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Importance of the hierarchical core@multi-shell nanostructure in obtaining white light emission in Ln(III)-doped ZrO2 nanoparticles Cristine Santos de Oliveira, Jefferson Bettini, Fernando Aparecido Sigoli, and Italo Odone Mazali Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00883 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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Crystal Growth & Design

Importance of the hierarchical core@multi-shell nanostructure in obtaining white light emission in Ln(III)-doped ZrO2 nanoparticles Cristine Santos de Oliveira†, Jefferson Bettini‡, Fernando Aparecido Sigoli† and Italo Odone Mazali†* † ‡

Institute of Chemistry, University of Campinas – UNICAMP, P. O. Box 6154, 13081-970, Campinas, SP, Brazil Brazilian Nanotechnology National Laboratory, P. O. Box 6192, 13083-970, Campinas, SP, Brazil

Supporting Information Placeholder ABSTRACT: The design of the hierarchically tailored 4.4 nm nanoparticles was based on the alternation between un-doped and Ln(III)-doped layers (Ln(III) = Eu(III), Tb(III)), ensuring the isolation of different Ln(III) ions in order to prevent energy transfers between them, which may lead to long term loss of quality of the emission due to one-sided cascading of the excitation energy to a single emitter. Photoluminescence emission and excitation spectra have shown the importance of this design in obtaining white light: for the alternating layer design there are no energy transfer between Tb(III) and Eu(III) ions, and emission spectra with λexc = 260 nm of Tb(III),Eu(III)-codoped ZrO2 nanoparticles over a ZnO-coated silica host show a combination of Tb(III) green, Eu(III) red, and silica defects blue emissions resulting in pure white emission, and warm white with an additional Eu(III)-doped layer. While the ZnO coating layer was vital in improving Eu(III) emission intensity by suppresion of SiO2 surface (-OH) oscillators, nanoparticles constructed with gradually less features of the main hierarchical nanostructure resulted in Tb(III)→Eu(III) energy transfers, and loss of fine control over the final emission color.

Solid state lighting has received considerable attention in the recent years especially for the development of white light-emitting diodes (LEDs), due to its several advantages such as lower energy consumption, high light output and long device lifetime1. Although great focus is put on the development of white LEDs with higher luminance efficiency values, this focus tends to drift away from color quality of the emission, resulting in low color rendering index (CRI) values and subsequent biological issues recently observed such as changes in the human circadian rhythm2 and damage to the photosensitive cells of the retina due to their low sensitivity to the strong blue component of commercial LEDs3. In order to obtain good quality and control over the final emission, two main aspects must be approached: energy transfers between different emitters may lead to long term loss of quality of the emission due to onesided cascading of the excitation energy4-8, which points to the use of Red-Green-Blue (RGB) LEDs instead of phosphor-converted LEDs (pcLEDs), and indicates the need to isolate different phosphors from one another; RGB LEDs however, may present several interfaces between the different phosphors, which may undesirably scatter light and are also rich in defects that are a source of nonradiative decay for the excited phosphors9. Approaching this matter from nanomaterials perspective and the countless methods and strategies available nowadays for their preparation, a nanoparticle design was proposed: using the method of impregnationdecomposition cycles (IDC), which consists of an association of metallo-organic decomposition (MOD) with the use of a support

host, where one IDC consists of impregnation of this host with a given metallo-organic single-source precursor followed by its thermal decomposition in order to obtain the compound of interest, would allow the construction of hierarchical core@multi-shell nanoparticles, according to previous studies that confirmed its applicability for controlled growth and size of both pure10,11 and hierarchically organized core-shell oxide nanoparticles12,13 with small size (1-10 nm), through variation of the number of IDC and precursor alternation. The present work focused on constructing Ln(III)-doped ZrO2 nanoparticles, combining ZrO2 transparency in the 300-8000 nm range, high diffraction index, large band gap (5.25.8 eV) and low phonon energy (470 cm-1)14-17, considerably lower than other matrices such as SiO2 (1100 cm-1) and Al2O3 (870 cm1 14 ) , with the luminescence properties of lanthanide ions Eu(III) and Tb(III), with their long lifetimes and narrow emission bands in the visible18, a combination that promotes applications in photonics14,17, solid state lighting16 and other optical devices. Using silica as support host, the nanoparticle hierarchical core@multi-shell structure was based on alternating un-doped and Ln(III)-doped layers (Ln(III) = Eu(III), Tb(III)) alternating precursors in the IDC method, in order to isolate different Ln(III) ions to prevent energy transfers between them, while also preventing non-radiative decay through interfaces since the several thermal treatments keep nanoparticles monocrystalline19 within the protected environment of the porous structure. The downside to the use of a silica host for such application, however, is the radiative energy loss through non-radiative decay by the surface hydroxyl oscillations (~3600 cm-1). On the previous work by Oliveira et al.20, using the same method to grow pure ZnO on porous silica Vycor glass (PVG) resulted in a majority of Si-O-Zn species at the surface instead of ZnO nanoparticles, meaning ZnO could be used as a coating layer to suppress the hydroxyl groups and thus improve the luminescence of the nanoparticles. In the present work, we study the importance of the hierarchical core@multi-shell design of alternating pure ZrO2 and Eu(III)- or Tb(III)-doped ZrO2 layered nanoparticles grown inside ZnO-coated pores of porous Vycor glass (PVG)21, in obtaining white light emission from the combined emissions from PVG (blue), Eu(III) (red) and Tb(III) (green). Materials and Methods. Synthesis of Eu(III)- and Tb(III)acetylacetonate: Europium(III) 2-acetylacetonate hydrate (Eu(acac).xH2O), and Terbium(III) 2-acetylacetonate hydrate (Tb(acac)3.xH2O), were synthesized using the following method: Eu2O3 (Sigma-Aldrich) and Tb2O7 (Sigma-Aldrich) were dissolved in an aqueous solution of HCl (Synth) at boiling temperature under stirring, with addition of a few drops of H2O2 29.0 %

