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Creating Recoverable Mechanoluminescence in Piezoelectric Calcium Niobates through Pr3+ Doping Jun-Cheng Zhang, Yun-Ze Long, Xu Yan, Xusheng Wang, and Feng Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01550 • Publication Date (Web): 24 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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Chemistry of Materials

Creating Recoverable Mechanoluminescence in Piezoelectric Calcium Niobates through Pr3+ Doping Jun-Cheng Zhang,*,† Yun-Ze Long,† Xu Yan,† Xusheng Wang,§ and Feng Wang*,‡,# †

College of Physics, Qingdao University, Qingdao 266071, P. R. China

‡

Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong SAR, P. R. China §

Functional Materials Research Laboratory, Tongji University, Shanghai 201804, P. R. China

#

City University of Hong Kong Shenzhen Research Institute, Shenzhen 518057, P. R. China

Supporting Information ABSTRACT: Recoverable mechanoluminescence (ML), characterized by nondestructive and repetitive luminescence in response to a mechanical stress, has shown considerable promise in a variety of practical applications including lighting, display, as well as stress sensing and imaging. However, the progress in utilizing ML processes has been constrained by the difficulties in developing new ML materials and in ascertaining ML mechanism. In this paper, we report a new strategy to constructing recoverable ML materials by doping piezoelectric hosts with luminescent lanthanide ions, which simultaneously create luminescent centers and carrier traps in the host lattice. The viability of the strategy has been confirmed by assessing a series of Pr3+ activated calcium niobates composed of mCaO•Nb2O5 (m = 1, 2, and 3). Furthermore, systematical characterizations of the series of calcium niobates also reveal an unusual host dependent ML phenomenon. Our results are expected to expand the scope of designing ML materials and to deepen our understanding of ML mechanism, thereby promoting further utilization of recoverable ML.

■ INTRODUCTION Recoverable mechanoluminescent (ML) materials are an emerging class of mechano-optical phosphors that repeatedly respond to mechanical stimuli and produce light emissions.1-12 As recoverable ML materials generally preserve a high structural integrity during the luminescence process, they are advantageous over conventional destructive ML materials in practical applications requiring a continuous response. Since the first demonstration of using recoverable ML phosphors in artificial skin by Xu and coworkers in 1999,1,2 there has been an increasing focus on both fundamental research and technological application of recoverable ML materials.13-29 It is now generally accepted that an important prerequisite for recoverable ML is the need for carrier traps to store energies. Another common factor in satisfying recoverable ML is the presence of stress-induced piezoelectric field that triggers the release of the trapped energy as light emissions,30,31 except for a few non-piezoelectrics showing recoverable ML due to triboelectricity.4,32 Accordingly, previous attempts to search for recoverable ML materials are primarily based on screening of existing trap-controlled luminescent materials (i.e.; persistent phosphors) displaying piezoelectricity.1-12,33,34 Given the limited number of

available candidate persistent phosphors, the family of recoverable ML materials known to date only comprises about a dozen members, of which an extremely small portion shows high ML performance.14,35 The shortage of material systems has become a major obstacle to the understanding and application of recoverable ML. In order to enrich the family of recoverable ML materials, here we present a new strategy for the discovery of recoverable ML in piezoelectric materials instead of persistent phosphors. We demonstrate that through use of luminescent lanthanide ions as substitutional dopants, emission centers and carrier traps can be simultaneously created in the host lattice (Figure 1). The effect enables one to convert non-luminescent piezoelectrics into recoverable ML materials with high flexibility and designability, thereby largely expanding the horizons of exiting ML materials. ■ RESULTS AND DISCUSSION As a proof-of-concept experiment, we employed a series of calcium niobates composed of mCaO•Nb2O5 (m = 1, 2, and 3) as the model host materials, which have never been reported to show persistent luminescence or recoverable ML in literatures.36-40 These calcium niobates

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Figure 1. Schematic illustration showing the design strategy to creating recoverable ML in piezoelectrics.

display different levels of piezoelectricity due to their distinct crystal structures,41-47 which provide a united platform for verification of the design strategy and for systematical evaluation of the structure-luminescence relationship. Luminescent centers were introduced by doping Pr3+ ions that replace Ca2+ ions in the host lattice.48 Due to a non-equivalent substitution, two kinds of defects can be simultaneously created as described by equation (1). 2Pr 3Ca → 2 Pr •• (1)

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group, which is ascribed to inefficient energy transfer through the host lattice.36,37,49 ThL spectra exhibit a broad peak in the range of 25-225 oC (Figure 2b and Figure S5), indicating the existence of carrier traps in all the Pr3+ activated systems. By comparing the integral intensity of ThL, the relative trap concentrations in the host lattice were found to decrease in the order Ca2Nb2O7:Pr3+ > Ca3Nb2O8:Pr3+ > CaNb2O6:Pr3+. The depths of traps have also been estimated by using Chen’s method (Figure S5 and Table S1), which reveal a sub-peak near room temperature for Ca2Nb2O7:Pr3+ (0.60 eV). The result suggests the existence of persistent luminescence in Ca2Nb2O7:Pr3+, which is confirmed by the experimental observation (Figure S6).

