Europium(III)-Doped ZnO Obtained by a Hierarchically Nanostructured

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Europium(III)-doped ZnO obtained by a hierarchically nanostructured multilayer growth strategy Cristine Santos Oliveira, Jefferson Bettini, Fernando Aparecido Sigoli, and Italo Odone Mazali Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00712 • Publication Date (Web): 14 Oct 2015 Downloaded from http://pubs.acs.org on October 20, 2015

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Crystal Growth & Design

Europium(III)-doped ZnO obtained by a hierarchically nanostructured multilayer growth strategy Cristine Santos Oliveira†, Jefferson Bettini‡, Fernando Aparecido Sigoli† and Italo Odone Mazali†,* †

Functional Materials Laboratory, Institute of Chemistry, University of Campinas - UNICAMP, P. O. Box 6154, 13083-970, Campinas, SP, Brazil ‡

Brazilian Nanotechnology National Laboratory, P.O. 6192, 13083-970, Campinas, Brazil

ABSTRACT A layer-by-layer growth method, named herein as impregnation-decomposition cycles (IDC), was used to build hierarchically nanostructured multilayered nanoparticles, alternating undoped ZnO and Eu(III)-doped ZnO layers in order to thermally induce Eu(III) migration into the ZnO structure. Porous Vycor Glass (PVG) was used as a mesoporous silica host, where the pores were used as a confined environment for preparation of ZnO nanoparticles. The proposed method allowed the control over the growth and size of the nanoparticles and prevented their coalescence. HRTEM images and XRD data revealed that ZnO nanoparticles are spheroidal with wurtzite crystal structure and an average diameter of 4.8 nm for 10 IDC. UV-Vis DRS data indicated that the formation of ZnO nanoparticles is favored by europium(III) doping, indicated by the band-gap absorption detection that also undergoes red-shift by increasing the IDC number. Photoluminescence emission spectra have shown Eu(III) intra-configurational 4f-4f and ZnO excitonic emissions. Excitation spectra have shown energy transfers from ZnO to the silica as well as an energy transfer from ZnO to Eu(III) ions indicating the migration of these ions into the ZnO structure. *Italo Odone Mazali Functional Materials Laboratory, Institute of Chemistry, University of Campinas (UNICAMP). P.O. Box 6154, Zip Code 13083-970 Campinas/SP - Brazil. Phone: +55 19 3521-3164. Fax: +55 19 3521-3023 e-mail address: [email protected]

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Europium(III)-doped ZnO obtained by a hierarchically nanostructured multilayer growth strategy Cristine Santos Oliveira†, Jefferson Bettini‡, Fernando Aparecido Sigoli† and Italo Odone Mazali†,* †

Functional Materials Laboratory, Institute of Chemistry, University of Campinas (UNICAMP), P.O. Box 6154, Zip Code 13083-970, Campinas-SP, Brazil.

E-mail: [email protected] Phone: +55 19 35213164 / Fax: +55 19 35213023 ‡

Brazilian Nanotechnology National Laboratory, P.O. 6192, 13083-970, Campinas, Brazil

ABSTRACT A layer-by-layer growth method, named herein as impregnation-decomposition cycles (IDC), was used to build hierarchically nanostructured multilayered nanoparticles, alternating undoped ZnO and Eu(III)-doped ZnO layers in order to thermally induce Eu(III) migration into the ZnO structure. Porous Vycor Glass (PVG) was used as a mesoporous silica host, where the pores were used as a confined environment for preparation of ZnO nanoparticles. The proposed method allowed the control over the growth and size of the nanoparticles and prevented their coalescence. HRTEM images and XRD data revealed that ZnO nanoparticles are spheroidal with wurtzite crystal structure and an average diameter of 4.8 nm for 10 IDC. UV-Vis DRS data indicated that the formation of ZnO nanoparticles is favored by europium(III) doping indicated by the band-gap absorption detection that also undergoes red-shift by increasing the IDC number. Photoluminescence emission spectra have shown Eu(III) intra-configurational 4f-4f and ZnO excitonic emissions. Excitation spectra have shown energy transfers from ZnO to the silica as well as an energy transfer from ZnO to Eu(III) ions indicating the migration of these ions into the ZnO structure.

