Materials Research Bulletin 46 (2011) 2515–2522

Rare earth (Eu3+, Tb3+) centered composite gels Si–O–M (M = B, Ti) throughhexafluoroacetyl-acetone building block: Sol–gel preparation, characterizationand photoluminescence

Chang Wang, Bing Yan *

Department of Chemistry, Tongji University, Shanghai 200092, China

A R T I C L E I N F O

Article history:

Received 25 May 2011

Received in revised form 5 August 2011

Accepted 15 August 2011

Available online 22 August 2011

Keywords:

A. Amorphous materials

A. Optical materials

B. Sol–gel chemistry

D. Luminescence

A B S T R A C T

This report focuses on the syntheses of a series of novel photoactive composite xerogels materials in

which the functionalized hexafluoroacetylacetone (HFAASi) organic components are grafted into the

different inorganic networks (SiO2–B2O3 or SiO2–TiO2) via covalent bonds through a sol–gel process.

Subsequently, the physical characterization and especially photoluminescent properties of the resulting

xerogel materials are studied in detail. Except for composite xerogels linked to SiO2–TiO2 networks, all of

these composite xerogels exhibit homogeneous microstructures and morphologies, suggesting that

molecular-based materials are obtained with strong covalent bonds between the organic b-diketone

ligand and inorganic matrices. In addition, the ternary rare earth composite gels present stronger

luminescent intensities, longer lifetimes, and higher luminescent quantum efficiencies than the binary

ones, indicating that the introduction of the second ligands (phen) can sensitize the luminescent

emission of the rare earth ions in the ternary hybrid systems. It should be especially noted that these

composite xerogels based on Si–O–B networks possess not only higher thermal stability but also

stronger luminescent intensities than the other systems linked to different inorganic networks.

� 2011 Elsevier Ltd. All rights reserved.

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

1. Introduction

It is well known that rare earth complexes can exhibit sharp,intense emission lines under ultraviolet light irradiation, which isdue to abundant transition energy level of rare earth ions and theeffective intra-molecular energy transfer from the organic ligandscoordinated to them [1–3]. As a result, many of the rare earthcomplexes possess potential applications in such fields as efficientlight conversion molecular devices, spectroscopic structuralprobes, lasers, optical amplification fibers and organic light-emitting diodes, etc. [4–9]. Of all rare earth complexes, rare earthb-diketonate complexes are the most intensively investigated asluminescent species which ascribes principally to high absorptioncoefficient of b-diketone ligands and excellent luminescentproperties of them.

However, up to now, they still have a great distance frompractical applications, essentially putting down to their poorstabilities under high temperature or moisture conditions and lowmechanical strength [10]. Therefore, in recent years, the researchon luminescent rare earth materials has focused on hybridluminescent materials, which can combine the photophysical

* Corresponding author. Tel.: +86 21 65984663; fax: +86 21 65982287.

E-mail address: byan@tongji.edu.cn (B. Yan).

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.08.015

properties of the organic component with the favorable thermaland mechanical characteristics of inorganic component [11,12].Luminescent rare earth composite xerogels obtained by integrat-ing rare earth complexes with some matrices, for example silica-based materials [13–19], polymers [20–24] or mesoporous hybrids[25–28] exhibit excellent optical properties and good thermal andcompressive stabilities [29–31] in comparison to the pure rareearth complexes. According to the interaction between differentcomponents in composite xerogels, these materials can be dividedinto two major classes, the physically doped hybrids and thechemically bonded ones [32]. The physically doped hybrids cannotovercome the problems such as the quenching effect of lumines-cent centers, the inhomogeneous dispersion of two phases, andleaching of the photoactive molecules for the high energy vibrationaroused by the surrounding hydroxyl groups and weak interac-tions. Instead the chemically bonded hybrids belong to the wholecomplicated system with molecular scale, which can displayexcellent chemical stability and solve the article above-mentionedproblems effectively [13–28].