The Pr3+ related defects Pr carrying one positive charge act as the trapping centers of electrons to participate in the trap-related luminescent processes.6 The calcium niobate phosphors were synthesized through a solid-state reaction among CaCO3, Nb2O5, and Pr6O11. The phosphor compositions were controlled by tuning the ratio of the precursor metal oxides. SEM characterizations reveal a similar size and morphology of all the phosphors with a high uniformity (Figure S1). XRD patterns confirm the formation of relevant calcium niobates with high crystallinity (Figure S2). The assynthesized CaNb2O6:Pr3+ system has a columbite structure belonging to the Pbcn space group (Figure S3a). The structure possesses a low symmetry in spite of the centrosymmetric class mmm,42 suggesting a weak piezoelectricity. The structure of the as-synthesized Ca2Nb2O7:Pr3+ system belongs to the P21 space group.43,44 The noncentrosymmetric structure indicates the direct piezoelectricity (Figure S3b). A complete determination on the structure of Ca3Nb2O8 has not been undertaken in any reports, possibly due to the disturbance of massive vacancies.45-47 The space group of Ca3Nb2O8 is probably Fm3m or P4/nnc, indicating a centrosymmetric structure. However, an indirect piezoelectricity is inferred in view of the distortions of vacancies on structure.41 The local piezoelectric behavior of the samples has been confirmed by the PFM response of individual grain (Figure S4). The as-synthesized phosphors were then characterized by photoluminescence (PL) and thermoluminescence (ThL) to verify the formation of luminescent centers and carrier traps in the host lattice. Under ultraviolet (UV) excitation, PL spectra of the samples all show the characteristic emission peaks of Pr3+ attributed to the 3 P2,1I6,3P1,3P0,1D2→3H4 transitions (Figure 2a), confirming the successful activation of the host lattice by Pr3+ dopants. The broad emission band observed in CaNb2O6:Pr3+ is assigned to radiative recombination within the NbO6

Figure 2. (a) PL spectra and (b) ThL curves of the 3+ 3+ CaNb2O6:Pr (0.25 mol%), Ca2Nb2O7:Pr (0.1 mol%) and 3+ Ca3Nb2O8:Pr (0.075 mol%).

The phosphors were next incorporated into composite disks and examined under mechanical stress. As anticipated, CaNb2O6:Pr3+, Ca2Nb2O7:Pr3+ and Ca3Nb2O8:Pr3+ composites all present bright ML whose intensities grow linearly with increasing the compressive load (Figure 3). Figure 3a compares the ML responses (1D2→3H4 transition of Pr3+) of the optimized samples (the insets show the corresponding ML photographs at the peak load). The red ML presents a fusiform light distribution which is consistent with the stress distribution.2,13 The ML peak brightness was estimated referring to a standard red LED light source (620±10 nm). The transient ML brightness of the optimized samples is in the order Ca2Nb2O7:Pr3+ (~35 mcd m-2) > Ca3Nb2O8:Pr3+ (~11 mcd m-2) > CaNb2O6:Pr3+ (~3 mcd m-2). This relationship is consistent with that of the trap concentrations aforementioned, confirming the contribution of traps created by Pr3+ doping to ML process. The ML brightness of Ca2Nb2O7:Pr3+ is roughly 11 000 times higher than the eye sensitivity in dark environment.50 The luminescence is clearly visible to the naked eye in a dim room when the composite disk is pressed by hand. The intense ML from Ca2Nb2O7:Pr3+ guarantees the collection of dynamical ML images during the compressing and releasing process (Figure 3b). The dynamical ML

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3+

3+

Figure 3. Recoverable ML in Pr activated calcium niobate systems. (a) ML responses of the CaNb2O6:Pr (0.25 mol%), 3+ 3+ Ca2Nb2O7:Pr (0.1 mol%) and Ca3Nb2O8:Pr (0.075 mol%) composites under a compressive load. Insets show (i) optical image in 3+ 3+ 3+ a lit environment as well as ML photographs of the (ii) Ca2Nb2O7:Pr , (iii) Ca3Nb2O8:Pr , and (iv) CaNb2O6:Pr composites in a 3+ dim environment. (b) A sequence of transient ML images of the Ca2Nb2O7:Pr (0.1 mol%) composite during a triangle loading: (i) 0 s, 0 N; (ii) ~2.5 s, ~400 N; (iii) ~3.5 s, ~600 N; (iv) ~4.5 s, ~800 N; (v) ~5 s, ~1000 N; (vi) ~5.5, ~200 N; (vii) ~7.5 s, ~100 N. (c) 3+ Recoverable ML in the Ca2Nb2O7:Pr (0.1 mol%): (i) ML decay behavior; (ii) ML recovery behavior; (iii) Reproducibility of the recoverable ML.