Keywords: Zinc oxide, nanoparticle, europium(III), luminescence, core@shell.


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1. INTRODUCTION Zinc oxide has received considerable attention towards its optical and electronic applications, in the fabrication of LEDs, transistors, energy harvesting materials, and others[1-5]. This is due to ZnO particular properties: it is a wide band gap (Eg ≈ 3.3 eV)[1] semiconductor with high exciton binding energy (~60 meV)[1], and several defectrelated electronic levels within its band gap, due to its intrinsic non-stoichiometry[5-9]. However, requirements for nowadays applications often involve the combination of ZnO properties with those of specific dopants in order to boost its applicability even further. A few examples would be: Mn(II) and Co(II), which promote ferromagnetic properties[10]; Al(III), Ga(III) and In(III) [11], n-type dopants that increase conductivity and overall optical quality of ZnO thin films. La(III) is also used as a doping ion for photocatalytic applications[12] and the combination of ZnO with ZnS improves photoconductivity[13] and photoluminescence properties[13-15]. The doping with lanthanide ions, such as Eu(III)[16-19], Tb(III)[20] and Yb(III)[21], has also received considerable attention due to the possibility of applications in electronics and optics systems,

by

the

combination

of

lanthanides

optical

properties

with

ZnO

electroluminescence potential. These ions have narrow emissions bands that may also be split or intensified through modification of their chemical environment and microsymmetry [22,23]. Dorenbos and van der Kolk[24] have built an energy diagram combining the La(II) and La(III) charge transfer (CT) levels and the valence and conduction bands of ZnO. The latter authors suggest, based in the work done by Yang et al.[17], that Eu(III) behaves as a shallow electron trap with the Eu(II) fundamental energy level slightly below the ZnO conduction band. This means that in terms of bands/levels energy positioning, a ZnO→Eu(III) energy transfer shall be expected. The insertion of lanthanide ions into ZnO, however, is not an easy task due to differences of ionic radii, charge and coordination number

[21,25]

. Several attempts to successfully introduce

Eu(III) into the ZnO crystal structure have been reported: Pechini method[7,19], hydrothermal synthesis[26], use of Li(I) or F- as charge compensation for Eu(III)-doped ZnO[27,28], and ion implantation[29]. Simple methods that just mix the respective reagents together normally lead to ZnO containing Eu2O3 as segregated phase. Actually, addition of co-dopants as electrical charge compensators in lanthanide(III)-doped ZnO may result in electronic defects in the ZnO band gap, such as oxygen vacancies or interstitial Zn(I), while ion implantation can result in crystal lattice strains that may be undesirable for optical applications.



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Approaching this matter from nanomaterials perspective, there are various advantages and possibilities: not only do these materials present distinct optical properties from that of their bulk counterparts, but also there are several methods and strategies for their preparation, which may provide control over size and/or shape of nanoparticles, leading to promising new properties and materials[2,30,31]. For example, a system comprised by a host in which different compounds and/or structures are inserted into, or supported on, can lead to innovative properties, resulting of a combination of host and guest materials properties and the interaction between them[32]. The impregnation-decomposition cycles (IDC) method consists of a simple association of metallo-organic decomposition (MOD) with the use of a porous support host, where one IDC consists of impregnation of this host with a given metallo-organic single-source precursor followed by its thermal decomposition leading to compounds of interest

[32]

.

Previous studies have confirmed the applicability of the IDC as a layer-by-layer method for growth and size control of nanoparticles and core@shell oxide nanostructures (1-10 nm)

[32-37]

