Synthesis, structure, EPR studies and up-conversion luminescence of ZnO:Er³⁺–Yb³⁺@Gd₂O₃ nanostructures

Abstract

ZnO:Er³⁺–Yb³⁺@Gd₂O₃ nanostructures were obtained by “wet” chemistry methods – the sol–gel technique for the preparation of ZnO and ZnO:Er³⁺–Yb³⁺ nanoparticles (NPs), and the seed deposition method for obtaining Gd₂O₃. The crystal structure, morphology, phase and elemental composition, resonant microwave absorption of rare earth ions, point defects in the ZnO:Er³⁺–Yb³⁺@Gd₂O₃ crystal structure and up-conversion luminescence were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) spectroscopy, and optical spectroscopy. The crystallization temperature (600 °C) of the Gd₂O₃ phase on the ZnO surface was found. As-obtained ZnO:Er³⁺–Yb³⁺ NPs (with size ∼7 nm) are highly crystalline and monodispersed. ZnO:Er³⁺–Yb³⁺ NPs annealing at 900 °C leads to the formation of highly polydispersed ZnO:Er³⁺–Yb³⁺ NPs, covered by a Gd₂O₃ shell. The process of the incorporation of the rare earth ions into the ZnO structure, as well as the effect of Gd₂O₃ content on the morphology and visible up-conversion (UC) luminescence in ZnO:Er³⁺–Yb³⁺ matrices were studied.

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1. Introduction

Zinc oxide (ZnO) is a well-known wide-band gap semiconductor, with a band gap of 3.37 eV at room temperature and a large exciton binding energy (60 meV).^1,2 Un-doped and doped ZnO nanoparticles with different activators are well-known optical materials and therefore they have a wide variety of applications such as photodetectors, sensors, solar cells, light-emitting diodes and p–n heterojunctions.^3–5 Furthermore, due to their good biocompatibility, low-cost, non-cytotoxicity and antibacterial properties ZnO is a very important material for medical and biological applications, mainly for food packaging, wound dressings, restorative dental materials, etc.^6,7

It is known that ZnO exhibits two luminescence bands: a short-wavelength band (UV region) and a broad long-wavelength band with a maximum in the green spectral range.⁸ Additionally, the wide band gap of ZnO allows the incorporation of luminescent centers such as rare earth (RE) ions.⁹ The presence of the activator and co-activator (sensitizer) in the inorganic materials (e.g. ZnO, NaGd(Y)F₄) leads to formation of new donor–acceptors interactions, resulting in efficient down-conversion (DC) or up-conversion (UC) processes.^10,11

Up-conversion materials are able to convert low-energy photons to higher-energy ones (UV and visible) using near infrared excitation. If luminescence of ZnO matrix depends on size and form of the materials, the emission wavelength of the UC nanoparticles is not size-dependent. Trivalent erbium ion (Er³⁺) in the different host matrices shows efficient UC emission in the green and orange-red spectral range. To increase the absorption cross-section and the pump efficiency, co-activation is needed. For trivalent rare earth ions such as Er³⁺, terbium (Tb³⁺) and thulium (Tm³⁺), the ytterbium ions (Yb³⁺) are an efficient sensitizer to obtaining blue, green and red emission.^12,13 Thus, due to their unique electronic structure, rare earth based materials had been extensively studied, especially from the point of view of their interesting luminescence, as well as magnetic properties.

Recently, one of the most rapidly developing areas in nanotechnology is the creation and study of the hybrid core–shell heterostructures. The fabrication of the core–shell structures based on two or more types of materials as well as the variation of their properties allows developing new multifunctional platforms for wide scientific applications.^14,15

