Journal of Colloid and Interface Science 401 (2013) 34–39

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Journal of Colloid and Interface Science

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Effect of structure-directing agents on facile hydrothermal preparationof hierarchical c-Al2O3 and their adsorption performance toward Cr(VI) and CO2

Jinrong Ge a, Kejian Deng b, Weiquan Cai a,⇑, Jiaguo Yu a, Xiaoqin Liu c, Jiabin Zhou d

a School of Chemical Engineering, State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, 205 Luoshi Road,Wuhan 430070, PR Chinab College of Chemistry and Materials, South-Central University for Nationalities, 708 Minyuan Road, Wuhan 430074, PR Chinac State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing, Jiangsu 430074, PR Chinad School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China

a r t i c l e i n f o

Article history:Received 4 January 2013Accepted 14 March 2013Available online 4 April 2013

Keywords:Sodium aluminateHierarchical c-Al2O3

Hydrothermal methodStructure-directing agentAdsorption performanceCr(VI)CO2

0021-9797/$ - see front matter � 2013 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2013.03.028

⇑ Corresponding author. Fax: +86 27 87859019.E-mail address: caiwq@whut.edu.cn (W. Cai).

a b s t r a c t

Hierarchical flower-like and sphere-like mesoporous c-Al2O3 microparticles were successfully preparedby a facile hydrothermal method followed by a calcination process using sodium aluminate as aluminumsource, urea as precipitating agent, and Pluronic F127 (EO106PO70EO106), polyacrylic acid sodium (PAAS),and mixed F127–PAAS as structure-directing agents (SDAs), respectively. Effects of the SDAs on the phasestructure, morphology, textural properties, surface alkaline, and the adsorption performance towardCr(VI) and CO2 of the as-prepared samples were comparatively studied by X-ray diffraction (XRD), scan-ning electron microscope (SEM), transmission electron microscope (TEM), N2 adsorption–desorption, CO2

temperature programmed desorption (CO2-TPD), and UV–Vis spectrophotometric method. The resultsindicate that the sphere-like c-Al2O3 obtained by using F127 as the SDA shows the best adsorption per-formance toward Cr(VI) with a high adsorption rate of 95% and adsorption capacity of 5.7 mg/g when theadsorption reaches equilibrium for 4 h at room temperature. However, the flower-like c-Al2O3 obtainedby using PAAS as the SDA has the biggest CO2 adsorption capacity of 1.04 mmol/g at room temperature.This work provides a simple and practical way to prepare potentially bifunctional c-Al2O3 adsorbent forthe removal of pollutants in water and air treatment from cheap sodium aluminate by using differentSDAs.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

With the rapid development of modern industries, wastewaterpolluted by heavy metal ions, e.g., Cr(VI), Cd(II), Hg(II), and globalwarming caused by greenhouse effect are becoming one of themost formidable environmental challenges of the global society[1,2]. On the one hand, when foods and drinking water are pollutedby heavy metal ions, they can accumulate to a high concentrationin human body which leads to carcinogenic teratogenesis andmutation. On the other hand, the increasing concentration of CO2

resulting from different manufacturing processes, especially thecoal based power plants, is the main source which causes green-house effect. The development of novel nanotechnology andnanomaterials is expected to play an important role for the reme-diation of such water pollution and air pollution problems. An idealgoal is to develop multi-functional nanomaterials which can effi-ciently and fastly remove these contaminants via a simple andcheap adsorption separation process [3].

ll rights reserved.

Hierarchical nanostructured metal oxides consisting of buildingblocks in multiple length scales at different levels provide desirableproperties, e.g., high surface area, high surface-to-bulk ratio, andfacile species transportation path. Therefore, they are widely usedin catalysis, energy conversion, storage, separation, and other areas[1,4]. The formation of hierarchical structures with designed chem-ical components and controlled morphologies, which strongly af-fect the application performance of nanomaterials, is generallyconsidered to be a self-assembly process. During the process, thecrystallites form regular higher level structures that often resemblenatural organisms [1,5–7]. As one of the most important metal oxi-des, alumina, especially c-Al2O3 has many environmental applica-tions as catalysts support and adsorption or separation material,etc. [8–10] because of its unique thermal, chemical, and mechani-cal stability [11]. Its application performance can be adjusted viacontrolling its crystallinity, morphology, textural properties, andother physicochemical properties. So far, hierarchical boehmitewhich is an important precursor for c-Al2O3 via an isomorphoustransformation between 400 and 700 �C, and alumina nanomateri-als such as nanotubes and nanorods [12], nanoplatelets and nano-wires [13], bunches of nanowires [14], flower-like structure [15],