. Santos et al.[38] have controlled the growth of MoO3 nanoparticle sizes that

were followed by Raman and UV-Vis spectroscopies. In the work done by Corrêa et al.[39] a series of core@shell nanoparticles of TiO2 and CeO2 were investigated by Raman spectroscopy correlating the energy changes of Eg (TiO2) and T2g (CeO2) bands with particle sizes indicating the possibility of controlling both the core diameter and shell thickness by the IDC layer-by-layer method. Using the IDC method it is possible to prepare nanoparticles by alternating layers (IDC) of pure ZnO metallo-organic precursor and mixed lanthanide and ZnO precursors, that after thermal treatments will lead to Eu(III)-doped ZnO. Using alternating layers it would be possible to avoid segregation of Eu2O3 by minimizing the amount of Eu(III) that will migrate into ZnO layers during thermal treatment. In the present work, the IDC method was used to prepare hierarchically nanostructured multilayered nanoparticles, alternating pure ZnO and europium(III)containing ZnO layers in order to thermally induce the Eu(III) migration into the ZnO structure. These nanoparticles were prepared and hosted into the mesoporous structure of the Porous Vycor Glass (PVG), in order to control their growth preventing the coalescing of nanoparticles. Pure ZnO nanoparticles were first synthesized in order to understand their growth behavior as a function of each IDC and the energy transfer and interactions with the silica host. Eu(III)-doped ZnO nanoparticles were then synthesized by alternating layers of pure ZnO and Eu(III)-containing ZnO. In order to understand


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the initial formation of ZnO clusters a set of Al(III) and Sr(II)-doped ZnO nanoparticles were synthesized and characterized. Photoluminescence data indicate a possible energy transfer from ZnO to Eu(III) ions.

2.EXPERIMENTAL PROCEDURE 2.1 Materials: Zinc(II) 2-ethylhexanoate, Zn[OOCCH(C2H5)C4H9]2, europium(III) 2ethylhexanoate,

Eu[OOCCH(C2H5)C4H9]3,

strontium(II)

2-ethylhexanoate,

Sr[OOCCH(C2H5)C4H9]2, hereafter named Zn(hex)2, Eu(hex)3 and Sr(hex)2, respectively, were purchased from Strem Chemicals, and aluminum(III) acetylacetonate, Al(C5H8O2)3, denominated Al(acac)3 was purchased from Sigma-Aldrich. These metallo-organic compounds were used without further purification.

2.2 Syntheses of un-doped ZnO bulk samples: Two ZnO bulk samples were obtained submitting 1: pure Zn(hex)2 reagent and 2: its [1.10 mol L-1] hexane solution to a thermal treatment in a furnace under static air atmosphere at 600 ºC for 4 h.

2.3 Syntheses of hierarchically nanostructured multilayer nanoparticles: Disks of Porous Vycor® glass 7030 (PVG) of 0.5 cm diameter x 0.1 cm thickness were used as substrate. Undoped ZnO nanoparticles in PVG were synthesized by impregnation of the PVG disks with Zn(hex)2 [1.10 mol L-1] in hexane for 18 h. After impregnation, the disks were submitted to thermal treatment in a furnace under static air atmosphere at 600 ºC for 4 h. This procedure consists of one impregnation-decomposition cycle (IDC). The samples obtained are PVG/xZnO, where x = (1 to 10) corresponds to the number of IDCs. Using the same conditions a PVG/xEu2O3 composition, where x = (1 to 10), was synthesized using a [0.028 mol L-1] Eu(hex)3 hexane solution. Doped hierarchically nanostructured samples were synthesized using three main compositions:

(i) PVG/5ZnO@x(ZnO/D)@yZnO, hereafter described as ZDZ 5/x/y, where D (0.5 mol%) = Eu(III), Al(III), Sr(II), resulting in ZEuZ, ZAlZ and ZSrZ, respectively, with x and y describing the IDC numbers.

(ii) PVG/5ZnO@w[2(ZnO/Eu(III))@2ZnO]@α(2ZnO/Eu(III)), hereafter described as Z[EuZ]Eu 5t/α, where t describes how many (2ZnO/Eu(III))@2ZnO layers are present,



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and α describes if there is a (2ZnO/Eu(III)) layer after them. This set of samples also contain an extra set of ZEuZ samples 5/0/0, 5/1/0, 5/2/0, 5/2/1 for comparison.

(iii) PVG/2(ZnO/Eu(III))@vZnO, hereafter described as EuZ 2/v, where v is the IDC number.

In Table 1 are found the illustrations for the strategies employed for the nanoparticles growth. This last synthesis was performed to verify the maximum amount of ZnO that could be impregnated, and simultaneously observe the layer-by-layer growth of the nanoparticles through means of the band gap absorption red-shift. Parameters for the IDCs were the same used for PVG/xZnO, using C = 1.10 mol L-1 as standard, except for EuZ 2/v where C = 0.50 mol L-1. All samples of a given composition were submitted to the same total thermal treatment, i. e. samples ZEuZ 5/0/0 and ZEuZ 5/2/10 were both submitted to 17 thermal treatments.


