preparation, channel surface hydroxyl characterization and photoluminescence properties of...

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Microporous and Mesoporous Materials 85 (2005) 355–364

Preparation, channel surface hydroxyl characterizationand photoluminescence properties of nanoporous

nickel phosphate VSB-1

Xiuli Wang, Qiuming Gao *, Chundong Wu, Juan Hu, Meiling Ruan

State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Graduate School, Shanghai Institute of Ceramics,

Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China

Received 15 September 2004; accepted 4 December 2004Available online 15 August 2005

Abstract

VSB-1 is one of the most important thermostable nanoporous transition metal phosphates. The synthetic condition was discussedin detail. Hydroxyl groups on the channel surface of VSB-1 play a critical role in the reactions requiring Bronsted acidity. The typesof hydroxyl groups of VSB-1 material were distinguished by in situ FT-IR measurements for the samples before and after silylation.In addition, ion exchange studies showed that the hydroxyl groups in VSB-1 were acidic. The acidic site density and acid strength ofVSB-1 were measured by NH3-TPD. Violet-blue photoluminescence (PL) emission bands at 412, 436 and 470 nm were observed forVSB-1 at ambient temperature. Enhanced PL intensities were obtained by encapsulating CuO, NiO and CoO nanoclusters in thechannels of VSB-1.� 2005 Elsevier Inc. All rights reserved.

Keywords: Preparation; Thermostable nanoporous VSB-1; Acidic hydroxyl groups; Photoluminescence

1. Introduction

Zeolites have attracted much attention since the wideapplications in the fields of ion exchange, adsorption,and catalysis [1]. Along with quick developments onzeolitic materials, research has been gradually shiftedtowards the exploration of microporous structures withother elements besides of silicon. The discovery ofmicroporous aluminum phosphates by Flannigen andco-workers in 1982 represented the typical familycomposed of PO4 tetrahedra instead of SiO4 tetrahedrain the open frameworks [2]. Since then researchersextended studies to microporous phosphates in whichother elements completely replaced aluminum. Forexample, Parise [3] described the syntheses of several

1387-1811/$ - see front matter � 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.micromeso.2004.12.027

* Corresponding author. Tel.: +86 21 52412513; fax: +86 2152413122.

E-mail address: [email protected] (Q. Gao).

gallophosphates and Xu and co-workers [4] undertooka systematic study in the gallium system to produce aseries called GaPO4-Cn (n = 1–12). Following thesuccessful introduction of transition metals into zeoliticAlPOs, GaPOs, etc., scientists attempted to synthesizephosphates containing exclusively transition metals.Because transition metals may have different valencesand abundant co-ordination numbers, they may havenovel structures and utilizations, such as redox catalystsas well as optical, electronic and magnetic materials,besides traditional zeolitic properties. In the earlier1990s, vanadium and molybdenum phosphates wereprepared [5] and then a large number of transition metalphosphates with open frameworks were synthesized [6].Their poor thermal stabilities led to the collapse of thepore structures on calcination, thus rendering themunsuitable for applications that require porosity. Thesuccessful syntheses of nanoporous nickel phosphatesVSB-1 [7] and VSB-5 [8] with high thermal stability

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356 X. Wang et al. / Microporous and Mesoporous Materials 85 (2005) 355–364

brought excellent prospects of applications in manyareas, such as alkali and alkaline-earth metal ions ex-change [7a], hydrogen deposits [9], as well as excellentcatalytic properties in the systems of butadiene conver-sion to ethylbenzene over VSB-1 [7b] and hydrogenationof 1,3-butadiene and decomposition of 2-methyl-3-butyn-2-ol over VSB-5 [8]. The understanding of thesynthetic effect factors are becoming urgent and therelated information will be beneficial for scientists tooptimize and explore this kind of materials.

The hydroxyl groups on the pore or channel surfaceof zeolitic catalysts, which play an important role inthe reactions of transformation of hydrocarbons requir-ing Bronsted acidity [10–12], were widely investigated byvarious techniques, such as IR, NMR [13–17], and soforth. However, the detailed properties of hydroxylgroups of VSB-1 are hardly known. VSB-1 has onedimensional hexagonal nanopores with a diameter ofabout 0.9 nm. There are 24 polyhedra and 12 P–OHgroups in each window of the channels. To shed somelight on hydroxyl groups in VSB-1, the in situ FT-IRabsorptions of the hydroxyl groups were determinedbefore and after silylation by chlorotrimethylsilane(CTMS). In addition, the acidity of the hydroxyl groupson the nanochannels or nanopores of VSB-1 were stud-ied by ion exchange and NH3-TPD measurements.