J. Ge et al. / Journal of Colloid and Interface Science 401 (2013) 34–39 35

hollow nanospheres [16], microspheres constructed by nanosheets[17], hollow core/shell and hollow microspheres [18], single nano-flakes to nanoflake assemblies, flower-like structures, and hollowmicrosphere constructed by nanoflakes [6] have been reported.Among the methods to prepare them, hydrothermal method iswildly used to achieve a fine control of their morphologies over awide range of controllable synthesis conditions.

In our previous works, hierarchical spindle-like c-Al2O3 parti-cles with adsorption capacity of 6.7 mg/g Cr(VI) at 6 h has beenprepared by a nontemplate hydrothermal synthesis and sequentialcalcination route using aluminum nitrate or aluminum chloride asprecursors and urea as precipitating agent. However, its adsorptionkinetics is a little slow [19]. Herein, hierarchical flower-like andsphere-like mesoporous c-Al2O3 microparticles with bifunctionaladsorption properties toward Cr(VI) and CO2 were successfullyprepared by using cheap sodium aluminate as aluminumsource, and F127 (a nonionic surfactant with structural formulaEO106PO70EO106, where EO represents the ethylene oxide blockand PO represents the propylene oxide block) and polyacrylic acidsodium salt (PAAS) as structure-directing agents (SDAs) via a facilehydrothermal reaction–calcination method. Importantly, wereport the first example of mesoporous spherical-like c-Al2O3

particles with notably small mesopores centering at 3.7 nm andvery fast adsorption kinetics toward Cr(VI) prepared by usingF127 Pluronic block copolymer as the SDA.

2. Experimental section

2.1. Sample preparation

All reagents are of analytical grade except sodium aluminatewhich is a chemical pure and used without further purification.In a typical experiment, 2.38 g of sodium aluminate, 20.23 g ofurea, and 1 g F127 were simultaneously dissolved in distilled waterto form a mixed solution (70 ml) under vigorous stirring. The solu-tion was then transferred into a 100 ml Teflon-lined, stainless steelautoclave, sealed, and maintained at 140 �C for 24 h. After the sys-tem was naturally cooled to room temperature, the white precipi-tate was separated from solution and thoroughly washed severaltimes with deionized water and ethanol successively and thendried in a vacuum oven at 80 �C for 12 h. The powder was calcinedat 450 �C for 4 h in static air to obtain c-Al2O3. Similar experimentswere performed by using 0.47 g PAAS and mixed 1 g F127 and0.47 g PAAS as SDAs, respectively.

2.2. Characterization

The X-ray diffraction (XRD) measurement was carried out by aRigaku D/Max-RB diffractometer (Japan) using Cu Ka radiation at ascan rate (2h) of 0.05� per second. The accelerating voltage and ap-plied current were 40 kV and 80 mA, respectively. The averagecrystallite sizes of the samples were quantitatively calculatedusing Scherrer formula (d = (0.9k)/(Bcosh), where d, k, B, and hare crystallite size, Cu Ka wavelength of 0.15418 nm, thehalf-height width of the isolated (440) and (400) strong peaksfor c-Al2O3 [20], and Bragg’s diffraction angle, respectively).Morphological analysis was performed by a JSM-5610LV scanningelectron microscope (JEOL, Japan) with an acceleration voltage of20 kV. The powdered sample was also ultrasonically dispersed inethanol and then transferred onto a copper grid covered withcarbon film for transmission electron microscopy (TEM) analysisperformed on a TECNAI G220 S-TWIN electron microscope at anaccelerating voltage of 200 kV. N2 adsorption/desorption isothermswere measured on a Micromeritics ASAP2020 adsorption analyzer(USA). The Brunauer–Emmett–Teller (BET) surface areas of the

samples were calculated from a multipoint BET method using theadsorption data in the relative pressure (P/P0) range of 0.05–0.3.A desorption isotherm was used to determine the pore size distri-bution (PSD) by the Barrete–Joynere–Halender (BJH) method,assuming a cylindrical pore model. The N2 adsorption volume atP/P0 of 0.994 was used to determine the pore volume and the aver-age pore size.

