Optical Haze Nanopaper Enhanced Ultraviolet Harvesting for

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Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2019, 7, 9966−9975

Optical Haze Nanopaper Enhanced Ultraviolet Harvesting for Direct Soft-Fluorescent Emission Based on Lanthanide Complex Assembly and Oxidized Cellulose Nanofibrils Sufeng Zhang,†,‡ Gang Liu,†,‡ Hui Chang,∥ Xinping Li,†,‡ and Zhao Zhang*,†,‡,§,∥

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†

Shaanxi Provincal Key Laboratory of Papermaking Technology and Specialty Paper Development, College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi’an 710021, Shaanxi, China ‡ National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi University of Science and Technology, Xi’an 710021, Shaanxi, China § State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China ∥ Northwest University, Xi’an 710069, Shaanxi, China S Supporting Information *

ABSTRACT: Optical haze nanopapers were fabricated based on a self-assembly process of lanthanide complexes with tris(2benzimidazolylmethyl) amine (NTB) [(Tb(NTB)Cl3 and/or Eu(NTB)Cl3] and TEMPO-oxidized cellulose nanofibrils (tCNFs), using a subsequent simple blending-vacuum filtration method. These optical haze nanopapers were characterized in terms of visible light transmittance, efficiency of ultraviolet (UV) light harvesting, and soft-fluorescence emission. The lanthanide complexes acted as a light-scattering source to enhance optical haze (∼100%) and a light emitter to emit soft-fluorescence under a UV light-emitting diode (UVLED, λex, 365 nm). Due to excellent light-scattering behavior induced by the optical haze nanopaper, UV harvesting efficiency was increased from 53% to 98%, and the absolute quantum efficiency was enhanced from 21% to 25%. The optical haze nanopaper with high efficiency UV harvesting and soft-fluorescent emission presented potential applications in solar cells, lampshades, light diffusers, UV-blocking devices, anticounterfeiting devices, and organic light-emitting diodes. KEYWORDS: Oxidized cellulose nanofibrils, Lanthanide complexes, Optical haze with light diffusion, Ultraviolet harvesting, Soft-fluorescent LED

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INTRODUCTION

after physical or chemical modification, have been widely used in biosensors, catalysis, and supercapcitors.9−11 For CNFbased optical materials, a low coefficient of thermal expansion (CTE, 12−28 ppm·K−1), high tensile strength, thermal stability, and recyclability alow them to be more easily processed at higher temperatures than petroleum-based polymer substrates.12 Hence, optical materials prepared from CNF substrates have profound and meaningful influence in the fluorescent material field. Miao et al. have reported a method for rapidly fabricating transparent and multiluminescent, TEMPO-oxidized, nanofibrillated cellulose nanopaper.13 Kulpinski et al. have published a method for manufacturing luminescent nanopaper by incorporating ZrO2, stabilized by Y2O3, into fiber-structured particles.14 Zhu et al. have introduced an advantageous strategy for fabricating highly

In recent years, polymer-based optical materials have been widely used in flexible organic light-emitting diodes, solar cells, and sensors.1−4 However, most optical material substrates have been petroleum-based derivatives that face challenges, such as environmental problems, service life, and economic cost. As the most extensively used polymer-based material, cellulose nanofibrils (CNFs) are nanosized cellulose fibers extracted from plant cell walls, are the most abundant renewable materials on Earth, and have attracted intense attention due to their superior optical and physical properties.5 Cellulose nanofibrils (CNFs) have been prepared by many methods, such as chemical pretreatment, high speed shearing, high pressure homogenization, and enzyme treatment.6,7 CNFs possess a self-assembly ability because of their abundant hydroxyl groups. Generally, nanopaper with pure CNF selfassembly exhibits high crystallinity in a dense packing mode, which effectively avoids light scattering and yields high transparency.8 Functional materials prepared from CNFs, © 2019 American Chemical Society

