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Colloids and Surfaces A: Physicochem. Eng. Aspects 453 (2014) 142–148

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

j ourna l h om epa ge: www.elsev ier .com/ locate /co lsur fa

nfluence of various anions (Cl−, NO3−, and CH3COO−) of europium

alts on the thermal decomposition behavior of Eu3+-modified,8-naphthalic anhydride hybrid mesoporous silica

eng Wang, Jihong Sun ∗, Shiyang Bai, Xia Wu, Jinpeng Wangollege of Environmental & Energy Engineering, Beijing University of Technology, Beijing 100124, PR China

i g h l i g h t s

Thermal stability of Eu3+-modifiedNA loaded BMMs was enhanced.Influences of various anions ofeuropium salt on decompositionbehavior and kinetic property ofchromophore were remarkable.Dynamic-based control played mainrole during the whole process.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 19 November 2013eceived in revised form 7 April 2014ccepted 12 April 2014vailable online 21 April 2014

a b s t r a c t

Hybrid bimodal mesoporous silica grafted 1,8-naphthalic anhydride (NA) as a luminescent tracer wasmodified by Eu3+ ions, whereas the influence of the interaction between europium salts with vari-ous anions (Cl−, NO3

−, and CH3COO−) and NA molecules grafted onto the mesopores on their thermaldecomposition behaviors and kinetic properties were studied in detail in accordance with the thermo-gravimetric analyzer (TGA) profiles obtained at different heating rates under flowing N2 atmosphere.

eywords:esoporous silica

pparent activation energyhermal decomposition behavioru3+ ions,8-Naphthalic anhydride

Meanwhile, Kissinger and Flynn–Wall–Ozawa algorithms were used to calculate the apparent activationenergies of Eu3+-modified NA existing in the mesoporous channels and therefore the thermal decom-position mechanism was discussed on the basis of the Coats-Redfern method. Moreover, XRD, TEM,N2 adsorption–desorption isotherms and FT-IR techniques were employed to analyze the structuralcharacteristics and textural properties of the resultant samples.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Drug sustained/controlled delivery system is one of the mostopular research subjects and has been applied widely in a numberf fields especially in biology, pharmaceutical, clinical medicine and

∗ Corresponding author at: College of Environmental & Energy Engineering, Bei-ing University of Technology, 100 PingLeYuan, Chaoyang District, Beijing 100124,R China. Tel.: +86 10 67396118; fax: +86 10 67391983.

E-mail address: jhsun@bjut.edu.cn (J. Sun).

ttp://dx.doi.org/10.1016/j.colsurfa.2014.04.036927-7757/© 2014 Elsevier B.V. All rights reserved.

so on during the past decades [1,2]. For the first time, Vallet-Regíet al. used 3-aminopropyl groups to modify MCM-41 and demon-strated the feasibility of controlling the delivery performance ofloaded-drugs in the amine-modified mesoporous channels [3–5], ithas been found that the suitable textural parameters and structuralproperties of the mesoporous silica-based carriers in mesoporousmatrix are an important strategy and a feasible pathway to achieve

the desired drug delivery profile [6–10].

Presently, we reported that the functional bimodal mesoporoussilica having well-defined small mesopores of 3 nm and a broad dis-tribution of tubular mesopores with a controlled pore size between

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2

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F. Wang et al. / Colloids and Surfaces A: P

0 and 30 nm, and the varying synthesis conditions allow the largerrug molecules easier accessibility to the active sites and thereby

mprove the desired drug loading/release efficiencies [11–14] dueo the disappearance of diffusion limitation. All these features ofMMs could provide an ideal carrier for drug storage and controlledelease.

However, during the progress of drug-delivery system, to find suitable way to track and detect the performance of the con-rolled drug storage and release from mesoporous silica will be ofrime importance, but on the other hand the related loading andelease mechanism, which underlies the adsorption or diffusion ofhe loaded drug molecules in the confined delivery system, has noteen essentially probed.