2.3. Adsorption experiment for Cr(VI)

Adsorption experiment for Cr(VI) at room temperature was car-ried out in a batch mode by mixing 0.15 or 0.4 g c-Al2O3 samplewith 80.0 ml K2Cr2O7 solution (30 mg/l, guarantee reagent) in a250 ml stoppered erlenmeyer flask. The flask was shaken via adesktop oscillator at 200 rpm for 4 h, and the suspension wasimmediately centrifuged to remove the c-Al2O3. Concentration ofCr(VI) in the filtrate was analyzed by a UV/Vis spectrophotometer(UVMINI-1240, Shimadzu, Japan) at k = 540 nm via the diphenylc-arbazide method [21]. The pH of Cr(VI) solution was adjusted to2–12 using 1.0 M HCl and 1.0 M NaOH solutions, respectively.Analytical sample was taken from the suspension after variousadsorption times and separated by microfiltration membrane.The percentage of Cr(VI) adsorbed by c-Al2O3 was determined asthe difference between the initial C0 and final concentration Ct ofCr(VI) in the solution. The adsorption rate of Cr(VI) was calculatedas follows:

% adsorption ¼ C0 � Ct

C0� 100

2.4. Adsorption experiment for CO2

Adsorption experiment toward CO2 was carried out using ultrahigh purity CO2 in a pressure range of 0–1 bar at 25 �C on a Tri Star-II3020 analyzer (Micromeritics Instrument Corporation). All thesamples were degassed at 300 �C for 4 h before analysis.

The temperature programmed desorption of CO2 (CO2-TPD)measurement was carried out on a Chemisorb 2720 analyzer underflow rate of 30 mL min�1 N2. Prior to the CO2 adsorption, all sam-ples were heated to 550 �C for degassing, cooled to 25 �C, and thenexposed to a flow of 30 mL min�1 CO2 for 30 min. Desorption pro-ceeded at a heating rate of 10 �C min�1 up to the final temperatureof 450 �C.

3. Results and discussion

3.1. Structural properties

Effects of different SDAs on the phase structures of the samplesare shown in Fig. 1.

It shows that all the reflection peaks of the three samples areidentified as cubic c-Al2O3 with lattice constants of a = 7.9,b = 7.9, and c = 7.9 Å (JCPDS Card No. 10-0425), indicating a com-plete conversion via calcination [20]. The broad diffraction peaksfor the samples reveal their nanosize nature. Their correspondingdiffraction intensity becomes a little higher according to the orderof the resulting c-Al2O3 using PAAS, mixed F127–PAAS and F127 asthe SDAs, respectively. Their corresponding crystallite sizes are 5.6,5.8, and 6.3 nm, respectively. Therefore, the introduction of F127and PAAS in the hydrothermal system does not affect the crystal-line microstructure of the resulting c-Al2O3, but only acts on thecrystallite organization. In addition, no characteristic peaks fromother crystalline impurities are present in Fig. 1, indicating highpurity of the samples.

10 20 30 40 50 60 70 80

(c)

(b)

(440

)

(511

)

(222

)

(400

)

(311

)

(111

)

Rel

ativ

e in

tens

ity (

a.u.

)

2Theta (degree)

(a)

Fig. 1. XRD patterns of the obtained c-Al2O3 using different SDAs (a) F127, (b)mixed F127–PAAS, and (c) PAAS.

36 J. Ge et al. / Journal of Colloid and Interface Science 401 (2013) 34–39

3.2. Morphologies

The morphologies and microstructures of the samples werefurther investigated by SEM and TEM. Fig. 2a and b shows thatthe c-Al2O3 prepared by using F127 as SDA presents hierarchicalspherical-like particles with average size of ca. 1.5 lm in a veryhigh yield. The inset of Fig. 2b also shows that these microspheresare comprised of a large number of well-aligned nanoflakes withthickness of 20 nm. When the c-Al2O3 was prepared by using