Received: February 18, 2019 Revised: April 11, 2019 Published: May 8, 2019 9966

DOI: 10.1021/acssuschemeng.9b00970 ACS Sustainable Chem. Eng. 2019, 7, 9966−9975

Research Article

ACS Sustainable Chemistry & Engineering emissive thin films by combining quantum dots and cellulose nanofibers.15 In our previous study, irreversible solvatochromic nanopapers have been developed for the first time using CNFs films.16 A functionalized nanopaper has also been developed for ultraviolet blocking and anticounterfeiting security materials by grafting europium(III) complexes 12 and ytterbium(III) carbon quantum dots onto CNF surfaces prior to formation of nanopaper.17 Thus far, there have been few reports regarding optical nanopaper with soft-fluorescent organic light-emitting diodes (OLEDs). Nanopaper with optical haze based on functionalized cellulose nanofibrils has received growing interest due to its intriguing traits in solar cells, light diffusers, UV-blocking devices, and flexible OLEDs.18 Haze describes the percent of transmitted light that is diffusely scattered, such that the higher the haze is, the lower the material gloss and image transmission are. Some materials, such as the lampshade of a fluorescent lamp, show excellent luminosity and high haze, maintaining high luminous flux while reducing irritation for naked eyes. Hazy materials can cause normal incident light to become extremely diffusive, increasing the light path length, and thus improving the possibility of a photon being captured within the material’s active region.19,20 However, there has been a tradeoff between light transmittance and optical haze. The optical haze of nanocellulose nanopaper depends on diffuse transmittance caused by light scattering at the nanocellulose scale and its surface morphology porosity and roughness.21,22 Xu et al. have prepared low-haze, hybrid nanopaper by adjusting the mixing ratio of CNCs and CNFs.23 Zhang et al. also have prepared controllable optical haze cellulose nanopaper through an alcohol-related stimulated with wrinkling and swelling.24 Zhu et al. have prepared haze nanopaper through a top-down method including lignin removal and ambient pressureinduced rearrangement of cellulose.25 Isobe et al. have prepared haze nanopaper through high-humidity drying method.26 The above methods had drawbacks with more or less. However, few optical haze nanopapers via the co-selfassembly method of lanthanide complexes with tCNFs have been produced and, in particular, no nanopaper possessing soft-fluorescent emission ability. This method has many advantages, including spontaneous reaction, mild and controllable reaction conditions, high reproducibility, and few byproducts. In this study, optical haze nanopapers were fabricated based on a self-assembly process of lanthanide complexes with tris(2benzimidazolylmethyl) amine (NTB) [(Tb(NTB)Cl3 and/or Eu(NTB)Cl3] and TEMPO-oxidized cellulose nanofibrils (tCNFs), with a subsequent simple blending−vacuum filtration method. Compared with the traditional petroleumbased haze materials, the nanopaper prepared with tCNFs as the substrate in this study avoided a trade-off effect and achieved both high transmission and haze. By controlling the lanthanide complex amount, the tailored haze index of the resulting nanopapers was determined. These nanopapers exhibited translucence and a haze degree that combined the advantages of lanthanide complexes and tCNFs, emitting softfluorescence under UV-LED excitation at 365 nm. Due to excellent light scattering behaviors induced by the optical haze nanopaper, the UV harvesting efficiency was increased from 53% to 98%, and the absolute quantum efficiency was enhanced from 21% to 25%. The preparation method of these soft-fluorescent emission cellulose nanopapers was efficient as well as mild and could be a meaningful reference

for subsequent studies. This novel material has potential for use in applications such as in light diffusers, anticounterfeiting devices, and organic light-emitting diodes.

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EXPERIMENTAL SECTION

Reagents and Materials. All chemicals were commercially available and used without further purification. Nitrilotriacetic acid (NTA, 98.5%), o-phenylenediamine (OPD, 98.5%), methanol, dimethyl sulfoxide, and propanediol were of analytical reagent grade and were all purchased from Da Mao Chemical Reagents Co., Ltd. (Tianjin, China). There were lanthanide complexes including europium chloride hexahydrate [EuCl3(H2O)6, 99.9%] and terbium hexahydrate chloride [TbCl3(H2O)6, 99.9%], which were obtained from Dickman’s Reagent Co., Ltd. (Shenzhen, China). 2,2,6,6Tetramethylpiperidine-1-oxyl (TEMPO, 98%) was purchased from Aladdin Reagents Co., Ltd. (Shanghai, China). Sodium hypochlorite (NaClO, AR) and sodium bromide (NaBr, AR) were purchased from Kermel Reagents Co., Ltd. (Tianjin, China). The deionized water was prepared in the laboratory during the experiment process. Cellulose nanofibrils were prepared via a high-pressure homogenization process using a laboratory cotton pulp board as a raw material. Preparation of tCNFs Aqueous Suspensions. The tCNFs aqueous suspensions was acquired by a method of a previously published literature;13 this method achieved the induced oxidation of CNFs C-6 primary hydroxyl groups to carboxyl groups by using the TEMPO/NaClO/NaBr oxidation system. At the beginning of the experiment, a certain amount of pulp was dispersed in deionized water for 15 min via ultrasonic treatment to form a CNFs suspension, and then, a given ratio of TEMPO and NaBr solution was added to the above system successively. Then, a certain amount of NaClO solution was gradually added as an oxidant to start the oxidation reaction and continuously stirred for 3 h under ultrasonic. The pH was adjusted to 7 with the addition of NaOH aqueous solution during the entire process. Ultimately, hydrochloric acid was added to adjust the pH to 7. The resulting mixed system was subjected to repeated centrifugation and deionized water washing to remove the supernatant to obtain a filter cake which was redispersed in water to prepare 0.5 wt % tCNFs aqueous suspensions. FT-IR (KBr, cm−1): 3336 (s), 2908 (w), 2356 (w), 1631 (m), 1423 (w), 1367 (w), 1315 (w), 1159 (w), 1105 (w), 1055 (s), and 551 (m). Preparation of Tris(2-benzimidazolylmethyl) Amine. Tris(2benzimidazolylmethyl) amine (NTB) was prepared according to a previous procedure.27 The nitrilotriacetic acid and o-phenylenediamine (molar ratio: 1:3) were mixed into a round-bottomed flask, and then, propylene glycol (60 mL) was added as a solvent. Next, the system was continuously heated at 140 °C and stirred for 12 h until no bubbles were released and then cooled to room temperature. There was ice water (50 mL) added to precipitate an amount of viscous tan solid, which was recrystallized five times in hot methanol to give pinkish needle crystals (yield, 80%). 1HNMR (400 MHz, CDCl3): δ(ppm) 12.47 (s, 3H, N−H), 7.59−7.56 (d, 6H, Ar−H), 7.19 (s, 6H, Ar−H), 4.15 (s, 6H, CH2). FT-IR (KBr, cm−1): 3450 (m), 1627 (m), 1535 (w), 1487 (w), 1436 (m), 1352 (w), 1271 (m), 1215 (w), 1120 (w), 1024 (w), 968 (w), 844 (w), and 740 (m). Preparation of Ln(NTB)Cl3 (Ln = Eu or Tb). Ln(NTB)Cl3 (Ln = Eu or Tb) was prepared according to ref 28. A solution, prepared by adding NTB (80 mg, 0.2 mmol) and TbCl3(H2O)6 (90 mg, 0.24 mmol) to anhydrous methanol (20 mL), was refluxed at 65 °C for half an hour. The resulting solution was filtered to remove insoluble residue. Then, the filtrate was poured into the vessel, and white crystals were formed gradually after 24 h with methanol evaporation (yield, 65%). Then, EuCl3(H2O)6 was replaced with TbCl3(H2O)6 to prepare another lanthanide complex by the same procedure. Preparation of NTB-Ln3+-tCNFs Nanopapers (Ln = Eu and/ or Tb). The above-prepared Tb(NTB)Cl3 methanol solution (2 mL) and 0.5 wt % tCNFs aqueous suspensions (20 mL) were mixed and stirred at room temperature for 10 min under ultrasonic conditions. The NTB-Tb3+-tCNFs nanopaper was made with vacuum filtration using a glass sand core suction filter device and vacuum drying at 40 9967