Previous efforts to generate luminescent materialsusing crystalline porous materials were via either incor-porating organic dyes or doping with metal activators[18]. For example, laser action was observed whenorganic laser dye molecular, pyridine-2-{1-ethyl-4-[4-(p-dimethylaminophenyl)-1,3-butadienyl] pyridiniumperchlorate}, was inserted into the pores of AlPO4-5crystals [19]. Reports on dye-free or metal-activator-freeopen framework phosphates that display photolumines-cence (PL) properties were few previously [20]. PL prop-erties of other porous materials, such as porous silica,porous alumina, porous zirconia, etc., have been re-ported [21–25]. The interaction on the interface betweenguest and host can significantly modify the electronic andphotonic response of both guests and hosts [22,26–31].Here, we described the violet-blue emitting propertiesof nanoporous nickel phosphate VSB-1 at room tem-perature. Metal oxide nanoclusters or nanoparticlesencapsulated in the channels or pores of VSB-1 cansignificantly affect the PL intensities of host VSB-1.

2. Experimental

2.1. Materials preparation

VSB-1 was synthesized from both aqueous and non-aqueous systems in which water and ethylene glycol(EG) were used as the solvents under hydrothermaland solvothermal conditions, respectively. Nickel (II)

dichloride hexahydrate, nickel acetate tetrahydrate andnickel fluoride tetrahedrate have been used as the nickelsources, respectively. Phosphoric acid (85 wt.%) wasused as the phosphorous sources. Amines and inorganicbases were chosen as the templates or pH adjusters.Hydrogen fluoride (40 wt.%) was the pH adjuster andmineralization agent. The typical synthetic steps wereas follows: one kind of nickel source was added to thedeionized water, followed by adding amine or ammo-nium salt and an amount of hydrogen fluoride, and thenphosphoric acid was added dropwise. The whole mix-ture with the molar ratios of 1.0NiCl2 Æ 6H2O:1.0H3-PO4:1.0HF:1.0NH3 Æ H2O:40.0H2O, pH = 2.5 was keptunder vigorous stirring for 0.5 h. The final mixturewas sealed in Teflon-lined stainless steel autoclaves at180 �C for 6 d under autogenous pressure. After this,autoclaves were removed from the oven and quenchedin cold water. The yellowish green powders were filteredand washed with plenteous deionized water at roomtemperature, then dried at 160 �C for 6 h in air and keptdrying at room temperature.

Silylation was carried out by a reflux method in amixture of VSB-1 (1.0 g), chlorotrimethylsilane (CTMS,10.0 mL) and toluene (60.0 mL) at 80 �C for 50 h withstirring. The solid product was recovered by centrifug-ing, then washed with superfluous toluene and dried atambient temperature.

Metal oxide encapsulated VSB-1 samples were pre-pared by three steps. First, the metal ion exchanged sam-ple (Me-VSB-1) was obtained via an ion exchangeprocess. Typically, 0.5 g VSB-1 sample was added to asolution of 0.1 M copper (II) acetate and the mixturewas stirred for 24 h at room temperature. The precipitatewas separated by centrifuging and washed off the unre-acted copper (II) acetate with plenteous deionized water,followed by drying at 313 K overnight. Then, the Cu-VSB-1 sample was treated with NaOH aqueous solutionat room temperature. The color of the sample changedfrom yellow-greenish to gray-greenish after the treat-ment, which may be in correspondence with the for-mation of copper hydroxide within VSB-1. Aftertreatment for 0.5 h, the sample was recovered by centri-fuging, followed by washing thoroughly with deionizedwater until the pH value of the filtrate decreased to about7, and then dried at 313 K. Last, in order to obtain CuOencapsulated VSB-1 (CuO-VSB-1), the NaOH treatedCu-VSB-1 sample was calcined at 573 K for 3 h in air.The other metal oxides encapsulated VSB-1 samples(called MeO-VSB-1, Me = Co, Ni, Zn, Ca) were pre-pared using the same methods as that of CuO-VSB-1.

