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Yb3+ nanofibers by electrospinning
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Highly temperature-sensitive and blue upconversion luminescence properties of Bi2Ti2O7:Tm3+/Yb3+ nanofibers by electrospinning ⁎
Wanyin Gea, , Meimei Xua, Jindou Shia, Jianfeng Zhua, Yongxiang Lib a School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, China b School of Engineering, RMIT University, Melbourne, VIC 3000, Australia
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Ti O :Tm /Yb nanofibers were • Bisuccessfully synthesized by electro2
2
7
3+
3+
Blue upconversion luminescence of electrospun Bi2Ti2O7:Tm3+/Yb3+ nanofibers and a highly temperaturesensing properties for the surface temperature detection.
spinning.
nanofibers possess excellent up• All conversion blue luminescence properties.
high sensitivity value of • The was achieved at 300 K. 0.024 K The surface temperature detection • properties of nanofibers were studied. −1
A R T I C LE I N FO
A B S T R A C T
Keywords: Upconversion luminescence Contactless temperature-sensing Bi2Ti2O7:Tm3+/Yb3+ Nanofibers Electrospinning
Pure pyrochlore phase Bi2Ti2O7 matrix nanofibers with various molar ratios (Tm3+/Yb3+ =1: 2–20) were synthesized by electrospinning technology. The nanofibers were about 200–300 nm in diameter and tens of micrometer in length with a rough surface. The emission bands in the range of 400–800 nm were realized and derived from the transitions of 1G4 → 3H6, 1G4 → 3F4, 3F2,3 → 3H6, and 3H4 → 3H6, respectively. The strong blue emission can be seen with the naked eye, revealing an excellent blue upconversion feature. In addition, the temperature sensing properties of Bi2Ti2O7:Tm3+/Yb3+ fibers (Tm3+/Yb3+ = 1:8) were investigated in the temperature range of 300–505 K, showing a relative high sensitivity value of 0.024 K−1 at 300 K. For comparison, both contactless methods, thermal infrared imager and fluorescence intensity ratio (FIR) techniques were used to detect the temperature of a designed object coated with Bi2Ti2O7:Tm3+/Yb3+ nanofibers. Our results indicated that the FIR technique possessed enhanced temperature-sensing properties beyond the thermal infrared imager for surface temperature detection. This work will arouse extensive interest in upconversion studies in bismuth-based oxides and pave the way for further development of contactless FIR temperaturesensing applications of upconversion materials.
⁎
Corresponding author. E-mail address: [email protected] (W. Ge).
https://doi.org/10.1016/j.cej.2019.123546 Received 23 August 2019; Received in revised form 25 October 2019; Accepted 18 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Wanyin Ge, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123546
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1. Introduction
better illustrate the practicality of our prepared nanofibers, the FIR technology and another noncontact measuring device (infrared thermal imager) were used to characterize the surface temperature of a designed object at the same time. Our results showed that the thermal infrared camera could clearly display the temperature distribution of the object, but it had obvious disadvantages in detecting the surface temperature limiting its applications in many scenarios. This work will provide an integration of blue upconversion emission and high sensitivity for noncontact thermometric sensing.
Lanthanides activated upconversion luminescence (UCL) materials have gained extensive attention own to their unique properties, such as weak autofluorescence, high penetration depth in biological tissues and high detection sensitivity [1–5]. In the view of temperature sensing, the contactless optical thermometric method using fluorescence intensity ratio (FIR) technology has attracted considerable interest due to its being noninvasive [6], more accurate [7], high sensitive etc. [8–11], which overcomes the disadvantages of conventional contact thermometry. Furthermore, Yb3+/Tm3+ sensitizer/activator ions pairs are commonly used to obtain blue upconversion luminescence and has relatively wide spectrum range which can be used in various technical fields [12,13]. For example, Yb3+/Tm3+ co-doped fluorides with temperature-sensing properties were reported by many groups [14–17]. However, fluorides suffer from structure instability at higher temperature and may cause damage to cell, restricting their temperature sensing range and applications [18,19]. In order to solve this issue, an alternative strategy is to substitute the fluorides by oxides [20], because of the outstanding chemical and structure stability of metal oxides [21,22]. In the view of chemical composition, most of the matrices reported contain rare earth elements, which are unfavorable for mass production [23]. Therefore, exploring for a non-rare-earth based upconversion host is highly demanded. It is well known that bismuth is a plentiful element with nontoxic and harmless features [24], This makes bismuth-based oxides suitable candidates for potential upconversion host materials and thermometric sensing materials. Specifically, Bi2Ti2O7 host, a kind of functional material, has widely used in catalysis, ferroelectricity and ceramic capacitors [25–27]. In addition, the Bi2Ti2O7 matrix possesses lower photon energy (710 cm−1), which is beneficial in improving the quantum efficiency of upconversion luminescence [28]. Therefore, the strong luminescence intensity is favorable for high temperature-sensing property. Nevertheless, only a few studies on the upconversion luminescence of Bi2Ti2O7 matrix based compounds have been reported, and there has been no temperature-sensing property available yet [29–32]. Our group explored the upconversion luminescence by doping lanthanides ions into Bi2Ti2O7 nanoparticles via the sol-gel method in previous work [33]. Besides, the morphologies of Bi2Ti2O7 matrix available from literatures were mainly emphasized on opal structure or films. The onedimensional (1D) structured materials are highly desirable in both theoretical and practical application due to their high surface-to-volume [34,35]. The unique applications of nanofibers were reported for drug delivery and cell imaging [36,37]. Especially, nanofibers are advantageous for improving optical properties and thus may enhance the temperature-sensing performances [38]. It is vital to explore the 1D structures of Bi2Ti2O7 host matrix with highly temperature-sensitive and upconversion luminescence properties, despite that the synthesis of Bi2Ti2O7 composition pose a serious challenge due to the existence of other impurity phases and complex preparation process [39]. In this work, we present a successful approach for obtaining pure Bi2Ti2O7 host nanofibers by an electrospinning process for the first time. The Tm3+/Yb3+ co-doped Bi2Ti2O7 host does not contain any secondary phase after annealing at 800 °C. With various molar ratios (Tm3+/Yb3+ = 1:2–20), the Bi2Ti2O7 host luminescent nanofibers with strong blue UC luminescence were realized, which can be observed by the naked eye under the excitation of 980 nm laser. In addition, taking the Bi2Ti2O7:Tm3+/Yb3+ (1:8) as an example, the intensity of the spectrum significantly responds with variation of ambient temperature. The results demonstrated that Tm3+/Yb3+ co-doped Bi2Ti2O7 nanofibers possessed excellent optical thermometry property. Thus, the Bi2Ti2O7 nanofibers host can also be used as a promising sensing material for monitoring the temperatures in vivo biological tissue applications. It is especially desirable to probe the surface temperature of the object with enhanced accuracy by such contactless method. In order to