To obtain effective functional materials (phosphor, collar cells, biomarker, etc.) it is needed to control the materials characteristics, such as crystal structure, phase and chemical composition, particles morphology, type and concentration of the activator. “Wet” chemistry methods (sol–gel, co-precipitation, hydrothermal, microemulsion, etc.) are well-known methods, which make possible to obtain highly crystalline nanoparticles below 10 nm with controlled shape, size and dispersity.^16,17 Moreover, an increasing number of publications are devoted to the synthesis and characterization of the micro- or nanomaterials, doped with luminescent ions or dopants, which modify the crystal structures.^18,19 However, the mechanisms of the activator incorporation into the crystal lattice are not fully understood and the aqueous synthesis of the nanoparticles with strictly stoichiometry, especially when the synthesis occurs in the multi-component system, needs the strict control of the synthesis process. Non-controlled synthesis can lead to formation of undesired phases or groups, adversely affecting the functional properties of the final material. Theoretical ions concentration of precursors and real ions concentration in final materials may be different, because part of the activator/dopant ions are incorporated in the crystal structure), as well as part of the ions remains in solution (supernatant) or in the “dead volume” (walls and bottom of the laboratory glassware). Therefore, the incorporation coefficients (segregation coefficient in case of the single crystals) of the activator/dopant for each material is different^20,21 and it is very important to know the real ion concentration in the final material.

At the same time, it should be noted, that the surface of the nanoparticles, including ZnO, is full of defects, so adsorption of OH⁻ or CO₃²⁻ groups (from surrounding synthesis environment) is evident. To prevent absorption processes the surface of nanocrystals needs to be modified by a protective shell based on metal oxide, e.g. Gd₂O₃.²²

Obtained ZnO:Er³⁺–Yb³⁺@Gd₂O₃ nanoparticles, due to their up-conversion properties, as well as biocompatibility and non-toxicity of the ZnO matrix, can find wide applications in biomedicine. They can be good alternative to well-known organic biomarkers, based on lipids or semiconductors quantum dots (QDs), which consist of toxic metals. Unlike UV excitation, which is harmful for living organisms, up-conversion nanoparticles produces near-infrared (NIR) light-excited fluorescence which is not toxic for in vivo studies. Also, due to the paramagnetic properties of the gadolinium ions, they can be used as contrast agents, useful for magnetic resonance imaging, MRI. Furthermore, electron paramagnetic resonance spectroscopy (EPR) will allow to detect rare earth ions and paramagnetic structural defects in the crystal structure. The presence of structural defects can be an indicator of the synthesis quality of ZnO structures, which could be an important factor when applied in vivo experiments.

In this paper we report the sol–gel synthesis of the nanostructures, based on ZnO nanoparticles, doped with Er³⁺ and Yb³⁺ up-conversion ions and covered by Gd₂O₃ shell. The process of the incorporation of rare earths ions into the ZnO structure, as well as the effect of Gd₂O₃ content on the crystalline structure, morphology, visible up-conversion (UC) luminescence in the ZnO:Er³⁺–Yb³⁺ matrices are studied.

2. Results and discussion

2.1. Structural characteristics

2.1.1. Morphology analysis. Un-doped ZnO NPs and ZnO@Gd₂O₃ core–shell nanostructures were previously obtained and thoroughly studied by our group.²³ ZnO NPs, doped with Er³⁺ and co-doped with Yb³⁺ as well as ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell structures were obtained by analogy with un-doped nanostructures. The ZnO crystal phase was obtained by hydrolysis of Zn salts (in our case Zn(Ac)₂) in a basic solution (NaOH) at 60 °C in methanol. After separation, washing and drying, obtained ZnO:Er³⁺–Yb³⁺ NPs were covered by Gd³⁺ in aqueous solution at different times (6 h, 48 h and 96 h) and subsequently annealed to obtain the Gd₂O₃ shell. For convenience, samples nomenclature and the synthesis conditions of the samples preparation are presented in Table 1.

Table 1 Nomenclature and synthesis conditions of the samples preparation

Samples title	Samples preparation
S-I	As-obtained ZnO NPs (synthesis temperature 60 °C)
S-II	ZnO NPs annealed at 900 °C for 1 h
S-III	ZnO:Er^3+–Yb^3+ NPs annealed at 900 °C for 1 h
S-IV	ZnO:Er^3+–Yb^3+ NPs modified by Gd^3+ shell for 6 h and annealed at 900 °C for 1 h (ZnO:Er^3+–Yb^3+@Gd2O3 core–shell)
S-V	ZnO:Er^3+–Yb^3+ NPs modified by Gd^3+ shell for 48 h and annealed at 900 °C for 1 h (ZnO:Er^3+–Yb^3+@Gd2O3 core–shell)
S-VI	ZnO:Er^3+–Yb^3+ NPs modified by Gd^3+ shell for 96 h and annealed at 900 °C for 1 h (ZnO:Er^3+–Yb^3+@Gd2O3 core–shell)

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The as-obtained ZnO NPs (S-I) obtained by sol–gel technique have spherical shape, are high crystalline and monodispersed with the mean size around 7 nm. Doping ZnO matrix with Er³⁺ and Yb³⁺ does not change the form and size of the nominal pure ZnO NPs (Fig. S1, ESI†). On the other hand, the annealing of the ZnO, ZnO:Er³⁺–Yb³⁺, and ZnO:Er³⁺–Yb³⁺@Gd₂O₃ samples at 900 °C for 1 h leads to a nanoparticle sintering and to a strong particle growth due to recrystallization process.