So far, there are sufficient researches on luminescent hybridmaterials using inorganic silica-based networks as matrices. Zhang[14,16], Carlos [13,18,19] and our team [23,24,33] have alreadydone a great deal of fruitful work. However, the hybrids derivedfrom the sol–gel process of other element alkoxyl compounds havenot been extensively involved [34–38]. The main reason for this

+ 2O=C=N(CH2)3Si(OEt )3

reflux

Hydrolysi s

C

O

C

H

70ºC

H

C

O

NaH TH F

CF3

Condensation

F3C

stir

DMF

room tempera ture

RE

CF3

F 3C

O

O

O

O

OOCCC

C

CH3

C C

C

C

H2OOH2 C

F 3CF 3

CF3 F3C

HN

OC (C

H 2)3

(CH2)3

HNOC

O

Si

O

X

Si

O O

SiO

O

CONH

CO

NH

CONH

HNOC(CH 2) 3

Si

(CH 2) 3(CH

2)3

(CH

2) 3

Si

Si

X

X

X X

X

A:TEOS or TBT orTBB; X:Si or Ti or B; RE :Eu or Tb .

A

Fig. 1. The scheme for the synthesis of the HFFASi bridge and the final hybrids

composite xerogels.

C. Wang, B. Yan / Materials Research Bulletin 46 (2011) 2515–25222516

fact may mainly be due to the fewer species of alkoxyl compoundsthan silane derivatives and the difficult control of their hydrolysisand copolycondensation process. But it can be expected to realizethe assembly of the hybrid systems with different inorganicnetwork [34,35] and even the multi-component hybrid hostnetwork [36–38]. So it is necessary to study the construction ofrare earth hybrid material systems with other hybrid xerogels.

In this paper, hexafluoroacetylacetone (HFAA) is grafted to 3-(triethoxysilyl)propyl isocyanate (TESPIC) to achieve a molecularprecursor HFAASi through a hydrogen-transfer nucleophilicaddition reaction between the methylene of HFAA and isocyanategroup of TESPIC. Subsequently, a series of binary compositexerogels (RE–HFAA–Si–O–X, RE = Eu, Tb, X = Si, Ti, B) are synthe-sized through linking molecular precursor HFAASi to inorganicnetworks in the sol–gel process. Meanwhile, we also prepare thecorresponding ternary hybrid luminescent materials by introduc-ing of the second ligands 1,10-phenanthroline (phen) into theabove system. Moreover, the luminescent properties, microstruc-ture, and thermal stabilities of these materials are analyzed indetail.

2. Experimental

2.1. Materials

Hexafluoroacetylacetone (HFAA) is obtained from ShanghaiChemical Plant (99.9%) and 3-(triethoxysilyl)-propyl isocyanate(TESPIC) is purchased from the Lancaster Company. Tetraethylorthosilicate (TEOS), titanium butoxide (TBT) and tributyl borate(TBB) are commercially available and used without purification.The solvents tetrahydrofuran (THF) and dimethylformaide (DMF)used are distilled before utilization according to the literatureprocedures [39]. Europium nitrate and terbium nitrate areobtained by the dissolution of corresponding oxide in concentratednitric acid. Other starting (e.g. NaH) reagents are used as received.

2.2. Synthesis

Synthesis of HFAASi precursor. The precursor HFAASi is preparedaccording to a known procedure [40] and characterized by 1H NMRand FTIR spectra. Firstly HFAA (1 mmol, 0.21 g) is dissolved inanhydrous tetrahydrofuran (THF, 20 mL) by stirring and then2 mmol NaH (0.08 g, 60%) is added to the solution. After 2 h,2 mmol (0.49 g) TESPIC is added to the above mixed solution bydroplet. The whole mixture is refluxing at 70 8C for 12 h and allsynthetic manipulations are performed under an atmosphere ofargon in three-necked bottle containing a magnetic stirrer. Ayellow oily liquid product HFAASi (IUPAC nomenclature bis((-triethoxysilyl)propyl amide) hexafluoroacetylacetone, C25H44O10

F6N2Si2) is furnished through isolation and purification. Theresulting HFAASi is characterized by 1H NMR spectrum and itsdata are as follows: d 0.51 (4H, t, CH2), d 1.28 (18H, t, OC2H5), d 1.59(4H, m, Si–CH2), d 3.29 (4H, m, O–CH2), d 3.80 (12H, m, NCH2), d8.05 (2H, t, NH). It can be proved that the TESPIC has grafted ontothe ligand HFAA successfully based on the above data.