Recently, we used 1,8-naphthalic anhydride (NA) as the fluores-ent tracer to graft onto the amine-modified mesoporous surface ofMMs. The influence of different synthesis methods including theodification of amine groups and the amount of loaded NA on the

uminous behavior of the resulting hybrid silica have been demon-trated in detail. The preliminary results indicated that the factorsffecting on the luminous molecules and modified silica matrixnside mesoporous channels should be critical for their controlleduminous performance [15–17].

Although, due to the photostability and good biocompatibility,rganic NA molecules and its derivatives have been widely usedn the applications of biological and medicinal fields [18–20], onef the major drawbacks was that their fluorescence intensity inybrid luminescent material still remained a great challenge [21].o solve this problem, more recently, an enhanced luminescentesoporous BMMs via grafting with Eu3+-modified NA onto the

mine-functionalized mesoporous surface have been prepared andhe related mechanism was elucidated in our group [22]. Partic-larly, the preliminary results demonstrated that the influencesf various kinds of europium salts with different anions (NO3

−,H3COO−, and Cl−) on the luminescent intensities of characteristiceaks of the resultant hybrid mesopore silica were remarkable.

But, up to now, the thorough investigations on the controlleduminescent molecules with mesoporous silica by a convenient andontrollable way are very limited. Therefore, it is very necessaryo explore the adsorption performance between amine-modified

esoporous surface and grafted NA molecules with Eu-enhanceduorescent intensity and particularly, to elucidate thermal stabilityf the luminescent NA molecules with the help of thermal decom-osition behavior of luminescent molecules.

On the basis of our previous studies [22–25], the influence ofhree anions (Cl−, NO3

−, and CH3COO−) of europium salts on ther-al stability and decomposition behavior of the Eu-modified NAolecules were investigated in detail via thermogravimetric anal-

sis (TGA) profiles under N2 with three heating rates (5, 10 and5 K/min, respectively). Meanwhile, the apparent activation ener-ies for their thermal decomposition properties were calculatedn accordance with Kissinger and Flynn–Wal–Ozawa algorithmsnd the corresponding evaluation was demonstrated accordingo Coats–Redfern method. The X-ray diffraction (XRD), Transmis-ion electron microscopic (TEM), N2 sorption isotherms, Fourierransform infrared (FT-IR) and TGA were employed to character-ze their structure and properties. These results are very useful forurther understanding of the dispersion environments of luminous

olecules inside mesopores.

. Experimental

.1. Chemicals

The NA and europium salts (Europium acetate, Europiumitrate, and Europium chloride), and (3-aminopropyl)

chem. Eng. Aspects 453 (2014) 142–148 143

triethoxysilane (APTES) were supplied from Alfa Aesar Com-pany. N, N-dimethyl formamide (DMF) were provided by BeijingChemical Factory. BMMs were synthesized by our group. All thematerials were of A.R. grade. Doubly distilled water was used in allexperiments.

2.2. Synthesis of Eu3+-modified NA grafted into the APTES-BMMs

Synthesis procedure of APTES-BMMs was similar to the previouswork of Li et al. [16] and luminescent hybrid bimodal mesoporoussilica was further modified, as follows: 0.5 g AP-BMMs was taken ina 100 ml three-necked round-bottom flask containing 50 ml DMFsolution. This mixture was kept for stirring and 0.005 g NA wasadded to it. One of three kinds of Europium salts was added tothe above solution. The molar ratio of the constituents was NA:EuX = 1:1, where x = Cl−, NO3

−, CH3COO−, respectively. After stir-ring for 5 min at room temperature, the mixture was heated at403 K for 3 h in an oil bath under reflux. Then it was cooled toroom temperature, the resulting precipitates were collected byfiltration, washed with petroleum ether and DMF, and dried at353 K. Finally, the samples were obtained and remarked as LHMS-EC (anions was Cl−), LHMS-EN (anions was NO3

−), and LHMS-EA(anions was CH3COO−), respectively.

