Materials Research Bulletin 48 (2013) 5013–5018

Hydrothermal synthesis, characterization and luminescent properties ofGdPO4�H2O:Tb3+ nanorods and nanobundles

Hejuan Song, Liqun Zhou *, Ling Li, Fei Hong, Xinru Luo

Ministry of Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei Collaborative Innovation Center for Advanced Organic Chemical

Materials, College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, PR China

A R T I C L E I N F O

Article history:

Received 13 January 2013

Received in revised form 15 May 2013

Accepted 16 May 2013

Available online 2 June 2013

Keywords:

A. Nanostructures

C. X-ray diffraction

D. Luminescence

A B S T R A C T

In this paper, the Tb3+-doped GdPO4�H2O nanorods and nanobundles have been synthesized by the

hydrothermal method with and without glycine, respectively. The X-ray powder diffraction (XRD),

thermogravimetric and differential thermal analysis (TG–DTA), Fourier transform infrared spectroscopy

(FT-IR), transmission electron microscopy (TEM), energy-dispersive spectra (EDS) and photolumines-

cence (PL) were employed to characterize the as-obtained products. It was found that the addition of

glycine and the pH value have crucial influences on the formation of the resulting morphologies and

sizes. The possible formation mechanisms for GdPO4�H2O:Tb3+ nanorods and nanobundles were put

forward. A detailed investigation on the photoluminescence of GdPO4�H2O:Tb3+ different samples

revealed that the luminescent properties of products are strongly correlated with the morphologies,

sizes, coordination environment and crystal field symmetry.

� 2013 Elsevier Ltd. All rights reserved.

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Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

With the advanced tools of characterization and manipulation,nanoscale materials have received widespread attention for theirnovel size- and shape-dependent properties, as well as theirunique applications that complement those of the bulk counter-parts [1–4]. In this regard, luminescent nanomaterials doped withrare-earth ions are of immense importance because of theirtechnological applications in lighting, displays, X-ray photography,lasers, and amplifiers for fiberoptic communication [5–9]. Inparticular, the optical properties of luminescent materials at thenanoscale are enormously affected by their shapes [10–14].Therefore, controlling the sizes and morphologies of thesestructure and function materials is still of great significant.

In recent years, hydrothermal method as a typical solution-based approach has been proven an effective and convenientprocess in preparing various inorganic materials with diversecontrollable morphologies and architectures in terms of cost andpotential for large-scale production [15–17]. Furthermore, duringthe hydrothermal process, one of the promising and popularstrategies of controlling the shape and size of a targeted material isto select carefully an appropriate organic additive with functionalgroups that selectively adheres to a particular crystal facet andeffectively slows the growth of that facet relative to others, leading

* Corresponding author. Tel.: +86 27 88662747; fax: +86 27 88663043.

E-mail addresses: songhejuan0708@163.com (H. Song), zlq@hubu.edu.cn,

zhoulq2003@yahoo.com.cn (L. Zhou).

0025-5408/$ – see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2013.05.067

to the morphological modification of the crystals [18]. To our bestknowledge, many important nanoscale luminescent materialshave been prepared in the form of nanorods or nanowires via thehydrothermal method, and their optical properties have beenintensively studied. The EDTA-assisted hydrothermal process tosynthesize uniform LnPO4�xH2O (Ln = Y, La–Nd, Sm–Lu) nanocrys-tals with 0D, 1D, and 2D structures had been reported by Li and hisgroup [19,20], and Lin et al. [21] had prepared LnPO4 (Ln = La, Gd,Y) nanocrystals by a CTAB-assisted hydrothermal process. As aresult of their studies, it was proved that the optical properties ofthese nanocrystals are strongly dependent on their morphologiesand sizes. Up to now, the morphology- and size-controlledsynthesis of Tb3+-doped GdPO4�H2O luminescent materials via aglycine-assisted hydrothermal process has not been reported.

In our paper, we obtained Tb3+-doped GdPO4�H2O nanorods andnanobundles with and without glycine as the capping agent via ahydrothermal process at 180 8C and 48 h. By adjusting the pH valueof solution, the morphologies and sizes of products can becontrolled. The possible formation mechanisms and luminescentproperties of the as-synthesized samples with different morphol-ogies have been investigated in detail.