Synthesis of RE(III)-centered hybrid material with composite Si–O–

B network host (RE = Eu, Tb). A certain amount of HFAASi isdissolved in DMF (20 mL) with stirring in the Teflon beaker andthen a corresponding quantity of Eu(NO3)3�6H2O is added to thesolution. Two hours later, a proper amount of TBB is added in thereaction solution and then the mixture is agitated strongly for 8 hto obtain a single phase in a the covered beaker at roomtemperature (The mole ratio of Eu(NO3)3�6H2O/HFAASi/TBB/H2Ois 1:3:12:48.). Finally, the beaker is transferred to the 80 8C ovenfor about 5 days ageing to form gelation. Whereafter, the binaryhybrid product (named Eu-HFAASi-O-B) is taken out and grinded

into powder for optical characterization studies. The procedure ofpreparing ternary composite xerogels (named phen-Eu-HFAASi-O-B) is similar to the above except that the mixed Eu(NO3)3�6H2O andphen replace Eu(NO3)3�6H2O, and the molar ratio ofEu(NO3)3�6H2O:phen is 1:1. A series of binary and ternarycomposite xerogels containing Tb(III) is synthesized based onthe same method when Eu(NO3)3�6H2O is replaced byTb(NO3)3�6H2O.

Synthesis of RE(III)-centered hybrid material with composite Si–O–

Ti network host (RE = Eu, Tb). According to the same method, binaryand ternary rare earth composite xerogels with Si–O–Ti networkhost are prepared by employing titanium TBT to replace TBB.However, differently, much faster hydrolysis rate of TBT than TBBresulted in often producing an opaque precipitate. So, in order toobtain homogeneous reaction system, Qiu’s research strategy isadopted. The molar ratio of reactants is the same to the above [41].Meanwhile, RE(III)-centered hybrid material with composite Si–O–Si network host is synthesized for comparison. Fig. 1 shows thescheme of the reaction procedure and predicted composition of thecomposite xerogels.

2.3. Measurements

All measurements are completed under room temperature. 1HNMR spectra are recorded in CDCl3 on a BRUKER AVANCE-500spectrometer with tetramethylsilane (TMS) as internal reference.FT-IR spectra are obtained with a Nicolet Nexus 912AO446spectrophotometer (KBr pellet) in the 4000–400 cm�1 region.The ultraviolet absorption spectra (using chloroform as solvent)are taken with an Agilent 8453 spectrophotometer. The ultravio-let–visible diffuse reflectance spectra are acquired by a BWS003spectrophotometer. The X-ray powder diffraction patterns arerecorded on a Bruker D8 diffractometer (40 mA–40 kV) using

C. Wang, B. Yan / Materials Research Bulletin 46 (2011) 2515–2522 2517

monochromated Cu Ka1 radiation (k) (1.54 A) over the 2u range of10–708. Thermogravimetric (TG) curves are measured with aNetzsch, model STA 409C, under nitrogen atmosphere crucibles inthe Al2O3 crucibles at a rate of 10 8C/min from 30 to 1000 8C. Theluminescent excitation and emission spectra are obtained on a RF-5301 spectrophotometer. Luminescent lifetime measurements arecarried out on an Edinburgh FLS920 phosphorimeter using a 450 Wxenon lamp as the excitation source. The microstructures arechecked by the scanning electronic microscope (Philips XL30).

3. Results and discussion

The IR spectra of the ligand (HFAA) and precursor (HFAASi) areshown in Fig. 2A ((a) for HFAA and (b) for HFAASi). Comparing (a)and (b) it can be observed that –CH2– stretching vibration peak ofHFAA at 3251 cm�1 has been replaced by a strong broad bandcentred at 2956 cm�1, which derives from the three methylenegroups of TESPIC. Moreover, in curve (c), vibration peak at 1274and 1085 cm�1 assigned to n(C–Si) and n(Si–O) are stretchingvibration absorption bands respectively, and the band centred at3420 cm�1 corresponds to the stretching vibration of grafted –NH–group. Meanwhile, the bending vibration (dNH, 1530 cm�1) furtherproves the formation of amide groups. New bands at 1770 and1687 cm�1 in curve (b) which attribute to the C55O absorption of

5001000150020002500300035004000

HFAASi

HFAA

Wavenumber / cm-1

Abs

orba

nce

400350300250

303 nm

Wavelength / nm

c

a b

Rel

ativ

e In

tens

ity /

a.u.

266 nm

A

B

Fig. 2. The FT-IR spectra of organic ligand HFAA and HFAASi bridge (A); the

ultraviolet absorption spectra of (a) HFAA, (b) HFAASi and (c) the excitation

spectrum of Eu-HFAASi complex (B).