2.3. Characterization

Powder XRD were performed on a BRUKER/AXSD8 ADVANCEdiffractometer with Cu K� radiation at 35 kV and 20 mA, the mea-surements were recorded in a 2� range from 1◦ to 10◦ and ascan speed of 1.0◦/min. The TEM images were obtained on anelectron microscope (JEM-2010 F) under 200 kV accelerating volt-age. N2 adsorption/desorption isotherms were collected at 77 K ona Tristar 3020 system. All samples were pretreated at 353 K for3 h before measurements. Specific surface area was calculated bythe Brunauer–Emmett–Teller (BET) method and the correspondingpore size distribution was evaluated from the data points over a rel-ative pressure range of 0.05–0.15 P/P0 of the desorption branchesusing the Barrtt–Joyner–Halenda (BJH) model. The FT-IR spectra ofthe samples were recorded with TENSOR27 BRUKE spectrometer(resolution of 2 cm−1) using KBr pellets technique. The TG-DTG pro-files were measured via a PerkinElmer Pyris1 TG instrument witha heating rate of 5, 10 and 15 K/min respectively, in a range from300 to 1073 K with the rate of 20 mL/min.

3. Results and discussion

3.1. XRD and TEM analysis

The XRD patterns of related samples modified by differentanions and particular amount of Eu3+ ion additive were depicted inFig. 1. As shown in Fig. 1a–c, all diffraction peaks around 2� = 2.3◦

(2.30◦ for LHMS-EC, 2.34◦ for LHMS-EN and 2.36◦ for LHMS-EA)were corresponding to typical reflection (1 0 0) of BMMs as reportedinvestigates [26], meaning that the ordered mesoporous structurescould be preserved after post treatment, which can be further con-firmed by TEM images. Fig. 1 (insert) revealed the abundant smallpores of around 2 nm and the accumulated large pores with a size of20 nm. The corresponding d-spacing values of the (1 0 0) diffractionpeak were 3.98 nm to LHMS-EC, 3.78 nm to LHMS-EN and 3.75 nmto LHMS-EA, respectively with decreasing tendency. Therefore itwas evidenced that the Eu3+-modified NA molecules were success-fully grafted onto the mesoporous channels as compared with that

of BMMs [16] and LHMS [22]. Evidently, the reasons that the d-spacing is dependent on the anion are related to several influences,including the steric hindrance of anions, chemical binding forcebetween Eu3+ ions and anions, the interaction between Eu3+ ion and

144 F. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 453 (2014) 142–148

Frr

Ns

3

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FeE

4000 350 0 300 0 250 0 200 0 150 0 100 0

c

b

C-NC=O

Tra

nsm

issi

on

(%

)

Wav enumber (cm-1)

C-H

a

ig. 1. XRD patterns of samples with different anions of europium salts and cor-esponding TEM images (insert), (a) LHMS-EC, (b) LHMS-EN and (c) LHMS-EA,espectively.

A molecules and moreover their distribution on the mesoporousurface of BMMs [22,25].

.2. N2-sorption isotherms

N2 adsorption/desorption isotherms of samples and relevantexture parameters were presented in Fig. 2. As can be seen, allsotherms of the samples represented a IV type along with H1 typeysteretic loops similar to that of the reported BMMs [26], whichas ascribed to the capillary condensation of N2 with the typi-

al mesoporous characteristic. Two sharp adsorption steps wereeen clearly including the first step at a relative pressure rangef 0.1–0.3 P/P0 and the second one at 0.8–1.0 P/P0, correspond-ng bimodal pore distribution with the narrow small mesoporesround 2.0 nm and the larger broaden pores with a mean size of0.0 nm [16]. Meanwhile, after modification with EuCl3, EuNO3nd Eu(CH3COO)3, respectively, the isotherms for three obtainedamples revealed a decreasing tendency in N2 absorbed volumeFig. 2a–c), resulting in a decreased surface area and pore volume

22], which clearly demonstrated the presence of Eu-modified NA

olecules and the effectiveness of various anions. The pore distri-utions (Fig. 2 insert) revealed that the large mean pore size was

ig. 2. N2 adsorption/desorption isotherms of samples with different anions ofuropium salts and corresponding pore distribution (insert), (a) LHMS-EC, (b) LHMS-N and (c) LHMS-EA, respectively.