DOI: 10.1021/acssuschemeng.9b00970 ACS Sustainable Chem. Eng. 2019, 7, 9966−9975

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Preparation Process of NTB-Ln3+-tCNFs Nanopaper

°C. The NTB-Eu3+-tCNFs nanopaper was prepared by replacing the Tb(NTB)Cl3 system with the Eu(NTB)Cl3 system. Furthermore, the double-component Ln3+ nanopapers (Eu: Tb = 1:10) were also synthesized by compositing Tb(NTB)Cl3, Eu(NTB)Cl3, and tCNFs using the similar blending−vacuum filtration method. FT-IR (KBr, cm−1): NTB-Tb3+-tCNFs, 3336 (m), 2902 (w), 2355 (w), 1600 (m), 1431 (w), 1315 (w), 1276 (w), 1157 (w), 1107 (w), 1010 (m), 947 (w), 900 (w), 736 (w), 657 (w), 592 (w), 553 (w), and 441 (w); NTB-Eu3+-tCNFs, 3336 (m), 2902 (w), 2355 (w), 1600 (m), 1429 (m), 1367 (w), 1315 (w), 1278 (w), 1159 (w), 1107 (w), 1010 (s), 948 (w), 900 (w), 734 (w), 703 (w), 659 (w), and 553 (m); NTBTb3+/Eu3+-tCNFs, 3336 (m), 2908 (w), 2356 (w), 1600 (m), 1429(m), 1367 (w), 1315 (w), 1203 (w), 1157 (w), 1107 (w), 1101 (s), 948 (w), 900 (w), 661 (w), 605 (w), and 553 (m). Characterization. The structural properties were measured by using a Fourier-transform infrared spectrometer (FTIR, VECTOR-22, Bruker Company, Germany) with the wavelength range of 4000 to 400 cm−1 (the KBr pellet method was used to characterize solid samples, and the ATR accessory was applied to tCNFs nanopapers). Surface morphologies were characterized by scanning electron microscopy (SEM, Vega 3 SBH, TESCAN Company, Czechia). The chemical state of the element was measured with X-ray photoelectron spectroscopy (XPS, AXIS SUPRA, Shimadzu Company, U.K.). The transmittance of the nanopapers was measured with a UV−visible spectrophotometer (UV−vis, Cary 5000, PerkinElmer, U.S.A). Fluorescence properties were measured with a fluorospectrophotometer (FS5, Edinburgh, US). Visible luminescence quantum yield (Φoverall) of the nanopaper was determined using an absolute integrating-sphere method (Edinburgh Instrument FLS980). XRD analyses were investigated with an X-ray diffractometer (XRD, D8 Advance, Bruker Company, Germany). The tensile strength was measured by using a universal stretcher (AI-7000-NGD, Gotwell Co., Ltd., Dongguan, China). The haze of nanopapers was measured by using a chroma spectrophotometer (ci7800, X-Rite, U.S.A.), and the thickness of the nanopapers was measured with a paper thickness meter (thickness gauge, Qingtongboke Co., Ltd., Hangzhou, China).

