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One-step synthesis and morphology evolution of luminescent Eu2+ dopedstrontium aluminate nanostructures

Dajie Si, Baoyou Geng* and Shaozhen Wang

Received 15th October 2009, Accepted 23rd February 2010

DOI: 10.1039/b921613h

Several shapes of Eu2+ doped strontium aluminate have been simply obtained by hydrolysis of Sr2+ and

Al3+ in hydrothermal and solvothermal systems under mild conditions. The influence of reaction

conditions on the formation of the samples has been considered. Flower-like nanoparticles are

synthesized in glycol, when the solvent is substituted by H2O, and/or adding NaOH or stirring for 30 min

before injecting the prepared Eu(NO3)3, needle cluster-like, sheet-like and lantern-like structures are

synthesized. At the same time, the doped Eu3+ is reduced to Eu2+ at much lower temperature, and the

emission peaks result from the transition of 4f65d–4f7 of Eu2+ around 405 nm, 435 nm and 530 nm are also

observed, no typical peak of Eu3+ is found, which indicates the complete reduction from Eu3+ to Eu2+. The

results of this article may give a guide to synthesize the similar Eu2+ doped luminescent materials.

1. Introduction

As a well known host materials, strontium-based aluminate

(SAO) phosphors have attracted much attention in recent years

due to their excellent luminescent features such as high quantum

efficiency, long-lived after glow, good chemical stability and so

on.1–11 From the point of the structure, the SAO can be considered

as SrO–Al2O3 systems, in which Eu3+ ions can substitute Sr2+ sites

in the lattices and form a stable dopant compounds. However, as

a luminescent material, Eu3+ ions can not act as good activators,

which should be reduced to Eu2+ ions for better luminescence. In

order to achieve the above object, many reduction methods have

been introduced. For example, the [H2 + N2] system,12 [H2 + Ar]

system,13,14 g-ray irradiation15 combustion reduction,16 etc., but

all these methods need a secondary management and high

temperature. Here, we report a simple one-step hydrothermal

approach to the synthesis of several shapes of Eu2+ doped stron-

tium aluminate by the hydrolysis of Al3+ and Sr2+ with the pres-

ence of Eu3+. The reduction of Eu3+ to Eu2+ happened at a lower

temperature in the process.

In this case, we obtained the products with different

morphologies, such as nano-scale needle-like clusters (S1), ellipse

plates (S2), lanterns (S3), and flowers (S4). The products are

organized by primary nanoparticles or one-dimensional nano-

structures. It should be contributed to the assistance of surfac-

tant or solvent. For example, hexadecyltrimethylammonium

bromide (CTAB) has different effects on the crystallographic

facets, which makes the crystals grow preferentially along the

c-axis and then the received nanorods aggregated into needle-like

clusters (S1), thin ellipse plates (S2) and lanterns (S3).17 The

received flakes could curl into flowers (S4) because of the exis-

tence of glycol.18 The Eu2+ doped strontium aluminate may grow

according to a ‘‘nucleation-growth-assembly’’ process.

College of Chemistry and Materials Science, Anhui Key Laboratory ofFunctional Molecular Solids, Anhui Laboratory of Molecular-BasedMaterials, Anhui Normal University, Wuhu, 241000, P. R. China.E-mail: bygeng@mail.ahnu.edu.cn; Fax: +86-553-3869303; Tel: +86-553-3869303