Fig. 1 presents the SEM images of the ZnO NPs doped with Er³⁺ and Yb³⁺ and ZnO@Gd₂O₃ which show nanostructured morphologies. After the annealing, the ZnO:Er³⁺–Yb³⁺ (S-III) particles are polydispersed with sizes around 30-300 nm. Surface modification of ZnO:Er³⁺–Yb³⁺ NPs by Gd(NO₃)₃ water solution (for 96 h) and following annealing at 900 °C for 1 h leads to blocking of the particles growth and formation of monodispersed nanoparticles with sizes around 50 nm (S-IV–VI). Thus, we can conclude, that the morphology of the obtained annealed ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell nanostructures is not depended from Gd₂O₃ content.

Fig. 1 SEM images of the samples S-III (a), S-IV (b), S-V (c), S-VI (d).

2.1.2. X-ray diffraction (XRD) analysis. Traditionally, the Gd₂O₃ crystal phase can be obtained by the decomposition of gadolinium nitrate or oxalate in the temperature range of 800–1000 °C.²⁴ To determine the optimal temperature of crystallization of the Gd₂O₃ phase, the core–shell structures (un-doped ZnO nanoparticles, modified with Gd(NO₃)₃ water solution for 96 h) were annealed in the temperature range of 500–900 °C (Fig. 2).

Fig. 2 The phase composition of ZnO@Gd₂O₃ nanostructures, annealed at different temperatures: a – 500 °C, b – 600 °C, c – 900 °C for 1 h, inset – range at 27–30 2θ degrees at a higher magnification.

The diffraction peaks in whole temperature range, correspond to the main (100), (002), (101), (102), (110), (103), (112), (201), and weak (200), (004) and (202) crystal planes, belonging to hexagonal wurtzite ZnO structure with P6₃mc space group (corresponding to the standard crystallographic data in the JCPDS-ICDD index card no. 36-1451). For all samples the most intense peak was the one attributed to the (101) planes of the wurtzite structure. At 500 °C no additional phases were found. Annealing of the sample at 600 °C (Fig. 2b and more detailed in inset) leads to the appearing of the peak with low intensity attributed to the (222) planes, corresponding to the beginning of the cubic Gd₂O₃ phase crystallization. At 900 °C the peak attributed to the (222) planes is more intense and a new small peak corresponding to the (400) plane of the Gd₂O₃ phase is observed. Thus, for further experiments 900 °C was chosen as the optimal temperature.

Fig. 3 presents the XRD patterns of the ZnO NPs (as-prepared), as well as ZnO NPs, ZnO:Er³⁺–Yb³⁺ NPs, and ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell nanostructures, annealed at 900 °C. The broad peaks for the sample (S-I) are due to the small particle size of the as-obtained ZnO NPs.²³ Diffraction peaks for all annealed samples (where the NPs size increases up to 50 nm, samples S-II–S-VI) are intense and narrow indicating that these particles are highly crystalline. Doping ZnO matrix with Er³⁺ and Yb³⁺ ions leads to a small shift (within the measurement error) of the diffraction peaks to lower angles. For the ZnO:Er³⁺–Yb³⁺ sample (S-III) some small peaks attributed to Yb₂O₃ are found. But after the Gd₂O₃ shell modification (S-IV–VI), the diffraction pattern consists of only the ZnO and Gd₂O₃ crystalline structures. The Gd₂O₃ cubic phase was observed after just 6 h of ZnO NPs surface modification by Gd³⁺ and annealing (S-IV). The peak positions for all samples did not change, indicating that Gd³⁺ ions are not being incorporated into the ZnO structure, in other words, the Gd₂O₃ shell covered the ZnO NPs surface with formation of the two independent ZnO and Gd₂O₃ crystalline phases.