Fig. 2. SEM (a, c and e) and TEM (b, d and f) images of the obtained c-Al2O3 using

mixed F127–PAAS as the SDAs (Fig. 2c and d), well-dispersedasymmetric flower-like particles with an average size of ca.0.5 lm were obtained. The inset of Fig. 2d also shows that theseflower-like particles assemble from some well-aligned and inter-connected nanoflakes with a thickness of ca. 15 nm and width ofca. 23 nm. In contrast, the flower-like c-Al2O3 particles preparedby using PAAS as the SDA (Fig. 2e and f) present a larger averagesize of ca. 0.8 lm. However, when the material was prepared with-out any SDA, a product consisting of irregular particles was ob-tained. It was reported that the hydrogen bonds between theboehmite (precursor of c-Al2O3) surface and SDAs molecules canreduce the free energy of the crystallites to form low dimensionalnanoflakes [22–24]. These nanoflakes tend to aggregate in order todecrease the surface energy by reducing exposed areas. Conse-quently, hierarchical sphere-like and flower-like mesoporous c-Al2O3 microparticles were formed through oriented self-assemblymediated by F127 and PAAS, respectively. In contrast, F127 can dis-perse the hierarchical structure well and make them assemblefrom more nanoflakes. This different effect of F127 and PAAS existsin their different chemical structure, and they do not interact in thesame way with the aluminum precursor. F127 is nonionic amphi-philic molecule belonging to the PEO–PPO–PEO family [11]. F127molecules interact with metal oxide surfaces by hydrogen bondingthrough their PEO groups. By contrast, anionic PAAS containshighly hydrophilic groups, and it has been established that carbox-ilic acids strongly react with the hydroxyl groups to form carbox-ylate bonds with surface aluminum ions [25–28]. Carboxylateions can coordinate to metals in a number of ways. The three basicpossibilities are monodentate, chelating and bridging bidentatestructures. Complex formation between aluminum and PAAS

different SDAs (a and b) F127, (c and d) mixed F127–PAAS, and (e and f) PAAS.

Table 1Pore structure parameters of the c-Al2O3 prepared by using different SDAs: (a) F127,(b) mixed F127–PAAS, and (c) PAAS, respectively.a

Sample SBET (m2/g) Vt (cm3/g) w (nm)

(a) 238.3 0.33 5.6(b) 177.6 0.32 7.3(c) 229.1 0.33 5.8

a Notation: SBET—BET specific surface area; Vt—single-point pore volume; w—adsorption average pore width by BET.

J. Ge et al. / Journal of Colloid and Interface Science 401 (2013) 34–39 37

makes the inorganic precursor more bulky, and by consequence,this most probably reduces its degree of crystallization in c-Al2O3, as evidenced by the XRD pattern in Fig. 1.

3.3. Textural properties

Effects of the three SDAs on the textural properties of the sam-ples were further analyzed by N2 adsorption–desorption in Fig. 3.Fig. 3A shows that N2 adsorption isotherms of all the samples dis-play type IV according to the International Union of Pure and Ap-plied Chemistry classification [29]. Curve a of the c-Al2O3

prepared by using F127 as SDA shows an obvious condensationstep related to small mesopores (centering around 3.7 nm formedbetween primary crystallites) and a narrow hysteresis loop at P/P0

between 0.45 and 0.92. However, Curves b and c of the c-Al2O3 pre-pared by using mixed F127–PAAS and PAAS as SDAs, respectively,show weak condensation steps related to big slit-like mesoporesformed between plate-like particles [19] and a little wide hystere-sis loops at P/P0 between 0.6 and 0.98. Furthermore, at low P/P0

(<0.2), the isotherms show small adsorption values, indicatingthe presence of type I micropores [30]. Fig. 3B shows that the c-Al2O3 prepared by using F127 as SDA presents a comparativelysharper mesopore size distribution centering around 3.7 nm. How-ever, the c-Al2O3 prepared by using mixed F127–PAAS and PAAS asSDAs respectively present similar curves with wide PSD, and theirbig mesopore peaks center around 44.2 nm and 48.2 nm, respec-tively. The pore structure parameters of the three c-Al2O3 samplesare also listed in Table 1 which shows that all the samples havehighspecific surface area, moderate pore volume, and small aver-age pore size. However, the c-Al2O3 prepared by using F127 asSDA shows the highest specific surface area of 238.3 m2 g�1 andnearly the same pore volume of 0.33 cm3 g�1 with the c-Al2O3 pre-pared by using PAAS as SDA. It is obvious that there exists a com-petition effect between F127 and PAAS, resulting in less effect thanany of the single template. F127 can significantly improve the tex-tural characteristics of the final c-Al2O3 and make its PSD sharper.A mechanism to explain the improvement of the textural charac-teristics of mesoporous c-Al2O3 includes adsorption of copolymeronto boehmite nanoparticles, below the critical micelle concentra-tion (CMC) of F127, and the formation of micelles which act asspace fillers, above the CMC [11]. In contrast, PAAS can signifi-cantly prevent this adjustment effect.