2722 | CrystEngComm, 2010, 12, 2722–2727

All the products exhibit a typical broad band emission peak of

Eu2+, which indicated the reduction of Eu3+ cations in the SAO

lattices. As mentioned above, Sr2+ sites are easily substituted by

Eu3+ because of their similar radius. However, in order to keep

the charge balance, two Eu3+ ions should be needed to substitute

for three Sr2+ ions. The non-equivalence substitution leads to the

appearance of vacancy defects EuSr� (positive) and VSr

0 0(nega-

tive) in the matrix, which act as accepter and donor of electrons,

respectively. The VSr0 0

vacancy might be released and swim in the

host lattice by thermal stimulation. The electrons in the vacancy

defects of Sr2+ would be transferred to Eu3+ sites and reduced

Eu3+ to Eu2+. It is also reported that the rigid three-dimensional

network AlO4 tetrahedral are necessary for the reduction of

trivalent rare earth. We also found that the peak wavelength of

the phosphorescence does not vary with the morphology of the

samples, this implies that the crystal field which affects the 5d

electron states of Eu2+ is not changed by the morphology of the

samples, but the intensity of the photoluminescence varies with

the morphologies, which should be attributed to the crystallinity

and size of the products.

2. Experimental

Sample preparation

All reagents were of analytical grade and were purchased and

used as received without further purification. Before experiment,

Eu2O3 (99.95%) was dissolved in HNO3 (A. R.) and a Eu(NO3)3

(0.01M) solution was prepared. In a typical procedure, 1 mmol

AlCl3, 1 mmol Sr(NO3)2 and 1 g CTAB were dissolved into

20 mL glycol at room temperature, then adding 3 mL the of

prepared Eu(NO3)3 (0.01M) to it, and the solution was then

transferred into a Teflon-lined 40 mL autoclave and maintained

at 180 �C for 12 h. This sample was denoted as sample 4 (S4). For

synthesizing of S1–S3, the solvent was substituted by H2O, and/

or adding 1 mL NaOH (0.01M) or stirring for 30 min before

injecting the prepared Eu(NO3)3.

The products was then collected by centrifugation and washed

with deionized water and absolute ethanol repeatedly, then dried

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in air at 60 �C for further characterization. The detailed experi-

mental steps for the synthesis of some typical samples are listed

in Fig. 1.

Characterization

X-Ray powder diffraction (XRD) was carried out on an XRD-

6000 (Shimadzu Corporation, Japan) X-ray diffractometer with

Cu Ka radiation (l ¼ 1.54060 �A) at a scanning rate of 0.05� s�1.

Scanning electron microscopy (SEM) micrographs were taken

using a Hitachi S-4800 scanning electron microscope (Hitachi

Corporation, Japan) attached with Energy dispersive X-ray

spectroscopy. Transmission electron microscopy (TEM) micro-

graphs were performed using JEM 2010 F microscopes (JEOL,

Japan). Photoluminescence measurements were performed on

a Hitachi F-4500 fluorescence spectrofluorometer (Hitachi

Corporation, Japan) at room temperature.

Fig. 2 SEM images of S1 (a, b), S2 (c, d), S3 (e, f), S4 (g, h).

3. Results and discussion

The general morphology of the as-synthesized product was

examined by SEM. The typical morphologies of the obtained

products are shown in Fig. 2, which shows that the as-prepared

products mainly exhibit four kinds of morphologies. Each

product has high yield (> 90%) and high purity. The needle

cluster-like sample (S1) was synthesized by adding 1 mL NaOH

(0.01M) into the solvent (H2O) (Fig. 2a). It can be seen from the

high-magnification SEM in Fig. 2b that the sample is composed

of nanorods with lengths of about 1 mm and diameter of about

100 nm. The pure phase of ellipse plates (S2) was generated in the

solvent of pure H2O. From Fig. 2c and 2d, it can be seen that

the product is composed of copious amounts of thin plates with

the average thickness of 50 nm, width of 250 nm and length of

1 mm, respectively. When adding the process of stirring before

injecting Eu(NO3)3 into the solvent (H2O), the lantern-like

particles (S3) were synthesized. The particle is composed of flakes

and has a average diameter of about 1mm (as shown in Fig. 2e

and 2f). Fig. 2g and 2h shows the SEM images of the products

(S4) obtained in glycol solution. The low-magnification SEM

image in Fig. 2g reveals that the obtained product is high-yield

flower-like structures. The obtained structures are connecting

together without well dispersity. The corresponding high-

magnification SEM (Fig. 2h) indicates that the product is

composed of curled flakes with the typical thick of about 20 nm.