2.2. Characterization

Powder X-ray diffraction (XRD) measurement wascarried out on a Rigaku X-ray diffractometer (CuKaradiation, k = 0.15418 nm, 40 kV, 40 mA). The scan

X. Wang et al. / Microporous and Mesoporous Materials 85 (2005) 355–364 357

was in a 2h/h continuous mode with the scan scope be-tween 3� and 60� and a step length of 0.02�. The soliddiffuse reflectance ultraviolet visible (UV–vis) spectrawere measured on a Shimadzu UV-3101PC spectrome-ter. In situ FT-IR absorption was measured on a highresolution Thermo Nicolet FT-IR spectrometer withthe error of ±0.2 cm�1. Temperature-programmeddesorption (TPD) of NH3 was carried out on a PTC-1temperature-programmed instrument and a 102G gaschromatogrameter as detector. Before the TPD experi-ment the sample was activated at 500 �C for 3 h in flow-ing He (30 cm�1/min). The treated sample was saturatedwith pure ammonia at 120 �C. After purging with pureHe for 2 h, it was heated (10 �C/min) to 500 �C. ThepH values of the solutions before and after ion-exchangewere measured by a PHS-2F pH Meter. The propertiesof the porous structures were determined from N2-sorp-tion measurements at 77 K using a Micromeritics ASAP2020M system. The samples were outgassed under vacu-um at 523 K for 18 h before the adsorption of nitrogen.High resolution transmission electron micrographs(HRTEM) with X-ray energy-dispersive spectroscopicanalysis (EDX) was carried out on a JEM200CX elec-tron microscope operating at 120 kV. The sample forHRTEM was prepared by dispersing a certain amountof the product particles via the slurry in acetone ontoa holey carbon film on a Fe grid. X-ray photoelectronspectroscopy (XPS) was measured on a Microlab 310FScanning Auger Microprobs with a Dual Anode(MgKa, AlKa) XPS, using monochromatic MgKa radi-ation (hm = 1253.6 eV). The background pressure in theanalytic chamber was lower than 1 · 10�8 Pa. The bind-ing energy was calibrated by using C1s photoelectronpeak at 285 eV as a reference standard. Photolumines-cence (PL) spectra were obtained on a Shimadzu RF-5301PC spectroflurophotometer with a Xe lamp as theexcitation source at room temperature.

10 20 30 40 50 60

5000

cps

f

e

d

c

b

a

Inte

nsity

(a.

u.)

2 theta (deg.)

Fig. 1. XRD patterns at different temperatures ((a) 25 �C, (b) 100 �C,(c) 200 �C, (d) 300 �C, (e) 400 �C and (f) 500 �C).

3. Results and discussion

3.1. Preparation and thermal stability of VSB-1

The selection of nickel sources is mainly according totheir solubility in the chosen solvents. Nickel (II) dichlo-ride hexahydrate, nickel acetate tetrahydrate and nickelfluoride tetrahedrate have very good solubility in waterand alcohol. Nickel difluoride tetrahedrate includingboth nickel and fluoride sources is a convenient materialfor the formation of VSB-1. The preferable source ofphosphorous is orthophosphoric acid, which was widelyused in the syntheses of other metal phosphates, such asaluminum, gallium, iron, or cobalt phosphates, etc., [3–6]. The ester of phosphoric acid is inert in the reaction.P2O5, another excellent phosphorous source, is not suit-able in the reaction because of the co-product of phos-

phites. The polarity of the solvent is very importantfor the synthesis of VSB-1. Water is a good solvent forthe formation of VSB-1. The EG solvent, which has alower polarity than that of water, may be medium forthe formation of VSB-1. In the solvents with much lowerpolarity, such as ethanol, n-propanol, cyclohexanol,benzene, cyclohexane, etc., VSB-1 was not able to beprepared and condensed phase Ni3(PO4)2 was found.Solution acidity is another main effect factor for the for-mation of VSB-1. VSB-1 was synthesized from weaklyacidic and/or neutral systems. No solid product wasproduced in strong acid systems, while in basic systems,another kind of condensed nickel phosphate, Ni12-(HPO4)6(PO4)2(OH)6, formed. Addition of F� ions isquite important for the formation of VSB-1. Onlycondensed nickel phosphate Ni12(HPO4)6(PO4)2(OH)6appeared in the systems without F� ions. This phenom-enon is similar to the synthesis elaboration of cloverite[32], a gallophosphate with an extra large opening porecomprising 20 member rings and with three dimensionalchannel structure. The amines are beneficial to preparethe excellent crystallized crystals of VSB-1, while itwas also able to be synthesized with inorganic bases,such as NH3 Æ H2O, LiOH, NaOH, KOH, or RbOH, etc.

Thermal stability is quite important for the applica-tion of VSB-1. Fig. 1 shows the XRD patterns ofVSB-1 at different temperatures. The diffraction peakpositions were almost the same as that of VSB-1 at roomtemperature for all of the samples with the reflectionsfor VSB-1 assigned and listed in Table 1, while the inten-sities decreased along with the increase of temperaturefrom room temperature to 500 �C. The high thermal sta-bility gives VSB-1 potential prospects in many applica-tion areas that require porosity.