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w/v (Synth) in the dissolution of Tb2O7 to reduce Tb(IV) to Tb(III). After complete dissolution of precursors, the solutions’ pH were adjusted to 5 by evaporating the remaining HCl and then cooled to room temperature and reserved. On another recipient acetylacetone (Sigma-Aldrich) (3:1 acac:Ln molar ratio) was dissolved in a minimal amount of ethanol 99.5 % (Synth), followed by the addition of an equimolar amount of KOH (Sigma-

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Aldrich) (1:1 acetylacetone:KOH) and this solution was kept at 50 °C under stirring for 1 hour for acetylacetone deprotonation. These solutions were mixed at ambient temperature and kept under stirring for 24 h. The formation of fine white powders followed, and these solids were filtered off under vacuum using a ME 24 membrane with 0.2 µm diameter pores, washed thoroughly with distilled water and then dried in oven at 60 °C for 30 min.

Table 1. Strategy employed for the synthesis of the hierarchically nanostructured multilayer nanoparticles (ZnZrH), represented sample by sample. The numbers represent the number of IDC, while the black line under the particles represents the internal surface of the PVG pores and the dark gray line above it the ZnO coating layer - 3ZnO//aZrO2/bZrO2:Eu(III)/cZrO2/dZrO2:Tb(III)/eZrO2/fZrO2:Eu(III)/gZrO2*

A–3

B – 3//1

C – 3//3

D – 3//3/1

E – 3//3/2

F – 3//3/2/1

G – 3//3/2/2

H – 3//3/2/2/1

I – 3//3/2/2/1/1

J – 3//3/2/2/1/2

K – 3//3/2/2/1/2/1

L – 3//3/2/2/1/2/2

M – 3//3/2/2/1/2/2/1

N – 3//3/2/2/1/2/2/3

*The nanoparticle multi-shell is filled from left to right, after the ZnO-coated PVG (A - 3). Whenever an index (a, b, c,…) is not specified, it is 0. ACS Paragon Plus Environment

