Preparation of multicolor luminescent cellulose fibers containing lanthanide doped inorganic nanomaterials

Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Preparation of multicolor luminescent cellulose fibers containing lanthanide doped inorganic nanomaterials Aleksandra Erdman a, Piotr Kulpinski a,n, Tomasz Grzyb b, Stefan Lis b,nn a b

Department of Man-Made Fibers, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland Department of Rare Earths, Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan, Poland

art ic l e i nf o

a b s t r a c t

Article history: Received 15 October 2014 Received in revised form 10 February 2015 Accepted 24 February 2015

In this paper, the UV sensitive optically active cellulose fibers contained of 0.5% w/w Sr2CeO4, Gd4O3F6:5% Eu3 þ and CeF3:5%Tb3 þ with respectively blue, red and green emission are described. The fibers were formed from an 8% by weight cellulose spinning solution in N-methylmorpholine-N-oxide (NMMO). The modifiers were chosen because of their specific color of emitted light. Photoluminescent particles were introduced into the polymer matrix during the dissolution process of cellulose in NMMO. The emission intensity and excitation energy of the final cellulosic luminescent products were examined by photoluminescence spectroscopy. The degree of dispersion of the nanoparticles in the polymer matrix was evaluated using transmission electron microscopy (TEM). The influence of modifier's particles on the mechanical properties of the fibers was determined. & 2015 Elsevier B.V. All rights reserved.

Keywords: Optically active cellulose fibers Nanoparticles Lanthanides Luminescent materials

1. Introduction Nanomaterials doped with lanthanide ions (Ln3 þ ) are intensively investigated in the last years due to their applications in the fields of material sciences (new materials with luminescent properties), physics (diodes and lasers techniques) and medicine (diagnostic and therapy) [1–5]. From the many years their unique properties have been used for designing phosphors, lasers, optoelectronics, security markers, solar cells or catalysts [6–12]. Nowadays also life sciences, i.e., biology and medicine, use the spectroscopic properties of nanomaterials doped with Ln3 þ ions [4,13–15]. The main reasons of applications of these materials are the spectroscopic properties of the Ln3 þ ions and character of f–f transitions that are responsible for the observed luminescence. These transitions are partially forbidden, by selection rules, which results in relatively long luminescence lifetime (ms), narrow line width (several nm) and low absorption cross section of the Ln3 þ ions. Shielding properties of 5s and 5p outer orbitals have additional effects on these transitions. Furthermore, emission of Ln3 þ ions is characterized by high quantum yields and the possibility of a large difference between the wavelengths of absorbed and emitted radiation what also influences their applications [3,9,16]. The wavelength of emitted light is strongly depended on the Ln3 þ ion used and the most of these ions can emit in the visible region. n

Corresponding author. Tel.: þ 48 42 631 3359. Corresponding author. Tel.: þ 48 61 829 1679. E-mail addresses: [email protected] (P. Kulpinski), [email protected] (S. Lis). nn

Therefore, nanocrystalline materials doped with Ln3 þ ions were chosen as good candidates for luminescence activators in cellulose fibers. As the hosts for luminescent Tb3 þ and Eu3 þ , ions, inorganic compounds were chosen: CeF3 and Gd4O3F6. As the blue emitting material, Sr2CeO4, was used, without any Ln3 þ dopants. These materials were previously investigated and were selected as stable phosphors with the high emission intensity [17–19]. The use of above mentioned Sr2CeO4 compound and two Ln3 þ ions gave the possibility to cover the visible range by the three basic colors: blue, green (Tb3 þ ) and red (Eu3 þ ). Present research describes a method of obtaining luminescent cellulose fibers. This kind of optically active material can be produced by an NMMO method which is well known as a method of manufacturing “Lyocell” and “Tencel” type fibers [20]. The use of N-Methylomorpholine-N-Oxide (NMMO) as a direct solvent of cellulose is one of the newest, promising and environmental friendly method of making regenerated cellulose fibers. One of the most important advantages is the possibility of modification of Lyocell fibers. It can be achieved by introduction of the modifier into the spinning dope during the cellulose dissolution process in NMMO [21]. The conditions of the cellulose dissolution process are relatively rigorous (highly alkaline environment with relatively high temperature), so the chosen modifier must be stable and inert for them. In present research, added modifiers contains the lanthanide ions, which according to our study [22] seems to be very promising materials for obtaining photoluminescent cellulose fibers and fulfills the strict conditions of the preparation of the solution and fibers formation. The modifiers are chosen as to obtain fibers with different colors of emitted light. This paper