Table 1The assignation of XRD reflections for VSB-1

2h d hkl

5.188 17.019 10015.608 5.673 30018.784 4.720 31022.453 3.957 21125.350 3.511 22127.462 3.245 40129.907 2.985 41132.622 2.743 33134.530 2.595 61036.550 2.457 44037.336 2.407 20239.041 2.305 61140.692 2.215 31241.761 2.161 40242.808 2.111 32244.842 2.020 50245.832 1.978 42246.479 1.952 64048.077 1.891 90049.172 1.851 81150.557 1.804 61251.759 1.765 74052.945 1.728 83054.690 1.677 92055.128 1.665 10356.116 1.638 66056.683 1.623 80257.240 1.608 84058.219 1.583 22359.310 1.557 66159.722 1.547 5528.99 9.826 11017.661 5.018 00119.851 4.469 11122.757 3.904 32025.893 3.438 31127.714 3.216 42031.515 2.837 60032.837 2.725 52035.759 2.509 00236.748 2.444 43137.525 2.395 52139.230 2.295 30240.869 2.206 44142.108 2.144 63043.490 2.079 72045.336 1.999 54145.992 1.972 63146.806 1.940 51248.395 1.879 60249.330 1.846 52251.009 1.789 73152.061 1.755 44253.242 1.719 62254.693 1.677 71255.698 1.649 11356.258 1.634 83156.828 1.619 21357.387 1.604 30358.495 1.577 31359.317 1.557 403

Table 1 (continued)

2h d hkl

59.993 1.541 64210.387 8.510 20018.041 4.913 22020.531 4.322 20123.652 3.759 30126.159 3.404 50028.952 3.082 32131.740 2.817 50133.054 2.708 42136.159 2.482 10236.942 2.431 70038.098 2.360 62039.962 2.254 71041.226 2.188 70142.284 2.136 62143.494 2.079 41245.500 1.992 81046.154 1.965 55047.284 1.921 72148.708 1.868 43249.790 1.830 55151.159 1.784 65052.358 1.746 53253.822 1.702 10,0,054.842 1.673 00355.836 1.645 54256.398 1.630 75056.960 1.615 10,1,057.522 1.601 72258.625 1.573 93059.583 1.550 75113.755 6.433 21018.419 4.813 10120.861 4.255 40023.941 3.714 41027.205 3.275 33029.192 3.057 51031.961 2.798 43034.324 2.611 51136.353 2.469 60136.947 2.431 11238.482 2.337 21240.331 2.234 22241.403 2.179 54042.457 2.127 80044.000 2.056 71145.504 1.992 33246.318 1.959 80147.442 1.915 73049.016 1.857 82050.097 1.819 64151.610 1.770 90152.503 1.742 82154.547 1.681 65155.121 1.665 74155.982 1.641 20356.402 1.630 63257.101 1.612 10,0,157.936 1.590 92159.176 1.560 81259.719 1.547 11,0,0



3.2. Studies on channel surface hydroxyls

3.2.1. Types of OH Groups in VSB-1

VSB-1 has 24 member ring channels with a diameterof about 0.9 nm along with the c axis. On the surfaceof the nanochannels, there are many OH groups as ter-minal groups of PO4 tetrahedra extending into the chan-nels, which are filled with ammonium ions and adsorbedwater or hydronium ions. These OH groups may act asfunctional groups in chemical reactions and can bemodified by reacting with other functional molecules.Although the crystal structure of VSB-1 was reported[7], the details of the OH groups on the nanochannelsurface of VSB-1 were hardly known. To make clearof this, we monitored the in situ FT-IR absorptions ofthe OH groups in VSB-1 before and after silylation byCTMS.

The FT-IR spectrum between 4000 and 650 cm�1 isdetermined for VSB-1 at room temperature. The samplewas evacuated to 1.0 Pa before experiment. The O–Hbending vibration of H2O can be observed at1649 cm�1. This mode of N–H is detected at1458 cm�1. The asymmetrical stretching vibration ofP–O appears at 1219, 1057 and 1021 cm�1. The symmet-rical stretching vibration of P–O is detected at 895 cm�1.The bending vibration of P–O can be observed at 753and 684 cm�1.