three rounds of high-speed shearing (1500 rpm, 15 min), highpressure homogenization, and TEMPO/NaBr/NaClO system oxidization, with the last step under ultrasonic conditions. The oxidization system selectively oxidized cellulose primary hydroxyl groups without affecting nanofiber morphology and crystallinity.19 Ultrasonic-assisted TEMPO oxidation was beneficial for increasing the carboxylation degree. This effect was mainly attributed to greater contact of ultrasonically treated fiber surfaces with the reagent.30 The resulting tCNFs were dispersed in aqueous solution without flocculation because of the presence of a certain amount of anions, which provided beneficial conditions for nanopaper preparation. There were abundant hydroxyl groups and carboxyl groups at the C6 site of tCNFs, which coordinated with lanthanide ions to provide stronger self-assembly ability.31 The average width of the prepared tCNFs was 80 nm (Figure S1). The reason was that compared with water molecules the carboxyl group of tCNFs had stronger coordination ability, which prevented the coordination effect between water molecules and Ln(NTB)Cl3. Finally, three tCNFs fluorescent nanopapers were prepared with different lanthanides, including NTB-Tb3+tCNFs, NTB-Eu3+-tCNFs, and NTB-Tb3+/Eu3+-tCNFs. The lanthanide ions formed coordinate bonds with nitrogens of NTB and oxygens of tCNFs. The chosen mass proportions of tCNFs aqueous suspensions and lanthanide complexes determined a paper’s fluorescent properties and tensile strength. By mixing 20 mL of 0.5 wt % tCNFs aqueous suspension and 2 mL of methanolic lanthanide complex solution, soft-fluorescent nanopapers were obtained. Taking into account the results mentioned above, a certain mass proportion was employed to prepare NTB-Ln3+-tCNFs nanopaper. SEM characterization. Nanopaper morphology was characterized by scanning electron microscopy (SEM). During the nanopaper drying process, after water was evaporated, nanosized tCNFs were observed to be densely packed without significant air voids inside the nanopaper, which caused its low optical haze. The surfaces of pure tCNFs nanopaper were extraordinarily smooth and without any roughness (Figure 1a). Further observations at the 10 μm scale revealed that the nanopaper was dense and tCNFs distribution relatively uniform, which could have been generated by hydrogen bonding between fibers. The surfaces of NTB-Ln3+-tCNFs nanopapers were slightly rough and exhibited few voids (Figure 1b and c). Compared with previous images, nanocellulose fibers were clearly seen to be intertwined to form a dense three-dimensional structure, which thus weakened light

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RESULTS AND DISCUSSION Preparation of tCNFs and NTB-Ln3+-tCNFs Nanopapers. NTB, as a ligand with a simple preparation and excellent energy conversion efficiency for lanthanide complexes, has been developed to fabricate multicolored fluorescent materials.29 Here, NTB-Ln3+-tCNFs nanopapers (Ln = Eu and/or Tb) were prepared (Scheme 1). First, Ln(NTB)Cl3 lanthanide complexes were prepared under mild conditions, in which ions were coordinated with ligands that provided four coordination sites to satisfy the polydentate requirements of lanthanide ions. Multiple empty orbitals of lanthanide ions also provided the basis for subsequent coordination with tCNFs. Next, cotton pulp board was used as a raw material to prepare a tCNFs aqueous suspension using 9968

DOI: 10.1021/acssuschemeng.9b00970 ACS Sustainable Chem. Eng. 2019, 7, 9966−9975

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ACS Sustainable Chemistry & Engineering

position of C−O shifted from 1055 to 1010 cm−1. The above results might have been due to carboxyl and lanthanide ion coordination. NTB characteristic peaks were stretching vibrations of Ar−N−H and CN−CC (3450 and 1436 cm−1, respectively), stretching vibrations of tertiary amine (1352 cm−1), and bending vibrations of C−N (740 cm−1). After lanthanide complexes were composited into paper, the three nanopaper types showed emerging peaks nearby 947 cm−1. According to the above analysis, lanthanide complexes were demonstrated to have been embedded into tCNFs matrices to form NTB-Ln3+-tCNFs nanopapers. XRD Characterization. The crystallinity properties of the present tCNFs and NTB-Tb3+-tCNFs nanopapers were investigated by X-ray diffractometry. Diffraction peaks were observed at 2θ angles around 15.2° (101), 16.5° (10̅1), and 22.6° (002), which indexed well to typical cellulose (Figure S3).33 After being composited with terbium ion complexes, NTB-Tb3+-tCNFs nanopaper also presented diffraction peaks similar to pure tCNFs nanopaper. However, an accompanying and distinct decrease in peak intensity (at 22.6°) was attributed to the influence of coordination bonds between lanthanide complexes and tCNFs. Notably, XRD patterns of these complex-containing fluorescent nanopapers did not exhibit any new diffraction peaks, which indicated that lanthanide complex addition had no great effect on these nanocellulose matrices. XRD spectra of NTB-Eu3+-tCNFs and NTB-Tb3+/ Eu3+-tCNFs nanopapers were similar to that of NTB-Tb3+tCNFs. Overall, the type of lanthanide ions used here appeared to have no effect on diffraction properties. Tensile Strength Characterization. Good stress−strain properties are generally required in paper-based materials, particularly for applications in flexible optical materials. Using a servo universal testing machine, nanopaper tensile strengths were explored. A noteworthy feature of these nanopapers was that NTB-Ln3+-tCNFs nanopapers showed good mechanical

Figure 1. Surface and cross-section SEM images of pure tCNFs nanopaper (a, d), NTB-Tb3+-tCNFs nanopaper (b, e), and NTBEu3+-tCNFs nanopaper (c, f). All were prepared by mixing 30 mL of 0.5 wt % tCNFs aqueous suspensions and 2 mL of methanolic lanthanide complexes, with hybrid nanopaper prepared from 1 mL each of methanolic Tb(NTB)Cl3 and Tb(NTB)Cl3 solutions.