TESPIC prove that TESPIC is grafted onto HFAA. Fig. 2B showsultraviolet absorption spectra of HFAA (a) and HFAASi (b). Aobvious red shift of the major p–p* electronic transitions a ! b(from 266 to 303 nm) can be observed by comparing theabsorption spectrum of HFAASi (b) with that of HFAA (a), whichindicates that the electron distribution of the organic ligand’sconjugated system has changed. TESPIC grafting to HFAA leaded tothe change which made the energy differences between p and p*decrease, therefore, we predicted that HFAA has participated inreaction with TESPIC. Fig. 2B(c) exhibited excitation spectrumcurve of Eu–HFAASi (detected at 613 nm), which superposes theabsorption curve of HFAASi (b) largely. The overlap between theabsorption band of HFAASi (curve (b)) and excitation band of Eu–HFAASi (curve (c)) signified that the ligand HFAA can sensitize thecentral Eu3+ ion in the hybrid material efficiently, namely antennaeffect. Consequently, we can reach to the conclusion that there isthe intra-molecular energy transfer occurred between the HFAASiand Eu3+ ion, which will be further proved in the later the emissionspectra of corresponding materials.

Fig. 3A shows the FTIR spectra of three binary compositexerogels based on different inorganic networks. These vibrationpeaks appearing in the high frequency range (3400–1600 cm�1)can be ascribed to the stretching vibration of the O–H group [38].The absorption peaks within 1050–1355 cm�1 originated from thestretching vibrations of Si–O–M bonds (M = Si, B, Ti), respectively

5001000150020002500300035004000

Abs

orba

nce

Wavenumber / cm-1

Eu-HFAA-Si-Si

Eu-HFAA-Si-Ti

Eu-HFAA-Si- B

5001000150020002500300035004000

Wavenumber / cm-1

Abs

orba

nce

phen-Eu-HFAA-Si-O- B

phen-Eu-HFAA-Si-O-Si

phen-Eu-HFAA-Si-O-Ti

A

B

Fig. 3. The FT-IR spectra of binary (A) and ternary (B) composite xerogels based on

different inorganic networks.

70605040302010

Eu-HFAA-phen-Si-B

Eu-HFAA-phen-Si-Ti

Eu-HFAA-phen-Si-Si

Eu-HFAA-Si-B

Eu-HFAA-Si-Ti

Eu-HFAA-Si-Si

Eu-HFAA

Inte

nsity

/ a.

u.

2θ / degrees

Fig. 4. The selected XRD patterns of different europium composite xerogels.

C. Wang, B. Yan / Materials Research Bulletin 46 (2011) 2515–25222518

[39]. The FTIR spectra of ternary composite xerogels are showed inFig. 3B. The twisting bending vibrations resulted by the absorptionof hydrogen atoms of phen at 840 cm�1 cannot be hardly observed,which provides a convincing evidence that phen can effectively becoordinated to the rare earth ions [40]. According to Fig. 3, the mostintense vibration signal around 3300 cm�1 and presence of a signalaround 1650 cm�1, are characteristic of the H2O molecule. Not onlybinary composite xerogels but also ternary, there is not apparentdistinction among the different composite Si–O–M networks,which suggests the homogeneous sol–gel process to form theuniform composite xerogels system.

Fig. 4 shows the room-temperature X-ray diffraction patterns ofthe selected europium composite xerogels from 108 to 708. Generallyspeaking, there is usually sharp and strong peak in the diffractioncurve of a material containing a large crystalline region [41].However, all the diffraction curves of these materials do not exhibitsuch diffraction peaks but similar broad peaks, with angle 2u centredaround 218 which is characteristic of amorphous silica materials[42], and which shows all these materials are amorphous. Theabsence of any crystalline regions in these samples is due to thepresence of organic moiety in the host inorganic framework [43]. Itseems that the second ligand phen introduced has no influence onthe disorder structure of the inorganic skeleton by makingcomparisons among these diffraction curves. In addition, thereare no measurable amounts of phases corresponding to the organic

100080060040020050

60

70

80

90

100

Eu-HFAA-Si-Ti

Eu-HFAA-Si-Si

TG /

%

Temperature / ºC

Eu-HFAA-Si-B

Fig. 5. The selected TG–DSC curves of europium composite xerogels.

compound (silylated precursor HFAASi or phen) or free RE nitrate inthese composite xerogels, which proves, to a certain extent, theformation of the true covalent-bonded molecular compositexerogels [44].