To clearly know the types and thermal stabilities ofhydroxyls of VSB-1, in situ FT-IR high resolution spec-tra with an error of ±0.2 cm�1 between 3800 and2500 cm�1 of VSB-1 evacuated to 1.0 Pa at various tem-peratures were obtained (Fig. 2). At room temperature,the spectrum exhibited a broad band between 3610 and3000 cm�1, a relatively sharp peak at 3566 cm�1 and aless distinct shoulder at 3610 cm�1. Along with the tem-

3800 3600 3200 3000 280026000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

36103566

g

h

f

ed

c

b

a

Abs

orba

nce

(a.u

.)

Wavenumber (cm-1)

Fig. 2. In situ FT-IR spectra of VSB-1 between 3800 and 2500 cm�1

at different temperatures ((a) 25 �C, (b) 100 �C, (c) 150 �C, (d) 200 �C,(e) 300 �C, (f) 400 �C, (g) 500 �C and (h) 600 �C).

perature increased, the broad band became weaker andthe shoulder became more distinctive. The peak at3610 cm�1 is attributed to isolated OH groups, whichis related to the relatively highest energy among the hy-droxyl groups. The broad band from 3610 to 3000 cm�1

is associated with the overlap between hydrogen-bondedOH groups and hydrogen-bonded ammonium ions. Thepeak at 3566 cm�1 belongs to those OH groups hydro-gen-bonded only with ammonium ions or adsorbedwater/hydronium ions in the channels. Since the OHgroups in VSB-1 interact with one another to give abroad IR band, it is almost impossible to distinguishthem.

When VSB-1 was silylated with CTMS, the in situFT-IR high resolution spectra were changed (Fig. 3).At room temperature, there were three sharp peaks at3576, 3559 and 3541 cm�1, which can be attributed tothree types of hydrogen-bonded OH groups [33–35].When the temperature increased to 200 �C, there werenew peaks present at 3137, 3047 and 2800–2900 cm�1.The methyl in CTMS formed very weak hydrogen-bonds with remained ammonium ions, so there wereno sharp peaks of methyls and ammonia ions presentat room temperature. At 200 �C the weak hydrogen-bonds between methyls and ammonia ions were broken,thus the asymmetric and symmetric stretching absorp-tion of ammonium ions and the absorption peak ofmethyls are present at 3137, 3047 and 2800–2900 cm�1, respectively. We also found that the peakat 3541 cm�1 became weaker and at last disappearedalong with the temperature increased, which is corres-ponding to the dehydration due to the relatively stronghydrogen-bond effects. Strong hydrogen-bonds lead totheir relatively weak O–H bonds, related to the FT-IRabsorption at 3541 cm�1 with low energy, comparedwith those of other types of hydroxyls.

3800 3600 3200 3000 2800 2600

0.0

0.5

1.0

1.5

2.0

2.5

3.0

354135593576

h

gf

edc

b

a

Abs

orba

nce

(a.u

.)

Wavenumber (cm-1)

Fig. 3. In situ FT-IR spectra of silylated VSB-1 between 3800 and2500 cm�1 at different temperatures ((a) 25 �C, (b) 100 �C, (c) 150 �C,(d) 200 �C, (e) 300 �C, (f) 400 �C, (g) 500 �C and (h) 600 �C).

Scheme 1. Different types of hydroxyl groups before and after silylation by chlorotrimethylsilane. Type 1 is the isolated OH groups corresponding tothe 3610 cm�1 absorption; type 2 is the OH groups hydrogen-bonded only with ammonium ions or adsorbed water/hydronium ions in the channelsassociated with the band at 3566 cm�1; type 3 is the OH groups with the absorptions at 3559 cm�1, in which a H atom forms a hydrogen bond withthe O atom of an adjacent OH group; type 4 is the OH groups responsible for the 3576 cm�1 peak, in which an O atom forms a hydrogen bond withthe H atom of an adjacent OH group; and type 5 is the chains of OH groups that contain more than one pair of mutually hydrogen-bonded OHgroups with the absorption peak at 3541 cm�1, in which both O and H atoms form hydrogen bonds with the H and O atoms of adjacent OH groups.Number 6 is the converted-POSi(CH3)3 group.