transmission. This might have been caused by the two types of lanthanide complexes. By observing cross sections, the addition of lanthanide complexes was found to lead to the formation of layer−layer morphology (Figure 1d−f). FT-IR Characterization. FT-IR spectra showed that NTBLn3+-tCNFs (Ln = Eu and/or Tb) nanopapers of composite lanthanide complexes and pure tCNFs nanopaper exhibited characteristic bands of 3336 and 2908 cm−1, which indicated stretching vibrations of −OH and −CH, respectively (Figure S2).32 For tCNFs, the characteristic peak at 1631 cm−1 was attributed to CO stretching vibrations in carboxyl groups of oxidized cellulose, which indicated that some CNF hydroxyl groups were oxidized to carboxyl groups. For the three tCNFs nanopapers composited here with lanthanide complexes, the characteristic band at 1600 cm−1 was blue-shifted by 30 cm−1, compared to pure tCNFs, and the stretching vibration peak

Figure 2. XPS patterns of NTB-Tb3+-tCNFs and NTB-Eu3+-tCNFs nanopaper (a). High-resolution XPS spectra of Tb 4d (b), Eu 3d (c), C 1s (d, g), N 1s (e, h), and O 1s (f, i). 9969

DOI: 10.1021/acssuschemeng.9b00970 ACS Sustainable Chem. Eng. 2019, 7, 9966−9975

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ACS Sustainable Chemistry & Engineering

Table 2. Specific Elemental Data of NTB-Eu3+-tCNFs Nanopaper

properties. The tensile strength of pure tCNFs reached 12.5 MPa. With composite lanthanide complexes, the tensile strengths of NTB-Tb3+-tCNFs and NTB-Eu3+-tCNFs nanopapers were increased to 15 and 15.1 MPa, respectively, which was attributed to Ln(NTB)Cl3’s reinforcing effects (Figure S4). Furthermore, the stress index of the NTB-Tb3+/Eu3+tCNFs nanopaper was 15.5 MPa. The mechanical strength of tCNFs nanopapers doped with two lanthanide complexes was concluded to be basically similar, and the mixed lanthanide soft-fluorescent nanopapers showed excellent mechanical properties. The results of stress−strain testing indicated that the original nanopaper had a good Young’s modulus (2.1 GPa), as determined by fitting σ−ε curves to the equation E = σ/ε. The Young’s modulus of NTB-Ln3+-tCNFs decreased slightly to 1.5−1.6 GPa. XPS Characterization. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis was used to determine the surface elemental composition of NTB-Ln3+-tCNFs nanopapers. The spectra revealed that the two types of fluorescence nanopapers were mainly composed of carbon, nitrogen, and oxygen and also contained Tb or Eu (Figure 2a). The C-1s peaks of NTBTb3+-tCNFs nanopapers were deconvoluted into four parts at 282.8, 284.5, 286.3, and 287.9 eV and attributed to CN, C− C/C−H, C−O, and CO/O−C−O bonds, respectively (Figure 2d). Compared to NTB-Eu3+-tCNFs nanopapers, the binding energy of C-1s was almost identical (Figure 2g). The N-1s regions showed characteristic peaks assigned to nitrogen (C−N−C, C−NC, and N−C3 at 398.2, 400.3, and 403 eV, respectively; Figure 2e). All these peaks were attributed to nitrogen of NTB. Similar to carbon, the nitrogen binding energy of the two films was also basically the same (Figure 2h) and oxygen derived from the cellulose matrix. The binding energy of Tb-4d was 151.5 eV, which was lower by ∼1.1 eV than the generally accepted data (Tb-4d, 152.6 eV; Figure 2b), which was ascribed to the coordination bonds of Tb−O between Tb3+ and carboxyl in tCNFs that led to a decrease in binding energy.34 The XPS spectrum of Eu-3d revealed that the binding energy was 1133.2 and 1163 eV for 3d5/2 and 3d3/2, respectively (Figure 2c). The integrated area of the 3d5/2 orbit (emitting red light) was larger than that of 3d3/2 (emitting orange light). Thus, tCNFs were concluded to have combined with lanthanide complexes by coordination effects between O atoms and Tb3+ or Eu3+.35 The lanthanide ion contents of the final materials have a critical influence on the luminescence performance, and thus, it needs to be accurately quantified. The atomic content of elements in the final material was semiquantitatively analyzed by XPS, and the obtained lanthanide ions content was 7.20% (Tb3+) and 7.91% (Eu3+) (Tables 1 and 2). Optical Haze Nanopapers Induced Light Softening. The transmittance and haze of pure tCNFs and fluorescent NTB-Ln3+-tCNFs nanopapers were investigated. The transmittance of pure tCNFs nanopaper was observed to reach 90%