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Table 1. Strategies employed for the synthesis of the hierarchically nanostructured multilayer nanoparticles. The numbers represent the number of IDC. The black line under the particles represents the internal surface of PVG. ZDZ 5/x/y

Z[EuZ]Eu 5/t/α α

EuZ 2/v

ZDZ 5/0/0

EuZ 2/0

ZDZ 5/1/0

EuZ 2/1

ZDZ 5/2/0

EuZ 2/2

ZDZ 5/2/1

EuZ 2/3

ZDZ 5/2/3

Z[EuZ]Eu 5/1/1

EuZ 2/4

ZDZ 5/2/5

Z[EuZ]Eu 5/2/0

EuZ 2/5

ZDZ 5/2/7

Z[EuZ]Eu 5/2/1

EuZ 2/27

ZDZ 5/2/10

Z[EuZ]Eu 5/3/0

EuZ 2/28



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2.4 Characterization: HRTEM images were obtained using a JEOL 3010 microscope (300 kV, 1.7 Ǻ point resolution). EDS analysis was obtained in STEM mode using a JEM 2100F microscope (200kV). The samples were prepared by suspending the powders in ethanol and placing a drop of this suspension on a holey carbon coated Cu grid. High-intensity synchrotron X-ray powder diffraction (XRD) data were collected using X-rays of λ = 1.77 Å. Diffuse reflectance UV-Vis spectra (DRS) were obtained on a CARY 5 UV-Vis spectrophotometer. The disks were grounded and the powdered samples were smoothly compacted into a quartz sample holder which was mounted in an integrating sphere. The data were collected between 200 and 800 nm with a 2 nm spectral bandwidth. BaSO4 powder was used as standard for instrumental background correction. Photoluminescence data was acquired using a Horiba Jobin Yvon FL3-22iHR320 model at stationary state and room temperature, using a 450 W Xenon lamp as excitation source, front-face mode (22.5º angle), and the photomultiplier Hamamatsu R298 was used as a detector. Emission spectra were obtained from 290 to 740 nm with a 2 nm step, and excitation spectra in the range from 240 to 400 nm with a 1 nm step . All spectra were corrected automatically according to lamp emission, optics and detectors responses. Emission spectra in the near-infrared region, 940-1200 nm, were obtained with a 2 nm step using the same excitation source and a photomultiplier Hamamatsu 10330-75 for detection. For emission lifetime measurements, a pulsed 150 W Xenon lamp was used as an excitation source, in front-face mode (22.5º angle) using a TCSPC system and 1024 channels.

3. RESULTS AND DISCUSSION The IDC method led to a linear mass increase for all samples, estimated using the mass differences of the PVG disks before and after each IDC, reaching values as high as 1.9 % per IDC. Undoped ZnO samples PVG/xZnO were weighed one by one showing an increase of 1.6 % per IDC (Figure 1a). This mass increase is considered high in comparison with similar growth conditions of different oxides such as TiO2[33,34], CeO2[34,35], NiO[36] and γ-Fe2O3[37], but smaller than obtained for SnO2[32,40]. The ZDZ 5/x/y samples have a similar mass increase among themselves (1.8-1.9 %) indicating that doping does not affect mass gain (Figure 1b). A maximum of 42% mass increase was achieved for the EuZ 2/v composition (Supporting Information - Figure S1), which is about half of what a rough estimation based on PVG superficial area and ZnO density and volume would give (~ 90 %). This is attributed to the PVG porous


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structure becoming gradually more restricted and clogged with an increasing amount of ZnO that does not only hinder the impregnation but also compromises the decomposition of precursors.

Figure 1. Percentual cumulative mass gain versus number of IDCs obtained for (a) PVG/xZnO and (b) ZEuZ, ZAlZ, ZSrZ, with their respective linear regressions.