http://dx.doi.org/10.1016/j.jlumin.2015.02.049 0022-2313/& 2015 Elsevier B.V. All rights reserved.

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Fig. 1. TEM images of Gd4O3F6:5%Eu3 þ nanocrystals in polymer matrix.

describes the criteria of choosing the proper modifiers and methods of preparation of them. Also method of making photoluminescent cellulose fibers and their properties is described. The inspiration of present research was the system of protection for garments and papers that is still not perfect nowadays. The luminescent Lyocell fibers that are presented in this paper can be also used as unique and advanced material to manufacture yarns and threads for textile protection. These fibers can be incorporated into fabrics in small amounts and in addition, may be a useful tool in the management of production to mark the batch production in different plants with similar range. The luminescent fibers are excellent materials for documents protection. It is possible to produce a paper containing certain amount of staple or filament fibers with luminescent properties. Such materials are relatively difficult to be counterfeited and therefore can be used for production of special purpose papers (notes, passports, ID tags etc.). The authenticity of such documents can be easily proven by simple lighting them up with UV radiation. Under UV irradiation the luminescent fibers appear as gloving lines. The additional advantage of the cellulose fibers is their high compatibility to the paper pulp. It means that the fibers can be very well bonded to the paper, what makes them extremely hard to be removed from the product during e.g. printing, processing and handling.

2. Materials and methods 2.1. Instrumentation The cellulose solutions were obtained on the high efficiency laboratory-scale IKAVISC kneader type MKD 0.6 H60 and the fibers were formed with the use of a dry–wet spinning method on the laboratory-scale piston-spinning device with spinneret equipped with 18 orifices of 0.4 mm diameter.

The mechanical properties of fibers were checked on a Zwick Z2.5/TN1S tensile testing machine, in accordance with ISO standard ISO 5079:1995. The linear density of the fibers was measured according to ISO standard ISO 1973:1995. The evaluation of size and distribution of modifiers' nanoparticles in the polymer matrix were performed with the use of the transmission electron microscope (TEM) technique. TEM images were measured at JEM 1200 EXII, JOEL electron transmission microscope, using an accelerating voltage of 80 kV. The excitation and emission spectra of all samples were recorded on a HITACHI F-7000 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Excitation and emission spectra were corrected for the instrumental response.

3. Experimental 3.1. Preparation of the modifiers 3.1.1. Blue luminescent Sr2CeO4 nanocrystals To prepare the Sr2CeO4, Sr(NO3)2 (POCh S.A., ACS grade 98þ %, Poland), CeCl3  6H2O (Sigma-Aldrich, 99.9%, Poland) the 1 M solutions of the reagents were prepared and their stoichiometric amounts (volumes) were dissolved in distilled water in order to synthesize 1 g of the product. Then the citric acid monohydrate (CHEMPUR, p.a. grade, Poland) and ethylene glycol (CHEMPUR, p.a. grade, Poland) were added to the solution, as chelating and polymerizing agents, respectively (12 g citric acid and 2 mL of glycol per 1 g of the final product). The total volume of 100 mL of the final mixture was heated and stirred on the electric stove for about 30 min. Afterwards, solution was concentrated by slow evaporation (about 8 h) in drying oven, hence a transparent brown gel was obtained. The precursor was fired at 1000 °C for 3 h in

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muffle furnace at the air atmosphere. Obtained compound was ground in agate mortar into fine powder.

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Agglomerates size (nm) Fig. 2. Diameter distribution of Gd4O3F6:5%Eu3 þ nanocrystals in fiber (a) single crystals (b) agglomerates.