3.4. Adsorption performance of Cr(VI)

Herein, we used the as-prepared c-Al2O3 with unique hierarchi-cal morphologies and mesoporous structures to investigate their

50

100

150

200

250

300

350

a

b

Relative pressure (P/P0)

Vol

ume

adso

rbed

(cm

3 ST

P/g)

c

(A)

0.0 0.2 0.4 0.6 0.8 1.0

Fig. 3. N2 adsorption–desorption isotherms (A) and the corresponding pore size distribF127–PAAS, and (c) PAAS.

applications in water treatment. Cr(VI) is a primary highly toxicpollutants to human body tissue owing to its oxidizing potentialand easy permeating of biological membranes. Thus, it is ofincreasing importance to develop an effective adsorbent to removeit. In solution at pH higher than 5.5, Cr(VI) is present in form ofCrO2�

4 ions. At lower pH values, the Cr(VI) is present either asHCrO�4 or as Cr2O2�

7 depending on the Cr concentration [31,32].Obviously, not very low pH promotes the adsorption of Cr(VI),but alkaline media or excessive acid will destroy its anionic chem-ical species. Furthermore, the adsorption property of the surface ofalumina strongly depends on pH in bulk solution. If the pH is belowthe isoelectric point of alumina, its surface is charged positively,and electrostatic attraction between the charged alumina surfaceand ions of an opposite charge occurs in bulk solution [33]. Thus,adsorption rates of Cr(VI) at different pH on different amount ofc-Al2O3 (0.15 g and 0.4 g, respectively) prepared by using differentSDAs were firstly studied. Fig. 4 shows that all the samples exhibita highly pH-dependent behavior because the properties of both thec-Al2O3 surface (charge and potential) and the solution composi-tion of Cr(VI) change with pH, and all of them reach the highestadsorption rate at pH = 3. When the pH is lower than 8.5, surfaceof the alumina is positively charged (the isoelectric point of alu-mina is about 9.0 [30]), and thus, they have bigger capacity to ad-sorb the metal anion. Fig. 4 also shows that in comparison withF127–PAAS and PAAS, addition of F127 results in the biggestadsorption rate of 90% and 58% while adopting 0.4 g and 0.15 gc-Al2O3, respectively. Therefore, 0.4 g adsorbent and a pH of 3 wereselected for subsequent work.

As shown in Fig. 5, the c-Al2O3 prepared by using F127 as theSDA has the best adsorption performance toward Cr(VI). It nearlyreaches adsorption balance with adsorption rate of 88% in 2 minand then reaches adsorption balance with adsorption rate of 95%and adsorption capacity of 5.7 mg g�1 in 240 min. Its good adsorp-tion performance may be due to the ion exchange and surface com-plexation between the rich hydroxyl groups on the surface of thehierarchically mesoporous c-Al2O3 with high surface area and

10 1000.0

0.5

1.0

1.5

2.0

c

b

a

dV/d

log(

w)

(cm

3/g

.nm

)

Pore width (nm)

(B)

utions curves (B) of the obtained c-Al2O3 using different SDAs: (a) F127, (b) mixed

0.0

0.1

0.2

0.3

0.4

0.5

0.6

PAAS

mixed F127-PAAS

Ads

orpt

ion

rate

(%

)

pH

F127 (A)

2 4 6 8 10 12 2 4 6 8 10 120.0

0.2

0.4

0.6

0.8

(B)

PAAS

mixed F127-PAAS

Ads

orpt

ion

rate

(%

)

pH

F127

Fig. 4. Effects of pH, SDAs and amount of adsorbent (A) 0.15 g, and (B) 0.4 g on adsorption rates of Cr(VI).

0 100 200 300 400 5000.0

0.2

0.4

0.6

0.8

1.0

bc

Ads

orpt

ion

rate

(%

)

t (min)

a

Fig. 5. Adsorption rates of Cr(VI) at different times on c-Al2O3 prepared by usingdifferent SDAs: (a) F127, (b) mixed F127–PAAS, and (c) PAAS.

0 100 200 300 400 500 600 700

0.15

0.30

0.45

0.60

0.75

0.90

1.05

b

aQ

uant

ity a

dsor

bed

(mm

ol/g

)

Absolute pressure (mmHg)

c

Fig. 6. CO2 adsorption isotherms of the c-Al2O3 prepared by using different SDAs:(a) F127, (b) mixed F127–PAAS, and (c) PAAS.