2. Experimental 2.1. Materials Acetic acid (HAC, ≥99%), N, N-dimethylformamide (DMF, ≥99%), tetrabutyl titanate (C16H36O4Ti, TBOT, ≥99%), polyvinylpyrrolidone (PVP, 1,300,000), Ytterbium nitrate pentahydrate (Yb(NO3)3·5H2O, ≥99.9%), Thulium nitrate pentahydrate (Tm(NO3)3·5H2O, ≥99.9%), bismuth nitrate pentahydrate (Bi(NO)3·5H2O, ≥99.9%) were used as received without further purification. 2.2. Preparation of Bi2Ti2O7:Tm3+/Yb3+ nanofibers Bi2Ti2O7:Tm3+/Yb3+ nanofibers were synthesized via an electrospinning process according to our previous work with some modification [40]. The schematic diagram of the preparation process of Bi2Ti2O7:Tm3+/Yb3+ nanofibers is illustrated in Fig. S1 (Supporting information, SI). The prepared electrospinning precursor was applied with a high voltage of 25 kV. The flow rate of the liquid was controlled to be 15 μL/m and the distance between the needle tip and the collector was fixed at 12 cm. Under the application of the electric field, white filaments could be collected on the collector. Then, the collected filaments were dried in an oven (6 h) and placed in a muffle furnace for annealing to improve their crystallization. The sample was sintered at 800 °C for 1 h with a heating rate of 5 °C /min, and then cooled down to room temperature naturally. The molar ratios of Tm3+ and Yb3+ were tuned to be 1:2, 1:4, 1:8, 1:10, 1:15, 1:20 by changing the initial molar ratio of Tm3+ and Yb3+ precursors. 2.3. Characterizations The morphology and elemental composition were carried out by a high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2) equipped with an energy dispersive X-ray spectroscope (EDS, APOLLO XLT2, USA). The crystal structure of the synthesized nanofibers was evaluated by an X-ray diffraction (XRD) on the DX-2007 diffractometer (Dandong Haoyuan Instrument Co. Ltd.) with Cu-Kα1 radiation (λ = 1.5406 Å). A scanning electron microscope (SEM, Hitachi, S4800) was used to observe the surface microstructure of the samples. The upconversion emission spectra were obtained by a spectrometer with a wavelength resolution of 0.75 nm under a 980 nm emission laser (Ocean Optics, USA). The optical imaging of visible light and in-situ upconversion fluorescence imaging were investigated by an imaging integrated spectroscopy system (Nikon, Japan). A temperature control system was used to monitor the working temperature of Bi2Ti2O7:Tm3+/Yb3+ nanofibers with an accuracy of ± 0.1 °C. A thermal infrared imager was used to detect temperature and record thermal images (FOTRIC, USA). 3. Results and discussion The morphology and elemental composition of a typical Bi2Ti2O7:Tm3+/Yb3+ sample were characterized by TEM (Tm3+/ Yb3+ = 1:8) in Fig. 1. The TEM images confirm the fibrous structure feature of the sample in Fig. 1a. The fibers are made up of small nanoparticles. The HRTEM image (Fig. 1b) shows the distinct diffraction 2
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Fig. 1. (a) TEM and (b) HRTEM images of the Bi2Ti2O7:Tm3+/Yb3+ sample (Tm3+/Yb3+ = 1:8) and (c) the corresponding elemental mapping.
fringe, indicating a good crystallinity of the calcined Bi2Ti2O7:Tm3+/ Yb3+ nanofibers. The measured d-spacing of 0.60 nm matches well with the (1 1 1) planes of cubic pyrochlore structure. Fig. 1c presents a STEM images of a typical nanofiber, combining with the elemental mapping. The EDS elemental mapping suggests that the five elements (Bi, Ti, O, Tm and Yb) distributed uniformly in the Bi2Ti2O7:Tm3+/Yb3+ nanofiber. It is worth mentioning that there are no chemical compositional fluctuations even if the Yb and Tm elements are trace amounts. These results revealed that the electrospinning route was versatile for the trace doping and Bi2Ti2O7:Tm3+/Yb3+ nanofibers fabrication as well. Fig. 2a–d depict the SEM images of electrospun Bi2Ti2O7:Tm3+/ 3+ Yb nanofibers (Tm3+/Yb3+ = 1:8) unannealed and annealed samples. These nanofibers are about 200–300 nm in diameter and tens of micrometer in length. In particular, the surface of the as-prepared nanofibers are smooth (Fig. 2a and b), then, they become slightly rough with some pores after annealing process (Fig. 2c and d), which can be assigned to the volatilization/carbonization of PVP binder during the annealing at high temperature. These results showed that the nanofibers with a high aspect ratio were successfully synthesized via the electrospinning route. A formation mechanism of Bi2Ti2O7:Tm3+/Yb3+ nanofibers fabrication is proposed, as depicted in Fig. 2e. In a typical nanofiber growth process, the precursor Bi(NO3)3, Tm(NO3)3, Yb(NO3)3 and Ti(C4H9O)4 are added sequentially to the mixed solution of HAC and DMF. HAC can produce an acidic environment to inhibit the hydrolysis of Ti4+. Since DMF is a good solvent, the precursors were mixed up to form a uniform transparent solution after agitation. Subsequently, PVP was added to provide appropriate viscosity due to its feature of long molecular chains. After a vigorous stirring, a transparent electrospinning solution was obtained, revealing the uniform elemental distribution. The solution for the electrospinning process was placed in an electrostatic field. The droplet at the tip of the needle is deformed by the electric force. Taylor cone can be formed at the tip of the needle when the electric force overcomes the surface tension of the droplet. Droplet jet is generated under the optimized electrospinning parameters. Simultaneously, the white filaments are collected on the collector. Finally, the obtained product is annealed at 800 °C for 1 h in the air. The diameters of obtained fibers shrink slightly and the surface became rough with many pores. This phenomenon can be ascribed to the decomposition of PVP, HAC, DMF, NO3− and crystallization of Bi2Ti2O7:Tm3+/Yb3+. The annealed samples have strong blue luminescence characteristics
Fig. 2. (a, b) and (c, d) present the SEM images of the unannealed and annealed Bi2Ti2O7: Tm3+/Yb3+ nanofibers (Tm3+/Yb3+ = 1:8), respectively. (e) Proposed formation mechanism of Bi2Ti2O7:Tm3+/Yb3+ upconversion nanofibers.