3.1.2. Green luminescent CeF3:5%Tb3 þ nanocrystals Luminescent nanocrystals were synthesized as follows: first terbium oxide, Tb4O7, was dissolved in HNO3 (POCh S.A., ultrapure, Poland) and evaporated to dryness several times to remove the excess of HNO3. From the obtained nitrate and CeCl3  6H2O (Sigma-Aldrich, 99.9%, Poland) 1 M solution was prepared. Next, glycerin (POCh S.A., pure, 99.5%, Poland), CeCl3 and Tb(NO3)3 solutions were mixed and diluted by distilled water using amounts necessary to synthesize 1 g of the product. The concentration of glycerin in the prepared solution was chosen to be 25% and its volume to 100 mL. Then the second solution consisted of NH4F (POCh S.A., ACS grade 98þ%, Poland) and glycerin was prepared in the same way (50 mL). Afterwards, the second solution was slowly added to the first one, in the temperature of 50 °C. The mixture was stirred over 30 min. Finally the white precipitate was collected and washed with water. 3.1.3. Red luminescent Gd4O3F6:5%Eu3 þ nanocrystals Gd4O3F6:5%Eu3 þ was synthesized by the modified Pechini method. The starting materials were gadolinium and europium oxides, Gd2O3 and Eu2O3 (Stanford Materials 99.99%, United States), nitric acid HNO3 (POCh S.A., ultra-pure, Poland), ammonium fluoride NH4F (POCh S.A., ACS grade 98 þ%, Poland), citric acid monohydrate (CHEMPUR, p.a. grade, Poland), and ethylene glycol (CHEMPUR, p.a. grade, Poland). To synthesize material, the gadolinium and europium oxides were dissolved in HNO3 and evaporated several times in order to remove an excess amount of HNO3. Obtained Gd(NO3)3 and Eu(NO3)3 were filled up to 100 mL by distilled water and 24 g of citric acid following with 4 mL of ethylene glycol were added. A large excess of citric acid was used to prevent precipitation of lanthanide fluorides (24 g of citric acid and 4 mL of ethylene glycol per 1 g of product). During intensive stirring, an aqueous solution of ammonium fluoride in 10 mL of water was slowly dropped into the solution (with 25% excess due to the stoichiometric amounts of Gd3 þ and Eu3 þ ions). The

Fig. 3. TEM images of CeF3:5%Tb3 þ nanoparticles in cellulose matrix.

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spinning dope. The cellulose dissolution process takes usually about 100 min, and as a result the thick, viscous and amber-like colored spinning dope was obtained.

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3.3. Preparation of cellulose fibers

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The cellulose fibers spinning process was carried out as follows: the prepared spinning dope was extruded in the temperature of 115 °C through nozzle holes with the speed of 1 m/min into the air gap and then into the solidification bath containing water (bath temperature 20 °C). The fibers were draw up with the speed of 55 m/min then washed and dried in the room temperature.

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4. Results and discussion To obtain the fibers with special properties first of all, the chemical compound introduced to the spinning dope, which finally become part of fibers structure should have special and unique properties e.g. antibacterial by generating e.g. silver nanoparticles [23] in to the polymer matrix, thermochromic (e.g. thermoactive pigments [24]), magnetic by introducing e.g. barium ferrite particles [25], luminescent (e.g. zirconium oxide doped with Eu3 þ ions [22]) etc. In fact, the introduced to the fibers' structure modifier functionalizes the fibers. The specific conditions of the process of spinning dope preparation as well as specific conditions of fibers spinning process make some limitations for the shape and chemical properties of modifier. The modifier should be chemically stable under the conditions of the cellulose dissolution and spinning process. So, the potential modifier must meet several criteria listed below:

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Agglomerates size (nm) Fig. 4. Diameter distribution of CeF3:5%Tb3 þ nanoparticles in fiber (a) single crystals (b) agglomerates.