38 J. Ge et al. / Journal of Colloid and Interface Science 401 (2013) 34–39

HCrO�4 at the liquid/solid interface in acidic conditions [34]. Themesopores are not a limiting factor for Cr(VI) diffusion into theinterior of the mesoporous c-Al2O3, which is consistent with otherstudies using mesoporous materials [35]. Furthermore, its adsorp-tion kinetics and adsorption capacity toward Cr(VI) is compara-tively faster and higher than some other 3D metal oxidesnanostructures, such as flower-like a-Fe2O3 (4.47 mg g�1 in180 min), c-Fe2O3 (3.86 mg g�1 in 180 min), Fe3O4 (4.38 mg g�1

in 180 min) prepared by using an ethylene glycol-mediated refluxprocess [34], flower-like ceria (5.8 mg g�1 in 300 min) based onethylene glycol mediated self-assembly process [3], hierarchicalstructured SiO2@c-AlOOH spheres (4.57 mg g�1 in 330 min) pre-pared by using silica colloidal spheres in one step [36], the c-AlOOH nanobelts (5.6 mg g�1 in 240 min), and nanoplates(4.0 mg g�1 in 240 min) prepared by hydrothermal reactions of c-AlOOH nanoparticles in the presence of CH3COOH and HCl [21].Therefore, the hierarchically mesoporous c-Al2O3 with high surfacearea in this paper has faster adsorption kinetics and higher adsorp-tion capacity, indicating its promising applicability in watertreatment.

3.5. Adsorption performance of CO2

The CO2 adsorption isotherms of the three c-Al2O3 samples atroom temperature were further studied, as shown in Fig. 6. It canbe seen that the c-Al2O3 prepared by using PAAS as the SDA pre-

sents the highest adsorption capacity (1.04 mmol/g) as comparedto the c-Al2O3 prepared from mixed F127–PAAS and from F127which adsorb 0.98 mmol/g and 0.85 mmol/g CO2, respectively.Although the c-Al2O3 prepared by using F127 as the SDA has thehighest surface area among the three samples, it has the lowestadsorption capacity of CO2, indicating that the surface area is notthe decisive factor that affects the adsorption capacity of the sam-ples. It is known that the adsorption capacity of the sample towardCO2 at low temperatures strongly depends on physical adsorptionof a metal oxide which is accentuated when alkaline sites are pres-ent on the surface [37]. Although high surface area of the samplemay provide more basic sites, the strong basic sites cannot be acti-vated at low temperatures. Thus, the c-Al2O3 prepared by usingPAAS as the SDA may has more alkaline sites at room temperaturedue to the weak alkaline of the dilute PAAS solution which maygenerate basic sites and suppress acidic sites. The nature of the sur-face sites of aluminum oxides is expected to be influenced by theSDA and this determines their acid–base properties [38]. The alu-minum ions affect the acidity, whereas the oxide ions contributeto the alkalinity. When aluminum precursor is calcined to obtainc-Al2O3, the dehydration of physically and chemically adsorbedwater occurs followed by the formation of oxygen bridges betweensurface aluminum ions leading to higher acidity at the surfacebinding sites. In the case when the polymer PAAS is used as SDA,the mesoporous alumina is not well-crystallized and more oxygen isexpected to be involved in the surface bonding; this strengthens thesurface alkalinity. By contrast, when the sample is well-crystallized,

100 200 300 400-0.578

-0.576

-0.574

-0.572

-0.570

-0.568

-0.566

-0.564

c

b

TC

D s

igna

l (a.

u.)

Temperature ( )

a

Fig. 7. CO2-TPD profiles for the c-Al2O3 prepared by using different SDAs: (a) F127,(b) mixed F127–PAAS, and (c) PAAS.

J. Ge et al. / Journal of Colloid and Interface Science 401 (2013) 34–39 39

the more aluminum bonded hydroxyls lead to a stronger surfaceacidity.