under the excitation of 980 nm laser. The XRD patterns of the sample Bi2Ti2O7:Tm3+/Yb3+(Tm3+/ 3+ Yb = 1:8) annealed at various temperatures (25–900 °C) are presented in Fig. S2a. It is evident that the unannealed sample is amorphous, and the crystallinity enhance after annealing process. Accordingly, the UCL intensity increases with the increasing of annealing temperature (Fig. S2b). However, the luminescence intensity of the nanofibers annealed at 800 °C is close to that of the sample annealed at 900 °C. Thus, the optimal annealing temperature was identified to be 800 °C. In addition, we analyzed the crystal structure and crystallinity of all the samples with different doping concentrations by powder XRD. It can be seen from Fig. 3a that all samples are pure phase with good crystallinity. In particular, for pure Bi2Ti2O7 nanofibers, its diffraction peaks can be indexed well with the standard card (PDF: 97-016-1101), suggesting that the nanofibers belong to the cubic pyrochlore structure and have a high degree of crystallinity. A shift of 2θ toward larger angle can be seen clearly for the (4 0 0) peak as shown in Fig. 3b. This phenomenon is ascribed to the lattice contraction due to the smaller ionic radius of Yb3+ and Tm3+ than that of Bi3+ according to Bragg's Law [41]. 3.1. Upconversion luminescence properties (UCL) The upconversion luminescence properties (UCL) of the prepared samples were studied under the excitation of 980 nm laser as shown in Fig. 4a (Bi2Ti2O7:Tm3+/Yb3+ = 1:2, 1:4, 1:8, 1:10, 1:15, 1:20). The spectra of different samples were measured while maintaining the same 3
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non-radiative. Then, the Tm3+ ions located at the 3F4 state are excited to the 3F2,3 level by absorbing other photons and resulting in the red emission at 700 nm. Alternatively, the Yb3+ ion can absorb two infrared photons subsequently and the Tm3+ ions can be pumped to 3F2,3 level (ET2). The third channel (ET3) is that the Yb3+ ion absorbs three infrared photons and then the Tm3+ ion can be excited to the 1G4 level. Part of the ions at the 1G4 level return to the ground state by releasing energy in the form of blue light (483 nm) and some relax to 3H4 level non-radiatively (797 nm), whereas the other relax radiatively to the 3F4 level and emit weak red light (653 nm). 3.2. Optical temperature sensing
Fig. 3. XRD patterns of (a) pure Bi2Ti2O7 and Bi2Ti2O7:Tm3+/Yb3+ fibers (Tm3+/Yb3+ = 1:2, 1:4, 1:8, 1:10, 1:15, 1:20); (b) the magnified patterns of (4 0 0) peak.
The upconversion spectra of the Bi2Ti2O7:Tm3+/Yb3+ (Tm3+/ Yb = 1:8) sample were collected in the temperatures range of 300–505 K under excitation of 980 nm laser with a minor pump power of 60 mW, as shown in Fig. 5a. The position of the two bands (700 nm, 797 nm) do not show any shift during the whole range of temperature. The emission intensity of 797 nm ascribed to the 3H4 level decreases with the increase of temperature, while the other emission band assigned to 3F2,3 level increases correspondingly. The phenomenon can be seen clearly in Fig. 5b. As it is all known, the 3H4 and 3F2,3 levels are thermal coupled levels and many groups studied its temperature sensing according to the FIR technique [44–46]. The value of LnFIR decreased gradually with the increase of temperature and the experimental data can be fitted by the followed formula [47]: 3+
experimental conditions and an infrared power of 100 mW. It can be observed that the spectra of all samples are similar in the range of doping concentrations of this study, indicating the doping concentration does not affect the luminescence output characteristics. It is noteworthy that all samples emitted intense blue light that can be seen by the naked eye. In addition, the blue and red emission intensities of the nanofibers is the strongest when the doping ratio is 1:8. Fig. 4b shows a typical upconversion spectrum of the Bi2Ti2O7:Tm3+/Yb3+ sample at various exciting powers. The emission intensity (I) increases with increasing pump power (P) and the relationship between them can be described as [42]:
I ∝ Pn
FIR =
where n is the number of low-energy photons to achieve UC process. All the samples show two distinguishable bands during the test wavelength range and correspond to the transition 1G4 → 3H6 and 1G4 → 3F4. A plot of Ln(I) versus Ln(P) shows a straight line with slope n as displayed in Fig. 4c. The slopes for emission position at 483 nm, 653 nm are 2.42 and 1.97. The results indicated that the blue and red emission were three and two-photon processes, respectively [43]. Fig. 4d illustrates the possible upconversion luminescence mechanism of an Yb3+/ Tm3+ system. The Yb3+ ions are excited from 2F7/2 → 2F5/2 by absorbing an infrared photon and then having three channels to achieve energy transfer (ET) to Tm3+ ions. The 3H5 level can be populated by the first channel (ET1) and part of these Tm3+ ions reach 3F4 level via
IH = Aexp( −ΔE/ k T) IF
(1)
where IH and IF represent the H4 and F2,3 levels, respectively. ΔE is the energy gap between the thermal coupled levels, k is the Boltzmann constant, T and A refer to absolute temperature and constant. The slope and intercept of the fitting result are 2170.5 and 1.88 (Fig. 5(c)). Furthermore, the result is useful for studying the relative sensitivity (Sr) which is a significant parameter to value the temperature sensing performance. The relationship between FIR and relative sensitivity can be defined as the formula as follows [48]: 3
Sr =
1 dFIR FIR dr
3
(2) Fig. 4. (a) UC spectra of Bi2Ti2O7:Tm3+/Yb3+ samples with different molar ratio, and (b) emission intensity dependence of the pump power density under excitation of 980 nm (Tm3+/Yb3+ = 1:8); (c) Ln-Ln plots of upconversion luminescence intensity of Bi2Ti2O7:Tm3+/Yb3+ sample versus various excitation powers; (d) the energy level diagram of Yb3+, Tm3+ and the possible upconversion luminescence mechanism.