Analyzing the XRD patterns of samples PVG/10ZnO and ZEuZ 5/2/10 (Figure S2), the three main diffraction peaks of wurtzite ZnO are observed only for the latter. A similar behavior was reported by Yao et al.[41] and may be attributed to peaks positions being coincident to the non-crystalline halo of the silica host, small nanoparticles (~4.5 nm, as seen in HRTEM images - Figures 2a-b) and high dispersion within the host. Representative HRTEM images were obtained for PVG/10ZnO and ZEuZ 5/2/10 samples are shown in Figure 2. In all cases disperse spheroidal ZnO nanoparticles with diffraction planes of the wurtzite structure are observed. Similar



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result was published by Panigrahi et al.[42] who synthesized silica-coated ZnO nanoparticles. The preparation of nanoparticles with high dispersion and small sizes is a result of the efficient combination of nanoparticles growth control by the IDC method, and the coalescence prevention by the PVG mesoporous structure. Nanoparticle size distribution histograms obtained from HRTEM images show a small increase in average size from 4.5 to 4.8 nm for 7 and 10 IDCs, respectively, attributed to nanoparticle growth with each IDC (Figure S3 a-b). However, the ZEuZ 5/2/10 sample show a smaller average particle size (4.2 nm, Figure S3c), that may indicate a fast formation of seeds, which was also observed by UV-Vis spectroscopy, combined with a decrease in growth kinetics due to the presence of Eu(III) as reported by Pires et al.[43] An EDS analysis was also performed in STEM mode for sample ZEuZ 5/2/10 over ZnO:Eu(III) nanoparticles supported inside PVG (Figure S4), and it was possible to detect an amount of Zn of 25%/Si, in agreement with cumulative mass gain results, as well as 1% Eu/Zn, in agreement with the amount of Eu(III) used in doping.


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Figure 2. HRTEM images of samples (a) PVG/10ZnO and (b) ZEuZ 5/2/10. The insets show the ZnO wurtzite diffraction planes.

UV-Vis absorption spectra were obtained through diffuse reflectance (DRS) converted to F(R) = (1-R)2/2R, the Kubelka-Munk function, where R is the absolute reflectance. The band gap energies (Eg) of samples were obtained by extrapolation of the linear range of the absorption spectrum for y = 0 in a curve of (F(R).hν)2 vs. hν for a direct band gap[44,45]. The spectra of undoped ZnO bulk samples are shown in Figure 3. The ZnO samples have lower band gap absorptions (2.82 and 3.04 eV) than expected



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for ZnO bulk (~ 3.3 eV). This may be attributed to the formation of defects such as VO and Zni which are of lower energy than the band gap[4], causing its absorption to be slightly red-shifted.

Figure 3. UV-Vis absorption spectra obtained by diffuse reflectance (DRS) converted by the Kubelka-Munk function of bulk ZnO samples. The inset illustrates how Eg was obtained.

The UV-Vis spectra of PVG/xZnO samples were obtained for each IDC (Figure 4). The spectra are similar and show an absorption band around 200 nm attributed to the Zn2+←O2- charge transfer that is slightly red-shifted as a function of the IDC number due to the increase in size of nanoparticles that are under quantum confinement. On the other side, the absorption band in the region of 245-250 nm does not shift with the IDC number. As observed by Yao et al.[41], the interaction between non-bridging oxygen (SiO-) from the silica network and the ZnO surface may result in the formation of Zn-O-Si species, increasing the absorption energy of ZnO. Yao et al.[41] suggest that for a porous host with high surface area there is a possibility of non-bridging oxygen formation even using low temperatures of thermal treatment, and considering that in the present work decomposition temperature used was 600 ºC, higher than that used by the latter authors, this effect would be even more prominent. This is also in agreement with the work published by Tkachenko et al.[46], who have grown ZnO and CuO nanoparticles inside silica hosts and observed certain difficulty for the formation of ZnO clusters due to strong interaction between zinc ions and SiO2 surface. Tkachenko et al.[46] state that ZnO clusters grow primarily as monodisperse sites, similarly to a monolayer, and consider the possibility of zinc silicate formation at the host surface. In another similar work, Gärtner et al.[47] have synthesized powders of TiO2 with silica and confirmed an


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increase in the band gap as the TiO2:SiO2 molar ratio decreased. The absorption band observed at 270 nm and edge at around 320 nm (Figure 4), although similar to the one seen by Zhang et al.[48] who attribute it to the ZnO band gap, is in this current work attributed to a defect-related absorption of PVG, given its similarity with the spectrum obtained for pure PVG (Figure S5).