images indicate the promising application in real-time observation of stress concentrations (Figure S7). ML from CaNb2O6:Pr3+, Ca2Nb2O7:Pr3+ and Ca3Nb2O8:Pr3+ all display a similar recoverable performance. Figure 2c depicts a representative recoverable ML recorded from Ca2Nb2O7:Pr3+. The peak intensity of ML has a significant decay under the consecutive compressing-releasing cycles. However, the attenuated ML can be fully recovered after the UV irradiation. The recoverable ML possesses a high reproducibility during the repeated loading-UV irradiation cycles. These results indicate that, as designed, the recoverable ML created in this work stems from the coupling of piezoelectricity and trap-controlled luminescence.51,52 The piezoelectric field-triggered detrapping process is responsible for the dynamical ML, while the recharging of traps by UV irradiation accounts for the recoverable ML.2,3,6,8-10 As CaNb2O6:Pr3+ and Ca3Nb2O8:Pr3+ do not show afterglow, our findings clearly indicate that recoverable ML is no longer limited to persistent phosphors. We also prepared Ca4Nb2O9:Pr3+ that shows no piezoelectricity (Figure S8). Although the sample displays strong PL and persistent luminescence, no ML was observed at all. The results suggest that piezoelectricity is indispensable for recoverable ML in this study. A reasonably high

Figure 4. (a-c) Spectra and images of PL and ML from the (a) 3+ 3+ CaNb2O6:Pr (0.25 mol%), (b) Ca2Nb2O7:Pr (0.1 mol%), and 3+ (c) Ca3Nb2O8:Pr (0.075 mol%), respectively. (d) Schematic ML illustration of the proposed tunneling pathway in the ML process.

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piezoelectricity is expected to facilitate detrapping of carriers, thereby enabling a sensitive response of the ML materials to mechanical stimuli.11,23,26-29 It is worth noting that the complicate interactions between the luminescent centers and host such as host tolerance and quenching should be premediated and optimized when selecting the dopant/host combinations.53 For example, ML was not successfully created by using Tb3+ and Dy3+ dopants, probably due to luminescence quenching between the dopant ions and the host lattice. Another important observation in our study is the discrepancy between PL and ML spectra. Previously reported ML spectra are either in good agreement with the corresponding PL spectra or only display a marginal shift in the emission wavelengths, which have been understood by indirect transfer of carriers between traps and luminescent centers, such as through conduction band or charge transfer state [Figure S8f].1-3,6-12,26-28 By stark contrast, the ML spectra of the calcium niobates differs significantly from the PL spectra in that the emission peaks in the blue and green spectral regions are completely missing (Figure 4 and Figures S9, S10). We attribute the difference in the PL and ML spectra of the calcium niobates to direct coupling (i.e.; tunneling) between the trap state and the 1D2 excited state of the Pr3+ dopants (Figure 4d), owing to a closely matched energy of the two states.33,34 As a result of the tunneling effect, population of higher-lying emitting states is bypassed in the detrapping process, leading to the missing of emission peaks in the short wavelength region. In a further set of experiments, we assessed the luminescence intensities as a function of the dopant concentration of Pr3+. Figures 5a-c show the dependences of the PL, ThL and ML intensities on Pr3+ concentration in the CaNb2O6:Pr3+, Ca2Nb2O7:Pr3+ and Ca3Nb2O8:Pr3+, respectively. The results reveal a concentration quenching effect