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Synthesis of the ZnO-coating layer on Porous Vycor Glass (PVG): Clean 0.5 cm diameter x 0.1 cm thickness PVG discs were submitted to three successive impregnation-decomposition cycles (3 IDC), each consisting of: immersion of the PVG discs in a hexane solution of Zinc(II) 2-ethylhexanoate (Strem Chem.) [1.0 mol L-1] for 18 h, the impregnation step, followed by thorough washing with pure hexane and then thermal treatment at 600 °C for 4 h with 10 °C /min in oven under static air atmosphere, the decomposition step. Synthesis of ZrO2 nanoparticles: Hierarchically nanostructured Ln(III)-doped ZrO2 nanoparticles synthesis followed, using the same method with the now ZnO-coated PVG as growth support, and pure or 0.5 mol% Eu(III)- (Eu(acac)3.xH2O) or 1.0 mol% Tb(III)- (Tb(acac)3.xH2O) doped, or (0.4 mol% Eu(III))-(0.2 mol% Tb(III)) co-doped Zr(acac)4 [0.25 mol L-1] in dichloromethane as precursors. The IDC sequence used to grow the nanoparticles and their designations are described: - ZnZrH: Main nanoparticles with complete hierarchical structure; IDC Sequence: 3 IDC ZrO2, 2 IDC Eu(III)-doped ZrO2, 2 IDC ZrO2, 1 IDC Tb(III)-doped ZrO2, 2 IDC ZrO2, 2 IDC Eu(III)doped ZrO2, 3 IDC pure ZrO2; - ZnZrC: Nanoparticles without intermediary un-doped layers, to evaluate their influence over energy transfers and emission color; IDC Sequence: 2 IDC Eu(III)-doped ZrO2, 1 IDC Tb(III)-doped ZrO2, 2 IDC Eu(III)-doped ZrO2; - ZrM: Nanoparticles with simultaneously doped Eu(III) and Tb(III), without a hierarchical structure, to evaluate the luminescence performance of a simple mixture of ions compared to the main hierarchical design; IDC Sequence: 5 IDC Eu(III)-Tb(III) co-doped ZrO2; Samples were taken for each IDC, starting from 3ZnO for ZnZrH and ZnZrC, and 1 ZrO2:(Eu(III),Tb(III)) for ZrM, in order to study the influence and importance of each layer over the final emission. Main samples ZnZrH illustrations and nomenclature can be found in Table 1, and comparative samples illustrations and nomenclature can be found in Tables S1 (ZnZrC) and S2 (ZrM) in the Supplementary Material. All samples were submitted to the same total thermal treatment. Characterization: TEM images were obtained using a JEOL 3010 microscope (300 kV, 1.7 Ǻ point resolution), and STEM and EDS measures were obtained in a JEM 2100F microscope (200kV, 1.4 Ǻ point resolution) using an Oxford SDD X-Max 80 mm2 windowless detector. Samples were prepared by suspending the powders in ethanol and placing a drop of this suspension on a holey carbon coated Cu grid. The powder X Ray diffraction measures were recorded in a Shimadzu XRD 7000 diffractometer using CuKα radiation (λ = 1.5418 Å) in the 2θ range of 5 – 80° at a 2°/min rate. Diffuse reflectance UV-Vis spectra (DRS) were obtained on a CARY 5 UV-Vis spectrophotometer, using the powdered disks smoothly compacted into a quartz sample holder which was mounted on an integrating sphere, and data was collected between 200 and 800 nm with a 2 nm spectral bandwidth. Powdered PVG was used as standard for instrumental background correction. Photoluminescence data was acquired using a Horiba Jobin Yvon FL3-22-iHR320 model at stationary state and room temperature, using a 450 W Xenon lamp as excitation source, front-face mode (22.5° angle), and the photomultiplier Hamamatsu R298 was used as a detector. Emission spectra were acquired within the range of 290 to 720 nm and excitation spectra were acquired within the range of 240 to 580 nm, with a 1 nm step. All spectra were corrected automatically according to lamp emission,

optics and detectors responses. For emission lifetime measurements, a pulsed 150 W Xenon lamp was used as excitation source, in front-face mode (22.5° angle) using a TCSPC system and 1024 channels. CIE (Commision Internationale de l’Eclairage) chromaticity diagram results were obtained using the Spectra Lux Software v. 2.022. RESULTS AND DISCUSSION: The mass increase of the PVG discs for ZnZrH samples was estimated using their mass differences before and after each IDC. A two-step linear mass increase was observed, starting at 2.14±0.09% per IDC and reaching a saturation degree at 8 IDC where it goes down to 1.17±0.04 % per IDC (Figure S1 - Supplementary Material) due to clogging of the pore structure. The XRD patterns obtained for samples ZnZrH A, C, H and N (Figure S2) have shown the PVG non-crystalline halo peaking at ~24°, while diffraction peaks of wurtzite ZnO weren’t observed and the main peak for tetragonal ZrO2 appears for sample C and increases in intensity as the nanoparticles are grown further. This peak is also large, due to small nanoparticle size (average of 4.4 nm, obtained in the TEM results following) and high nanoparticle dispersion within the silica. TEM images were obtained for sample ZnZrH N and representative images are shown in Figure 1. In all cases well-disperse spheroidal ZrO2 nanoparticles with diffraction planes characteristic of the tetragonal crystal structure were observed, and the nanoparticle size distribution histogram has shown a size distribution between 2 and 8 nm with an average size of 4.4±0.1 nm (Figure S3). EDS measures obtained in STEM mode (Figure 2(a)) for a single 5 nm ZrO2 particle of the same sample have shown Si and Zr peaks with major intensity, and a low intensity Zn peak (Figure 2(b)). The intensity ratio between the Zr and Zn peaks suggest that Zn is found only as surface species, and not as part of the nanoparticles. Peaks for Fe, Co and Cr are attributed to the sample holder and microscope pole piece, and Cu to the sample grid used. Further-

Figure 1. TEM images of sample N. The insets show the nanoparticles diffraction planes. more, with enough accumulation it was possible to observe the Eu peak for a concentration of ~1 % in a single nanoparticle (Figure 2(c)), a novel result for EDS detection, showing that the nanoparticles are effectively doped.



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Figure 2. (a) STEM image of ZnZrH N indicating the nanoparticle from which EDS measures were taken, (b) main EDS results obtained and (c) Cr count normalized EDS result emphasizing the Eu peak. UV-Vis absorption spectra were obtained through diffuse reflectance (DRS) converted to F(R) through the Kubelka-Munk function F(R) = (1-R)2/2R, where R is the absolute reflectance (Figure 3). The spectra show three distinct absorption bands, the ZrO2 band gap at 210 nm slowly red-shifted as a function of the IDC number (Figure S4), the absorption attributed to Zn-O-Si species at 245 nm14 which is red-shifted with the growth and development of the ZrO2 nanoparticles, and a large absorption at 300 nm attributed to PVG surface defects. According to Oliveira et al.14 the Zn-O-Si species band does not shift with the number of IDC, and in the present work this shift was attributed to interaction between ZrO2 nanoparticles and these species at the surface.