2. Experimental procedures

2.1. Synthesis

All chemicals were directly used as received without furtherpurification. Deionized water was used throughout. In a typical

Fig. 1. XRD patterns of GdPO4�H2O:Tb3+ at different pH values: (a) pH = 2, (b) pH = 7,

(c) pH = 10 without glycine and (d) pH = 2, (e) pH = 5, (f) pH = 8 with glycine.

Fig. 2. TG–DTA curves of the GdPO4�H2O:Tb3+ obtained at pH = 2 with glycine.

Fig. 3. FT-IR spectra of the GdPO4�H2O:Tb3+ obtained at pH = 2 with glycine.

H. Song et al. / Materials Research Bulletin 48 (2013) 5013–50185014

process, Gd(NO3)3 and Tb(NO3)3 solutions were prepared bydissolving 0.95 mmol of Gd2O3 (99.99%) and 0.05 mmol of Tb4O7

(99.99%) with concentrated nitric acid. Then 6 mmol glycine wasadded into the above solution to form a clear and homogeneoussolution of Gadolinium (Terbium)–glycine complex at roomtemperature. After vigorous stirring for 30 min, 10 mL solutioncontaining 4 mmol of (NH4)2HPO4 was added dropwise into theabove mixture under continuous stirring. Subsequently, PO4

3�

reacted with Gadolinium (Terbium)–glycine complex to form awhite precipitate. The pH of mixed solution was rapidly adjusted toa designated value through the addition of NH3�H2O solution undercontinuous stirring. After additional agitation for another 60 min,the colloidal solution (with precipitated GdPO4) was transferredinto a 50 ml Teflon-lined autoclave (filled up to 80% of its totalvolume), and the autoclave was sealed and maintained at 180 8Cfor 48 h. After the autoclave was cooled down to the roomtemperature naturally, the precipitates were collected by centri-fugation, washed with deionized water and absolute alcoholseveral times, and dried at 80 8C. The final white product wasobtained and kept for further characterization.

2.2. Characterization

The purity and phase structure of the samples were examinedby X-ray powder diffraction (XRD) using a Rigaku D/max-3C X-raydiffractometer with Cu Ka (40 kV, 40 mA) radiation(l = 0.15406 nm). Infrared spectra (FTIR) were recorded in therange of 4000–500 cm�1 on a Fourier transform spectrometer(Perkin-Elmer, Spectrum 1, USA) with a resolution of 1 cm�1.Thermogravimetric and differential thermal analysis (TG–DTA)was performed up from room temperature to 700 8C at the heatingrate of 10 8C/min under nitrogen gas flow (Perkin-Elmer, DIA-MOND, USA). The morphologies and structures of the productswere investigated by using field emission transmission electronmicroscopy (TEM, FEI Tecnai G20) equipped with an energy-dispersive spectra (EDS). The photoluminescence (PL) wascharacterized with a JASCO FP-6500 fluorescence spectrophotom-eter using a Xe lamp as the excitation source at room temperature.

3. Results and discussion

3.1. Crystallite structure analysis

The crystalline structure and phase purity of the as-formedGdPO4�H2O:Tb3+ products through the hydrothermal process werecharacterized by X-ray diffraction (XRD) and showed the samecrystalline structure, as shown in Fig. 1. All samples exhibitedsimilar diffraction peaks that correspond to the pure hexagonalphase with body-centered structure of GdPO4�H2O (lattice con-stants a = b = 0.6905 nm, c = 0.6326 nm, a = b = 908, g = 1208,JCPDS 39-0232), and space group is P3121. It is more interestingto note that the peak intensities of the products Fig. 1d–f obtainedwith glycine are much stronger than those of the other threesamples Fig. 1a–c obtained without glycine. The reason may bethat glycine as a capping agent can make the crystallinity ofsamples increase. In addition, no peaks of impurity phase weredetected in all samples, which indicates that Tb3+ ions had beeneffectively built into the host lattices and the GdPO4�H2O:Tb3+ withgood crystallinity could be obtained under the present hydrother-mal conditions.