Thermo gravimetric analysis (TGA) is performed on thematerials selected in N2 atmosphere from 30 to 1000 8C. Fig. 5((a) for Eu-HFAASi-O-Si, (b) for Eu-HFAASi-O-Ti, (c) for Eu-HFAASi-O-B) shows the TGA traces of these materials. It can be viewed thatthese three samples behave the similar change trends in weightloss and three main degradation steps from the TGA curves.Compared to materials (a) and (c), (b) exhibits sharp weight lossunder the lower temperature (150 8C). We infer that fast hydrolysisand condensation speed of TBT did not form homogeneous anduniform materials and then resulted in worse thermal stability ofmaterial containing Si–O–Ti networks. Meanwhile, material (c) didnot lose much mass obviously until the temperature rose to about400 8C, which indicates hydrolysis and condensation are higherbetween hetero-alkoxy compounds than in a single alkoxy silane[44]. In general, there is one main purpose reached by introductioninorganic networks to organic complex, which is the increase ofthermal stability. As is clearly observed in the whole mass lossprocess of these materials, the thermal stabilities of all thesematerials are largely improved compared with the organic ligand,HFAA which is liquid at room temperature. Besides, it also revealsthat thermal stability caused by different inorganic host networksis different. From their mass loss curves, we could conclude that Si–O–B network is more excellent inorganic matrix comparing to theother two inorganic matrices, which is the result we wanted inthese experiments.

Fig. 6 gives the selected scanning electron micrographs (SEM) ofterbium composite xerogels. ((A) for Eu-HFAA-Si-O-B, (B) for Eu-HFAA-Si-O-Si, (C) for phen-Eu-HFAA-Si-O-B, (D) for phen-Eu-HFAA-Si-O-Si, (E) for Eu-HFAA-Si-O-Ti, and (F) for phen-Eu-HFAA-Si-O-Ti)The uniform surface of these composite xerogels (except (E) and (F))suggests that hydrolysis and co-condensation process might occurand homogeneous materials are obtained with strong covalentbonds between the inorganic networks and organic phases [42].Comparing to the composite xerogels with doped rare earthcomplexes which generally experience phase separation phenome-na [43], the two phases in these hybrids (except (E) and (F)) withchemical covalent bonds (Si–O) can exhibit their distinct propertiestogether [44]. There are many regular and uniform dendritic stripesmicrostructure on the skin of these materials, which are mostlyattributing to the sol–gel treatment. In the hydrolysis andcondensation processes of silica and titanium/boron, we reckonthat there is one-dimensional inorganic polymeric chain formed.Therefore, it is seen from the final structure and morphology thatthere are the homogeneous bulk trunks or stripes of treemicrostructure on the surfaces of each kind of these compositexerogels. As for material (E) and (F), the hydrolysis rate of titaniumalkoxides leads to poor condensation between Ti and Si, and thusresults in phase separation. We conjecture these two materials fail toexhibit fine luminescent properties and the conjecture will beproved by results discussed behind. Moreover, on the surface ofthese materials there are many granules which are residual solvent.

Fig. 7A and B shows the diffuse reflectance spectra of Eu3+ andTb3+ composite xerogels, respectively. It is observed that all of thesematerials exhibit a similar broad absorption band in the UV–visrange (200–600 nm) which corresponds to transition from theground state of the organic ligand to the first excited state (S0! S1)from Fig. 7A ((a): EuHFAA, (b): Eu-HFAA-Si-O-Si, (c): Eu-HFAA-Si-O-Ti, (d): Eu-HFAA-Si-O-B, (e): phen-Eu-HFAA-Si-O-Si, (f): phen-Eu-HFAA-Si-O-Ti and (g): phen-Eu-HFAA-Si-O-B). The luminescenceintensity of material rests with the matching degree between thetriplet state energy of ligand and excited-state energy of rare earthion, we can primarily predict that the energy level difference

Fig. 6. The selected scanning electron micrographs of terbium composite xerogels: (A) Eu-HFAA-Si-O-B, (B) Eu-HFAA-Si-O-Si, (C) phen-Eu-HFAA-Si-O-B, (D) phen-Eu-HFAA-

Si-O-Si, (E) Eu-HFAA-Si-O-Ti, and (F) phen-Eu-HFAA-Si-O-Ti.