Figure 3. UV-Vis absorption spectra of ZnZrH samples obtained by diffuse reflectance (DRS) converted by the Kubelka-Munk function. Photoluminescence emission spectra were obtained with λexc = 260 nm (O2-→Ln3+ charge transfer), λexc = 350 nm (Tb(III) 7 F6→5L9) and λexc = 393 nm (Eu(III) 7F0→5L6). Excitation spectra were obtained using λem = 438 (PVG defects), λem = 543 nm (Tb(III) 5D4→7F5) and λem = 613 nm (Eu(III) 5D0→7F2). Emission spectra obtained for ZnZrH samples with λexc = 260 nm (Figure 4(a)) show a series of wide emission bands in the 300-450 nm range, attributed to PVG defects, Eu(III) 5D0→7FJ (J = 0, 1, 2, 3, 4) and Tb(III) 5D4→7FJ (J = 6, 5, 4, 3). The Eu(III) emission profile is in agreement with its presence in a distorted tetragonal cation site (D4h→D2d), hence the increase of 5D0→7F2 in comparison to 5D0→7F1. The corresponding CIE diagram (Figure 4(b)) shows the shift in emission color with the construction of the nanoparticle: from blue (ZnZrH A-C), to magenta for Eu(III)-

doped (ZnZrH D-G) to pure white after doping with Tb(III) (H-J) and then to yellowish or warm-white after the second Eu(III)doped layer (ZnZrH K-M), and then to yellow for ZnZrH N, an effect better understood in the lifetime results following. The D-G emission is shifted towards red with increasing doping (D→E) and then back to purple with the ZrO2 coating layer (E→G), due to ZrO2 defects emitting in the 450 nm region. It is noted that there is a delicate balance of emissions with only 2 emitters participating in the final emission, but once the third emitter - Tb(III) - is present (ZnZrH H-N) the control over the final emission is much more accurate, even with the increment of Eu(III), and made it possible to obtain pure and warm-white emissions, both interesting for applications in solid state lighting. A comparison with similar nanoparticles grown on PVG without the ZnO coating layers was performed for nanoparticles with D and H structure, and the results can be found in Figures S5 and S6, respectively. The improvement of the Eu(III) emission intensity was attributed to the effective suppression of surface (–OH) groups by the ZnO coating layer, which are responsible for one of the main non-radiative decay routes for Eu(III) ions. The decrease in intensity of the PVG defect emissions was also attributed to coating, since these are probably surface defects direct or indirectly related to (–OH) groups. It was also observed there is a slight decrease in intensity for the Tb(III) emission, attributed to a non-radiative energy transfer to ZnO acceptor defects V’’Zn/V’Zn (‘ = negative charge). The emission spectra with λexc = 350 nm (Tb(III) 7 F6→5L9) (Figure S7(a)) show two broad bands at 414 and 438 nm and a shoulder located at 456 nm attributed to PVG defect emissions, which is expected since 350 nm also excites PVG defect bands, and Tb(III) 5D4→7FJ (J = 6, 5, 4, 3) emissions. The corresponding CIE diagram (Figure S7(b)) shows no considerable change in emission, remaining in the blue region. The lack of Eu(III) emissions in Tb(III) excited emission spectra is an indication there is no Tb(III)→Eu(III) energy transfer. The emission spectra with λexc = 393 nm (Figure S8(a)) show the same broad bands at 414 and 438 nm and shoulder at 456 nm, and Eu(III) 5 D0→7FJ (J = 0, 1, 2, 3, 4). The corresponding CIE diagram (Figure S8(b)) shows a gradual shift of the emission from blue to purple with the increment of Eu(III) in the nanoparticles. Excitation spectra (Figure S9(a)) with λem = 438 nm show that there are non-radiative energy transfers from Zn-O-Si species to PVG, to which the absence of ZnO defect emissions was attributed. A slight shift broadening of this band to lower energies is in agreement with the results obtained in UV-Vis absorption, where the interaction of the Si-O-Zn species with the ZrO2 nanoparticles would shift this band towards lower energies. The excitation spectra with λem = 543 nm (Tb(III) 5D4→7F5) (Figure S9(b)) show a band around 254 nm attributed to the O2-→Tb3+ charge transfer,