3.2. Thermal analysis

Fig. 2 shows a typical TG–DTA curve for the sample heated at arate of 10 8C/min under nitrogen gas flow. The weight loss of thesample terminated at about 350 8C and three weight-loss regions

H. Song et al. / Materials Research Bulletin 48 (2013) 5013–5018 5015

occurred at 25–135 8C, 135–260 8C and 260–350 8C. The weightloss in the temperature range 25–135 8C corresponds to theremoval of adsorbed water. The weight loss in the temperaturerange 135–260 8C, which is accompanied by an endothermic peakat about 170 8C in the DTA curve, is supposed to be related with thedecomposition of crystal water. And the slight weight loss in thetemperature range 260–350 8C, which is assumed to be constitu-tion water. Within the limits of 350 8C, the total weight loss wasabout 6.7%, which is in agreement with the XRD data analysis. Theweight loss phenomenon is similar to the literature [22]. Thus, theas-synthesized hexagonal nanostructures are believed to beGadolinium phosphate hydrates as GdPO4�H2O. Certainly, the

Fig. 4. TEM images of GdPO4�H2O:Tb3+ at different pH values: (a) pH = 2, (b) pH = 5

existence of water molecule is necessary to stabilize the hexagonalphase.

3.3. IR characterization

Fig. 3 gives the FT-IR spectra of as-prepared GdPO4�H2O:Tb3+

obtained at pH = 2 with glycine, the bands located at 3526, 1618,1075, 984, 624, and 546 cm�1 are observed. The bands centered at1618 and 3526 cm�1 can be due to the bending and stretchingvibrations for hydration water (O–H bond) of the hydrogen phasein the as-prepared powders, respectively [23]. The other bandspresent on the FT-IR spectra of the as-prepared powder are

, (c) pH = 8 with glycine and (d) pH = 2, (e) pH = 7, (f) pH = 10 without glycine.

Fig. 5. EDS spectrum of the product obtained at pH = 2 with glycine.

H. Song et al. / Materials Research Bulletin 48 (2013) 5013–50185016

assigned to the phosphate groups. The band centered at 1075 cm�1

is a characteristic of the asymmetry stretch vibration of the P–O,while the band at 984 cm�1 can be assigned to the bend vibrationof P–O in PO4

3� groups (called n3 region). The bands located at 624and 546 cm�1 are attributed to the bend vibration of O–P–O inPO4

3� groups (commonly known as n4 region) [24–26]. The aboveXRD, TG–DTA, and FTIR analysis show that the as-synthesizedproduct GdPO4�H2O:Tb3+ is hydrated due to the presence of watermolecule.

3.4. Morphologies analysis

Fig. 4 shows the morphologies and sizes of the productsobtained at different pH values with and without glycine. Thetypical TEM images of the GdPO4�H2O:Tb3+ obtained at differentpH values (pH = 2, 5, 8) with glycine are showed in Fig. 4a–c. It isclearly shown that the three samples are well dispersed, and theirshapes almost completely consist of nanorods, but a littlemorphology modulation could be easily realized by changingthe pH value of the solution. As shown in Fig. 4a, when the pH valuewas adjusted to 2, the product appeared as uniform nanorods withdiameters of 32–64 nm and lengths ranging from 112 to 400 nm(the average sizes for diameter and length are about 40 nm and228 nm, respectively). When the pH value was increased to 5, thesample was also totally composed of nanorods (Fig. 4b), but thesizes decreased apparently, and the average sizes for diameter andlength were about 28 and 125 nm, respectively. A further increaseof pH value to 8 led to the formation of smaller nanorods with 16–32 nm in width and 48–144 nm in length (the average sizes fordiameter and length are about 26 and 78 nm, respectively, Fig. 4c).Therefore, with the pH value increasing from 2 to 5 then to 8, thenanorods became even shorter and smaller, and the aspect ratio ofthe GdPO4�H2O:Tb3+ obtained with glycine was greatly decreased.Similarly, the sample prepared at pH = 2 without glycine was alsonanorods (see Fig. 4d). Compared with the sample formed at pH = 2with glycine, the sizes of the nanorods obtained at pH = 2 withoutglycine increased, and the average sizes for diameter and lengthwere about 42 and 282 nm, respectively. As exhibited in Fig. 4e andf, when the pH value increased to 7 and 10 without glycine, theGdPO4�H2O:Tb3+ phosphors appeared as nanobundles, which werecomprised of nanorods and the nanorods were tightly boundtogether.

Fig. 6. Illustration of the proposed formation mechanism

The results can suggest that both the presence of glycine andthe pH of the solution have a significant influence on the productsmorphologies and sizes. In the synthesis process, glycine as acapping agent could restrict the latitudinal and longitudinalgrowth of GdPO4�H2O:Tb3+ nanorods and make the superficial areaof products enlarge, while the increase of pH value could make thesizes of nanorods increase and the nanorods aggregated together toform bundle-like structure in the absence of glycine.