solution was heated at 80 °C for 24 h in order to evaporate water and to obtain a gel. Next, the prepared precursor was annealed at 500 °C at the air atmosphere within 2 h. The obtained compound was ground in agate mortar into a fine powder. 3.2. Preparation of the spinning dope To obtain the cellulose spinning dope the following procedure is required: the mixture of appropriate amounts of cellulose pulp (DP ¼1236) (Rayoniers), cellulose solvent (the 50% aqueous solution of N-Methylomorpholine-N-Oxide), manufactured by the Huntsman Co., propyl ester of gallic acid (Tenox PG) (Sigmas) as antioxidant and the modifier was placed into the kneader equipped in efficient stirring system and heating. The modifiers were added to the spinning dope in such a quantity as to reach a concentration of 0.5% w/w of each type of modifier in dry cellulose fibers. The mixture was stirred and slowly heated to about 115 °C. During the heating process, the excess of water was removed under the low-pressure. The process of dissolution was carried out to the point to reach 14 wt% of final concentration of water in the

1. should not react with the cellulose solvent (reaction with the solvent change the properties of the modifier and also can cause the decomposition of NMMO and cellulose), 2. must be stable in highly alkaline environment (pH about 10), 3. must be stable to relatively high temperature (the maximum process temperature 120 °C), 4. modifier cannot be soluble in water (which is present in the spinning dope preparation and in the solidification bath. The solubility in water causes that the modifier is washed off from the fibers), 5. the form of substances that are incorporated into spinning dope must not interrupt the spinning process. The particles of modifiers must be small enough not only to pass through the holes in the spinneret but also to pass the filter system, which is made of several layers of still mesh. Bigger particles stuck in spinneret orifices and block the filter making the spinning process unstable. Relatively big particles of the modifier introduced to the fibers structure deteriorate the mechanical properties of obtained fibers. According to the authors' experience, the size of the modifier particles should not exceed 2 μm, but the best way to avoid the above mentioned problems is to incorporate the modifier in the nano-size. The modifier can be incorporated into spinning dope as dispersion in organic or inorganic solvent but what is mentioned in 4th point, the modifier cannot be soluble in water. As it was shown elsewhere [26], the luminescent nanophosphors containing lanthanide ions meet all of above listed criteria and can be successfully incorporated into cellulose matrix to obtain the photoluminescent fibers.

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Fig. 5. TEM images of Sr2CeO4 nanoparticles in cellulose matrix.

4.1. Determination of the modifiers particles size in fiber' matrix

4.2. Mechanical properties

4.1.1. TEM examination of fibers modified with Gd4O3F6:5%Eu3 þ nanoparticles. To examine the distribution and size of nanocrystals and their agglomerates in polymer matrix, the TEM method was used. TEM images show that the luminescent nanocrystals incorporated into the fibers matrix form agglomerates. However, as it can be concluded from the TEM images, the occurring agglomerates are constructed of small and separate nanocrystals of modifier, predominantly with the size of 20–35 nm. The size of agglomerates is in the range of 100–300 nm (Figs. 1 and 2).

The mechanical properties are the one of most important features of the fibers. The tenacity of fibers and the elongation at the break as well as some others properties strongly depend not only on the degree of polymerization of the cellulose or the conditions of spinning process but also depend on the kind, concentration and degree of dispersion of the introduced modifier. Acceptable level of the mechanical properties of the fibers is also important from the textile manufacturing point of view. Previous to textile product with good quality is created, the fibers should withstand several processes as for example yarn formation, knitting or waving. During mentioned processes the fibers undergoing bending, stretching and twisting which in some extend deteriorate their mechanical properties. From this point of view, good mechanical properties of the fibers are important factor for their processing. The results of linear density and mechanical properties of obtained luminescent fibers are shown in Table 1. The results prove that the presence of 0.5 wt% of modifier in fibers' matrix does not influence on mechanical properties. This means that the modifiers in such quantity have not significant influence on the spinning process. The certain decrease of tenacity is observed only for the fibers contained Sr2CeO4 modifier. In this case, also the linear density is slightly higher in comparison to other fibers. This fact can be connected with the size of modifiers particles. According to the TEM examination, the CeF3:5%Tb3 þ and Gd4O3F6:5%Eu3 þ modifier particles are in nanometric size. The blue luminescent Sr2CeO4 modifier has rather the micrometer size of particles. The presence of relatively large inorganic particles in the polymer matrix disturbs the oriented fibers structure and relatively weak polymer-particles interactions weaken the mechanical properties of the fibers. Usually, the areas in the fibers structure with the presence of large particles are the weakest points of the fibers. Apparently, weak adhesion forces between the