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and several Tb(III) 4f transitions, as well as PVG defect broad bands because of its residual emission in the analysed λem. In a similar fashion, the excitation spectra with λem = 613 nm (Eu(III) 5 D0→7F2) (Figure S9(c)) show a band around 260 nm attributed to the O2-→Eu3+ charge transfer, and several Eu(III) 4f transitions, as well as residual PVG bands, but no Tb(III) transitions, indicating there is no energy transfer from these ions and thus Eu(III) and Tb(III) are effectively isolated from each other. When the intermediary un-doped layers were removed from the nanoparticles design (ZnZrC), different emission and excitation profiles were obtained: for λexc = 260 nm (Figure 4(c)), the relative emission intensities for Eu(III) and Tb(III) are higher than for PVG, and according to the CIE diagram (Figure 4(d)) pure and warm-white emissions are no longer achievable, instead the final emission shows broader shifts compared with ZnZrH, from blue towards red (ZnZrC A-C), and then towards green (ZnZrC C-D) and yellow (ZnZrC D-F). For λexc = 350 nm however (Figure S10), additionally to the previously observed PVG and Tb(III) emissions, Eu(III) emissions 5D0→7F2 and 5D0→7F4 were observed, meaning that Tb(III)→Eu(III) energy transfers begin to occur, even if at a low rate. For λexc = 393 nm on the other hand (Figure S11), cold white and pure white emissions were obtained for samples ZnZrC C, and E-F, respectively, compatible with commercial LEDs, albeit with a less predictable and thus less controllable emission shift with the number of IDC, since only PVG and Eu(III) emissions contribute to the final emission. Excitation spectra for λem = 438, 543 or 613 nm (Figure S12) were similar to the obtained for ZnZrH. Finally when the hierarchical nanostructure is completely removed and Tb(III)/ Eu(III) are simultaneously doped (ZrM), the emission profile changes again and energy transfer becomes slightly more evident: for λexc = 260 nm (Figure 4(e),(f)), the emission starts at green and shifts towards yellowish-green, with the increase in Tb(III) 5D4→7FJ and decrease in Eu(III) 5D0→7FJ and PVG relative emissions intensities, shifting further away from the previously obtained white emissions. For exclusive excitation of Tb(III) (λexc = 350 nm) (Figure S13), once again Eu(III) lines 5 D0→7F2,4 were observed plus additional lines 5D1→7F0 and 5 D0→7F3, indicating Tb(III)→Eu(III) energy transfers occur more

readily in ZrM than in ZnZrC samples. For λexc = 393 nm (Figure S14) a purplish white emission was also obtained for ZrM B, which in Ln(III) concentration terms would be similar to the ZnZrC D sample that has a light-blue emission. The excitation spectra (Figure S15) were very similar to the previously obtained for ZnZrH and ZnZrC, but for λem = 613 nm (Eu(III) 5D0→7F2) the Tb(III) line 7F6→5L9 is now observed, another evidence of the increase in Tb(III)→Eu(III) energy transfer. Emission lifetimes obtained for Eu(III) 5D0→7F2 and Tb(III) 5 D4→7F5 emission for samples ZnZrH, ZnZrC and ZrM are presented in Figure 5(a)/(b), 5(c)/(d) and 5(e)/(f), respectively, and corresponding decay curves can be found in Table S3 of the Supplementary Material. For all samples analyzed biexponential decays were obtained, with the shorter lifetime attributed to Eu(III) ions subject to lattice distortions and O-H oscillators, which contribute to a faster decay through non-radiative processes. The longer lifetime values are attributed to Eu(III) in nanoparticle environments that are far enough from O-H oscillators and experiencing less structural distortion. For ZnZrH the observed emission lifetime for Eu(III) 5D0→7F2 increases with the covering of the first doped layer with the pure ZrO2 layer due to suppression of surface defects, from 2.07±0.03 ms/ 0.74±0.02 ms (E) to 2.12±0.03 ms/ 0.82±0.02 ms (F), stabilizing until the next Eu(III)doped layer, where a decrease in the longer lifetime to 2.06±0.03 ms/ 0.80±0.03 ms (L) takes place, attributed to Eu(III) concentration quenching for a thinner layer, and then another lifetime increase takes place with the next surface covering layer, to 2.13±0.05 ms/ 0.88±0.03 ms (M). The emission lifetime for Tb(III) 5D4→7F5 on the other hand does not shift significantly, increasing slightly from 2.4±0.1 ms/ 0.9±0.1 ms (H) to 2.6±0.1 ms/ 1.1±0.1ms (I) due to the covering layer, but then decreases gradually with the last pure ZrO2 layer, to 2.4±0.1 ms/ 0.8±0.1 ms (M and N). This was attributed to the effective refractive index of zirconia affecting the Tb(III) emission as the layer above the Tb(III)-doped layer thickens, increasing the zirconia part of the optical path. The same effect could be observed for the Eu(III) emission comparing samples M and N, although minor at this point since one of the Eu(III)-doped layers is closer to the particle surface. This shows the high sensitivity of these ions emissions to the refractive index of the nanoparticles’ surroundings even at

Figure 4. Emission spectra with λexc = 260 nm/corresponding CIE diagrams of (a)/(b) ZnZrH samples A-N, (c)/(d) ZnZrC samples A-F and (e)/(f) ZrM samples A-C. (*) indicates the moment of insertion of the optical filter. ACS Paragon Plus Environment