The EDS spectrum (Fig. 5) of the product obtained at pH = 2 withglycine shows the presence of Gd, P, O and Tb, the atomic ratio ofGd, P, O and Tb is approximately 0.95:1:5:0.05. It suggests that theproduct may be GdPO4�H2O:Tb3+, and Tb3+ ion has been effectivelybuilt into the GdPO4�H2O host lattice. The result is good agreementwith the XRD analysis above.

3.5. Possible growth mechanism of GdPO4�H2O:Tb3+ crystals

The crystal growth mechanism in solution is so perplexing thatthe actual crystallization process remains mysterious. Neverthe-less, on the basis of the above results, the possible formationmechanism of such intricate rod-like structure could be carefullyelucidated as follows. When Gd3+ (Tb3+) ions were treated withappropriate concentration of glycine, no obvious precipitation

of the GdPO4�H2O:Tb3+ nanorods and nanobundles.

Fig. 7. Excitation spectra of GdPO4�H2O:Tb3+ at different pH values: (a) pH = 8, (b)

pH = 5, (c) pH = 2 with glycine and (d) pH = 2, (e) pH = 7, (f) pH = 10 without glycine.

Fig. 8. Emission spectra of GdPO4�H2O:Tb3+ at different pH values: (a) pH = 8, (b)

pH = 5, (c) pH = 2 with glycine and (d) pH = 2, (e) pH = 7, (f) pH = 10 without glycine.

H. Song et al. / Materials Research Bulletin 48 (2013) 5013–5018 5017

occurred before the addition of PO43� at room temperature and the

system remain homogeneous prior to hydrothermal treatment.The existence of Gd3+ (Tb3+) in the solution was in the form of theGd3+ (Tb3+)–glycine complex. After the introduction of PO4

3�, theGd3+ (Tb3+)–glycine complex gradually decomposed, releasing theGd3+ (Tb3+) into the solution. Both Gd3+ (Tb3+) and PO4

3� had highchemical potential, the GdPO4�H2O:Tb3+ crystalline particles wereformed gradually due to the high chemical reactivity between Gd3+

(Tb3+) to PO43�. Subsequently, the tiny particles diffused and

parallelled to a certain axis of the formed GdPO4�H2O:Tb3+ nuclei,the nuclei aggregated together to form rod-like morphology.

When the pH value of the solution increased, the Gd3+ (Tb3+)ions were prone to precipitate, potential was thereby reduced.However, when glycine was added to chelate the Gd3+ (Tb3+) at thebeginning, glycine would compete with the OH� to combine withGd3+ (Tb3+). Thus, the possible precipitation of Gd(OH)3 [Tb(OH)3]can be repressed even at high pH values, the chemical potentialwas thereby increased, promoting the formation ofGdPO4�H2O:Tb3+ crystalline nuclei. As a result, the aspect ratioof GdPO4�H2O:Tb3+ obtained in the presence of glycine at pH = 8reduced [27]. And the sizes of the GdPO4�H2O:Tb3+ samplesdecreased with the increase of pH values of the solutions. Theexperimental results demonstrate that the pH value and theaddition of glycine are both vital factors in leading the growth ofGdPO4�H2O:Tb3+ nanorods.

The formation process of GdPO4�H2O:Tb3+ nanobundlesobtained without glycine underwent a typical hydrothermallyinduced route. At first, direct mixing of two solutions containingGd(NO3)3:Tb and (NH4)2HPO4 yielded the formation of a largenumber of amorphous GdPO4�H2O:Tb3+ particles, which served asprecursors of crystal. Thus, tiny crystalline nuclei were formed inthe solution at elevated temperatures under hydrothermalconditions. Subsequently, GdPO4�H2O:Tb3+ particles diffused,further grew and tended to show rod-like morphologies. Withthe pH values increasing, the GdPO4�H2O:Tb3+ nanorods formednanobundles by oriented aggregation, and the sizes of theGdPO4�H2O:Tb3+ crystals greatly increased in neutral and alkalineconditions (pH = 7 and pH = 10). Based on the above results, themodels of rod-like and bundle-like architectures formation fromtiny particles are proposed, as illustrated in Fig. 6.