Fig. 3. FT-IR spectra of samples with different anions of europium salts (a) LHMS-EC,(b) LHMS-EN, and (c) LHMS-EA, respectively.

around 16.30 nm (16.36 nm for LHMS-EC, 16.34 nm for LHMS-ENand 16.40 nm for LHMS-EA), while the small mean pore size was allless than 2 nm. Both of them were smaller than that of the reportedBMMs (2.9 nm and 20.0 nm [16]). In addition, the specific surfacearea of samples: 647 m2/g for LHMS-EC, 643 m2/g for LHMS-ENand 564 m2/g for LHMS-EA were smaller than that of BMMs (about917 m2/g [16]) and LHMS (around 694 m2/g [22]). Whereas the porevolume were 1.03 cm3/g for LHMS-EC, 0.97 cm3/g for LHMS-EN and0.88 cm3/g for LHMS-EA, respectively, in contrast, that of BMMs was1.99 cm3/g [16] and LHMS was 1.01 cm3/g [22]. These results sug-gested that the Eu3+-modified NA molecules have been successfullygrafted onto the functionalized mesoporous surface.

3.3. FT-IR analysis

FT-IR spectra of obtained samples were shown in Fig. 3. Allthe characteristic peaks of hybrid mesoporous silicas exhibitedobviously in the spectra, including the asymmetric and symmetricSi-O stretching vibrations in 1030–1250 cm−1 range, at 800 cm−1

and the peak at 950 cm−1 aroused from the stretching of Si-OHin Fig. 3(a–c) [27]. Meanwhile, the presence of –CH2NH2 func-tional group could also be confirmed via the peaks at 1417, and2948 cm−1, which actually belonged to the C–H stretching vibra-tion and the C–N stretching, respectively [28,29]. Whereas, thepeaks at 1660 and1590 cm−1 were ascribed to the stretching vibra-tions of C O, which was derived from the characteristic absorptionband of NA, and implied that the luminescent groups have beenincorporated into the hybrid mesoporous BMMs matrix [30].

3.4. TG and thermal decomposition properties

3.4.1. TheoryA feasible algorithm about kinetic data evaluation has been gen-

erally accepted in terms of Kissinger, Ozawa, and Coats-Redfernmethods [31,32]. However, the most widely employed methods areKissinger and Ozawa equations, which can determine the apparentactivation energy without knowledge of reaction order [33]. Thedetailed descriptions are as following that:

Kissinger equation [34]:

Ln

(ˇ

T2P

)= − E

RTP+ Ln

(AR

E

)(1)

F. Wang et al. / Colloids and Surfaces A: Physico

F1

Ta

e

s

L

eoc

s

li

d

L

roe

s

fsm

3

wpo

ig. 4. TG curves of samples with different anions of europium salts at a rate of0 K/min (a) LHMS-EC, (b) LHMS-EN and (c) LHMS-EA, respectively.

where, is the heating rate, E is the apparent activation energy,P the maximum peak temperature, A the pre-exponential factornd R the gas constant.

The plot Ln ˇ

T2P

versus 1TP

, is a straight line, whose slope allows

valuation of the E value. Then Eq. (1) is rewritten as:

lope = d(Ln(ˇ/T2P ))

d(1/TP)= − E

R(2)

Ozawa equation [35,36]:

n(ˇ) = Ln(

AE

Rg(˛)

)− 5.331 − 1.052

E

RTP(3)

In Eq. (3), is the heating rate, E denotes the apparent activationnergy, A represents the pre-exponential factor, g(a) is integral formf function, TP the maximum peak temperature, and R is the gasonstant.

lope = d(Lnˇ)d(1/TP)

= −1.052E

R(4)

According to the above equation: the slope of the related straightine derived by the Ln versus 1/TP plot, the corresponding E values obtained.