C. Wang, B. Yan / Materials Research Bulletin 46 (2011) 2515–2522 2519

between HFAA and Eu3+ is suited so that the organic ligand canabsorb abundant energy in ultra-visible extent and transfer theenergy to the corresponding rare earth ion in terms of the abovephenomena. So it can reach a conclusion that the final compositexerogels can be expected to exhibit excellent luminescenceproperties, which is proved by the luminescence spectra analysis.Meanwhile, seen from Fig. 7A, curves of (e) and (g) show sharpernegative absorbtion peak compared to curves (b) and (d) atcharacteristic emission wavelength, of which indicates addition ofphen can bring Eu3+ ion more stable eightfold-coordinationenvironment without the participant of H2O molecule and thenstrengthen the luminescence of Eu3+ to a greater extent. However,there is not similar appearance in curve (f). We predict it may be theresult of the faster hydrolysis rate of titanium butoxide tetraethylthan orthosilicate and tributyl borate make condensation coursecannot take place at the molecular level and then prepareheterogeneity of materials which cause a negative impact in therespect of absorption of energy.

The luminescence spectra of all the hybrid luminescentmaterials are investigated at room temperature and are shownin the Figs. 8 and 9. The excitation spectra are obtained bymonitoring the emission of Eu3+ or Tb3+ at 614 or 545 nmrespectively. From Figs. 8A and 9A, it can be seen that all thesematerials exhibit strong and broad absorption band, which can beattributed to the organically modified Si–O–Si/Ti/B composite

matrix [45]. Here the organically modified Si–O–Si/Ti/B compositematrix play a dual role, both as a host and ligand for thecoordination bonds between HFAASi–O–B(Ti) and Eu3+ [46]. Theabsorption of not only photoactive organically modified HFAAgroup but also –Si–O–Si/Ti/B– network contribute to the energytransfer and luminescence of Eu3+ in the composite xerogelssystems. As far as the excitation spectra of Europium hybridmaterial are concerned, these composite xerogels excited at340 nm exhibit emissions characteristic of Eu ions. The emissionlines of the composite xerogels are assigned to the 5D0! 7FJ

transitions located at 578, 590 and 615 nm, for J = 1, 2 and 3,respectively. Among these emission peaks, the peak around615 nm assigned to 5D0! 7F2 emission is the most predominanttransition, which suggests that the chemical environment aroundEu(III) ions is lack of an inversion center [47]. The stronger theinteractions of europium ion with its local chemical environmentare, the more nonsymmetrical the complex becomes, the moreintense the electric-dipolar transitions becomes. Therefore,5D0! 7F1 transition (magnetic-dipolar transitions) decrease and5D0! 7F2 transition (electric-dipolar transitions) increase [48]. Bycomparing curves (b), (c) and (e), it can be observed clearly thatcurve (b) and curve corresponded to the hybrid material (e),especially curve (e) exhibit stronger luminous intensity than curve(c), which indicates Si–O–Si and Si–O–B are more favorable forEu3+–TTA complexes than Si–O–Ti. It is well known that B and Si

8007006005004003002000.0

0.2

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HFAA-Eu-phen-Si-Si

HFAA-Eu-phen-Si-TiHFAA-Eu-phen-Si-B

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lect

ance

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HFAA-Eu-Si-B

HFAA-Eu-Si-TiHFAA-Eu-Si-Si

Eu-HFAA

800700600500400300200

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HFAA-TbHFAA-Tb-Si-Si

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HFAA-Tb-ph en-Si-Ti

Wavelength / nm

Ref

lect

ance

HFAA -Tb-ph en-Si-B

A

B

Fig. 7. Selected ultraviolet–visible diffuse reflectance absorption spectra of

europium (A) and terbium (B) centered composite xerogels.

450400350300250

Eu-HFAAEu-HFAA-Si-Si

Eu-HFAA-Si-Ti

Eu-HFAA-Si-B

phen-Eu-HFAA-Si-Si

phen-Eu-HFAA-Si-Ti

phen-Eu-HFAA-Si-B

EX

Wavelength / nm

Rel

ativ

e In

tens

ities

/ a.

u.

625600575550

Eu-HFAA

Eu-HFAA-Si-Si

Eu-HFAA-Si-Ti

Eu-HFAA-Si-B

phen-Eu-HFAA-Si-Si

phen-Eu-HFAA-Si-Ti

phen-Eu-HFAA-Si-B

5 D0-

7 F 2

5 D0-

7 F 1

5 D0-

7 F 0

Rel

ativ

e In

tens

ities

/ a.

u.

Wavelength / nm

EM

A

B

Fig. 8. The excitation (A) and emission spectra (B) of europium composite xerogels.