4
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Fig. 5. (a) Temperature dependent spectra for Bi2Ti2O7:Tm3+/Yb3+(Tm3+/Yb3+ = 1:8) samples; (b) luminescence intensity of the two thermal coupled levels (3H4 ,797 nm; 3F2,3,700 nm) at various temperature and (c) the logarithm of FIR (LnFIR) as a function of inverse absolute temperature (1/T); (d) dependence of relative sensitive (Sr) on absolute temperature presents; (e) the relationship between calculated temperature and actual experimental temperature; (f) the value of FIR as a function of cycle number. Table 1 Method, transitions, temperature range and Sr of other upconversion materials. Samples
Method
Transitions
Temperature Range
SiO2@Yb3+, Tm3+/GZO Yb3+, Tm3+/Y2O3 Yb3+, Er3+, Tm3+, Ho3+/LiY9(SiO4)6O2 Yb3+, Er3+/NaY(WO4)2 Yb3+, Tm3, Ca2+/YF3 Yb3+, Er3+, Tm3+, Ho3+/K3Y(PO4) Yb3+, Tm3+/Y2Ti2O7 Yb3+, Tm3+/Bi2Ti2O7
Hydrothermal-stoken Solution combustion Co-precipitation Thermolysis-protocol Hydrothermal Solid-state reaction High-temperature solid state reaction Electrospinning
3
F2/3,3H4 → 3H6 F2/3,1G4(b) → 3H6 3 F2,3 → 3H6 (Tm3+) 2 H11/2, 4S3/2 → 4I15/2 3 F2, 1G4 → 3H6 3 F2/3, 3H4 → 3H6 (Tm3+) 1 G4(a), 3F2,3 → 3F4, 3H6 3 F3, 3H4 → 3H6
293 303 298 293 300 293 293 300
3
→ → → → → → → →
693 573 548 503 773 553 398 505
Sr(K−1)
Ref.
0.062 (693 K) 0.015 (445 K) 0.023 (298 K) or 0.013 (293 K) or 0.0084 (300 K) 0.022 (293 K) or 0.0081 (398 K) 0.024 (300 K) or
[49] [46] [7] [50] [51] [44] [52] This work
2046.9/T2 1127/T2 1910.1/T2 2170.5/T2
images recorded by the camera during the measurement process. The temperature distribution of the detection region coated Bi2Ti2O7:Yb3+/ Tm3+ nanofiner material can be observed clearly from the images. However, for the calibrated surface temperature, there is a large deviation of temperature obtained by the infrared thermal camera and the FIR technique, as shown in Fig. 6c, indicating that the infrared camera possesses deficiencies in the surface temperature detection of the object. It also further illustrates that the FIR technique possesses a better reliability and broad application prospects in surface temperature detections.
The corresponding curve of sensitivity verse absolute temperature is shown in Fig. 5d. The calculated FIR keeps on decreasing with the rise of temperature in the range of 300–505 K, obtaining its maximum value of 0.024 K−1 at 300 K. For comparison, some key parameters of other optical families are listed in Table 1. It is worth noting that the maximum Sr value of our Bi2Ti2O7:Yb3+/Tm3+ nanofiber sample is bigger than most reported materials. Fig. 5e illustrates the relationship between calculated temperature and actual experimental temperature, in which the calculated temperature was obtained according to Eq. (1) and the experimental temperature was read from thermocouples directly. The solid line represents calculated temperature from the FIR of our nanofibers. It can be observed that the calculated temperature matches well with the experimental temperature, indicating the FIR technique is reliable and possesses good accuracy. In addition, the FIR of I797/I700 possesses excellent repeatability after several cycles, as shown in Fig. 5f, making it suitable for temperature measurement. As well known, the infrared thermal camera is also a non-contact temperature detecting device. Here, we compare these two non-contact temperature measuring techniques as shown in Fig. 6a. The temperature of the heating surface was measured using the FIR technology and infrared thermal camera at the same time. Fig. 6b shows the thermal
4. Conclusions The Bi2Ti2O7:Tm3+/Yb3+ nanofibers with blue upconversion luminescence properties were successfully fabricated by an electrospinning process. The blue light can be seen by the naked eye even at 60 mW of the pump power under the excitation of 980 nm laser. The results showed that the strongest luminescence intensity was obtained for the sample with the doping ratio 1:8 of Tm3+ to Yb3+. The possible upconversion luminescence mechanism of Bi2Ti2O7:Tm3+/Yb3+ nanofibers were proposed based on its energy level diagram. By microscopic 5
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Fig. 6. (a) Schematic diagram of the measurement process; (b) infrared images of the samples under heating condition recorded by infrared thermal camera; (c) the comparison between detected and calculated temperature.
imaging of the samples under a near-infrared laser, the samples possessed excellent blue imaging characteristics. The sample showed outstanding optical sensing property in the range of 300–505 K and the maximum value of Sr was 0.024 K−1 at 300 K. The Bi2Ti2O7:Tm3+/ Yb3+ sample based on the FIR technique possessed evident advantages beyond the thermal infrared imager for the surface temperature detection. The results will promote not only the study of excellent upconversion in new matrix family materials, but also the applications of the new material in bioimaging and optical temperature sensing fields.
[7]
[8]
[9]
[10]
Declaration of Competing Interest [11]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
[12]
Acknowledgments [13]
This work was supported by Research Initiation Funds for the Academic Leaders from Shaanxi University of Science and Technology (2016XSGG03).