4.1.2. TEM examination of fibers modified with CeF3:5%Tb3 þ nanoparticles. According to our previous investigation [27], the TEM examination shows that the size of most of the particles of the modifier present in the fibers do not exceed 40 nm, with the dominance of particles in the range between 10 and 24 nm. Some particles agglomerate but the average size of agglomerates does not exceed 140 nm. TEM examination shows that the modifier is well distributed in polymer matrix (Figs. 3 and 4). 4.1.3. TEM examination of Sr2CeO4 modified fibers. For the confirmation of particles size and distribution of modifier Sr2CeO4 the TEM techniques was applied. The example of TEM image of modified cellulose fibers is shown in Fig. 5. The NisElements software analysis of TEM images show that the modifier's particles are larger than in case of the modifiers with red and green luminescence and also create relatively large agglomerates in the polymer matrix. The particles diameter distribution shows that the most of the particles of Sr2CeO4 have the size below 100 nm (Fig. 6), however about 60% of the agglomerates have a size in the range between 1–3 μm.

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Agglomerates size ( µm) Fig. 6. Diameter distribution of Sr2CeO4 crystals in fibers (a) single crystals (b) agglomerates.

the charge transfer (CT) between orbitals of O2  ions and the empty 4f shell of Ce4 þ ions. The excitation spectrum shows a broad asymmetric band, composed of two CT peaks with the maximum at around 290 and 350 nm. The higher energy band originated from O1-Ce4 þ transition, where O1 is the terminal oxygen ion in the structure of Sr2CeO4, and the peak at 350 nm resulted from the charge transfer transition between the equatorial oxygen ion and Ce4 þ ion (O2-Ce4 þ ) [28]. Emission in the range of 375–600 nm is connected with the radiative relaxation process from the excited CT state of CeO6 complex [29]. Fig. 8B shows luminescence properties of fibers doped with CeF3:5%Tb3 þ . In the excitation spectrum a wide and intensive band with maximum at around 255 nm is present what is a result of 4f1-4f05d1 transition of Ce3 þ ion [30]. The presence of this broad band is an evidence of energy transfer between Ce3 þ and Tb3 þ ions. The green emission of the obtained cellulose fibers (see Fig. 8B) results from transitions between excited electronic level 5 D4 of Tb3 þ ions and ground state 7FJ (J¼ 0  6). Characteristic narrow luminescence bands on the Tb3 þ luminescence spectra are well split and are related to the following transitions: 5D4-7F6 (490 nm), 5D4-7F5 (543 nm), 5D4-7F4 (584 nm) and 5D4-7F3 (622.5 nm). Fig. 8C presents luminescent properties of cellulose fibers doped by Gd4O3F6:5%Eu3 þ nanomaterial. Excitation spectrum presents a wide and intense band in the range of 225–300 nm connected with the O2  -Eu3 þ charge transfer (CT) in the structure [31]. The remaining excitation bands with maxima above 300 nm are related to the f–f transitions of Eu3 þ ions. The emission spectrum obtained under excitation by the UV light with the wavelength of 259 nm is presented in the Fig. 8C (solid line). All emission bands are due to the transitions from the 5D0 excited level to the 7FJ components of the ground state. The most intense emission line observed at 612 nm is connected with the one of components of 5D0-7F2 transition. In Eu3 þ ion, 5D0-7F2 transition is sensitive to the Eu3 þ local symmetry and becomes partially allowed when Eu3 þ ions occupy sites of low symmetry (without an inversion symmetry). The remaining emission bands are connected with transitions: 5D0-7F0 (580 nm), 5D0-7F1 (593 nm), 5 D0-7F3 (630 nm) and 5D0-7F4 (655 nm).