From the spectra of non-silylated and silylated VSB-1, five types of hydroxyl groups in VSB-1 can be distin-guished according to the absorption peaks. The strongerthe hydrogen-bonds are, the relatively weaker of the O–H bonds will be. Type 1 is the isolated OH groups cor-responding to the absorption at 3610 cm�1. Type 2 is theOH groups hydrogen-bonded only with ammonium ionsor adsorbed water/hydronium ions in the channels asso-ciated with the band at 3566 cm�1. Since the weakhydrogen-bond formed, the O–H bond of the hydroxylbecomes a little weaker than that of type 1. Type 3 isthe OH groups with FT-IR absorptions at 3559 cm�1,in which a H atom forms a hydrogen bond with the Oatom of an adjacent OH group; and type 4 is the OHgroups responsible for the 3576 cm�1 peak, in whichan O atom forms a hydrogen bond with the H atomof an adjacent OH group according to the literature13. Type 5 is the chains of OH groups [33–35] that con-tain more than one pair of mutually hydrogen-bondedOH groups with the FT-IR absorption peak at3541 cm�1, in which both O and H atoms form hydro-gen bonds with the H and O atoms of adjacent OHgroups, corresponding to the weakest O–H bond ofthe hydroxyls and the strongest hydrogen-bonds, similarto that of MCM-41 (Scheme 1). To clearly describe thedifferent types of OH groups for VSB-1, we list the FT-IR absorption peaks (in Table 2) compared with thoseof MCM-41, which has four kinds of OH groups inthe structure [13].

Considering that in the original non-silylated VSB-1,the interactions are present not only within each type of

Table 2The OH group types and their FT-IR absorption peaks (in cm�1) in VSB-1

Materials Isolated OH H-bonded OH offering O H-bonded OH o

VSB-1 3610 3576 3559MCM-41 3740 3690–3700 3620–3640

hydroxyl groups but also among all types. However,when VSB-1 is silylated, some of the OH groups are con-verted to P–O–Si(CH3)3 groups, which prevent the inter-actions between different OH group species due to theeffect of spatial resistance. Types 1 and 2 were easilyconverted to P–O–Si(CH3)3 groups, since their spatialresistances were small, which is corresponding to the dis-appearance of peaks at 3610 and 3566 cm�1 after silyla-tion. Some type 5 hydroxyls were converted to types 3and 4 after silylation, and then the amounts of hydroxyltypes 3, 4 and 5 were on the similar levels, which couldbe distinguished (Scheme 1). Type 5 hydroxyls are alsomore sensitive to temperature than the others; furtherdehydration will occur along with the increase of tem-perature due to the relatively strong hydrogen-bond ef-fects, which is related to that the intensities of the peakat 3541 cm�1 are becoming weaker and at last disappearat 400 �C. Thus, keeping the temperature at 80 �C, thedifferent types of OH groups could be clearly detectedvia the silylation method. In addition, in the spectra ofsilylated sample, the vibrational absorption peaks ofamine were also strong at 600 �C, while in the spectraof non-silylated sample, the peaks of amine were almostdisappeared at 500 �C, this perhaps is another proof ofthe spatial resistance effect of bulky P–O–Si(CH3)3groups, which leads to the amine being difficult to getaway from the confinement of the channels.

3.2.2. Acidity of OH groups in VSB-1

The acidity of OH groups in VSB-1 was investigatedvia a method of ion exchange. VSB-1 (0.1 g) was added

and MCM-41

ffering H OH chains H-bonded OH with NHþ4 ðH3O

þ;H2OÞ3541 35663600–3450

100 200 300 400 500

0.30

0.32

0.34

0.36

0.38

Inte

nsity

/ m

V

Temperature /˚C

Fig. 5. NH3-TPD curve of VSB-1.


to solutions of MeCl2 (Me = Co, Ni, Cu), respectively,with given concentrations, then stirred at room temper-ature for 5 h. pH values of the solutions were measuredbefore and after ion exchange. Table 3 and Fig. 4 givethe pH values before and after ion exchange with threekinds of metal ions at different concentrations with thesame amounts of VSB-1. Before ion exchange withVSB-1, the M2+ (M = Co, Ni, Cu) solutions are acidicas a result of hydrolyzation. Comparing the pH valuesbefore and after ion exchange for VSB-1 with those dif-ferent metal ions at the same concentration, we foundthat all the pH values decreased after ion exchange. Inthe process of ion exchange the following reaction tookplace: 2(VSB-1-OH) + M2+ ! (VSB-1-O)2M + 2H+.The H+ produced from the reaction led to the decreaseof pH. So, we conclude that the OH groups in VSB-1 areacidic.

In order to get the acid strength for OH groupsin VSB-1, the NH3-TPD (temperature-programmeddesorption) experiment was conducted and the NH3-TPD curve for VSB-1 is shown in Fig. 5. The densityof the acidic sites is relatively large and the acid strengthis in the medium, which is based on the fact that there isa broad band with a peak at 285 �C and the relevantamount of desorbed ammonia gas is 0.269 mmol/g.