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which is a common signature of doped luminescent materials and also guides the design and application of phosphors. However, the critical concentration of Pr3+ for the trap-controlled ML and ThL is relatively lower when compared to that for PL. We attribute the observations to the participation of traps in delivering the excitation energy to quenching centers responsible for concentration quenching (Figures 5d,e). With the assistance of traps, energy hoping in the host lattice can proceed over a longer interionic distance between adjacent Pr3+ dopants, corresponding to a lower activator concentration.27 ■ CONCLUSIONS In summary, our investigations on a series of calcium niobates suggest a new strategy for discovering recoverable ML phosphors. We show that piezoelectric materials can be rationally converted to recoverable ML phosphors even no persistent luminescence is present, which was previously considered as indispensable for recoverable ML. When one considers the larger number of piezoelectric materials than that of persistent phosphors, we expect that an increasing diversity of recoverable ML phosphors will become available for fundamental research and technological applications. ■ EXPERIMENTAL SECTION Material synthesis. Polycrystalline phosphors with the compositions (Ca1-xPrx)Nb2O6 (0 ≤ x ≤ 0.015, abbreviated as CaNb2O6:Pr3+ (0–1.5 mol%)), (Ca1-xPrx)2Nb2O7 (0 ≤ x ≤ 0.02, abbreviated as Ca2Nb2O7:Pr3+ (0–2 mol%)), (Ca13+ xPrx)3Nb2O8 (0 ≤ x ≤ 0.01, abbreviated as Ca3Nb2O8:Pr (0–1 mol%)), and (Ca1-xPrx)4Nb2O9 (0 ≤ x ≤ 0.01, abbreviated as Ca4Nb2O9:Pr3+ (0–1 mol%)) were synthesized by solid-state reaction among CaCO3 (Aladdin, 99.99%), Nb2O5 (Sinopharm Chemical Reagent Co., Ltd., 99.99%), and Pr6O11 (Alfa Aesar, 99.99%). A stoichiometric mixture was

3+

3+

Figure 5. Dependences of the PL, ThL and ML intensities on the dopant concentration of Pr in (a) CaNb2O6:Pr , (b) 3+ 3+ Ca2Nb2O7:Pr , and (c) Ca3Nb2O8:Pr , respectively. Schematic illustrations of the proposed energy migration in the (d) PL and (e) ML processes, respectively.


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thoroughly ground in ethanol, dried, calcined at 900 oC for 4 h in air, and re-ground. One part of powders were granulated with 8 wt% polyvinyl alcohol (PVA) binder, and pressed into pellets with a diameter of 10 mm and a thickness of 1 mm at 20 MPa. The white pellets were heated at 550 oC to burn off the PVA. Subsequently, the pellets and powders were sintered in an alumina crucible at 1400 °C (CaNb2O6:Pr3+ and Ca2Nb2O7:Pr3+) or 1450 °C (Ca3Nb2O8:Pr3+ and Ca4Nb2O9:Pr3+) for 4 h in a horizontal tube furnace under air atmosphere. After sintering, the pellet samples were collected for the examinations by scanning electron microscopy (SEM) and piezoresponse force microscopy (PFM), while the powder samples were ground and screened through a 20 μm sieve for the other characterizations as well as for the preparation of the mechanoluminescent (ML) composites. Physical characterization. The surface morphology was examined by SEM (JSM-7001F, JEOL Ltd.). PFM measurements were carried out on an atomic force microscope (Cypher S, Asylum Research) using Pt/Ir-coated tips to reveal the local piezoelectric behavior of individual grains. Prior to the measurement, the as-prepared pellets were polished to about 0.2 mm in thickness and then placed on a gold electrode. Powder XRD measurements were conducted on an X-ray diffractometer (D8 Advance, Bruker AXS GmbH). The diffuse reflectance spectra were measured using a UV/Vis/NIR spectrophotometer (V570, Jasco). The photoluminescence (PL) and afterglow (AG) spectra as well as AG decay curves were recorded on a fluorescence spectrometer (F-4600, Hitachi). Thermoluminescence (ThL) curves were measured using a ThL meter (FJ427A1, Beijing Nuclear Instrument Factory) at a heating rate of 1 o C s-1. To evaluate ML property, circular composite disks with a diameter of 25 mm and a thickness of 15 mm were prepared by mixing the screened powders and a transparent epoxy resin (SpeciFix, Struers GmbH) at a weight ratio of 1:9. A compressive load of 0-1000 N at the deformation rate of 3 min min-1 was produced with a universal testing machine (WDW-20, Jadarason Co. Ltd.). The ML spectra were recorded with a photon multi-channel analyzer system (QE65000, Ocean Optics). The ML intensity was measured using a photon-counting system that consists of a photomultiplier tube (H10682-01, Hamamatsu Photonics K.K.) and a photon counter (C8855-01, Hamamatsu Photonics K.K.). Before the AG, ThL and ML measurements, the samples were irradiated at 254 nm for 1 min using a 6W UV lamp. The PL, AG and ML photographs were recorded using a Nikon D3 camera with a Nikkor 50 mm f1.2 lens. All measurements except ThL were performed at room temperature.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional information pertaining to SEM, XRD, crystal structure, PFM, ThL, trap depth, afterglow, ML 3D

distribution, diffuse reflectance spectra, PL and PLE spectra (PDF)

AUTHOR INFORMATION Corresponding Author *(J. C. Zhang) Email: [email protected]; [email protected] *(F. Wang) Email: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (11404181, 51373082 and 51572195), the Taishan Scholars Program of Shandong Province (ts20120528), and the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province. F.W. acknowledges City University of Hong Kong for an applied research grant (grant number 9667109).

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