Binding Energy/eV

fwhm/eV

Area

At%

C 1s N 1s O 1s Tb 4d

283.00 399.00 531.00 151.00

5.028 4.455 4.004 6.698

434225.47 10421.46 368626.95 43873.50

71.22 0.95 20.63 7.20

Binding Energy/eV

fwhm/eV

Area

At%

C 1s N 1s O 1s Eu 3d

283.00 397.00 531.00 1133.00

3.825 2.940 3.287 4.465

450966.36 18630.60 430069.65 63873.50

72.64 1.35 18.10 7.91

in the visible light region (Figure 3a). With 600 nm stimulation, the transmittance of NTB-Tb3+-tCNFs and NTB-Eu3+-tCNFs nanopapers both decreased from 90% to 83%, which was attributed to likely visible light diffuse scattering from rough nanopaper surfaces.12 When a certain content of methanolic Tb(NTB)Cl3 was further added to NTB-Tb3+-tCNFs, the nanopaper transmittance was further reduced from 83% to 75% and enhanced light absorption observed. The refractive index mismatch between the tCNFs and air gaps endow light scattering through the nanopaper (Figure S5). For pure tCNFs nanopaper, denser tCNFs packings resulted in fewer air gaps, giving rise to limited haze (Figure 1d, 67%). However, the surface interaction of tCNFs was destroyed after grafting lanthanide complexes, which caused large amounts of cavities in the nanopaper film (Figure 1e and f), which gave rise to excellent optical haze (>88%). The haze of pure tCNFs, NTB-Tb3+-tCNFs, NTB-Eu3+tCNFs, and NTB-Tb3+/Eu3+-tCNFs nanopapers were 98%, 88%, 89%, and 67%, respectively, which indicated that optical haze was controlled by lanthanide complexes (Figure 3b). The fluorescence nanopaper containing double lanthanides had excellent haze and low light transmittance, which were indeed interesting results. The reason was that the amount of lanthanide complex in the double-component nanpaper was twice as much as that in the single-component nanopaper. The increase in lanthanide complex particles was bound to increase the scattering and absorption of light, which directly led to the decrease in light transmittance and the increase in haze. Accompanied with the incorporation of lanthanide complexes, nanopaper surfaces became rough and light scattering increased, in which the change in haze of a series of nanopapers was consistent with transmittance change. The light diffusion effects of tCNFs and NTB-Ln3+-tCNFs nanopapers were demonstrated by laser irradiation (Figure 3c and d). In general, the transmittance and haze of prepared nanopapers had value as references in the visible light range (Figure S6). The laser intensity did not weaken after penetrating the paper because of the high transparency of tCNFs, resulting in small radius screen lit areas. However, NTB-Ln3+-tCNFs nanopaper possessed strong light scattering, with the lit area from transmitted laser light becoming larger and more homogeneously illuminated. Optical Haze Nanopapers as UV-LED Enhanced Ultraviolet Harvesting for Direct Soft-Fluorescent Emission. To elucidate ultraviolet harvesting and softfluorescent emission of these nanopapers, nanopaper-doped Eu(NTB)Cl3 was compared with NTB-Ln3+-tCNFs nanopaper. For doped materials, Eu(NTB)Cl3 was spread on nanopaper surfaces, which were prepared using original tCNFs. In contrast, the NTB-Ln3+-tCNFs nanopaper was prepared using functional tCNFs via Eu(NTB)Cl3 assembly and tCNFs, and the Eu-central dispersed in the whole nanopaper. The usage amounts of tCNFs suspension (20 mL, 0.5 wt %) and Eu(NTB)Cl3 (2 mL) were uniform, and the thicknesses of the

Table 1. Specific Elemental Data of NTB-Tb3+-tCNFs Nanopaper Name

Name

9970

DOI: 10.1021/acssuschemeng.9b00970 ACS Sustainable Chem. Eng. 2019, 7, 9966−9975

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ACS Sustainable Chemistry & Engineering

Figure 3. Transmittance (a) and haze (b) of pure tCNFs nanopaper and fluorescent tCNFs nanopapers. Photographs of pure CNF (c) and NTBEu3+-tCNFs (d) nanopaper-induced light diffusion.

Figure 4. Images of transparent tCNFs nanopaper doped with Eu-complex when UV-LED is off (a) and on (b). Images of soft-fluorescent emission NTB-Eu3+-tCNFs nanopaper when UV-LED is off (d) and on (e). Mechanism diagram of pure-tCNFs nanopaper doped with Eu-complex (c) and soft-fluorescent emission NTB-Eu3+-tCNFs nanopaper (f).

prepared NTB-Eu3+-tCNFs nanopaper and pure-tCNFs nanopaper-doped Eu-complex were 55 and 50 μm, respectively. The capability of photon capturing was verified by examining the UV−visible spectrum of the two nanopaper types (Figure S7). Eu(NTB)Cl3-adapted tCNFs nanopaper was found to only absorb a small portion of UV light (200−400 nm), resulting in a certain UV light pass-through (50% at 300 nm). However, a large number of internal cavities absorbed and refracted UV light, which led to light path extension and decreased UV light transmittance by the optical haze nanopaper. This meant that excellent light scattering behavior induced by optical haze nanopaper gave rise to UV harvesting efficiency (at 300 nm) that was increased from 53% to 98%. Therefore, it was

determined that the nanopaper’s optical haze from light scattering indeed showed high efficiency UV harvesting, suggesting potential applications in light diffusers, solar cells, UV-blocking devices, and UV-LEDs. The effective application potential of NTB-Ln3+-tCNFs nanopaper in the soft-fluorescent-LED field was demonstrated by designing a simple device to investigate the ability of these materials to perform as UV-LEDs (λex, 365 nm, Figure 4). Meanwhile, pure-tCNFs nanopaper doped with Eu-complex and NTB-Eu3+-tCNFs nanopaper were placed together to highlight the light scattering ability of the latter. When the UVLED was turned off, the soft-fluorescent LED emission was hazy, but tCNFs doped with Eu-complex was quite clear 9971

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Figure 5. Excitation and emission spectra of NTB-Eu3+-tCNFs nanopaper (a), NTB-Tb3+-tCNFs nanopaper (b), and NTB-Tb3+/Eu3+-tCNFs nanopaper excitation at 336 nm (c). Energy transfer process of NTB-Eu3+-tCNFs, NTB-Tb3+-tCNFs, and NTB-Tb3+/Eu3+-tCNFs nanopaper.