C. Wang, B. Yan / Materials Research Bulletin 46 (2011) 2515–25222520

are diagonal elements with similar features and chemical behavior.This point can be verified from the sol–gel process of their alkoxycompounds [48]. Here the composite host of Si–O–B systemsshows little distinction as pure Si–O host, so the compositexerogels with composite Si–O–B host network cannot change theluminescence. On the other hand, Ti is a transition metal andpresents a chemical property different from Si [49]. Subsequently,the introduction of Ti–O is not favorable for the luminescence ofEu3+. Moreover, rapid hydrolysis and condensation of titaniumbutoxide may causes a certain degree of phase separation betweenrare earth complexes and inorganic network and further leads toluminescent quenching because of high local concentrations ofEu3+ complex.

According to Fig. 8B, the emission intensities of these kinds ofmaterials are determined in the order: phen-Eu-HFAA-Si-O-Si > Eu-HFAA-Si-O-Si; phen-Eu-HFAA-Si-O-B > Eu-HFAA-Si-O-B,which means the introduction of the second ligand can efficientlysensitize the luminescence of Eu3+ ions. We presume that phencan replace coordinated water molecules existing in complexesand thus avoid the energy loss caused by the vibration of theirhydroxyl groups. Besides, phen itself is an excellent ligand inaspect of energy absorption and transmission, which can transferenergy to central Eu ions effectively. For Terbium hybridmaterial, the emission peaks in the Fig. 9B centred at 490,543, 582 and 620 nm are assigned to 5D4! 7F6, 5D4! 7F5,5D4! 7F4 and 5D4! 7F3 transitions, respectively. The moststriking green luminescence (5D4! 7F5) is observed due to thefact that this emission is the most intense one. Corresponding tothe emission spectra of europium hybrid material, the lumines-cent intensities of terbium hybrid material do not change, that is

Tb-HFAA-Si-O-B > Tb-HFAA-Si-O-Si > Tb-HFAA-Si-O-Ti; phen-Tb-HFAA-Si-O-Si > Tb-HFAA-Si-O-Si; phen-Tb-HFAA-Si-O-B >

Tb-HFAA-Si-O–B.To investigate the luminescence lifetimes of these composite

xerogels, the decay curves of them are measured. All these typicaldecay curves can be described as a single exponential, indicating thatall the emitting centers (Eu3+ or Tb3+) lay down in the same averagecoordination environment. The lifetime data of the europium andterbium composite xerogels are present in Table 1. It is observed thatthe ternary composite xerogels exhibit longer luminescence life-times than the corresponding binary composite xerogels, whetherwhose emitting centers are Eu3+ or Tb3+. This phenomenon indicatesthat the introduction of the second ligand phen is of benefit to theluminescence properties of these composite xerogels, since thesecond ligand phen cannot only avoid the luminescence quenchingvia the removal of water molecules from the first coordinationsphere of emitting centers, but also absorb the light and transfer theenergy to the emitting centers efficiently [50,51]. It is interestinglyfound that both these composite xerogels with Si–O–Si and Si–O–Bmatrices show longer luminescence lifetimes than these compositexerogels with Si–O–Ti matrices. This indicates that both Si–O–Si andSi–O–B matrices are advisable choices to be used as the host toimprove the luminescence properties of these composite xerogels.

We also determine the emission quantum efficiencies of the 5D0

excited state of the europium ion for the europium compositexerogels based on the emission spectra and lifetimes of the 5D0

emitting level. The quantum efficiency of the luminescence step, hexpresses how well the radiative process (characterized by rate

450400350300250

Tb-HFAA

Tb-HFAA-Si-Si

Tb-HFAA-Si-Ti

Tb-HFAA-Si- Bphen-Tb-HFAA-Si-Si

phen-Tb-HFAA-Si- B

phen-Tb-HFAA-Si-Ti

EXR

elat

ive

Inte

nsiti

es /

a.u.

Wavelength / nm

650600550500450

phen-Tb-HFAA-Si-Tiphen-Tb-HFAA-Si-Si

Tb-HFAA-Si- B

Tb-HFAA-Si-Ti

Tb-HFAA-Si-Si

Tb-HFAA

5D4 - 7F3

5D4 - 7F4

5D4 - 7F5

5D4 - 7F6

EM

Wavelength / nm

Rel

ativ

e In

tens

ities

/ a.

u.

phen-Tb-HFAA-Si- B

A

B

Fig. 9. The excitation (A) and emission spectra (B) of terbium composite xerogels.