3.6. Photoluminescence properties

Fig. 7 shows the excitation spectra of the Tb3+-dopedGdPO4�H2O nanorods and nanobundles with a monitoringemission at 545 nm. The excitation spectra of all compoundsexhibit the similar features, but different intensities. In thewavelength region 300–500 nm, the spectra consist of many sharppeaks at 306, 312, 341, 352, 369, 378 and 488 nm, which areassigned to the transitions of 7F6! 5H6, 7F6! 5D0, 7F6! 5D1,7F6! 5D2, 7F6! 5L10, 7F6! 5D3 and 7F6! 5D4 of Tb3+ ions,respectively [28,29]. By comparing the intensities of excitationpeaks located at 369 nm of all compounds, the relative intensitiesof the samples (Fig. 7a–c) obtained with glycine are stronger thanthose of the other three samples (Fig. 7d–f) obtained withoutglycine. Furthermore, it can be observed that the intensity ofGdPO4�H2O:Tb3+ prepared with glycine at pH = 8 (a) is thestrongest, pH = 5 (b) follows, and pH = 2 (c) is the weakest. Onthe contrary, the excitation intensities of the samples (Fig. 7d–f)obtained without glycine decrease as the pH values of the solutionsincrease from 2 to 7 then to 10.

Upon the excitation of the Tb3+ 7F6! 5L10 transition at 369 nm,the obtained emission spectra (Fig. 8) of GdPO4�H2O:Tb3+

phosphors consist of f–f transition lines within 4f8 electronconfiguration of Tb3+, i.e., 469 nm (5D3! 7F3) and 490 nm(5D4! 7F6) in the blue region, 545 nm (5D4! 7F5) in the green

region, and 588 nm (5D4! 7F4) as well as 623 nm (5D4! 7F3) inthe red region. The strongest emission peak is located at 545 nm,corresponding to the magnetic-dipole-allowed transition of Tb3+,thus giving rise to the green emission [25].

From Fig. 8, it can be discovered that three remarkabledifferences in the emission spectra of all compounds: (1) theluminescent intensities at 545 nm of the products prepared withglycine in Fig. 8a–c is a little stronger than those of the other threesamples prepared without glycine in Fig. 8d–f. (2) Among the threesamples obtained with glycine, the luminescent intensities locatedat 545 nm enhance with the increase of pH values; while the resultis on the contrary in the other three samples obtained withoutglycine. (3) By comparing the emission peaks of GdPO4�H2O:Tb3+

formed at pH = 2 with (Fig. 8c) and without (Fig. 8d) glycine, theformer’s intensity located at 545 nm is clearly enhancement.

The reason of the three differences may be related to themorphologies, particle sizes, coordination environment and crystalfield symmetry of the products. (1) As the pH value was increasedfrom 2 to 5 then to 8, the nanorods obtained with glycine becameeven shorter and smaller. While the morphologies of the other

H. Song et al. / Materials Research Bulletin 48 (2013) 5013–50185018

three samples obtained without glycine aggregated together toform nanobundles with the increase of pH values. The smaller thesize of particle becomes, the stronger the emission intensity ofproduct is. (2) Under identical measurement condition, thenanorods at pH = 2 in the presence of glycine exhibited a strongeremission than the nanorods at the absence of glycine. We attributethe intensity change of the predominant peak to the distinctmicrostructures of these nanocrystals. The ratio of surface Tb3+

increased with the reduction of particle size, which lowered thelocal symmetry of the crystal field around the Tb3+ ions, thusenhancing the emission intensity.

4. Conclusions

In summery, the Tb3+-doped GdPO4�H2O nanorods and nano-bundles have been synthesized by the hydrothermal process at180 8C for 48 h with and without glycine, respectively. Well-dispersed GdPO4�H2O:Tb3+ nanorods were obtained at pH = 2. Withthe increase of solution pH value, the sizes of nanorods obtainedwith glycine reduced. When the pH value increased to 7 and 10without glycine, the GdPO4�H2O:Tb3+ phosphors appeared asnanobundles. The formation mechanisms of the nanorods andnanobundles have been investigated in detail. The PL properties ofGdPO4�H2O:Tb3+ with different sizes were measured. Under theexcitation wavelength of 369 nm, all the GdPO4�H2O:Tb3+ phosphorsexhibited a strong green emission at 545 nm, attributing to5D4! 7F5 transition of Tb3+ ion. It was demonstrated that theluminescence properties of different samples were stronglycorrelated with the morphologies, sizes, coordination environmentand crystal field symmetry.

Acknowledgements

This work was supported by the Hubei Province Nature ScienceFoundation (2010CDB04701) and the Hubei Province EducationOffice Key Research (D20101011) of China.

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