Figure 4. UV-Vis absorption spectra obtained by diffuse reflectance converted by the KubelkaMunk function of samples PVG/xZnO (x = 1 to 10). (*) indicates light source changing.

The UV-Vis spectra obtained for the first doped ZEuZ samples are shown in Figure 5. For samples ZEuZ 5/0/0 (no dopant) through ZEuZ 5/2/1 (first pure ZnO layer after the two Eu(III)-containing layers) the spectra profiles are similar to that obtained for undoped ZnO. However, for subsequent IDC the ZnO band gap is detected and redshifted as a function of the IDC number. The band gap energy of ZEuZ 5/2/3 sample is 3.29 eV and shifts regularly for each 2 IDC until 3.22 eV for ZEuZ 5/2/10 meaning these nanoparticles are under quantum confinement. Even though, the Eg values are still lower than expected considering usual ZnO bulk values (~3.3 eV) and the possibility of formation of Zn+i, a defect with slightly lower energy (~0.15 eV)[4] than the ZnO conduction band was taken into consideration. To sort out this possibility, compositions ZAlZ and ZSrZ were synthesized, replacing Eu(III) ions by Al(III) and Sr(II), respectively, where Al(III) would promote the formation of Zn+i while Sr(II) would not, and a band gap shift would be expected for Al(III)-doped samples. Both ZAlZ and ZSrZ samples show similar results, with the band gap band appearing for ZAlZ 5/2/1 sample with Eg = 3.29 eV to 3.15 eV for ZAlZ 5/2/10 (Figure S6a) and for ZSrZ 5/2 with Eg =



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3.29 eV to Eg = 3.12 eV for ZSrZ 5/2/10 (Figure S6b), indicating that the insertion of dopants only favor the growth of ZnO nanoparticles in addition to the dominant Zn-OSi species. The results obtained for Z[EuZ]Eu samples were similar as well (Figure S6c). In all cases, ZnO band gap appears only after the inclusion of a dopant, indicating the preference of Zn-O-Si species formation over Zn-O-Zn while the latter is favored in doped samples. It is worthy to stress that the doping ions are helping the formation of ZnO clusters/nanoparticles.

Figure 5. UV-Vis absorption spectra obtained by diffuse reflectance converted by the KubelkaMunk function for the ZEuZ 5/x/y samples. The inset shows the (F(R).hν)2vs.hν plots.

Using the theoretical equation of band gap energy as a function of nanoparticle diameter proposed by Schoenhalz et al.[49] (Eg = 3.41 + 3.87 d-1.83) based on DFT calculations and applied to several results from the literature[50], the band gap values obtained by UV-Vis spectra for all ZDZ 5/x/y and EuZ 2/v samples (Figure 5, S6 and S7), and the bulk band gap values obtained in the present work (2.82 and 3.04 eV, instead of 3.41 eV, Figure 3), it was possible to estimate the nanoparticles average sizes (Figure 6) and compare them with those obtained using HRTEM (Figure S3). For Eg = 3.04 eV the results indicate a wide distribution going as high as 10 nm that was not seen in HRTEM images. For Eg = 2.82 eV the largest average sizes are located within 3 to 4 nm, which is still smaller than most values obtained by microscopy. However, using an average between these two band gap energy values (average Eg = 2.93 eV), the largest nanoparticles (most ZDZ 5/2/10 samples) are located within the range of 4 to 5 nm, that is in good agreement with the average size of 4.2 to 4.8 nm obtained by HRTEM. This has shown that through a simple modification of a theoretical equation based on perfect


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ZnO nanocrystals, it was possible to apply it for several experimental ZnO nanoparticles.

Figure 6. Curves constructed using the original equation Eg = 3.41 + 3.87 d-1.83 initially proposed by Schoenhalz et al.[49], and the modified equations replacing the theoretical band gap energy of 3.41 eV by those obtained for bulk ZnO in the present work (2.82 eV e 3.04 eV and their average 2.93 eV), and plotted Eg values obtained from UV-Vis for ZEuZ, ZAlZ, ZSrZ, Z[EuZ]Eu and EuZ samples.

