 g NH4HCO3 and 20 ml high-purity water were sealed into a 100 ml Teflon equipped stainless steel autoclave followed by hydrothermal reaction at 180 °C for 4 h. Dry CDs were obtained by dialysis for 24 h and evaporation of the remaining water solution.
- 2. Synthesis of organophilic octadecylamine modified carbon dots (ODA-CDs): 5 ml hydrophilic CDs were dispersed in 15 ml distilled water and the pH was tuned to 5~7 with acetic acid. 20 ml toluene and 0.3 g octadecylamine (ODA, as surfactant) were added to the solution. The mixed suspension was then transferred to a 100 ml Teflon-lined autoclave and heated at 150 °C for 4 h. Then upper solution was removed and then centrifuged 2 times with distilled water.
- 3. Synthesis of Y2WO6: Eu3+ (10%): the samples were prepared by Pechini reaction. In a typical process, stoichiometric amounts of Y2O3 and Eu2O3 were pre-dissolved separately in hot HNO3 solution under vigorous stirring to form solution A, and the pH value was adjusted to 2-3. Meanwhile, the corresponding amount of (NH4)10H2(W2O7)6 was dissolved in 40 mL hot deionized water to form solution B with pH about 7-8. A weighed amount of citric acid monohydrate was dissolved in both of the solutions as chelating reagent, with n (CA): n (Ln3+) = 1: 1 and n (CA): n (W6+) = 4: 1 in molar ratio, respectively. After two solutions were mixed, highly transparent collosol was obtained under vigorous stirring for 1 h. The sol was dried in an oven for 4 h then pre-heated at 600 °C in air, and finally annealed at 1100 °C for 6 h to obtain the product.
- 4. Fabrication of ODA-CDs/Y2WO6: Eu3+ (10%) composite: 0.6 mg organophlilic ODA-CDs and 0.01 g Y2WO6: Eu3+ (10%) were fixed into 12 ml methyl methacrylate monomer (MMA), followed by the addition 0.01 g benzoyl peroxide (BPO) to initiate polymerization. The reaction was kept for about 40 min at 80 °C in water bath, then poured into a plastic box (radius, 1.4 cm; height, 1.3cm). The composite bulk was obtained after the complete polymerization at 60°C and then polished into translucent bulk.
Characterizations
The particle diameter was examined with a JEOL JEM-2100F transmission electron microscope with 200 kV. Morphologies of Y2WO6: Eu3+ (10%) were obtained by field emission scanning electronic microscope (FE-SEM, FEI Magellan 400). A drop of corresponding carbon dot aqueous solution was placed on a copper grid that was left to dry before transferring into the TEM sample chamber. The transmittance spectra were measured with Agilent Cary 5000 spectrophotometer. The absorption spectrum was carried out on Hitachi U-3010 spectrophotometer in reference to absolute ethanol. UV excitation and emission spectra were carried out at Hitachi F-4600 spectrometer. The scan speed was fixed at 240 nm/min, the voltage was 700 V and the slits were fixed at 2.5 nm. The transient decay time was recorded on an Edinburgh Instruments (FLS 980) spectrofluorimeter equipped with both continuous (450 W) and pulsed xenon lamps. To obtain the quantum yield (QY) of Y2WO6: Eu3+ (10%), a barium sulfate coated intergrating sphere (150 mm in diameter) was attached to the FLS 980. XRD data for phase identification was collected at ambient temperature with a Rigaku D/max 2500 diffractometer (Cu Kα radiation, λ = 1.54056 Å, 40 kV/ 200 mA). The 2θ ranges for XRD refinement are from 5 to 130 o with a step size of 0.02 o using a step-scan technique, and fixed counting time (t) of 1s/step. The crystal refinement was carried out through the program package Jana2006.
3. Results and discussion
Organophilic carbon dots were prepared by a surface-modified method according to our previous report [26]. The size of organophilic carbon dots are around 3 nm (Fig. 1(a)), the Fourier transform infrared (FTIR) spectra of CDs (Fig. 1(b)), ODA, and ODA-CDs indicate I and II bands of the amide linkage 1644 and 1552 cm−1, respectively, indicating a formation of amide linkages at about (-NHCO-) in the modified process [27]. Figure 1(c) shows the optical properties of the as-prepared CDs, the most intense emission centered at 410 nm are under 340 nm light excitation. A board absorption band below 400 nm shows that the visible-light region from the emission intensity is scarcely weakened by self-absorption, which is very similar to previous report [28]. Figure 1(d) show its decay time spectra, the monitoring excitation and emission wavelengths were fixed at 340 nm and 410 nm. The decay curve demonstrates well a double-exponential function, and the fitting result is τ1 = 1.66 ns, τ2 = 5.49 ns. Then, optimal red-emitting Y2WO6: Eu3+ (10%) was obtained by the Pechini sol-gel method. From the SEM image as shown in Fig. 2(c), the powder consists of numerous nanorods with 200-400 nm in length and 50-80 nm in diameter. The X-ray diffraction (XRD) pattern indicates the observed peaks coincide well with the standard data (ICSD No. 20955) without impurity phases. A further study of the structure was carried out through Jana2006 is shown in Fig. 2(a). The cell parameters were determined to a = 7.6052(2) Å, b = 5.3501(1) Å, c = 11.3989(2) Å, β = 104.3486(2) Å and V = 449.34(2) Å3 (further refinement result can be obtained from Table 1). The crystal structure of Y2WO6: Eu3+(10%) (shown in Fig. 2(b)), which was modeled by using the atomic coordinates obtained from the Rietveld refinement result, indicates the tungsten (W) atoms are surrounded by six oxygen atoms, forming the tetrahedral (WO6)6- group, while Y/Eu atoms have three different coordination, i.e. Y/Eu (1) and Y/Eu (2) occupy respectively 2e and 2f sites embraced by eight O atoms, and Y/Eu (3) occupies 4g sites coordinated by seven O atoms (Fig. 2(b)).