[14]
[15]
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123546.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Highly temperature-sensitive and blue upconversion luminescence properties of Bi2Ti2O7:Tm3+/Yb3+ nanofibers by electrospinning ⁎
Wanyin Gea, , Meimei Xua, Jindou Shia, Jianfeng Zhua, Yongxiang Lib a School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, China b School of Engineering, RMIT University, Melbourne, VIC 3000, Australia
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Ti O :Tm /Yb nanofibers were • Bisuccessfully synthesized by electro2
2
7
3+
3+
Blue upconversion luminescence of electrospun Bi2Ti2O7:Tm3+/Yb3+ nanofibers and a highly temperaturesensing properties for the surface temperature detection.
spinning.
nanofibers possess excellent up• All conversion blue luminescence properties.
high sensitivity value of • The was achieved at 300 K. 0.024 K The surface temperature detection • properties of nanofibers were studied. −1
A R T I C LE I N FO
A B S T R A C T
Keywords: Upconversion luminescence Contactless temperature-sensing Bi2Ti2O7:Tm3+/Yb3+ Nanofibers Electrospinning
Pure pyrochlore phase Bi2Ti2O7 matrix nanofibers with various molar ratios (Tm3+/Yb3+ =1: 2–20) were synthesized by electrospinning technology. The nanofibers were about 200–300 nm in diameter and tens of micrometer in length with a rough surface. The emission bands in the range of 400–800 nm were realized and derived from the transitions of 1G4 → 3H6, 1G4 → 3F4, 3F2,3 → 3H6, and 3H4 → 3H6, respectively. The strong blue emission can be seen with the naked eye, revealing an excellent blue upconversion feature. In addition, the temperature sensing properties of Bi2Ti2O7:Tm3+/Yb3+ fibers (Tm3+/Yb3+ = 1:8) were investigated in the temperature range of 300–505 K, showing a relative high sensitivity value of 0.024 K−1 at 300 K. For comparison, both contactless methods, thermal infrared imager and fluorescence intensity ratio (FIR) techniques were used to detect the temperature of a designed object coated with Bi2Ti2O7:Tm3+/Yb3+ nanofibers. Our results indicated that the FIR technique possessed enhanced temperature-sensing properties beyond the thermal infrared imager for surface temperature detection. This work will arouse extensive interest in upconversion studies in bismuth-based oxides and pave the way for further development of contactless FIR temperaturesensing applications of upconversion materials.
⁎
Corresponding author. E-mail address: [email protected] (W. Ge).
https://doi.org/10.1016/j.cej.2019.123546 Received 23 August 2019; Received in revised form 25 October 2019; Accepted 18 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Wanyin Ge, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123546
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1. Introduction
better illustrate the practicality of our prepared nanofibers, the FIR technology and another noncontact measuring device (infrared thermal imager) were used to characterize the surface temperature of a designed object at the same time. Our results showed that the thermal infrared camera could clearly display the temperature distribution of the object, but it had obvious disadvantages in detecting the surface temperature limiting its applications in many scenarios. This work will provide an integration of blue upconversion emission and high sensitivity for noncontact thermometric sensing.
Lanthanides activated upconversion luminescence (UCL) materials have gained extensive attention own to their unique properties, such as weak autofluorescence, high penetration depth in biological tissues and high detection sensitivity [1–5]. In the view of temperature sensing, the contactless optical thermometric method using fluorescence intensity ratio (FIR) technology has attracted considerable interest due to its being noninvasive [6], more accurate [7], high sensitive etc. [8–11], which overcomes the disadvantages of conventional contact thermometry. Furthermore, Yb3+/Tm3+ sensitizer/activator ions pairs are commonly used to obtain blue upconversion luminescence and has relatively wide spectrum range which can be used in various technical fields [12,13]. For example, Yb3+/Tm3+ co-doped fluorides with temperature-sensing properties were reported by many groups [14–17]. However, fluorides suffer from structure instability at higher temperature and may cause damage to cell, restricting their temperature sensing range and applications [18,19]. In order to solve this issue, an alternative strategy is to substitute the fluorides by oxides [20], because of the outstanding chemical and structure stability of metal oxides [21,22]. In the view of chemical composition, most of the matrices reported contain rare earth elements, which are unfavorable for mass production [23]. Therefore, exploring for a non-rare-earth based upconversion host is highly demanded. It is well known that bismuth is a plentiful element with nontoxic and harmless features [24], This makes bismuth-based oxides suitable candidates for potential upconversion host materials and thermometric sensing materials. Specifically, Bi2Ti2O7 host, a kind of functional material, has widely used in catalysis, ferroelectricity and ceramic capacitors [25–27]. In addition, the Bi2Ti2O7 matrix possesses lower photon energy (710 cm−1), which is beneficial in improving the quantum efficiency of upconversion luminescence [28]. Therefore, the strong luminescence intensity is favorable for high temperature-sensing property. Nevertheless, only a few studies on the upconversion luminescence of Bi2Ti2O7 matrix based compounds have been reported, and there has been no temperature-sensing property available yet [29–32]. Our group explored the upconversion luminescence by doping lanthanides ions into Bi2Ti2O7 nanoparticles via the sol-gel method in previous work [33]. Besides, the morphologies of Bi2Ti2O7 matrix available from literatures were mainly emphasized on opal structure or films. The onedimensional (1D) structured materials are highly desirable in both theoretical and practical application due to their high surface-to-volume [34,35]. The unique applications of nanofibers were reported for drug delivery and cell imaging [36,37]. Especially, nanofibers are advantageous for improving optical properties and thus may enhance the temperature-sensing performances [38]. It is vital to explore the 1D structures of Bi2Ti2O7 host matrix with highly temperature-sensitive and upconversion luminescence properties, despite that the synthesis of Bi2Ti2O7 composition pose a serious challenge due to the existence of other impurity phases and complex preparation process [39]. In this work, we present a successful approach for obtaining pure Bi2Ti2O7 host nanofibers by an electrospinning process for the first time. The Tm3+/Yb3+ co-doped Bi2Ti2O7 host does not contain any secondary phase after annealing at 800 °C. With various molar ratios (Tm3+/Yb3+ = 1:2–20), the Bi2Ti2O7 host luminescent nanofibers with strong blue UC luminescence were realized, which can be observed by the naked eye under the excitation of 980 nm laser. In addition, taking the Bi2Ti2O7:Tm3+/Yb3+ (1:8) as an example, the intensity of the spectrum significantly responds with variation of ambient temperature. The results demonstrated that Tm3+/Yb3+ co-doped Bi2Ti2O7 nanofibers possessed excellent optical thermometry property. Thus, the Bi2Ti2O7 nanofibers host can also be used as a promising sensing material for monitoring the temperatures in vivo biological tissue applications. It is especially desirable to probe the surface temperature of the object with enhanced accuracy by such contactless method. In order to