5. Conclusions polymer and modifier particles reduce also the elasticity of the fibers. Even for such small concentration as 0.5% w/w of the modifier with relatively large particles has negative influence on the mechanical properties of the fibers. 4.3. Luminescent properties of modified cellulose fibers The main purpose of the present study is to obtain the cellulose fibers with luminescent properties. As it was shown elsewhere [22,26,27] the luminescent properties of the fibers strongly depends not only on the type of modifier and quality (particle size and shape) but also on its concentration and distribution in polymer matrix. Luminescence of the prepared modifiers as well as modified cellulose fibers under UV irradiation is presented in the Fig. 7. This multicolor emission is pure and intense which makes cellulose as an ideal matrix for these inorganic, Ln3 þ -doped compounds. Absorption of excitation radiation or light emitted from the luminescence activators by cellulose is low and has not affected on the observed results. Fig. 8 shows excitation end emission spectra of the prepared luminescent fibers. Characteristic emission of the blue phosphor used as dopant: Sr2CeO4 presented in the Fig. 8A is connected with

Based on the NMMO process, the regenerated modified cellulose fibers were obtained. The results show that the one of the most important factors to obtain fibers with required properties is properly chosen modifier. However, there is possible to work out Lyocell type fibers with new luminescent properties, but first of all selection of the luminescent substances that fulfills the crucial criteria of the NMMO process should be done. Used in present research modifiers containing materials doped with lanthanide ions fulfill the strict conditions of the spinning dope and fibers preparation. The TEM observation shows that used photoluminescent nanocrystals are evenly distributed in cellulose matrix of fibers. As it was shown elsewhere, the presence of inorganic modifier in fibers matrix, influence on mechanical properties of obtained fibers. The mechanical properties strongly depend on the modifier particles size and modifier concentration. For relatively low concentration of modifiers e.g. 0.5% w/w, the value of tenacity of obtained fibers do close to the value of tenacity of standard, not modified cellulose fibers. The examination of optical properties of the fibers shows that the fibers have high intensity of luminescence emission under UV irradiation and the character of used compounds allows obtaining fibers that emit light with different colors. Based on the present study it can be stated that it is possible to obtain the cellulose fibers with specific

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Table 1 The linear density and mechanical properties of fibers with different luminescent modifiers. Modifier

Concentration Linear density [tex] Tenacity [cN/tex] Standard deviation for tenacity Elongation at break [%] Standard deviation for elongation

– Gd4O3F6:5%Eu3 þ CeF3:5%Tb3 þ Sr2CeO4

0.0 0.5 0.5 0.5

0.284 0.295 0.293 0.312

30.65 30.21 30.07 25.27

9.27 11.39 8.76 7.76

10.48 9.39 10.16 5.54

1.62 1.43 1.65 0.76

Fig. 7. Luminescence of the prepared nanopowders (A) and doped cellulose fibers (B–D) under ultraviolet irradiation (λex ¼ 254 nm); (B) Gd4O3F6:5%Eu3 þ ; (C) Sr2CeO4; (D) CeF3:5%Tb3 þ .

Luminescence intensity (arb. units)

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Financial support from the Polish Ministry of Science and Higher Education; Grant no. NN 508 623 240 is gratefully acknowledged. This work was partly supported by Grant no. NN508 0851 33 of Polish Ministry of Science and Higher Education.

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Wavelength (nm) Fig. 8. Excitation (left – dashed line) and emission (right – solid line) spectra of the cellulose fibers doped with (A) Sr2CeO4, λex ¼288 nm, λem ¼ 462 nm; (B) CeF3:5% Tb3 þ , λex ¼ 253 nm, λem ¼ 543 nm; (C) Gd4O3F6:5%Eu3 þ , λex ¼ 259 nm, λem ¼ 612 nm. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

and unique emission spectra by mixing two or more different luminescent materials in précised designed weight ratio. This kind of advanced cellulose fibers can be used for the protection of documents and in the textile industry as well.

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Please cite this article as: A. Erdman, et al., J. Lumin. (2015), http://dx.doi.org/10.1016/j.jlumin.2015.02.049i