Fig. 3 XRD analysis of the samples studied.

2.2. Elemental composition

2.2.1. Energy-dispersive X-ray spectroscopy (EDX) analysis. The analysis of the elemental composition of the ZnO:Er³⁺–Yb³⁺ and ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell nanostructures was performed using the dry powders. The energy dispersive X-ray spectra of the ZnO NPs, ZnO:Er³⁺–Yb³⁺ and ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell nanostructures contains the peaks attributed to main elements of present nanostructures (Fig. 4). After 6 h modification (S-IV) Gd peaks are already present in the spectra (Fig. 4c).

Fig. 4 EDX spectra of the S-II (a), S-III (b), S-IV (c), S-V (d) and S-VI (e) samples.

The EDX results show, that the incorporation coefficients for both rare earth ions (Er³⁺ and Yb³⁺) are very low and reach approximately 1%. This is due to the fact, that in “wet” synthesis after centrifugation and washing, part of the RE ions is incorporated into the crystal, but part of the RE ions remains in the solution (supernatant) or in “dead volume” (walls and bottom of the laboratory glassware). Depending on the modification time of the ZnO:Er³⁺–Yb³⁺ NPs in Gd(NO₃)₃ water solution and subsequent annealing, Gd as well as O contents are increased due to formation of the corresponding oxides.

Elemental mapping of Zn, Gd and O, shows homogeneous distribution of all components without any concentrated places at the surface (Fig. S2, ESI†). The resolution of the SEM mapping is not able to detect the presence of the Gd₂O₃ shell. By analogy with the previously obtained results, related to un-doped ZnO@Gd₂O₃ nanostructures,²³ we can conclude that shell formation of doped ZnO:Er³⁺–Yb³⁺@Gd₂O₃ nanostructures is similar and the Gd₂O₃ shell thickness is approximately of 3 nm (Fig. S3, ESI†). Moreover, the elemental analysis in the core and shell region indicates, that amount of the Gd is higher in the shell region (Fig. S4 and S5 ESI†).

2.2.2. X-ray photoelectron spectroscopy (XPS) analysis. XPS analysis was also used to study the elemental composition and the different bonding states of the elements present in the nanostructures. Fig. 5 shows the gadolinium Gd 3d, ytterbium Yd 4d and erbium Er 4d spectra of the ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell samples modified by Gd³⁺ for 6, 48 and 96 h (S-IV, S–V and S-VI), respectively. All samples modified by Gd³⁺ present the characteristic doublet signal of the Gd 3d spectra, with the Gd 3d_5/2 peak positions at around 1187.6 eV which can be attributed to Gd–O²⁵ bonds in Gd₂O₃ confirming the formation of the shell structure. The peak at around 1195.8 eV can be attributed to the Zn 2s signal from zinc. On the other hand, the Yb 4d and Er 4d show a relatively increase in intensity from sample S-IV to S-VI, which can be related to the nonuniform distribution of the activators in the ZnO crystal structure during synthesis process.

Fig. 5 XPS Gd 3d, Yb 4d and Er 4d spectra of the ZnO:Er³⁺–Yb³⁺ NPs modified by Gd³⁺ shell for 6 h (S-IV), 48 h (S-V) and 96 h (S-VI) and annealed at 900 °C for 1 h (ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell).

The Yb 4d and Er 4d peak positions are around 185.4 and 170.5 eV, respectively, which can be correlated to Yb–O and Er–O bonds in Yb₂O₃ and Er₂O₃ (ref. 26) materials, and are in agreement with the expected presence of Er³⁺ and Yb³⁺ ions. The Zn 2p signal from the zinc in the core structure was also recorded and showed in Fig. S6 (ESI†). The Zn 2p_3/2 peak position in the samples is located at around 1021.8 eV and can be correlated to Zn–O bonds in ZnO.²⁶ Thus, the XPS results are in agreement with the XRD data and confirm the presence of ZnO, Er³⁺, Yb³⁺ and Gd₂O₃ in the ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell samples. The relative chemical compositions of the S-IV to S-VI samples, listed in Table 2, obtained from the areas of the photoelectron peaks reveal that the Er and Yb elements are in a low proportion, up to 0.8 at% for the Er and 0.2 at% for the Yb. There is an increase in the Gd, Er and Yb relative compositions from the sample S-IV to the S-VI, which correlates well with the modification of the samples with the Gd³⁺ at different times.