The method of Coats-Redfern [37] is used to analyze the thermalecomposition mechanism. The equation in detail is as following:

n(

g(˛)T2

)= Ln

(AR

ˇE

)− E

RT(5)

In this equation, a is the degree of conversion, the heatingate, E the apparent activation energy, g(a) are functions dependingn the kinetic term model, T the absolute temperature, A the pre-xponential factor and R the gas constant.

The E values can be calculated from the equation (5)

lope = d(Ln(g(˛)/T2))d(1/TP)

= − E

R(6)

When E from the method of Coats-Redfern is equal to thoserom the Ozawa and Kissinger equations then the linear relation-hip is well, it can be concluded that the thermal decompositionechanism have been found.

.4.2. TGA performance

The TGA profiles of the samples at the heating rate of 10 K/min

ere shown in Fig. 4. Four stages of weight loss process wereresented, the first weight loss of less than 2 wt% was generallybserved below 373 K, which was attributed to decomposition of

chem. Eng. Aspects 453 (2014) 142–148 145

adsorbed water; the second one was around 2.69 wt% for LHMS-EN, 3.96 wt% for LHMS-EC, 3.24 wt% for LHMS-EA at 373–493 K,corresponding to the dehydration of silicon alcohol and partlyorganic materials [33]. The third weight loss of about 9.34 wt%for LHMS-EN, 8.31 wt% for LHMS-EC and 10.70 wt% for LHMS-EA,occurred in a temperature range of 493–703 K, corresponding to thedecomposition of the organic functional group molecules result-ing from APTES and partly from NA, and even the removal ofresiduary europium salts [38–40]. The last one with the weightloss of 4.65 wt% for LHMS-EN, 4.52 wt% for LHMS-EC, 6.30 wt% forLHMS-EA, and 4.94 wt% for LHMS respectively, at 703–1073 K, wasprobably associated with the complete decomposition of NA andAPTES molecules. Especially, CH3COO− ion with obvious steric hin-drance took part in the modification process of LHMS-EA, resultingin quite different weight losses in comparison with that of othersamples [25]. These facts suggested again that the behavior ofEu3+-modified luminescent molecules grafted into the mesoporouschannels of BMMs [16] were found in good agreement with that ofthe mentioned-above characterization.

3.4.3. Thermal decomposition analysisTypical DTG curves of the samples obtained at various heating

rates under N2 atmosphere were illustrated in Fig. 5. The E val-ues of each peak of a sample calculated by using the Kissinger andOzawa–Flynn–Wall methods and all were summarized in Table 1.As can be seen in Fig. 5(A–C), all samples presented three peaks,similar to that reported in Li et al. studies [23], while the heatingrate was found to be correlated directly with the rate of weightloss and the temperature at the maximum weight loss rate, show-ing that the temperature of decomposition was proportional to theheating rate.

According to our previous report [23], the main decompositiontemperature of pure NA was around 548 K. As seen in Fig. 5, peak-1 of all samples was ascribed to the decomposition of adsorbedwater on the basis of data mentioned in Fig. 4, which was notdiscussed further in detail. As depicted in Fig. 5A, peak-2 of LHMS-EC was corresponding to the dehydration of silicon alcohol andpartly organic materials [33], and the temperature of its maximumdecomposition was raised with the increasing heating rate (5, 10,and 15), showing 600 K, 610 K, and 618 K, respectively, whereas,that of peak-3 corresponded to 754 K, 776 K, and 792 K. In contrast,other samples presented a similar increased tendency in Figure 5Band 5 C, accordingly, the decomposition temperatures of peak-2were corresponding to 590 K, 599 K, and 603 K for LHMS-EN, while608 K, 617 K and 621 K for LHMS-EA, respectively. Evidently, themaximum decomposition temperatures of peak-2 for three sam-ples were all higher than that of LHMS (569 K, 584 K and 591 K,respectively) [23]. These results obviously indicated that the ther-mal stability of Eu3+-modified NA grafted BMMs was improved,the main reason was due to the intermolecular forces betweenEu-NA chromospheres and functionalized mesoporous surface ofBMMs [22,25]. Meanwhile, the temperatures of peak-3 for LHMS-EN (844 K, 858 K, and 871 K, respectively) were higher than thatfor LHMS (810 K, 828 K and 847 K, respectively), however, thatof LHMS-EC (754 K, 776 K, and 792 K, respectively) and LHMS-EA(780 K, 804 K, and 818 K, respectively) were lower than that ofLHMS (810 K, 828 K and 847 K, respectively) [23]. These results sug-gested that the decomposition processes of chromospheres werestrongly affected by both Eu3+ ions as well as their anions.