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Figure 5. Lifetimes obtained vs. total number of IDC for Eu(III) 5D0→7F2 (λem = 613 nm) and Tb(III) 5D4→7F5 (λem = 543 nm) emissions with λexc = 260 nm for samples: (a)/(b) ZnZrH, (c)/(d) ZnZrC and (e)/(f) ZrM. very short distances of ~1 nm, revealing possibilities for applications as nanosensors in studies of surface and short distance interactions23. Comparing the lifetime of Eu(III) and Tb(III) in nanoparticles with and without the ZnO coating layer, it was observed that although Eu(III) lifetimes do not suffer considerable change (Figure S16 (a)), the Tb(III) lifetime was almost halved (Figure S16 (b)), for both long and short lifetimes. This considerable change for Tb(III) was attributed to the non-radiative energy transfer from Tb(III) to ZnO acceptor defect levels V’’Zn/V’Zn that would be well aligned with the 5D4 level of Tb(III), while being far enough from Eu(III) 5D0 in order for a bridged energy transfer to occur. For ZnZrC samples the obtained lifetimes for both Eu(III) and Tb(III) ions were longer, likely due to both the absence of the initial ZrO2 core whose refractive index may lower the obtained lifetimes, and the existence of a Tb(III)→Eu(III) energy transfer which may increase Eu(III) emissions lifetime. A similar increase in lifetime related to the covering of the doped layers for ZnZrH was observed for ZnZrC B and C for Eu(III) 5 D0→7F2, and for ZnZrC D and E for Tb(III) 5D4→7F5. For ZrM samples, the starting lifetimes (ZrM A) are similar to the obtained for ZnZrH, but show a larger increase with the next 2 IDC (ZrM B), attributed to the smaller size of the nanoparticles. For ZrM C this increase persists for Eu(III) but the Tb(III) lifetime remains constant, due to the more prominent Tb(III)→Eu(III) energy transfer. Comparing the obtained lifetime results with some observed in the literature, the lifetimes obtained were shorter for Eu(III) than the obtained by Parma et al.24 for 4-5 nm nanocrystals doped with 3 %mol Eu(III) of tef = 3.7 ms, and similar to the obtained by Romero et al.25 with tef = 2.7 ms for 30-40 nm nanocrystals doped with 1 %mol Eu(III), but longer for Tb(III) compared with the obtained by Hardin et al.26, of 1.1 ms for ZrO2 ceramics with grain size of 100 nm and 1.5 %mol Tb(III). Finally, an energy level diagram was constructed based on several observations in this present work, comparing SiO2, Si-O-Zn and ZrO2 Valence and Conduction band locations, as well as Tb(III) 5 D4, Eu(III) 5D0 emitting levels, ZnO defects V’’Zn/V’Zn (‘ = negative charge), and a SiO2 defect absorption (Figure 6). The diagram construction strategy was as follows: i) The vacuum referred binding energies for SiO2 from Gupta et al.27 and ZrO2 from Dorenbos28 were aligned;

ii) Since our ZrO2 is under quantum confinement, the model of effective mass approximation used by Enright et al.29 was applied and estimated for our 4.4 nm ZrO2 nanoparticles (Figure S17) using me* = 0.9 and mh* = 1.6 from (30), where me* and mh* are the relative effective masses of electron and hole, respectively, and εr ~ 25 from (31), the ZrO2 vacuum-relative dielectric constant; iii) Assuming an absorption involving the fixed energy levels of the Zn-O-Si species and the shortening band gap of the quantum confined ZrO2 nanoparticles we estimated the ZrO2 conduction band shift (Figure S17) comparing the absorption maxima obtained for Si-O-Zn species (Figure S4); iv) With those values applied to the model used in 2, we could estimate the particle size and the valence band shift with the number of IDC (Figure S5), as well as place the excited state and ground state for the Si-O-Zn species; v) From Dorenbos et al.27 we could also position the Tb(III) 5D4 and Eu(III) 5D0 energy levels, and based on the work by Enright et al.29 applied to ZnO in an estimate comparison of absorption energy between a very small ZnO and Zn-O-Si sites, the ZnO intrinsic defect level positioning by Schmidt-Mende et al.32 and our experimental results for photoluminescence, we positioned the V’’Zn/V’Zn acceptor defect, which would be receiving energy from Tb(III) 5D4. vi) Finally, taking into consideration the observed energy transfer from Zn-O-Si sites to the silica and based on the work by Munekuni et al.33 we estimated that the referred absorption level would be well aligned with the Zn-O-Si species absorption levels, and that this same level could be attributed to the observed defect emissions of the silica peaking at 350 nm and 438 nm, based on the work by Cannas34.