Table 2 Chemical compositions of the of the samples studied

Sample	Gd (at%)	Zn (at%)	O (at%)	C (at%)	Er (at%)	Yb (at%)
S-IV	2.4	47.4	41.7	8.2	0.2	0.1
S-V	10.3	27.0	47.5	14.4	0.7	0.2
S-VI	10.9	28.2	45.8	14.2	0.8	0.2

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2.3. Electron paramagnetic resonance spectroscopy (EPR) analysis

Obtained ZnO:Er³⁺–Yb³⁺ NPs and ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell nanoparticles were studied by EPR. The system gives rise to multiple EPR signals of various sources, namely, Gd³⁺ ions (electron conf. [Xe] 4f⁷5s²p⁶, ground term ⁸S_7/2, electron spin S = 7/2) in the shell material (Gd₂O₃), Er³⁺ ions (electron conf. [Xe]4f¹¹5s²p⁶, ground term ⁴I_15/2, S = 3/2) and Yb³⁺ ions (electron conf. [Xe]4f¹³5s²p⁶, ground term ²F_7/2, S = 1/2) doped in ZnO, as well as ZnO defects.

The most dominant contribution in the EPR spectrum is coming from gadolinium oxide (Gd₂O₃).²⁷ The shell exhibits very broad EPR line of ∼260 mT at 295 K and gets broader to ∼300 mT at 4.2 K (S-IV) and 335 mT (S-VI), with effective g-factor ∼2.0. The dipol–dipol interactions between Gd³⁺ ions even are strong enough to broaden the line to the extent that the internal line structure is indistinguishable (Fig. 6). In this case, the situation is similar to that in copper(II) sulfate pentahydrate but due to the higher spin the interactions are stronger and leading to bigger line broadening than in the case of copper. Even, though 32 out of 80 atoms in the unit cell are Gd³⁺ ions, the interactions leading to collective magnetism were not observed. Field cooling and zero field cooling experiments showed no bifurcations until 2 K.²⁸ The small interactions between paramagnetic ions were simulated with a Curie–Weiss curve with small value of the Θ parameter.²⁸

Fig. 6 Normalized EPR spectra of Gd₂O₃ (black line) and ZnO (red line) powders recorded with polydopamine standard (5.5 × 10¹⁵ spins g = 2.0036, T = 295 K).

For comparison as EPR standard polydopamine radicals were used. The relative signal strength can be compared between ZnO defects and Gd₂O₃ (Fig. 6).

The X-band EPR spectrum usually consists of three main lines at g_e ≈ 6.0, g_e ≈ 2.8 and g_e ≈ 2.²⁹ The increasing amount of Gd³⁺ ions in the structure strongly broadens the lines, which end up as one broad line without resolved fine structure. It is showed that 1.95 × 10²¹ ions per cm³ is enough to broaden the lines at g_e ≈ 2.8 and g_e ≈ 2. In our case we do not observe even the last vanishing transition at g_e ≈ 6.0.²⁹

ZnO is visible in EPR due to the defect centres appearing in the structure during the synthesis of the nanocrystals. Possible defect centres in ZnO are: zinc vacancies, zinc on interstitial sites (visible Zn⁺), oxygen on interstitial sites (O⁻), and oxygen vacancies (visible: V_o²⁺).³⁰ The EPR linewidth of ZnO is 79 mT with effective g-factor of 2.1 (N = 6.3 × 10¹⁸ spins) (Fig. 6). The origin of the ZnO signal is related with thermally stable defects.³⁰

Fig. 7 depicts the transitions in the ZnO:Er³⁺–Yb³⁺ system, especially in the Er³⁺and Yb³⁺ spin system. It can be seen that the line structure is better resolved in liquid helium temperatures. The rare earth ions usually present fast relaxation times causing line broadening and are not resolved well at room temperatures. For example the EPR signal from Yb³⁺ ion is visible in liquid helium temperatures in systems like KZnF₃ where the Yb³⁺ ion substitutes Zn²⁺.³¹ The Er³⁺ ion was observed in liquid helium temperatures, 5.2 K, in SrTiO₃ single crystals³² and YAl₃(BO₃)₄ and EuAl₃(BO₃)₄ until 80 K.³³ It is assumed that the strong increase of the EPR signal intensity in liquid helium temperatures in comparison to the signal observed in room temperature is due to the lanthanides doping in the ZnO structure. The large mismatch and line broadening of lines can be the result of dipolar interaction between ions, different mixed atom coordination, and multiple phases like Er₂O₃ in the ZnO system.