2. Experimental 2.1. Materials Acetic acid (HAC, ≥99%), N, N-dimethylformamide (DMF, ≥99%), tetrabutyl titanate (C16H36O4Ti, TBOT, ≥99%), polyvinylpyrrolidone (PVP, 1,300,000), Ytterbium nitrate pentahydrate (Yb(NO3)3·5H2O, ≥99.9%), Thulium nitrate pentahydrate (Tm(NO3)3·5H2O, ≥99.9%), bismuth nitrate pentahydrate (Bi(NO)3·5H2O, ≥99.9%) were used as received without further purification. 2.2. Preparation of Bi2Ti2O7:Tm3+/Yb3+ nanofibers Bi2Ti2O7:Tm3+/Yb3+ nanofibers were synthesized via an electrospinning process according to our previous work with some modification [40]. The schematic diagram of the preparation process of Bi2Ti2O7:Tm3+/Yb3+ nanofibers is illustrated in Fig. S1 (Supporting information, SI). The prepared electrospinning precursor was applied with a high voltage of 25 kV. The flow rate of the liquid was controlled to be 15 μL/m and the distance between the needle tip and the collector was fixed at 12 cm. Under the application of the electric field, white filaments could be collected on the collector. Then, the collected filaments were dried in an oven (6 h) and placed in a muffle furnace for annealing to improve their crystallization. The sample was sintered at 800 °C for 1 h with a heating rate of 5 °C /min, and then cooled down to room temperature naturally. The molar ratios of Tm3+ and Yb3+ were tuned to be 1:2, 1:4, 1:8, 1:10, 1:15, 1:20 by changing the initial molar ratio of Tm3+ and Yb3+ precursors. 2.3. Characterizations The morphology and elemental composition were carried out by a high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2) equipped with an energy dispersive X-ray spectroscope (EDS, APOLLO XLT2, USA). The crystal structure of the synthesized nanofibers was evaluated by an X-ray diffraction (XRD) on the DX-2007 diffractometer (Dandong Haoyuan Instrument Co. Ltd.) with Cu-Kα1 radiation (λ = 1.5406 Å). A scanning electron microscope (SEM, Hitachi, S4800) was used to observe the surface microstructure of the samples. The upconversion emission spectra were obtained by a spectrometer with a wavelength resolution of 0.75 nm under a 980 nm emission laser (Ocean Optics, USA). The optical imaging of visible light and in-situ upconversion fluorescence imaging were investigated by an imaging integrated spectroscopy system (Nikon, Japan). A temperature control system was used to monitor the working temperature of Bi2Ti2O7:Tm3+/Yb3+ nanofibers with an accuracy of ± 0.1 °C. A thermal infrared imager was used to detect temperature and record thermal images (FOTRIC, USA). 3. Results and discussion The morphology and elemental composition of a typical Bi2Ti2O7:Tm3+/Yb3+ sample were characterized by TEM (Tm3+/ Yb3+ = 1:8) in Fig. 1. The TEM images confirm the fibrous structure feature of the sample in Fig. 1a. The fibers are made up of small nanoparticles. The HRTEM image (Fig. 1b) shows the distinct diffraction 2
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Fig. 1. (a) TEM and (b) HRTEM images of the Bi2Ti2O7:Tm3+/Yb3+ sample (Tm3+/Yb3+ = 1:8) and (c) the corresponding elemental mapping.
fringe, indicating a good crystallinity of the calcined Bi2Ti2O7:Tm3+/ Yb3+ nanofibers. The measured d-spacing of 0.60 nm matches well with the (1 1 1) planes of cubic pyrochlore structure. Fig. 1c presents a STEM images of a typical nanofiber, combining with the elemental mapping. The EDS elemental mapping suggests that the five elements (Bi, Ti, O, Tm and Yb) distributed uniformly in the Bi2Ti2O7:Tm3+/Yb3+ nanofiber. It is worth mentioning that there are no chemical compositional fluctuations even if the Yb and Tm elements are trace amounts. These results revealed that the electrospinning route was versatile for the trace doping and Bi2Ti2O7:Tm3+/Yb3+ nanofibers fabrication as well. Fig. 2a–d depict the SEM images of electrospun Bi2Ti2O7:Tm3+/ 3+ Yb nanofibers (Tm3+/Yb3+ = 1:8) unannealed and annealed samples. These nanofibers are about 200–300 nm in diameter and tens of micrometer in length. In particular, the surface of the as-prepared nanofibers are smooth (Fig. 2a and b), then, they become slightly rough with some pores after annealing process (Fig. 2c and d), which can be assigned to the volatilization/carbonization of PVP binder during the annealing at high temperature. These results showed that the nanofibers with a high aspect ratio were successfully synthesized via the electrospinning route. A formation mechanism of Bi2Ti2O7:Tm3+/Yb3+ nanofibers fabrication is proposed, as depicted in Fig. 2e. In a typical nanofiber growth process, the precursor Bi(NO3)3, Tm(NO3)3, Yb(NO3)3 and Ti(C4H9O)4 are added sequentially to the mixed solution of HAC and DMF. HAC can produce an acidic environment to inhibit the hydrolysis of Ti4+. Since DMF is a good solvent, the precursors were mixed up to form a uniform transparent solution after agitation. Subsequently, PVP was added to provide appropriate viscosity due to its feature of long molecular chains. After a vigorous stirring, a transparent electrospinning solution was obtained, revealing the uniform elemental distribution. The solution for the electrospinning process was placed in an electrostatic field. The droplet at the tip of the needle is deformed by the electric force. Taylor cone can be formed at the tip of the needle when the electric force overcomes the surface tension of the droplet. Droplet jet is generated under the optimized electrospinning parameters. Simultaneously, the white filaments are collected on the collector. Finally, the obtained product is annealed at 800 °C for 1 h in the air. The diameters of obtained fibers shrink slightly and the surface became rough with many pores. This phenomenon can be ascribed to the decomposition of PVP, HAC, DMF, NO3− and crystallization of Bi2Ti2O7:Tm3+/Yb3+. The annealed samples have strong blue luminescence characteristics
Fig. 2. (a, b) and (c, d) present the SEM images of the unannealed and annealed Bi2Ti2O7: Tm3+/Yb3+ nanofibers (Tm3+/Yb3+ = 1:8), respectively. (e) Proposed formation mechanism of Bi2Ti2O7:Tm3+/Yb3+ upconversion nanofibers.