In addition, it is noticeable that a downward trend of decom-position temperature at peak-2 was found in the following order:LHMS-EA > LHMS-EN > LHMS-EC, however, the decline tendency

at peak-3 was shown in the following order: LHMS-EN > LHMS-EA > LHMS-EC, which primarily indicated the decomposition of NAmolecules. These all observations mainly indicated that the influ-ence of various anions (Cl−, NO3

−, and CH3COO−) of europium

146 F. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 453 (2014) 142–148

Table 1Summaries of apparent activation energies (E) of thermal decomposition properties calculated by Kissinger and Ozawa methods for the samples at various Tp (K).

Sample Tp(K) Kissinger Ozawa

5 K/min 10 K/min 15 K/min E (kJ/mol) R2 E (kJ/mol) R2

LHMS-EC peak-2 600 610 618 179 0.9933 189 0.9940peak-3 754 776 792 140 0.9933 152 0.9946

LHMS-EN peak-2 590 599 603 235 0.9914 246 0.9921peak-3 844 858 871 258 0.9888 269 0.9901

1

8

sh

EctpEbwA[f2t4fdtcOEaEtobwpc

TS

LHMS-EA peak-2 608 617 62peak-3 780 804 81

alts on the thermal decomposition behavior of Eu3+-modified NAybrids were remarkable [22,25].

Moreover, it can be easily seen in Table 1 that both of the values derived from Kissinger and Ozawa methods were verylose each other, while the correlation coefficient R2 is higherhan 0.99. As discussed-above, Ek (peak-2) value in the tem-erature range (493–703 K) was concerned to the separation ofu-NA chromophores from the mesoporous surfaces, compara-ly, Ek (peak-3) value in the temperature range of 703–1073 Kas related to the thermal stability of Eu-modified chromophores.s shown in Table 1 and compared with our previous report

23], it was found that the Ek (peak-2) values were increasedrom 128 kJ/mol for LHMS [23] to 179 kJ/mol for LHMS-EC,35 kJ/mol for LHMS-EN and 252 kJ/mol for LHMS-EA, respec-ively. However, the Ek (peak-3) values were decreased from22 kJ/mol for LHMS [23] to 258 kJ/mol for LHMS-EN, 156 kJ/molor LHMS-EA and 140 kJ/mol for LHMS-EC. Obviously, both ofecomposition temperature and Ek values at peak-2 were higherhan that of LHMS [22], suggesting that the stability of thehromophores of Eu3+-modified NA molecules was enhanced.n the other hand, Ek values revealed the following order:k-peak-2 (LHMS-EA) > Ek-peak-2 (LHMS-EN) > Ek-peak-2 (LHMS-EC)nd Ek-peak-3 (LHMS-EN) > Ek-peak-3 (LHMS-EA) > Ek-peak-3 (LHMS-C), showing the same tendency as that of decompositionemperature at peak-2 and peak-3. The inductive forces of Eu3+ ionn NA molecule in the presence of various anions were responsi-

le for the above mentioned observations [25]. Besides that, it wasorth noting that the tendency towards the decomposition tem-erature at peak-3 (LHMS-EN > LHMS-EA > LHMS-EC) was found inlose agreement with the luminescent intensity of our samples in

able 2ummaries of the suggested Kinetic models and corresponding decomposition mechanis

Sample (K/min) Peak No. g(˛)