Fig. 7 Normalized spectra of ZnO:Er³⁺, Yb³⁺ recorded in 4.2 K (black) and 295 K (red).

2.4. Photoluminescence study

Fig. 8 shows the UC photoluminescence (PL) spectra of the ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell nanostructures. It is clearly seen that upon excitation by 980 nm S-III (ZnO:Er³⁺–Yb³⁺) and S-IV (ZnO:Er³⁺–Yb³⁺ modified with Gd₂O₃ for 6 h) revealed very weak up-conversion in the range of 515–570 nm and 630–685 nm. The increasing of the surface modification time (increasing Gd₂O₃ content), S-V and S-VI samples, leads to the enhance of the up-conversion emission. Pokhrel et al. have shown that these PL peaks correspond to radiative transitions from ⁴F_9/2 to ⁴I_15/2 (red emission) and from ⁴S_9/2, ²H_11/2 to ⁴I_15/2 (green emission) of Er³⁺ ion.³⁴ The splitting structure of PL peaks might be explained by the Stark effect (the splitting and shifting of energy levels due to presence of an electric field) of Er³⁺ energy levels induced by the crystal field of Gd₂O₃.²²

Fig. 8 UC luminescence of ZnO:Er³⁺–Yb³⁺ NPs and ZnO:Er³⁺–Yb³⁺@Gd₂O₃ hybrid nanostructures dependent on Gd content (top image) and energy level diagram showing possible excitation and emission mechanisms at 980 nm excitation (bottom image).

It is possible to describe the mechanism of the excitation and emission using the energy diagram (Fig. 8, bottom image). It is well-known that Yb³⁺ has a larger absorption cross-section at 980 nm than Er³⁺.³⁵ Thus, Er³⁺ can resonantly receive energy from Yb³⁺, so the transition of ⁴I_15/2–⁴I_11/2 and ⁴I_11/2–⁴F_7/2 are excited and the UC emission is significantly enhanced.³⁶ Afterward, non-radiative transitions to ²H_11/2 and ⁴S_3/2 produce green emission and to ⁴F_9/2 produce red emission.

There are two possible mechanisms to explain the increasing of UC emission due to the surface modification (the increase of the Gd₂O₃ content). Firstly, the surface modification annihilates the dangling bonds and attaching groups at the surface of nanoparticles which can quench the PL emission. Secondly, core/shell structure increases the distance between luminescent Er³⁺ ions and surface defects, thus blocking the non-radiative pathway.

We can also notice that the red emission prevails for the S-V sample, while in sample S-VI the red emission is suppressed and the green PL is enhanced. The same effects and their luminescence mechanisms were described by X. Meng and co-workers.²² Using the theory of multi-phonon relaxation rate, Meng et al. have shown that emission through ⁴S_9/2, ²H_11/2 transition is more influenced by the Yb³⁺ concentration. Therefore, the increasing of the Gd₂O₃ content leads to dominating of green PL.

3. Experimental

3.1. Synthesis of the ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell nanostructures

3.1.1. Materials. Zinc acetate dihydrate (Zn(CH₃COO)₂ × 2H₂O, Sigma-Aldrich), sodium hydroxide (NaOH, Stanlab), erbium(III) nitrate pentahydrate (Er(NO₃)₃ × 5H₂O, Sigma-Aldrich), ytterbium(III) nitrate pentahydrate Yb(NO₃)₃ × 5H₂O, Sigma-Aldrich), gadolinium(III) nitrate hexahydrate (Gd(NO₃)₃ × 6H₂O, 99.9, Aldrich), and methanol were used as starting materials.