Table 3The pH values before and after ion exchange of different metal ions atdifferent concentrations with VSB-1

Concentrations(M)

Co2+ Ni2+ Cu2+

Before After Before After Before After

1.00 4.21 3.38 3.78 3.60 2.55 2.350.50 4.58 3.65 4.23 3.94 3.15 2.790.25 5.34 3.90 4.85 4.20 3.60 3.080.10 6.07 4.21 5.09 4.43 4.03 3.230.05 6.24 4.31 5.18 4.56 4.16 3.36

0.0 0.2 0.4 0.6 0.8 1.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

pH

Concentration (mol/L)

Co2+(B)

Co2+(A)

Ni2+(B)

Ni2+(A)

Cu2+(B)Cu2+(A)

Fig. 4. The varieties of pH values before and after ion exchange withdifferent metal ions ((B) before ion exchange and (A) after ionexchange).

3.3. Optical properties of VSB-1

3.3.1. UV–vis spectra of VSB-1 at different temperatures

UV–vis absorption spectra between 350 and 550 nmat different temperatures are shown in Fig. 6. Theabsorption band at about 417 nm was clearly found be-tween ambient temperature and 200 �C, due to the elec-tron transition of Ni–O, which is the characteristicabsorption of the octahedral symmetry in co-ordinationenvironment [36]. The red-shift occurred for this bandfrom 419 to 432 nm when the temperature increasedfrom 300 to 500 �C in corresponding to the conden-sation of OH at higher temperature, which lead to avariety of co-ordination environments of P and Ni ions.The co-ordination field became weaker, co-ordinationsymmetry transformed gradually from six-coordinatedoctahedra to five- and/or four-coordinated polyhedraor the mixture of them [37]. At the same time the energydecreased and the band was red-shifted. At higher

350 400 450 500 5500.0

0.2

0.4

0.6

0.8

1.0

432

428419

417

417

417

f

e

dcb

a

Wavelength (nm)

Abs

orpt

ion

(a.u

.)

Fig. 6. UV–vis absorption spectra between 350 and 550 nm at differenttemperatures ((a) 25 �C, (b) 100 �C, (c) 200 �C, (d) 300 �C, (e) 400 �Cand (f) 500 �C).

Fig. 8. HRTEM images and corresponding SAED patterns of(a) CuO-VSB-1 and (b) pure VSB-1 samples.


temperature the color of the material became brownfrom yellowish green at room temperature, which is cor-responding to the red-shift.

3.3.2. VSB-1 encapsulated by different kinds of metal

oxidesMetal oxide nanoclusters or nanoparticles encapsu-

lated in the channels or pores of VSB-1 can significantlyaffect the PL intensities of host VSB-1. XRD patternswere used to examine the structures of MeO-VSB-1.CuO-VSB-1 is the typical example of metal oxide encap-sulated VSB-1. For the VSB-1 samples before and afterCuO encapsulation, all the peaks which were attributedto VSB-1 can be observed for CuO-VSB-1 and no dif-fraction peak of crystalline CuO was detected becauseof the small particle sizes, which is in accordance withthat of Ag encapsulated VSB-1 [38]. N2 adsorption iso-therms of VSB-1 and CuO-VSB-1 are shown in Fig. 7.The characteristic type I isotherms of microporousmaterials were found for both VSB-1 and CuO-VSB-1.The analytic results of N2 adsorption at 77 K showedthat the BET surface area was 95 m2/g for CuO-VSB-1, which is lower than 142 m2/g of pure VSB-1. Themaximum pore volume of 0.054 cm3/g for CuO-VSB-1is also lower than 0.077 cm3/g for pure VSB-1. The low-er surface area and pore volume of CuO-VSB-1 are incorrespondence with the encapsulation of copper oxidein the channels of VSB-1. HRTEM images of VSB-1and CuO-VSB-1 samples are given in Fig. 8 and the cor-responding selected-area electron diffraction (SAED)patterns are shown at the inset region. It can be foundthat the channels are well ordered for both VSB-1 andCuO-VSB-1 samples, which are in agreement with theabove XRD results. From the HRTEM image and thecorresponding SAED pattern of CuO-VSB-1 sample(Fig. 8a), no apparent crystalline CuO or large copper-rich particles on the external surface were observed.Comparing the HRTEM image of CuO-VSB-1 with thatof pure VSB-1 (Fig. 8b), one can obviously notice that