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Crystal Growth & Design 1

ACKNOWLEDGMENT

0 -1

-5 -6

4.8-5.1 eV

-4

5.06 eV

-3 9.0 eV

VRBE (eV)

-2 V''Zn/V'Zn

3+ 5

(Tb ) D4

5.76 eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-7 -8

3+ 5

(Eu ) D0

-9 -10

CSO is indebted to CNPq for a PhD fellowship. IOM is indebted to CNPq and FAPESP for financial support. Contributions from Brazilian Synchrotron Light Laboratory (LNLS, CampinasSP, Brazil) for HRTEM and STEM-EDS (LNNano-LNLS) analyses are also gratefully acknowledged. The authors would like to thank the Multiuser Laboratory of Advanced Optical Spectroscopy (LMEOA/IQ-UNICAMP FAPESP Proc. 2009/54066-7) for use of its equipment. This is a contribution of the National Institute of Science and Technology in Complex Functional Materials (CNPq-MCT/FAPESP).

SiO2

REFERENCES

Si-O-Zn

ZrO2

Figure 6. Energy level diagram proposed based on literature comparisons of energy level positioning, Effective Mass Approximation Model calculations for ZnO and ZrO2, and experimental results. In summary, hierarchically nanostructured core@multi-shell Tb(III),Eu(III)-codoped ZrO2 nanoparticles were successfully synthesized inside the pores of a ZnO-coated porous silica, with an average size of 4.4 nm. While the ZnO coating considerably improved the intensity of Eu(III) emissions due to suppression of silica (-OH) surface groups, the hierarchical layered structure was essential in allowing fine tuning of the materials emission, resulting in pure white and warm white emissions, both interesting for LEDs applications, while also preventing energy transfers between Tb(III) and Eu(III), ensuring long term stability of the emission color. With the gradual removal of the hierarchical structure, the control over the materials emission becomes less efficient, and as soon as the intermediary un-doped layers are removed Tb(III)→Eu(III) energy transfers begin to occur and become more prominent when the ions are doped simultaneously in doped layers, which also emphasizes the importance of the hierarchical nanostructure in providing a material with controlled emission for solid state lighting applications.

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ASSOCIATED CONTENT 11.

Supporting Information The Supporting Information PDF contains comparative nanoparticles designs and complementary experimental results: mass gain vs. number of IDC, powder XRD diffractograms, particle size distribution histogram, Zn-O-Si absorption band shift with IDC, excitation and emission spectra with corresponding CIE diagrams, luminescence decay curves and the model of effective mass approximation applied to ZrO2, contained in Figures S1 to S17 and Tables S1 to S3.

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The Supporting Information is available free of charge on the ACS Publications website.

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AUTHOR INFORMATION 16.

Corresponding Author Prof. Dr. Italo Odone Mazali Phone: +55 19 3521-3164. Fax: +55 19 3521-3023 E-mail: [email protected]

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Setlur, A. A., Phosphors for LED-based Solid-State Lighting, Electrochem. Soc. Interface, 2009, 32-36. Oh, J. H., Yang, S. J., Do, Y. R., Healthy, natural, efficient and tunable lighting: four-package white LEDs for optimizing the circadian effect, color quality and vision performance, Light Sci. Appl., 3, 2014, e141. Kuse, Y., Ogawa, K., Tsuruma, K., Shimazawa, M., Hara, H., Damage of photoreceptor-derived cells in culture induced by light emitting diode-derived blue light, Sci. Rep., 4, 2014, 5223. Xia, Z., Zhuang, J., Meijerink, A., Jing, X., Host composition dependent tunable multicolor emission in the single-phase Ba2(Ln1−zTbz)(BO3)2Cl:Eu phosphors, Dalton Trans., 42, 2013, 6327. Gómez, L. A., Menezes, L. S., Araújo, C. B., Gonçalves, R. R., Ribeiro, S. J. L., Messaddeq, Y., Upconversion luminescence in Er3+ doped and Er3+/Yb3+ codoped zirconia and hafnia nanocrystals excited at 980 nm, J. Appl. Phys., 107, 2010, 113508. Tu, D., Liang, Y., Liu, R., Li, D., Eu/Tb ions co-doped white light luminescence Y2O3 phosphors, J. Lumin., 131, 2011, 2569-2573. Lisiecki, R., VUV and UV-vis optical study on KGd2F7 luminescent host doped with terbium and co-doped with europium, J. Lumin., 143, 2013, 293-297. Ma, W., Shi, Z., Wang, R., Luminescence properties of full-color single-phased phosphors for white LEDs, J. Alloy Compd., 503, 2010, 118-121. Sohn, I. S., Unithrattil, S., Im, W. B., Stacked Quantum Dot Embedded Silica Film on a Phosphor Plate for Superior Performance of White Light-Emitting Diodes, ACS Appl. Mater. Interfaces, 6, 2014, 5744-5748. Strauss, M., Pastorello, M., Sigoli, F. A., Silva, J. M. S. S., Mazali, I. O., Singular effect of crystallite size on the charge carrier generation and photocatalytic activity of nano-TiO2, Appl. Surf. Sci., 319, 2014, 151-157. Cangussu, D., Nunes, W. C., Correa, H. L. D., Macedo, W. A. A., Knobel, M., Alves, O. L., Souza Filho, A. G., Mazali, I. O., gammaFe2O3 nanoparticles dispersed in porous Vycor glass: a magnetically diluted integrated system, J. Appl. Phys., 105, 2009, 013901. Corrêa, D. N., Souza e Silva, J. M., Santos, E. B., Sigoli, F. A., Souza Filho, A. G., Mazali, I. O., TiO2- and CeO2-Based Biphasic Core-Shell nanoparticles with Tunable Core Sizes and Shell Thicknesses, J. Phys. Chem. C, 115, 2011, 10380-10387. Santos, E. B., de Souza e Silva, J. M., Mazali, I. O., Raman spectroscopy as a tool for the elucidation of nanoparticles with coreshell structure of TiO2 and MoO3, Vib. Spectrosc., 54, 2010, 89-92. Rozo, C., Jaque, D., Fonseca, L. F., Solé, J. G., Luminescence of rare Earth-doped Si-ZrO2 co-sputtered films, J. Lumin., 128, 2008, 1197-1204. Liu, H.-Q., Wang, L.-L., Chen, S.-G., Zou, B.-S., Optical properties of nanocrystal and bulk ZnO:Eu3+, J. Alloy Compd., 448, 2008, 336339. Mukherjee, S., Dutta, D. P., Manoj, N., Tyagi, A. K., Sonochemically synthesized rare earth double-doped zirconia nanoparticles: probable candidate for white light emission, J. Nanopart. Res., 14, 2012, 814. Gómez, L. A., Menezes, L. S., Araújo, C. B., Gonçalves, R. R., Ribeiro, S. J. L., Messaddeq, Y., Upconversion luminescence in Er3+ doped and Er3+/Yb3+ codoped zirconia and hafnia nanocrystals excited at 980 nm, J. Appl. Phys., 107, 2010, 113508.