Fig. 3 shows the XRD patterns of as-prepared products with

different experimental conditions. All the prominent diffraction

peaks in this pattern can be indexed to Sr3Al2O6, which matches

well with the standard Joint Committee on Powder Diffraction

Fig. 1 Flow chart for the preparation of the samples.

This journal is ª The Royal Society of Chemistry 2010

Standards (JCPDS card file no. 24-1187). From Fig. 3a to 3c, the

peaks turn sharply in sequence. Fig. 3a suggests that S1 has

a moderate degree of crystallinity after adding NaOH, and the

sharp peaks at Fig. 3b and 3c indicate the formation of well-

crystallized Sr3Al2O6. After stirring 30 min, the corresponding

Fig. 3 XRD patterns of the as-prepared products. a (S1); b (S2); c (S3);

d (S4).

CrystEngComm, 2010, 12, 2722–2727 | 2723

Fig. 5 TEM images of S1 (a), S2 (b), S3 (c) and S4 (d).

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XRD pattern (Fig. 3c) shows much higher and narrower shapes,

which reveals high crystallinity of the product.

Further evidence for the composition was obtained by the

XPS. The spectra for the obtained nanostructures (sample 1 as

the example) are shown in Fig. 4. The XPS spectrum in Fig. 4a

shows main peaks corresponding to O 2s (24.0 eV), Al 2p

(75.5 eV), Al 2s (118.9 eV), Sr 3d (133.0 eV), Sr 3p (269.4 eV), and

O 1s (531.6 eV). The XPS spectrum in Fig. 4b shows the peaks

corresponding to Eu2+ 3d5/2 (1125.5 eV) and Eu2+ 3d3/2

(1157.3 eV). The XPS peaks for Eu3+ are nearly negligible, which

demonstrates that Eu3+ ions have been reduced to Eu2+ in this

case.

The TEM image of S1 in Fig. 5a illustrates that the as-

synthesized needle cluster-like nanostructures are uniform with

the size described in Fig. 2a and 2b. A typical TEM image in

Fig. 5b further reveals that the average length of the obtained

nano-sheet (S2) is about 1mm with the average thickness of

50 nm, and width is about 250 nm. Fig. 5c shows the TEM image

of the synthesized lantern-like nanostructures of S3. It can be

seen from the image that the lantern-like nanostructures are

assembled from the sheets. Fig. 5d is a typical TEM image of the

Fig. 4 The typical XPS spectrum of as-synthesized structures (S1). (a)

Overview XPS spectrum of the S1; (b) Eu2+ 3d5/2 and Eu2+ 3d3/2 spectrum.

2724 | CrystEngComm, 2010, 12, 2722–2727

as-prepared flowers (S1), in which the curled flakes can be clearly

observed, and we also have found several flower-like structures in

Fig. 5c.

To understand the formation mechanism of the products, the

time-dependent experiments have been performed. Fig. 6 shows

the SEM patterns of S1–S3 obtained after different reaction

times. The SEM images of S1 obtained after 4 h and 8 h in Fig. 6a

and 6b demonstrate that the sample is aggregated by nanorods

Fig. 6 SEM images of S1–S3 for 4 h (a, c and e), 8 h (b, d and f).

This journal is ª The Royal Society of Chemistry 2010

Fig. 7 Formation processes of the samples.

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and with the growth tendency of needle clusters. The SEM

images of S2 obtained after 4 h and 8 h (Fig. 6c and 6d) indicate

that the plate-like nanostructures are organized by nanorods.

The SEM images (Fig. 6e and 6f) of S3 obtained after 4 h and 8 h

show bundles and flakes aggregated by nanorods and lantern-

like nanostructures are organized by flakes. From above results,

we can find that the nanorods aggregate into flakes firstly, and

then the flakes assemble into lantern-like nanostructures.