Furthermore, the CO2-TPD profiles were comparatively re-corded to show this difference on the strength and amount of basicsites of the three samples under the same conditions, as shown inFig. 7. It shows that there exists a strongly broad desorption peak attemperatures around 70 �C and a very weak desorption peak above400 �C, indicating the existence of a wide range of basic sites at lowtemperatures on the surface of the samples [39]. The former wasattributed to weakly adsorbed CO2 species, whereas the latterwas assigned to strongly adsorbed CO2 species. Correspondingly,the c-Al2O3 prepared by using mixed F127–PAAS as the SDA hasthe strongest desorption peak, indicating that it has the highest ba-sic sites around room temperature. The desorption temperatures ofCO2 from these hierarchical c-Al2O3 were similar to those (73–86 �C) from the three-dimensionally ordered macroporous c-Al2O3 obtained with polymethyl methacrylate and F127 as thetemplate and Al(OiPr)3 as the Al source [40].

4. Conclusion

In conclusion, hierarchical flower-like and sphere-like mesopor-ous c-Al2O3 microparticles assembled from well-aligned nano-flakes have been successfully prepared by a facile hydrothermalmethod followed by a calcination process using cheap sodium alu-minate as aluminum source, F127, PAAS, and mixed F127–PAAS asSDA, respectively. Addition of F127 and PAAS has a significant ef-fect on the morphology and textural properties of the obtainedmesoporous c-Al2O3, and especially, F127 can make the PSD shar-per. The sphere-like c-Al2O3 microparticles prepared by using F127as the SDA show the best adsorption performance toward Cr(VI)with a high adsorption rate of 95% in 4 h at room temperature.However, the flower-like c-Al2O3 microparticles prepared by usingPAAS as the SDA has the highest CO2 adsorption capacity of1.04 mmol/g at room temperature. All of the encouraging resultsmean that this synthesis strategy can be extended to the prepara-tion of hierarchical metal oxides with controlled physicochemicalproperties and high added-value for a variety of applications suchas adsorption of heavy metal ions and CO2.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (51142002, 51272201 and

21277108), the 863 Program (2012AA062701), Wuhan YouthChenguang Program of Science and Technology(2013070104010002), Key Laboratory of New-Style Reactor andGreen Chemical Technology in Hubei Province, Wuhan Instituteof Technology (RGCT201104), the State Key Laboratory of Ad-vanced Technology for Materials Synthesis and Processing (WuhanUniversity of Technology, 2013-KF-5), the Key Laboratory of Catal-ysis and Materials Science of the State Ethnic Affairs Commission &Ministry of Education, Hubei Province, South-Central University forNationalities (CHCL11004), and the State Key Laboratory of Mate-rials-Oriented Chemical Engineering, Nanjing University of Tech-nology (KL11-01).

References

[1] J.S. Hu, L.S. Zhong, W.G. Song, L.J. Wan, Adv. Mater. 20 (2008) 2977.[2] Y.S. Bae, R.Q. Snurr, Angew. Chem. Int. Ed. 50 (2011) 11586.[3] L.S. Zhong, J.S. Hu, A.M. Cao, Q. Liu, W.G. Song, L.J. Wan, Chem. Mater. 19 (2007)

1648.[4] J.B. Fei, Y. Cui, X.H. Yan, W. Qi, Y. Yang, K.W. Wang, Q. He, J.B. Li, Adv. Mater. 20

(2008) 452.[5] J.S. Moore, M.L. Kraft, Science 320 (2008) 620.[6] W.Q. Cai, J.G. Yu, S.H. Gu, M. Jaroniec, Cryst. Growth Des. 10 (2010) 3977.[7] M.A.P. Christopher, W. Karen, F.L. Adam, Chem. Soc. Rev. (2013), http://

dx.doi.org/10.1039/C2CS35378D.[8] C. Márquez-Alvarez, N. Zilková, J. Pérez-Pariente, J. Cejka, Catal. Rev. 50 (2008)

222.[9] V.M. Gun’ko, G.R. Yurchenko, V.V. Turov, E.V. Goncharuk, V.I. Zarko, A.G.

Zabuga, A.K. Matkovsky, O.I. Oranska, R. Leboda, J. Skubiszewska-Zieba, W.Janusz, G.J. Phillips, S.V. Mikhalovsky, J. Colloid Interface Sci. 348 (2010) 546.

[10] S.C. Lee, Y.M. Kwon, C.Y. Ryu, H.J. Chae, D. Ragupathy, S.Y. Jung, J.B. Lee, C.K.Ryu, J.C. Kim, Fuel 90 (2012) 1465.

[11] R. Bleta, P. Alphonse, L. Pin, M. Gressier, M.J. Menu, J. Colloid Interface Sci. 367(2012) 120.