Photoluminescence emission spectra were obtained using excitations at 245 and 350 nm bands that were observed for Si-O-Zn species and the ZnO band gap, respectively. Excitation spectra were obtained using emissions at 435 and 613 nm attributed to band transitions of PVG and Eu(III) 5D0→7F2, respectively. Results obtained for ZEuZ samples are shown in Figures 7 and 8, and Z[EuZ]Eu in Figures S8 and S9. Emission spectra (Figure 7a) show a series of wide emission bands in the 300450 nm range attributed to PVG defects and Eu(III) 5D0→7FJ (J = 0 (577 nm), 1 (592 nm), 2 (613 nm), 3 (652 nm), 4(699 nm)). In addition, ZnO excitonic emission is also observed, in the 380-410 nm range, for samples ZEuZ 5/2/3 through 5/2/10 and Z[EuZ]Eu A 5/2/0 through 5/3/0 (Figure S8) and a red-shift is observed as a function of IDC number.



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Figure 7. Emission spectra of ZEuZ 5/x/y samples with (a) λexc = 245 nm and (b) λexc = 350 nm, and band attributions. (*) indicates the moment of insertion of the optical filter (399 nm and 450 nm, respectively) and (↓) indicates the excitonic emissions.

According to Chakrabarti et al.[51] the shifting of the excitonic band may be attributed to quantum confinement effect. The emission spectra obtained with excitation at 350 nm (Figure 7b) show two broad bands at 414 and 435 nm and a shoulder located at 456 nm attributed to PVG defect emissions along with very low intensity and narrow bands attributed toEu(III) 5D0→7F2,4 transitions at 613 and 699 nm, respectively. The presence of these bands is interesting and indicates an energy transfer from ZnO to Eu(III) ions since no Eu(III) 4f-4f transitions are found at 350 nm. Emission from ZnO electronic defects were not observed for any sample. Excitation spectra (Figure 8a) show that there are non-radiative energy transfers from both CT (Zn-O-Si species) and


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ZnO band gap to the silica host, which may be a reason to the absence of ZnO defect emissions.

Figure 8. Excitation spectra of ZEuZ samples with (a) λem = 435 nm, (b) λem = 613 nm and (c) λem = 613 nm with higher resolution over the ZnO band gap region, and band attributions.



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The excitation spectra for the Eu(III) 5D0→7F2 transition at 613 nm (Figure 8b) show one band around 260 nm attributed to the Eu3+←O2- charge transfer, and several other bands attributed to Eu(III) 4f-4f transitions. One may observe (Figure 8c) a slight baseline variation around the ZnO band gap (~380-390 nm) suggesting an energy transfer from the ZnO conduction band to Eu(III) excited levels, indicating a possible doping of ZnO, or from Zn-O-Si interface states, such as suggested by Hong et al.[52]. In a similar context, Mbule et al.[53] observed an improvement in Pr(III) emissions with the insertion of ZnO quantum dots, attributed to energy transfer from ZnO defect energy levels to praseodymium levels. Emission lifetimes obtained for samples PVG/10Eu, Z[EuZ]Eu 5/1/0, Z[EuZ]Eu 5/2/1 and Z[EuZ]Eu 5/3/0 are presented in Table 2 and their respective decay curves can be found in the Supplementary Material (Figure S10).

Table 2: Emission lifetime results obtained for samples PVG/10Eu2O3, ZEuZ 5/1/0, ZEuZ 5/2/1 and Z[EuZ]Eu 5/3/0 Eu(III) 5D0→7F2 τ(ms)