A brief summary of the detailed experimental conditions and

the corresponding morphologies of the products are listed in

Table 1. Four shapes of nanostructures have been synthesized

only by changing solvents and methods, the other conditions are

constant in the reaction system.

According to the above results, a possible formation mecha-

nism is proposed. The growth mechanism of shape-selected

synthesis of nanostructures can be attributed to nucleation-

growth-assembly process in the present of additives. The growth

process is described in following procedures, as illustrated in

Fig. 7. It is believed that pH played an important role in deter-

mining the crystal structure and morphology of the final

products. Recently, Wang and co-workers had reported

a nucleation–assembly–recrystallization mechanism in synthe-

sizing shape-controlled octahedral and truncated octahedral

crystals of zinc tin oxide (ZTO).18 Similarly, in our case, the

hydrolysis and recrystallization take place during the growth of

the products. As shown in Fig. 7, firstly, the hydrolysis of Sr2+

and Al3+ happened and the nuclei were formed in the solution. In

this process, pH will affect the amount of the nucleus and lead to

the different morphologies of the products. At higher pH (pH ¼8–9), large numbers of nuclei formed simultaneously, which

aggregated into clusters because of their high free energy. Then,

the hydrolysis took place continually and new formed nuclei

attached on the surface of the previous clusters. After that, the

nuclei recrystallized and grew along the active facets into nano-

rods in the present of CTAB. As the result, the needle cluster-like

nanostructures (S1) were synthesized. In contrast, at lower pH

(pH ¼ 6–7), less nuclei were formed and hardly any aggregation

had happened in this condition, similarly, the later formed nuclei

would attach on the active facets of the previous nuclei and grew

along the c-axis into nanorods with the assistance of CTAB.

Then, the prepared nanorods would assemble into flakes (S2)

because of the the hydrophobic groups of CTAB. In this

condition, if the solution was stirred for 30 min before trans-

ferring into the autoclave, the disturbed action would drive

nuclei to form many dispersed congeries, in which the nuclei did

not contact with each other. Subsequently, the nuclei in each

group grew into nanorods and further to aggregated sheets,

which finally developed into lantern-like structures in hydro-

thermal condition. In addition, we also found that the addition

Table 1 Experimental condition for the preparation of samples

Samples solvent AlCl3 Sr(NO3)2 Eu(NO3)3

S1 H2O 1 mmol 1 mmol 3 mL (0.01 M)S2 H2O 1 mmol 1 mmol 3 mL (0.01 M)S3 H2O 1 mmol 1 mmol 3 mL (0.01 M)S4 Glycol 1 mmol 1 mmol 3 mL (0.01 M)

This journal is ª The Royal Society of Chemistry 2010

of glycol leaded to the formation of curly flowers. Just as Zhu19

and co-workers have reported, the high viscous of the solvent

leads to the curling of flakes. Here, the received flakes could curl

into flowers (S4) because of the high viscosity of the existed

glycol.

Fig. 8a shows the PL spectra of the doped and undoped

strontium aluminate products excited by 375 nm. The doped

strontium aluminate products show two peaks around 405 and

435 nm, which should be attributed to the transition of 4f6 5d–4f7

of Eu2+.12,20–22 The position of 5d levels depends strongly on the

crystalline environment of Eu2+, the mixed states of 4f6 5d will be

split by the crystal field and will couple strongly to the lattice

phonons, resulting in broad band absorption and emission the

broad band emission region at about 400–460 nm. The undoped

strontium aluminate products do not exhibit obvious emission

peaks (magenta line). Fig. 8b shows PL spectra of the doped

products excited by 440 nm. The emission peaks around 530 nm

should also be attributed to the transition of 4f6 5d–4f7 of Eu2+.