[12] H.W. Hou, Y. Xie, Q. Yang, Q.X. Guo, C.R. Tan, Nanotechnology 16 (2005) 741.[13] X.Y. Chen, H.S. Hu, S.W. Lee, Nanotechnology 18 (2007) 285608.[14] J. Zhang, S. Wei, J. Lin, J. Luo, S. Liu, H. Song, E. Elawad, X. Ding, J. Gao, S. Qi, C.

Tang, J. Phys. Chem. B. 110 (2006) 21680.[15] J. Zhang, S.J. Liu, J. Lin, H.S. Song, J.J. Luo, E.M. Elssfah, E. Ammar, Y. Huang, X.X.

Ding, J.M. Gao, S.R. Qi, C.C. Tang, J. Phys. Chem. B 110 (2006) 14249.[16] D.H.M. Buchold, C. Feldmann, Nano Lett. 7 (2007) 3489.[17] L. Zhang, Y.J. Zhu, J. Phys. Chem. C 112 (2008) 16764.[18] W.Q. Cai, J.G. Yu, B. Cheng, B.L. Su, M. Jaroniec, J. Phys. Chem. C 113 (2009)

14739.[19] W.Q. Cai, J.G. Yu, M. Jaroniec, J. Mater. Chem. 20 (2010) 4587.[20] C.L. Ma, Y.L. Chang, W.C. Ye, W. Shang, C.M. Wang, J. Colloid Interface Sci. 317

(2008) 148.[21] L. Zhang, X.L. Jiao, D.R. Chen, M.X. Jiao, Eur. J. Inorg. Chem. 34 (2011) 5258.[22] L.X. Yang, Y.J. Zhu, H. Tong, Z.H. Liang, W.W. Wang, Cryst. Growth Des. 7 (2007)

2716.[23] Y.X. Zhang, Y. Jia, Z. Jin, X.Y. Yu, W.H. Xu, T. Luo, B.J. Zhu, J.H. Liu, X.J. Huang,

Cryst. Eng. Commun. 14 (2012) 3005.[24] W.Q. Cai, Y.Z. Hu, J. Chen, G.X. Zhang, T. Xia, Cryst. Eng. Commun. 14 (2012)

972.[25] M.R. Alexander, G. Beamson, C.J. Blomfield, G. Leggett, T.M. Duc, J. Electron.

Spectrosc. 121 (2001) 19.[26] W. Shen, M.M. Shi, M. Wang, H.Z. Chen, Mater. Chem. Phys. 122 (2010) 588.[27] M. Iijima, Y. Yonemochi, M. Tsukada, H. Kamiya, J. Colloid Interface Sci. 298

(2006) 202.[28] J.G. Yu, M. Lei, B. Cheng, X.J. Zhao, J. Solid State Chem. 177 (2004) 681.[29] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T.

Siemieniewska, Pure Appl. Chem. 57 (1985) 603.[30] W.Q. Cai, J.G. Yu, S. Mann, Microporours Mesoporous. Mater. 122 (2009) 42.[31] F. Granados-Correa, J. Jiménez-Becerril, J. Hazard. Mater. 162 (2009) 1178.[32] D. Mohan, C.U. Pittman Jr., J. Hazard. Mater. 137 (2006) 762.[33] B. Kasprzyk-Hordern, Adv. Colloid Interface Sci. 110 (2004) 19.[34] L.S. Zhong, J.S. Hu, H.P. Liang, A.M. Cao, W.G. Song, L.J. Wan, Adv. Mater. 18

(2006) 2426.[35] P. Wang, I.M.C. Lo, Water Res. 43 (2009) 3727.[36] Y.Q. Wang, G.Z. Wang, H.Q. Wang, W.P. Cai, C.H. Liang, L.D. Zhang,

Nanotechnology 20 (2009) 155604.[37] L.B. Sun, J. Yang, J.H. Kou, F.N. Gu, Y. Chun, Y. Wang, J.H. Zhu, Z.G. Zou, Angew.

Chem. Int. Ed. 47 (2008) 3418.[38] X.F. Yang, Z.X. Sun, D.S. Wang, W. Forsling, J. Colloid Interface Sci. 308 (2007)

395.[39] W.Q. Cai, J.G. Yu, M. Jaroniec, J. Mater. Chem. 21 (2011) 9066.[40] H. Li, L. Zhang, H.X. Dai, H. He, Inorg. Chem. 48 (2009) 4421.