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50

60

b

a

Vol

ume

adso

rbed

(cm

3 /g S

TP)

Relative pressure (P/P0)

Fig. 7. N2 adsorption isotherms of (a) VSB-1 and (b) CuO-VSB-1at 77 K.

the pore diameter (grayish or white part) becomesslightly smaller and the pore wall thickness (deep grayor black part) becomes slightly thicker for CuO-VSB-1sample, which look like the CuO film uniformly cover-ing on the channel surface of VSB-1. The simultaneousenergy-dispersive X-ray spectroscopy (EDX) analysisfor CuO-VSB-1 sample indicated that copper elementsuniformly dispersed and the content of CuO was about2.46 wt.%. XPS is a surface-sensitive technique and theanalytic sampling depth is less than 3 nm in general.However, because of the large voids of the zeolite, theeffective information, such as Co 2p, has been estimatedto be capable of reaching about 14 nm in depth, corre-sponding to about five unit cells of the zeolite belowthe surface [39]. Considering that VSB-1 is also a kindof microporous material with many voids (channels orpores), the information from XPS can be used to discussthe oxidation state of copper of the particles within thepores of VSB-1. Cu 2p XPS spectrum of CuO-VSB-1sample was determined. The binding energy of 934.5and 954.5 eV for Cu 2p3/2 and Cu 2p1/2, respectively,are characteristic of Cu(II) cations, and the regions forCu(II) at 944.6 and 963.8 eV [40] may result from elec-tron transfer to the core hole to yield d9 character [41].

400 450 500 550 6000

50

100

150

200

250

400 450 500 550 6000

1020304050

a

Inte

nsity

(a.

u.)

Wavelength (nm)

dcb

a

Inte

nsity

/ a.

u.

Wavelength / nm

Fig. 9. Photoluminescence spectra of pure VSB-1 (a), CoO-VSB-1(b), NiO-VSB-1 (c) and CuO-VSB-1 (d) samples under excitationat 350 nm with the inset of the magnification for trace (a).


Similar characterization has been carried out with thesame phenomena found for other MeO-VSB-1 samples.

3.3.3. Photoluminescence properties of VSB-1 and

MO-VSB-1 samplesAs shown in Fig. 9, there are three intrinsic PL bands

at about 412, 436 and 470 nm when excited at 350 nmfor pure VSB-1. Three emission bands at the same wave-lengths as those of pure VSB-1 were observed for CuO-VSB-1, NiO-VSB-1 and Co-VSB-1. It is obvious that therelative PL intensities for CuO-VSB-1, NiO-VSB-1 andCo-VSB-1 are stronger than those of pure VSB-1. WhenZnO and CaO were encapsulated in VSB-1 with thesame methods as CuO-VSB-1, the PL emission dis-appeared for ZnO-VSB-1 and CaO-VSB-1 at roomtemperature.

For pure VSB-1 sample, the emission bands at about412, 436 and 470 nm may be ascribed to 3d8 electrontransitions of the Ni2+ ions [42,43]. The enhanced inten-sities of CuO-VSB-1, NiO-VSB-1 and Co-VSB-1 arepossibly due to the existence of unoccupied d orbits oftransition metals, which is beneficial to reduce the prob-ability of irradiation transition. For ZnO-VSB-1 andCaO-VSB-1 samples, the fully occupied d orbits mayaccelerate the irradiation transition and result in thePL emission quenching. Further studies on mechanismof the emission properties are in the process.

4. Conclusions

The synthetic effect factors for thermostable VSB-1have been discussed in detail. Five types of hydroxylgroups in VSB-1 have been analyzed by in situ FT-IRmethod before and after silylation by CTMS. The acidicOH groups in VSB-1 have been confirmed by ion ex-change method. NH3-TPD experiments showed thatVSB-1 had relatively large density of acidic sites and

medium acid strength. The electron transition of Ni–Oof octahedral NiO6 has been found between ambienttemperature and 200 �C and the condensation of OHat higher temperature appeared, which led to a varietyof co-ordination environments of P and Ni ions withthe temperature increased from 300 to 500 �C, basedon the UV–vis absorption spectra analyses. The roomtemperature PL properties of VSB-1 have been ob-served. Encapsulating CuO, NiO and CoO nanoclustersinto the channels of VSB-1 resulted in the enhanced PLintensities.

Acknowledgements

This work was financially supported by the ChineseNational Science Foundation Grant 20201013, ‘‘Planof the Creative Funding’’ Grant SCX200404 and ‘‘Planof Outstanding Talents’’ of Chinese Academy ofSciences.

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