Soft-fluorescent emissive properties of NTB-Ln3+-tCNFs were further examined. UV−visible spectra of NTB-Ln3+tCNFs nanopapers exhibited relatively broader absorption bands than the Ln(NTB)Cl3 complex in methanolic solution, in which the absorptions at 249−253 and 272−275 nm were attributed to electronic transitions from tCNFs and the NTB ligand, respectively, with both of them directly coordinated to lanthanide ions (Figure S9). Five characteristic emission peaks were detected from NTB-Eu3+-tCNFs nanopaper when excited at 346 nm (Figure 5a). These peaks almost exhibited line-like emissions, located at 573, 591, 613, 650, and 670 nm, which were attributed to the 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 transitions of Eu3+, respectively. The strongest emission peak was located at 613 nm, which indicated that the nanopaper had relatively brilliant red emission, corresponding to Commission International De L’Eclairage (CIE, x = 0.66, y = 0.33).36 For NTB-Eu3+-tCNFs nanopaper, a broad band ranging from 300 to 450 nm (λem, 613 nm) was observed, which was attributed to ligand absorption. The NTB ligand was deduced to have successfully transferred absorbed UV energy to Eu3+ ions via the antenna effect. Excitation spectra exhibited a broad band in the UV region ranging from 300 to 400 nm with a 324 nm maximum (Figure 5b). Meanwhile, emission spectra presented a number of sharp peaks, including 489, 546, 585, and 623 nm from excitation at 324 nm. These peaks were attributed to 5D4 → 7 F6, 5D4 → 7F5, 5D4 → 7F4, and 5D4 → 7F3 transitions of Tb3+ ions, respectively.37 The strongest emission (λem, 546 nm), based on the hyper-sensitive 5D4 → 7F5 transition, endowed the pure green light with CIE chromatic coordinates x and y of 0.30 and 0.59, respectively, for NTB-Tb3+-tCNFs. This was an

(Figure 4a and d). UV-LED triggered red emission from nanopapers doped with Eu-complex produced a small radius light emission (Figure 4b and e). For NTB-Eu3+-tCNFs nanopaper, the prepared UV-LED lamp exhibited soft red light when turned on, suggesting its potential application in novel optical devices. The possible reason for these results was analyzed by spreading Eu-complex on pure tCNFs nanopaper, with a certain amount of fluorescent Eu-complex attached to the paper surfaces. Under UV excitation, most UV rays passed directly through the nanopaper and yielded weak red light emission (Figure 4c). However, observing NTB-Eu3+-tCNFs nanopaper, there was a large amount of Eu-complex centrally connected by coordination bonds inside the nanopaper, such that when passing through the nanopaper most UV rays were absorbed by complexes and converted to red light (Figure 4f). Electron energy transitioned from the steady state (S0) to the first excited singlet state (S1) and then to the triplet state (T1) via intersystem-crossing (ISC). The energy was then sensitized to Eu3+ and NTB-Eu3+-tCNFs to release intense red light after electron deactivation. Furthermore, the absolute quantum efficiency was enhanced from 19% (for nanopaper doped with Eu-complex) to 25% (for NTB-Eu3+-tCNFs), which was attributed to enhanced ultraviolet harvesting. Next, NTBEu3+-tCNFs and pure tCNFs nanopapers were compared under or free from an ultraviolet light source (excitation at 365 nm), respectively. Compared with pure tCNFs nanopaper, NTB-Eu3+-tCNFs nanopaper exhibited a higher haze value, which was caused by increased light scattering resulting from the pored and rough nanopaper surfaces, such that the abbreviation “SUST” was not clearly viewed (Figure S8). This was due to the fact that inner cavities absorbed and refracted light, resulting in greater scattering. 9972

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instead of tCNFs. The functional nanopaper with high efficiency UV harvesting and soft-fluorescent characteristic emission of Ln3+ ions (red emission for Eu3+ and green emission for Tb3+ ions) has potential applications in solar cells, lampshades, light diffusers, UV-blocking devices, anticounterfeiting devices, and OLEDs.