3.1.2. ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell synthesis. The un-doped ZnO nanoparticles (NPs), core, were obtained by adapted simple low-temperature sol–gel route.¹⁶ For typical ZnO:Er³⁺–Yb³⁺ NPs synthesis, Zn(CH₃COO)₂ × 2H₂O) (13.4 mmol) was dissolved in 125 ml methanol at a constant temperature of 60 °C. Doping with rare earth elements was realized via introduction of the RE metal salts (theoretical 2 at% Er³⁺ and 10 at% Yb³⁺) into the reaction mixture during ZnO NPs synthesis. Then, a solution of NaOH (23 mmol) in 65 ml of methanol was added to zinc acetate dihydrate under vigorous stirring. After about 3 h white ZnO nanoparticles were separated from mother liquor, washed with methanol twice and dried. The final product collected was a white powder.

Gd₂O₃ shell was prepared by seed deposition method.³⁶ Part of ZnO:Er³⁺–Yb³⁺ NPs were immersed into 0.1 mol l⁻¹ Gd(NO₃)₃ solution for a period of 6 h to 96 h. Obtained core–shell nanostructures were separated, washed with deionized water (18 MΩ cm) and dried in an excicator. To obtain gadolinium oxide phase (Gd₂O₃), nanoparticles were annealed at 900 °C in air for 1 h.

3.2. Characterization

The morphology and chemical composition of the samples were studied by scanning electron microscopy (SEM, JEOL, JSM-7001F) equipped with an energy dispersive X-ray (EDX) analyzer. Powder X-ray diffraction (XRD) studies of the powder samples were carried out on an Empyrean (PANalytical) diffractometer using Cu Kα radiation (λ = 1.54 Å), reflection-transmission spinner (sample stage) and PIXcel 3D detector, operating in the Bragg–Brentano geometry. The 2Theta scans were recorded at room temperature in angles ranging from 20 to 80 (°2Theta) with a step size of 0.006 (°2Th.) and continuous scan mode. XPS experiments were performed in a SPECS Sage HR 100 spectrometer with a non-monochromatic X-ray source (Aluminum Kα line of 1486.6 eV energy and 350 W). The samples were placed perpendicular to the analyzer axis and calibrated using the 3d_5/2 line of Ag with a full width at half maximum (FWHM) of 1.1 eV. An electron flood gun was used to compensate for charging during XPS data acquisition. The selected resolution for the spectra was 10 eV of pass energy and 0.15 eV per step for the high resolution spectra. All measurements were made in an ultra-high vacuum (UHV) chamber at a pressure below 8 × 10⁻⁸ mbar. The spectroscopic EPR measurements were performed with a RADIOPAN SX spectrometer equipped with an Oxford CF935 cryostat allowing measurements in the temperature range 4.2–300 K. The modulation amplitude was set to 0.1 mT, the microwave power was 11.38 mW. The visible UC emission was excited at 980 nm and at room temperature by means of the LSM 780 NLO confocal system (Zeiss) working in the lambda mode. Infrared light was delivered by tunable femtosecond Chameleon laser (Coherent) coupled to the LSM 780 system. Spectral analysis was performed on the light emitted by representative grains of the powder samples.

4. Conclusions

In summary, by using the sol–gel and seed deposition techniques ZnO@Gd₂O₃ and ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell nanostructures were obtained. The size of nanoparticles increased from 7 nm (individual as-obtained ZnO:Er³⁺–Yb³⁺ NPs) to 50 nm after Gd₂O₃ shell formation (core–shell nanostructures). XRD data showed, that Gd₂O₃ phase began to crystallize at 600 °C. In the ZnO:Er³⁺–Yb³⁺ NPs a secondary phase of Yb₂O₃ was found. Real RE concentrations in ZnO:Er³⁺–Yb³⁺@Gd₂O₃ crystal lattice were determined. EPR investigation allowed estimating the presence of three different signals which are listed here in descending order of appearance: gadolinium oxide Gd₂O₃, ZnO structure defects, and at liquid helium temperatures also Er³⁺,Yb³⁺ ions. The presence of the Gd₂O₃ protective shell on the ZnO:Er³⁺–Yb³⁺ surfaces led to an increase of the up-conversion luminescense intensity of the ZnO matrix due to a decreasing of the structural defects. The obtained ZnO@Gd₂O₃ and ZnO:Er³⁺–Yb³⁺@Gd₂O₃ core–shell nanostructures with effective green and red luminescence will have potential application as efficient phosphors for wide scientific and medical applications.

Acknowledgements

Financial support from the National Centre for Research and Development under research grant “Nanomaterials and their application to biomedicine”, contract number PBS1/A9/13/2012, is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18393j

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