under the excitation of 980 nm laser. The XRD patterns of the sample Bi2Ti2O7:Tm3+/Yb3+(Tm3+/ 3+ Yb = 1:8) annealed at various temperatures (25–900 °C) are presented in Fig. S2a. It is evident that the unannealed sample is amorphous, and the crystallinity enhance after annealing process. Accordingly, the UCL intensity increases with the increasing of annealing temperature (Fig. S2b). However, the luminescence intensity of the nanofibers annealed at 800 °C is close to that of the sample annealed at 900 °C. Thus, the optimal annealing temperature was identified to be 800 °C. In addition, we analyzed the crystal structure and crystallinity of all the samples with different doping concentrations by powder XRD. It can be seen from Fig. 3a that all samples are pure phase with good crystallinity. In particular, for pure Bi2Ti2O7 nanofibers, its diffraction peaks can be indexed well with the standard card (PDF: 97-016-1101), suggesting that the nanofibers belong to the cubic pyrochlore structure and have a high degree of crystallinity. A shift of 2θ toward larger angle can be seen clearly for the (4 0 0) peak as shown in Fig. 3b. This phenomenon is ascribed to the lattice contraction due to the smaller ionic radius of Yb3+ and Tm3+ than that of Bi3+ according to Bragg's Law [41]. 3.1. Upconversion luminescence properties (UCL) The upconversion luminescence properties (UCL) of the prepared samples were studied under the excitation of 980 nm laser as shown in Fig. 4a (Bi2Ti2O7:Tm3+/Yb3+ = 1:2, 1:4, 1:8, 1:10, 1:15, 1:20). The spectra of different samples were measured while maintaining the same 3
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non-radiative. Then, the Tm3+ ions located at the 3F4 state are excited to the 3F2,3 level by absorbing other photons and resulting in the red emission at 700 nm. Alternatively, the Yb3+ ion can absorb two infrared photons subsequently and the Tm3+ ions can be pumped to 3F2,3 level (ET2). The third channel (ET3) is that the Yb3+ ion absorbs three infrared photons and then the Tm3+ ion can be excited to the 1G4 level. Part of the ions at the 1G4 level return to the ground state by releasing energy in the form of blue light (483 nm) and some relax to 3H4 level non-radiatively (797 nm), whereas the other relax radiatively to the 3F4 level and emit weak red light (653 nm). 3.2. Optical temperature sensing
Fig. 3. XRD patterns of (a) pure Bi2Ti2O7 and Bi2Ti2O7:Tm3+/Yb3+ fibers (Tm3+/Yb3+ = 1:2, 1:4, 1:8, 1:10, 1:15, 1:20); (b) the magnified patterns of (4 0 0) peak.
The upconversion spectra of the Bi2Ti2O7:Tm3+/Yb3+ (Tm3+/ Yb = 1:8) sample were collected in the temperatures range of 300–505 K under excitation of 980 nm laser with a minor pump power of 60 mW, as shown in Fig. 5a. The position of the two bands (700 nm, 797 nm) do not show any shift during the whole range of temperature. The emission intensity of 797 nm ascribed to the 3H4 level decreases with the increase of temperature, while the other emission band assigned to 3F2,3 level increases correspondingly. The phenomenon can be seen clearly in Fig. 5b. As it is all known, the 3H4 and 3F2,3 levels are thermal coupled levels and many groups studied its temperature sensing according to the FIR technique [44–46]. The value of LnFIR decreased gradually with the increase of temperature and the experimental data can be fitted by the followed formula [47]: 3+
experimental conditions and an infrared power of 100 mW. It can be observed that the spectra of all samples are similar in the range of doping concentrations of this study, indicating the doping concentration does not affect the luminescence output characteristics. It is noteworthy that all samples emitted intense blue light that can be seen by the naked eye. In addition, the blue and red emission intensities of the nanofibers is the strongest when the doping ratio is 1:8. Fig. 4b shows a typical upconversion spectrum of the Bi2Ti2O7:Tm3+/Yb3+ sample at various exciting powers. The emission intensity (I) increases with increasing pump power (P) and the relationship between them can be described as [42]:
I ∝ Pn
FIR =
where n is the number of low-energy photons to achieve UC process. All the samples show two distinguishable bands during the test wavelength range and correspond to the transition 1G4 → 3H6 and 1G4 → 3F4. A plot of Ln(I) versus Ln(P) shows a straight line with slope n as displayed in Fig. 4c. The slopes for emission position at 483 nm, 653 nm are 2.42 and 1.97. The results indicated that the blue and red emission were three and two-photon processes, respectively [43]. Fig. 4d illustrates the possible upconversion luminescence mechanism of an Yb3+/ Tm3+ system. The Yb3+ ions are excited from 2F7/2 → 2F5/2 by absorbing an infrared photon and then having three channels to achieve energy transfer (ET) to Tm3+ ions. The 3H5 level can be populated by the first channel (ET1) and part of these Tm3+ ions reach 3F4 level via
IH = Aexp( −ΔE/ k T) IF
(1)
where IH and IF represent the H4 and F2,3 levels, respectively. ΔE is the energy gap between the thermal coupled levels, k is the Boltzmann constant, T and A refer to absolute temperature and constant. The slope and intercept of the fitting result are 2170.5 and 1.88 (Fig. 5(c)). Furthermore, the result is useful for studying the relative sensitivity (Sr) which is a significant parameter to value the temperature sensing performance. The relationship between FIR and relative sensitivity can be defined as the formula as follows [48]: 3
Sr =
1 dFIR FIR dr
3
(2) Fig. 4. (a) UC spectra of Bi2Ti2O7:Tm3+/Yb3+ samples with different molar ratio, and (b) emission intensity dependence of the pump power density under excitation of 980 nm (Tm3+/Yb3+ = 1:8); (c) Ln-Ln plots of upconversion luminescence intensity of Bi2Ti2O7:Tm3+/Yb3+ sample versus various excitation powers; (d) the energy level diagram of Yb3+, Tm3+ and the possible upconversion luminescence mechanism.