C. Wang, B. Yan / Materials Research Bulletin 46 (2011) 2515–2522 2521

constant Ar) shows, competes with non-radiative processes(characterized by overall rate constant Anr) [52,53]:

h ¼ Ar

Ar þ Anr(1)

Non-radiative processes influence the experimental lumines-cence lifetime by the equation [54]:

texp ¼ ðAr þ AnrÞ�1 (2)

So quantum efficiency can be calculated from the radiativetransition rate constant and experimental luminescence lifetime[55]:

h ¼ Artexp (3)

Table 1Luminescence decay times (t) and emission quantum efficiency (h) of europium hybri

Eu-HFAA-Si-O-Si Eu-HFAA-Si-O-B

I00a 1.5 24.6

I01a 30.5 98.4

I02a 401 535

A02 (s�1) 687 283

t (ms)b 456 412

Arad (s�1) 740 346

h (%) 33.7 14.2

a The integrated intensity of the 5D0! 7FJ emission peaks.b The luminescence decay times of 5D0! 7F2 transitions.

where Ar can be obtained by summing over the radiative rates A0J

for each 5D0! 7FJ transition of europium ion.

Ar ¼X

A0J ¼ A00 þ A01 þ A02 þ A03 þ A04 (4)

The transition 5D0! 7F1 is a magnetic dipole transition, whichis practically independent of the chemical environments aroundthe europium ion, and thus can be considered as an internalreference. The experimental coefficients of spontaneous emissionA0J can be calculated according to the equation [54,55]:

A0J ¼ A01I0J

I01

� �n01

n0J

� �(5)

Here A0J is the experimental coefficient of spontaneousemission and A01 is the Einstein’s coefficient of spontaneousemission between the 5D0 and 7F1 energy level. In vacuum, A01 hasa value of 14.65 s�1, when an average index of refraction n equal to1.506 is considered, the value of A01 can be determined to be 50 s�1

approximately (A01 = n3A01(vacuum)) [52,53]. n0J refers to the energybarrier and can be determined from the emission peaks of Eu3+’s5D0! 7FJ emission transitions. I is the emission intensity and canbe taken as the integrated intensity of the 5D0! 7FJ emissionbands of the 5D0! 7F0–4 emission curves.

The luminescence quantum efficiency of the europiumcomposite xerogels is shown in Table 1. On the basis of the abovediscussion, it can be seen that the value h mainly depends on thevalues of two quanta: one is lifetime and the other is I02/I01 (red/orange ratio). From Table 1, the similar phenomena to that ofluminescence lifetimes can be found is that the ternary europiumcomposite xerogels present higher luminescence quantum effi-ciency than the binary composite xerogels without the secondligand phen, since the introduction of the second ligand canaccelerate the energy-transfer process between the ligands andemitting centers. Moreover, both Si–O–Si and Si–O–B matricesseem to be more efficient matrices as the host to improve theluminescence quantum efficiency of the hybrid systems in thispaper, indicating that both the coordination environment ofemitting centers and the host have large influences on theluminescence quantum efficiency of the composite xerogels.

4. Conclusions

In summary, we have synthesized a series of hybrid xerogelsbased on silica and titanium/boron networks (Si–O–Si/Ti/B) by themodification of HFAA with TESPIC in the sol–gel process. Thesehybrid xerogels based on Si–O–Si/B are homogenous and no phaseseparation occur, which attribute to the formation of thecovalently bonded Si–O–Si/B network during the hydrolysis andpolycondensation reactions between the precursors (HFAASi) andtetraethyl orthosilicate/tributyl borate. The ternary hybrid mate-rial shows much more excellent luminescent properties such ashigher quantum yield, longer luminescence lifetimes. It should benoted that the composite xerogels based on Si–O–B possess higher

d materials.

phen-Eu-HFAA-Si-O-Si phen-Eu-HFAA-Si-O-B

8.1 55

98.4 100

711 978

376 505

810 876

430 582

34.8 50.9

C. Wang, B. Yan / Materials Research Bulletin 46 (2011) 2515–25222522

thermal stability and more excellent luminescent properties thanthat composite xerogels based on Si–O–Si/Ti which will bepromising candidate in many fields of applications. However,the mechanism how these different host matrices affect theluminescent properties in such materials needs further deepinvestigation.

Acknowledgements

This work is supported by the National Natural ScienceFoundation of China (20971100) and Program for New CenturyExcellent Talents in University (NCET-08-0398).

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