Sample PVG/10Eu2O3

ZEuZ 5/1/0

τ1 = 0.4±0.1 τ2 = 0.9±0.1 τ1 = 0.8±0.1 τ2 = 2.5±0.1

ZEuZ 5/2/1

τ1 = 1.3±0.1 τ2 = 2.8±0.2

Z[EuZ]Eu 5/3/0

τ1 = 1.6±0.1 τ2 = 3.0±0.1

For all samples analyzed, bi-exponential decays were obtained. The short emission lifetime value is attributed to surface located Eu(III) ion, where this ion is subject to O-H oscillators, which contribute to non-radiative decay routes. Long lifetime values are attributed to Eu(III) in inner nanoparticle environments that are far enough from O-H oscillators and experiencing less structural distortion. Pure Eu2O3 in PVG has the shortest lifetime values (Table 2) due to PVG (O-H)-rich surface and Eu(III)-Eu(III) energy transfer process. For sample Z[EuZ]Eu 5/1/0 a considerable increase of lifetime value is observed, attributed to the presence of ZnO protective “layers” between PVG


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surface and Eu(III)-doped ZnO species (PVG/ZnO@ZnO:Eu(III)), preventing direct interaction with PVG hydroxyl groups. The Z[EuZ]Eu 5/2/1 and Z[EuZ]Eu 5/3/0 samples,

with

ZnO

protective

layers

after

the

formation

of

ZnO:Eu(III)

(PVG/ZnO@ZnO:Eu(III)@ZnO) show an increase in lifetime values that may be attributed to compensation of surface defects and absence of high energy oscillators with the addition of ZnO protecting “layers” indicating a layer-by-layer nanoparticle growth. The obtained emission lifetime values are longer than other published results for Eu(III)-containing ZnO thin films[16] (0.1/0.5 ms for 0.1 %mol Eu(III):ZnO) and ZnO microrods[54] (0.03 ms), and shorter than ZnO powders[55] (3.4 ms) indicating a possible formation of Eu(III)-doped ZnO.

4. CONCLUSIONS Undoped and Eu(III)-doped ZnO nanoparticles with controlled size have been successfully synthesized, combining a layer-by-layer growth with the use of a silica host. The strong interaction between Zn(II) and non-bridging oxygen from silica surface leads primarily to the formation of Zn-O-Si species. The growth of ZnO nanoparticles is favored in the presence of dopant ions that probably favor the formation of ZnO clusters. The obtained Eg values were treated by a theoretical equation[36] of Eg vs. nanoparticle diameter, and by a simple adjustment of the band gap value in the original equation the nanoparticle sizes obtained from higher IDC numbers were in good agreement with those obtained by HRTEM. Photoluminescence spectra show that both Zn-O-Si species and ZnO nanoparticles transfer energy to the silica host and no ZnO electronic defect emissions were observed. Excitation spectra show a possible energy transfer from the ZnO band gap to europium excited levels. Emission lifetime results corroborate with a layer-by-layer growth reaching 3.0 ms, a quite long lifetime for Eu(III)-doped ZnO nanoparticles.

Supporting Material: The Supporting Material is available free of charge via the Internet at http://pubs.acs.org.

5. ACKNOWLEDGEMENTS CSO is indebted to CNPq for a PhD fellowship. IOM and FAS are indebted to CNPq and FAPESP for financial support. Contributions from Brazilian Center for Research in Energy and Materials (CNPEM) for XRD (LNLS – Brazilian Synchrotron



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Light Laboratory) and HRTEM (LNNano - Brazilian Nanotechnology National Laboratory) analyses are also gratefully acknowledged. The authors would like to thank the Multiuser Laboratory of Advanced Optical Spectroscopy (LMEOA/IQ-UNICAMP FAPESP Proc. 2009/54166-7) for use of its equipment.

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FOR TABLE OF CONTENTS USE ONLY

Europium(III)-doped ZnO obtained by a hierarchically nanostructured multilayer growth strategy Cristine Santos Oliveira†, Jefferson Bettini‡,Fernando Aparecido Sigoli† and Italo Odone Mazali†,* TABLE OF CONTENTS GRAPHIC

TABLE OF CONTENTS SYNOPSIS Using a controlled growth method, hierarchically multilayered nanoparticles were synthesized alternating layers of pure and Eu(III)-containing ZnO, and through thermal treatments inherent to the method used, thermally induce Eu(III) to migrate into the ZnO structure. Luminescence excitation spectra show an energy transfer from ZnO to Eu(III) ions, indicating the migration of the lanthanide ions into ZnO.


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Europium(III)-Doped ZnO Obtained by a Hierarchically Nanostructured

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