As we all know, the select excitation would be appeared when the

activators occupy in different lattice sites. Activators in some

sites would be preferentially excited upon the different excitation

wavelength, which results in the large difference in their emission

spectra. With 377 nm excitation, emissions around 405 nm and

435 nm are preferential. The Eu2+ prefers to be excited to higher

levels of 5d because the excitation at 377 nm located at the higher

energy level than 440 nm. When Eu2+ absorbs the energy from

the host, it transfers from ground state to higher levels of 5d and

makes an energy transfers to the lower 5d levels, then returns to

ground state via emission. The peak wavelength of the phos-

phorescence does not vary with the morphology of the samples,

this implies that the crystal field which affects the 5d electron

states of Eu2+ is not changed by the morphology of the samples,

but the intensity of the photoluminescence varies greatly with the

morphologies. The intensity of the PL should be related to

crystallinity and size of the products. Among four samples, S1

possesses the smallest size and moderate crystallinity, which

results in the highest emission intensity. Comparatively, S2 and

CTAB T/�C Method Morphology

0.1 g 180 NaOH Needle clusters0.1 g 180 Plates0.1 g 180 Stirring 30 min Lanterns0.1 g 180 Flowers

CrystEngComm, 2010, 12, 2722–2727 | 2725

Fig. 8 Emission spectra of S1 (black), S2 (blue), S3 (red), S4 (green);

magenta line in (a) shows the PL emission of the undoped strontium

aluminate products.

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S3 have larger size and S4 has worse crystallinity; therefore, the

PL intensities of S2–S4 are lower than that of S1.

All of the obtained emission peaks are attributed to Eu2+, the

typical peak at 612 nm of Eu3+ was not found, which consists

with the result of XPS. Here, we are trying to explain the

reduction phenomenon of Eu3+ / Eu2+ in Eu3+ doped strontium

aluminate prepared by hydrolysis of Al3+ and Sr2+ in solvent.

Strontium aluminate is a typical example of a stuffed tridymite;

Eu3+ was stuffed in the Sr2+ sites because of their similar radius.

In order to keep the charge balance, two Eu3+ ions should be

needed to substitute for three Sr2+ ions. The non-equivalent

substitution leads to the appearance of vacancy defects EuSr�

(positive) and VSr0 0

(negative) in the matrix, the defects act as

accepter and donor of electrons respectively. The VSr0 0

vacancy

might be released and swim in the host lattice by thermal stim-

ulation. At the same time, movement of defects in the interme-

diate layers would become stronger which further increased the

encounter probability between the relatively free electrons and

Eu3+. Consequently, by thermal stimulation, the electrons in the

vacancy defects of Sr2+ would be transferred to Eu3+ sites and

reduced Eu3+ to Eu2+. The whole process can be expressed by the

following equations:

2726 | CrystEngComm, 2010, 12, 2722–2727

3Sr2+ + 2Eu3+ / VSr00 + 2 EuSr� (1)

VSr00 / VSrx + 2e� (2)

2EuSr� + 2e�/2EuSrx (3)

From the point of the structure, the obtained Eu2+ ions locate

in three-dimensional (3D) network of AlO4 tetrahedra, which are

less likely to be attacked by oxidant. Therefore, Eu2+ ions are

stable in this lattice. In addition, the autoclaves used in this case

were sealed, which should generate high pressure during the

heating, accelerating the reaction and making it carry out easily.

4. Conclusions

In summary, several shapes of Eu2+ doped strontium aluminate

(SAO) have been simply prepared through hydrolysis of Sr2+ and

Al3+ in solvent, and the reduction of Eu3+ can successfully be

carried out at much lower temperature in Teflon-lined auto-

claves. The luminescence peaks sited at 405 nm, 435 nm and 530

nm should attributed to the transition of 4f65d–4f7 of Eu2+, no

typical peak of Eu3+ was found, which indicates the complete

reduction from Eu3+ to Eu2+.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China (20671003, 20971003), the Key Project of

Chinese Ministry of Education (209060), the Education

Department of Anhui Province (2006KJ006TD) and the

Program for Innovative Research Team in Anhui Normal

University.

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