4
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Fig. 5. (a) Temperature dependent spectra for Bi2Ti2O7:Tm3+/Yb3+(Tm3+/Yb3+ = 1:8) samples; (b) luminescence intensity of the two thermal coupled levels (3H4 ,797 nm; 3F2,3,700 nm) at various temperature and (c) the logarithm of FIR (LnFIR) as a function of inverse absolute temperature (1/T); (d) dependence of relative sensitive (Sr) on absolute temperature presents; (e) the relationship between calculated temperature and actual experimental temperature; (f) the value of FIR as a function of cycle number. Table 1 Method, transitions, temperature range and Sr of other upconversion materials. Samples
Method
Transitions
Temperature Range
SiO2@Yb3+, Tm3+/GZO Yb3+, Tm3+/Y2O3 Yb3+, Er3+, Tm3+, Ho3+/LiY9(SiO4)6O2 Yb3+, Er3+/NaY(WO4)2 Yb3+, Tm3, Ca2+/YF3 Yb3+, Er3+, Tm3+, Ho3+/K3Y(PO4) Yb3+, Tm3+/Y2Ti2O7 Yb3+, Tm3+/Bi2Ti2O7
Hydrothermal-stoken Solution combustion Co-precipitation Thermolysis-protocol Hydrothermal Solid-state reaction High-temperature solid state reaction Electrospinning
3
F2/3,3H4 → 3H6 F2/3,1G4(b) → 3H6 3 F2,3 → 3H6 (Tm3+) 2 H11/2, 4S3/2 → 4I15/2 3 F2, 1G4 → 3H6 3 F2/3, 3H4 → 3H6 (Tm3+) 1 G4(a), 3F2,3 → 3F4, 3H6 3 F3, 3H4 → 3H6
293 303 298 293 300 293 293 300
3
→ → → → → → → →
693 573 548 503 773 553 398 505
Sr(K−1)
Ref.
0.062 (693 K) 0.015 (445 K) 0.023 (298 K) or 0.013 (293 K) or 0.0084 (300 K) 0.022 (293 K) or 0.0081 (398 K) 0.024 (300 K) or
[49] [46] [7] [50] [51] [44] [52] This work
2046.9/T2 1127/T2 1910.1/T2 2170.5/T2
images recorded by the camera during the measurement process. The temperature distribution of the detection region coated Bi2Ti2O7:Yb3+/ Tm3+ nanofiner material can be observed clearly from the images. However, for the calibrated surface temperature, there is a large deviation of temperature obtained by the infrared thermal camera and the FIR technique, as shown in Fig. 6c, indicating that the infrared camera possesses deficiencies in the surface temperature detection of the object. It also further illustrates that the FIR technique possesses a better reliability and broad application prospects in surface temperature detections.
The corresponding curve of sensitivity verse absolute temperature is shown in Fig. 5d. The calculated FIR keeps on decreasing with the rise of temperature in the range of 300–505 K, obtaining its maximum value of 0.024 K−1 at 300 K. For comparison, some key parameters of other optical families are listed in Table 1. It is worth noting that the maximum Sr value of our Bi2Ti2O7:Yb3+/Tm3+ nanofiber sample is bigger than most reported materials. Fig. 5e illustrates the relationship between calculated temperature and actual experimental temperature, in which the calculated temperature was obtained according to Eq. (1) and the experimental temperature was read from thermocouples directly. The solid line represents calculated temperature from the FIR of our nanofibers. It can be observed that the calculated temperature matches well with the experimental temperature, indicating the FIR technique is reliable and possesses good accuracy. In addition, the FIR of I797/I700 possesses excellent repeatability after several cycles, as shown in Fig. 5f, making it suitable for temperature measurement. As well known, the infrared thermal camera is also a non-contact temperature detecting device. Here, we compare these two non-contact temperature measuring techniques as shown in Fig. 6a. The temperature of the heating surface was measured using the FIR technology and infrared thermal camera at the same time. Fig. 6b shows the thermal
4. Conclusions The Bi2Ti2O7:Tm3+/Yb3+ nanofibers with blue upconversion luminescence properties were successfully fabricated by an electrospinning process. The blue light can be seen by the naked eye even at 60 mW of the pump power under the excitation of 980 nm laser. The results showed that the strongest luminescence intensity was obtained for the sample with the doping ratio 1:8 of Tm3+ to Yb3+. The possible upconversion luminescence mechanism of Bi2Ti2O7:Tm3+/Yb3+ nanofibers were proposed based on its energy level diagram. By microscopic 5
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Fig. 6. (a) Schematic diagram of the measurement process; (b) infrared images of the samples under heating condition recorded by infrared thermal camera; (c) the comparison between detected and calculated temperature.
imaging of the samples under a near-infrared laser, the samples possessed excellent blue imaging characteristics. The sample showed outstanding optical sensing property in the range of 300–505 K and the maximum value of Sr was 0.024 K−1 at 300 K. The Bi2Ti2O7:Tm3+/ Yb3+ sample based on the FIR technique possessed evident advantages beyond the thermal infrared imager for the surface temperature detection. The results will promote not only the study of excellent upconversion in new matrix family materials, but also the applications of the new material in bioimaging and optical temperature sensing fields.
[7]
[8]
[9]
[10]
Declaration of Competing Interest [11]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
[12]
Acknowledgments [13]
This work was supported by Research Initiation Funds for the Academic Leaders from Shaanxi University of Science and Technology (2016XSGG03).
[14]
[15]
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123546.
[16]
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