Fig. 1 (a) TEM image of ODA-CDs (illustrated by arrows), (b) FTIR spectra of CDs, ODA, and ODA-CDs, (c) absorption, excitation (λem = 410 nm) and emission spectra (λex = 340 nm) and (d) decay time curve (λex = 340 nm, λem = 410 nm) of ODA-CDs in absolute ethanol.
Fig. 2 (a) X-ray Rietveld refinement, (b) schematic crystal structure of refinement result in one unit cell and (c) the SEM image of as-prepared Y2WO6: Eu3+(10%) powder (Table 1).
Table 1. Structural parameters for Y2WO6: Eu3+(10%) as determined by Rietveld refinement of powder XRD data at room temperature.
Figure 3(a) shows the excitation and emission spectra of Y2WO6: Eu3+ phosphor. The emission spectrum consists of a series of sharp peaks attributing to the 4f-4f transitions of Eu3+, while the excitation spectrum is composed of two parts: strong broad band centered at about 280 nm and sharp peaks originated from Eu3+ 4f-4f transitions [29]. In addition, the quantum efficiencies were calculated, as shown in Fig. 3(b), the area for this emission band is equal to the number of emitted photons, Nem. The areas for the excitation bands in the spectrum of an empty sphere correspond to the number of excitation photons, Nex0, and that of a sample-loaded sphere correspond to the number not absorbed by the sample, Nex, respectively [30]. From these values, the internal and external quantum efficiencies were calculated as IQE = Nem/(Nex0–Nex), and EQE = Nem/Nex0, respectively, and the results are as follow: IQE = 79.5%, EQE = 38.44%.
Fig. 3 (a) The excitation and emission spectra of Y2WO6: Eu3+ (10%) powder, the inset is the photograph of the powder under 254 nm light irradiation and (b) the quantum efficiencies determination under 280 nm excitation.
It is interesting that the change trend of Y2WO6: Eu3+ and carbon dots excitation band ranging in 280-340 nm show remarkable opposite correlation, which means the recombination luminescence can be tunable via varying excitation wavelength. Therefore, Y2WO6: Eu3+/carbon dots composite bulk in the PMMA matrix was firstly fabricated by mass polymerization. The transmittance spectra of 1 ml ODA-CDs/Y2WO6: Eu3+ (10%) inserted PMMA, as-compared pure and 1 ml ODA-CDs inserted PMMA were demonstrated in Fig. 4(a). It is clearly seen the transmittance of the samples without Y2WO6: Eu3+ can reach as high as 90% in the visible light region, and that of Y2WO6: Eu3+ mixed sample decrease to 65% due to the reasonable Rayleigh scattering. Figure 4(b) shows the luminescence spectra of the as-fabricated bulk under 295-340 nm UV light excitation. With different excitation wavelengths, the ratio of blue-emitting intensity originated from carbon dots and red-emitting intensity derived from Y2WO6: Eu3+ are varying because of the different sensitive excitation regions for the both components. From 295 to 340 nm, the proportion of blue-emitting is gradually increasing and the red-emitting is decreasing continuously. As a consequence, emission colors can be from red, across magenta into the blue as shown on a CIE color chart (Fig. 4(c)), which can be clearly observed photographs of the sample under different excitation wavelengths (Fig. 4(d)).
Fig. 4 (a) Transmittance spectra of pure PMMA (red line), ODA-CDs embedded PMMA (blue line) and ODA-CDs/Y2WO6: Eu3+ embedded PMMA (black line). The inset is the optical photograph of the as-fabricated sample, (b) emission spectra of as-fabricated composite bulk under various excitation wavelengths, (c) CIE color coordinates corresponding to the emission wavelengths shown in b and (d) photographs of the sample under different excitation wavelengths.
4. Conclusion
A novel composite bulk which exhibits tunable emission color upon varying the excitation wavelength. The composite bulk employs carbon dots together with Y2WO6: Eu3+ as blue and red emitting materials embedded in PMMA matrix. Due to the different excitation regions of the two materials, the emission color of the composite can be tuned via changing excitation wavelengths in the range from 280 to 340 nm. This results in emission colors from red, across magenta into the blue. At the same time the material is still considerably optically transparent (65%), which demonstrates clear potential for integration into optics applications.
Acknowledgments
This work is supported by National Natural Science Foundation of China (NSFC) (11104298, 51372228 and 51272270) and Shanghai Technical Platform for Testing and Characterization on Inorganic Materials (14DZ2292900), Shanghai Pujiang Program (14PJ1403900). The authors also wish to acknowledge Professor Xinyuan Sun (Gangshan Jing University, An Ji) for their supports with QY measurement, and Professor Václav Petříček (Institute of Physics, Prague, Czech Republic, the author of Jana2006) for his help in XRD refinement.
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