LHMS-EC 5 Peak-2 12 [− ln(1 − ˛)]25

Peak-3 14 [− ln(1 − ˛)]23

10 Peak-2 16 −ln(1 − ˛)

Peak-3 15 [− ln(1 − ˛)]34

15 Peak-2 16 −ln(1 − ˛)

Peak-3 15 [− ln(1 − ˛)]34

LHMS-EN 5 Peak-2 19 [− ln(1 − ˛)]3

Peak-3 30 ˛32

10 Peak-2 19 [− ln(1 − ˛)]3

Peak-3 30 ˛32

15 Peak-2 19 [− ln(1 − ˛)]3

Peak-3 29 ˛32

LHMS-EA 5 Peak-2 17 [− ln(1 − ˛)]32

Peak-3 15 [− ln(1 − ˛)]34

10 Peak-2 13 [− ln(1 − ˛)]12

Peak-3 10 [− ln(1 − ˛)]14

15 Peak-2 16 −ln(1 − ˛)

Peak-3 13 [− ln(1 − ˛)]12

252 0.9991 262 0.9991156 0.9959 169 0.9953

previous studies [22]. Depending on the nature of different anions,it was also confirmed that the effect of inductive forces of Eu3+ ionon thermal stability and luminescent intensity of Eu3+-modified NAmolecules was changeable [25].

In order to study the kinetic properties of decomposition ofEu3+-modified NA molecules existing in the mesoporous channels,peak-2 and peak-3 were selected and their results were displayedin Table 1. As can be seen, LHMS-EC at a heating rate of 10 K/min wastaken as an example, in which the E (Kissinger) of the peak-2 was179 kJ/mol and that of peak-3 (Kissinger) was 140 kJ/mol, wherethe E value at low temperature (610 K) was found larger than thatat high temperature (776 K). Therefore these results described thatin a low temperature range from 493 to 703 K, the decompositionbehavior of LHMS-EC was in dynamic-based control, while on theother hand the process at a high temperature range from 703 to1073 K comparably belongs to the diffusion-based control.

According to the TGA illustrations in Fig. 4, the proportion of theentire weight loss was around 44.9% for LHMS-EC, 52.3% for LHMS-EN and 48.6% for LHMS-EA in a temperature range of 493 K to 703 K(peak-2), which were higher than that of about 24.5% for LHMS-EC,26.0% for LHMS-EN and 28.6% for LHMS-EA in a temperature rangeof 703 K to 1073 K (peak-3). So from the overall above discussion itwas inferred that dynamic-based control played a key role in thewhole decomposition process and also found in good agreementwith the investigations reported for LHMS [22].

3.4.4. Evaluation of thermal decomposition kinetic functionIn accordance with the theoretical analysis in Section 3.4.1, the

detailed summaries of E values and related mechanisms of all sam-ples obtained by Coats-Redfern method with different heating rate

m of samples with various heating rate.

Kinetics model Decomposition mechanism

Avrami-Erofeev Random nucleation and growth

Avrami-Erofeev Random nucleation and growthMampel (first order) Random nucleation and growth

Avrami-Erofeev Random nucleation and growthMampel (first order) Random nucleation and growth

Avrami-Erofeev Random nucleation and growth

Avramie Erofeev Random nucleation and growth

Contracting Sphere Phase boundary reaction, spherical symmetryAvramie Erofeev Nucleation and growth

Contracting Sphere Phase boundary reaction, spherical symmetryAvramie Erofeev Random nucleation and growth

Contracting Sphere Phase boundary reaction, spherical symmetry

Avrami-Erofeev Random nucleation and growth

Avrami-Erofeev Random nucleation and growth

Avrami-Erofeev Random nucleation and growth

Avrami-Erofeev Random nucleation and growthMampel (first order) Random nucleation and growth

Avrami-Erofeev Random nucleation and growth

F. Wang et al. / Colloids and Surfaces A: Physico

Fh

(Dad

olo

Research Program of China (973 Program 2009CB930200), and

ig. 5. DTG curves of (A) LHMS-EC, (B) LHMS-EN and (C) LHMS-EA with variouseating rates (a) 5, (b) 10, and (c) 15 K/min.