excellent illustration that the NTB ligand also transferred UV energy to Tb3+ (CIE chromaticity graph, Figure S10). Hence, a mixed lanthanide complex of Tb(NTB)Cl3 and Eu(NTB)Cl3 was used to prepare a dual emission fluorescent nanopaper. This hybrid NTB-Tb3+/Eu3+-tCNFs nanopaper had excellent and diverse fluorescent properties under different excitation wavelengths, but it only exhibited characteristic Eu3+ ion emissions under 346 nm excitation. In contrast, the characteristic Tb3+ ion emission was barely detectable (Figure S11). With 324 nm excitation, the double fluorescence nanopaper emitted characteristic narrow peaks of Tb3+ and Eu3+ at 546 and 618 nm, respectively (Figure 5c). Due to highly efficient energy transfer from Tb3+ (5D4) to Eu3+ (5D0), the Tb3+ ion emission intensity was significantly less than that of the Eu3+ ion. The soft-fluorescent emissive mechanism of the NTB ligand involved it serving as an energy receiver sensitized to rare earth ions through ISC (Figure 5d). In addition, Tb(NTB)Cl3 and Eu(NTB)Cl3 powders were characterized to investigate fluorescence properties under the same conditions (Figures S12 and S13). NTB-Gd3+-tCNFs, displaying phosphorescence emission at 430 nm (τ of 7.11 μs, measured at 77 K), were chosen for comparison. The triplet (3ππ*) energy level of NTB was calculated to be 23,256 cm−1 and the singlet (1ππ*) energy level (34,482 cm−1) calculated roughly by referring to the UV−visible absorbance edge. Reinhouldt’s empirical rule38 illustrated that the energy gap ΔE (1ππ*−3ππ*) was more than 5000 cm−1, suggesting an effective ISC process. Next, the energy level matching degree between the triplet state energy state of the ligand NTB and the first excited state of Ln3+ (Eu3+/5D0, 17,826 cm−1 and Tb3+/5D4, 20,545 cm−1) was investigated. The energy gaps of the above energy states were 5430 and 2711 cm−1 for Eu3+ and Tb3+, respectively, which were suitable for improving the effective sensitization of Tb3+ and Eu3+ emissions. The decay lifetimes of Ln(NTB)Cl3 and NTB-Ln3+-tCNFs showed significant changes. Compared with the lifetimes of Ln(NTB)Cl3 before and after adding tCNFs, NTB-Tb3+-tCNFs became shorter (1502 μs → 575 μs), but NTB-Eu3+-tCNFs became longer (515 μs → 569 μs, Figure S14). This might have revealed that the energy levels of lanthanide compounds had shifted after connecting with tCNFs, and therefore, ligand NTB had different energy conversion efficiencies for Tb3+ and Eu3+. Furthermore, observing the decay lifetimes of typical emissions of Tb3+ and Eu3+ (546 and 613 nm, respectively) in NTB-Eu3+/Tb3+tCNFs, a decrease in the Tb3+ decay lifetime from 575 to 468 μs was clearly observed as well as an increase in the Eu3+ decay lifetime from 569 to 785 μs, compared with those for single lanthanide Eu3+- or Tb3+-containing nanopaper. Therefore, a possible explanation for the energy transfer process was suggested here in which, first, the NTB ligand in the nanopapers absorbed ultraviolet energy (Figure 5d). Then, the electronics transited to the singlet excited state (1ππ*) and subsequently transited to the triplet state (3ππ*) by means of ISC. The efficiency of energy transfer in NTB-Tb3+-tCNFs was more powerful than the efficiency in NTB-Eu3+-tCNFs. Using the energy transfer rate equation, kEn = 1/τq − 1/τu (τq and τu shorten lifetime of Tb3+ in NTB-Eu3+/Tb3+-tCNFs and NTBTb3+-tCNFs, respectively), the Tb → Eu energy transfer rate was estimated to be 3.6 × 105 s−1 in NTB-Eu3+/Tb3+-tCNFs nanopapers. A similar characteristic emission of Eu3+ and Tb3+ were present in lanthanide complexes, which was further confirmation that the energy mainly resulted from NTB

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CONCLUSIONS In this study, optical haze nanopapers with soft-fluorescent emission were fabricated based on a self-assembly process of lanthanide complexes and oxidized cellulose nanofibrils. A solution blending−vacuum filtration two-step approach used to produce these nanopapers was also efficient and relatively mild. After a series of characterizations, the resulting NTBLn3+-tCNFs nanopapers were found to possess excellent fluorescent properties, unique translucence, haze, and good cracking tensile strength. It was noteworthy that the observed fluorescent ability was attributed to appropriate intermolecular energy transfer via NTB’s “antenna effect.” Based on the above-mentioned points, lanthanide complexes were concluded to have been successfully linked to tCNFs substrate by self-assembly. The excellent optical haze (∼100%) and induced light-scattering behaviors in these functionalized nanopapers produced light diffusion and high efficiency UV harvesting. In addition, direct soft-fluorescent emission was present when functionalized nanopapers were triggered by a UV light-emitting diode (λex = 365 nm). Light diffusing nanopaper with soft-fluorescent emission possess potential applications in solar cells, light diffusers, UV-blocking devices, anticounterfeiting devices, and OLEDs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00970.

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Figures S1−S14. (PDF)

AUTHOR INFORMATION

Corresponding Author

* Z h a o Z h a n g : P h o n e : 8 6- 29 - 86 3 36 59 5. E- m a i l : [email protected]. ORCID

Sufeng Zhang: 0000-0002-0434-018X Zhao Zhang: 0000-0002-3688-5897 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Key scientific research plan (Key Laboratory) of Shaanxi Provincial Education Department (No. 17JS016), National Natural Science Foundation of China (No. 21703131), Doctoral Scientific Research Foundation of Shaanxi University of Science & Technology (No. 2016BJ40), State Key Laboratory of Pulp and Paper Engineering (No. 201821), Special Research Fund by Shaanxi Provincial 9973

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Department of Education (No. 18JK0122), and Chinese Postdoctoral Science Foundation (No. 2018M643707).

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