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18. Liu, L., Wang, Y., Su, Y., Ma, Z., Xie, Y., Zhao, H., Chen, C., Zhang, Z. Xie, E., Synthesis and White Light Emission of Rare Earth-Doped HfO2 Nanotubes, J. Am. Ceram. Soc., 94(7), 2011, 2141-2145. 19. Liang, R., Xu, S., Yan, D., Shi, W., Tian, R., Yan, H., Wei, M. Evans, D. G., Duan, X., CdTe Quantum Dots/Layered Double Hydroxide Ultrathin Films with Multicolor Light Emission via Layerby-Layer Assembly, Adv. Funct. Mater., 22, 2012, 4940-4948. 20. Oliveira, C. S., Bettini, J., Sigoli, F. A., Mazali, I. O., Europium(III)-Doped ZnO Obtained by a Hierarchically Nanostructured Multilayer Growth Strategy, Cryst. Growth. Des., 15, 2015, 52465253. 21. Strauss, M., Destefani, T. A., Sigoli, F. A., Mazali, I. O., Crystalline SnO2 Nanoparticles Size Probed by Eu3+ Luminescence, Cryst. Growth Des., 11, 2011, 4511-4516. 22. Santa-Cruz, P. A., Teles, F. S., Spectra Lux Software v.2.0, Ponto Quântico Nanodispositivos, UFPE, 2003. 23. Aubret, A., Pillonnet, A., Houel, J., Dujardin, C., Kulzer, F., CdSe/ZnS quantum dots as sensors for the local refractive index, Nanoscale, 8, 2016, 2317. 24. Parma, A., Freris, I., Riello, P., Enrichi, F., Cristofori, D., Benedetti, A., Structural and photoluminescence properties of ZrO2:Eu3+@SiO2 nanophosphors as a function of annealing temperature, J. Lumin., 130, 2010, 2429-2436. 25. Romero, V. H., De la Rosa, E., López-Luke, T., Salas, P., AngelesChávez, C., Brilliant blue, green and orange-red emission band on Tm3+-, Tb3+- and Eu3+-doped ZrO2 nanocrystals, J. Phys. D: Appl. Phys., 43, 2010, 465105. 26. Harding, C. L., Kodera, Y., Basun, S. A., Evans, D. R., Garay, J. E., Transparent, luminescent terbium doped zirconia: development of

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For Table of Contents Use Only Importance of the hierarchical core@multi-shell nanostructure in obtaining white light emission in Ln(III)-doped ZrO2 nanoparticles Cristine Santos de Oliveira†, Jefferson Bettini‡, Fernando Aparecido Sigoli† and Italo Odone Mazali†* †

Institute of Chemistry, University of Campinas – UNICAMP, P. O. Box 6154, 13081-970, Campinas, SP, Brazil ‡ Brazilian Nanotechnology National Laboratory, P. O. Box 6192, 13083-970, Campinas, SP, Brazil

SYNOPSIS: In our work we investigated the importance of the hierarchical core@multi-shell structure of sub 5 nm Ln(III)doped ZrO2 nanoparticles grown by a layer-by-layer method in obtaining white light emission. The design was based on alternating un-doped and doped layers, isolating Tb(III) and Eu(III) ions from each other, preventing energy transfers, and allowed us to obtain pure white and warm-white emissions.


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Importance of the Hierarchical Core@Multishell ... - ACS Publications

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