5, 10, and 15 K/min) can be found in Electrinoic Supplementaryata (seen in Appendix). Thereafter, the suggested kinetic modelsnd corresponding decomposition mechanism of hybrid BMMs atifferent heating rates are presented in Table 2.

As seen in Table 2 and Fig. 5, the decomposition process

f LHMS-EC and LHMS-EA under different heating rates fol-owed kinetic model of Avrami-Erofeev equation or Mampel (firstrder) in the temperature regions of peak-2 and peak-3 and the

chem. Eng. Aspects 453 (2014) 142–148 147

corresponding decomposition mechanism belonged to randomnucleation and growth model. Similarly, the kinetic model andthe related decomposition mechanism of LHMS-EN (peak-2) fol-lowed Avramie Erofeev function, random nucleation and growthin a low temperature range (493–703 K), but the kinetic modelof LHMS-EN (peak-3) abided by Contracting Sphere rule and therelated decomposition mechanism was Phase boundary reaction(spherical symmetry) in a high temperature region (703–1073 K).Comparably, both of the kinetic models and related decompositionmechanisms for LHMS-EC and LHMS-EA during the whole decom-position process, as well as for LHMS-EN in a low temperature range(peak-2) were well consistent with those of LHMS reported in lit-erature [23], however, at high temperature (peak-3), the results ofLHMS-EN were different from those of LHMS [23]. These investiga-tions indicated that the influence of NO3

− ion on thermal stabilityand decomposition performance of Eu3+-modified NA molecules athigh temperatures was obvious, which was found in good agree-ment with those of our more recent reported work [22].

4. Conclusions

In summary, the influence of various anions of europium saltson thermal decomposition behavior and kinetic properties ofEu3+-modified NA existing in the mesoporous channels of hybridmesoporous silica was investigated in detail on the basis of theTGA-DTG results along with other characterization methods. Threemethods (Kissinger, Ozawa–Flynn–Wall and Coats–Redfern) wereemployed to calculate and evaluate their E values of decompo-sition performances. The results showed that Eu3+-modified NAmolecules have been successfully grafted onto the functionalizedmesoporous surface, whereas the structural characteristics andthe textural parameters were changed depending on the natureof various anions of europium salts. On the other hand, the ther-mal stability of Eu3+-modified NA loaded BMMs was enhancedas compared to LHMS, specially, the tendency of decompositiontemperatures and Ek values was presented in the following order:LHMS-EA > LHMS-EN > LHMS-EC in a low temperature range of493–703 K (peak-2) and LHMS-EN > LHMS-EA > LHMS-EC at a hightemperature region of 703–1073 K (peak-3). These observationsindicated that the effect of Eu3+ ions and their anions on the decom-position processes of chromospheres were remarkable. Meanwhile,the decomposition performance of Eu3+-modified NA existing inprepared three samples (LHMS-EC, LHMS-EA and LHMS-EN) pre-sented dynamic-based control at a lower temperature periodand diffusion control at a higher temperature region, while, thedynamic-based control played main role during the whole process.In addition, the decomposition mechanisms for LHMS-EC, LHMS-EAand LHMS-EN during low temperatures (peak-2) followed randomnucleation and growth model while that of LHMS-EN at high tem-peratures (peak-3) followed Phase boundary reaction (sphericalsymmetry). Therefore it is inferred here that these investigates willbe beneficial to further understand the luminescent characteristicsof chromophore compounds existing in the mesoporous channelsof silica matrix and also make possible the high performance con-trollable synthesis of inorganic-organic hybrids.

Acknowledgments

This project was supported by the National Natural ScienceFoundation of China (21076003, 21276005), the National Basic

the Funding Project for Academic Human Resources Develop-ment in Institutions of Higher Learning under the jurisdictionof the Beijing Municipality (PHR 200907105, PHR201107104,

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05000541211019/20, and 005000543111517). We would also likeo thank Ms. Hamida for her fruitful suggestion.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.colsurfa.2014.4.036.

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