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Solid State Sciences 11 (2009) 829–835

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Solid State Sciences

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Synthesis and characterization of nanoparticles of a-Bi2Mo3O12 preparedby co-precipitation method: Langmuir adsorption parametersand photocatalytic properties with rhodamine B

A. Martınez-de la Cruz*, S. Obregon AlfaroFacultad de Ingenierıa Mecanica y Electrica, Universidad Autonoma de Nuevo Leon, Ciudad Universitaria, C.P. 66451 San Nicolas de los Garza, N.L., Mexico

a r t i c l e i n f o

Article history:Received 4 July 2008Received in revised form27 December 2008Accepted 22 January 2009Available online 31 January 2009

Keywords:AdsorptionPhotocatalysisRhodamine BLangmuir parameters

* Corresponding author. Tel.: þ52 81 83 29 40 20;E-mail address: azael70@yahoo.com.mx (A. Martın

1293-2558/$ – see front matter � 2009 Elsevier Masdoi:10.1016/j.solidstatesciences.2009.01.007

a b s t r a c t

Nanoparticles of a-Bi2Mo3O12 were prepared by co-precipitation method at calcination temperatures of250, 300, 400 and 480 �C. The characterization of a-Bi2Mo3O12 synthesized at different temperatures wascarried out by X-ray diffraction (XRD), thermal analysis (TGA/DTA), transmission electron microscopy(TEM), and diffuse reflectance spectroscopy (DRS). Adsorption parameters and photocatalytical activityunder visible light irradiation of a-Bi2Mo3O12 were evaluated using the rhodamine B (rhB) dye as model.The adsorption constant (K) and maximum amount of dye adsorbed (qmax) on the surface of the samplessynthesized were evaluated following the Langmuir isotherm. The sample calcinated at 250 �C showedthe maximum adsorption percentage of dye, which ranged between 20 and 46% for initial concentrationsof rhB from 5 to 15 mg L�1, with a K¼ 6.96� 105 L mol�1 and qmax¼ 2.73 mg g�1. All samples were able toinduce the oxidative photodegradation of rhB, however, the bleaching of dye solution was reached morequickly for the sample calcinated at 250 �C.

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�

1. Introduction

In the binary system Bi2O3–MoO3, the phases a-Bi2Mo3O12,b-Bi2Mo2O9 and g-Bi2MoO6 have been extensively studied due totheir interesting properties as catalysts for the conversion of olefinsto unsaturated aldehydes. For industrial purposes, these molyb-dates have been employed as catalysts on the ammoxidation ofpropene to acrylonitrile, for the selective oxidation of propene toacrolein, and for the oxidative dehydrogenation of butane tobutadiene [1]. Several efforts have been directed to synthesizethese oxides with higher surface area than those synthesized by theclassical solid state reaction in order to increase their catalyticactivity [2].

So far, the most common preparative method used to synthesizethese oxides is by the co-precipitation from bismuth nitrate andammonium molybdate on aqueous media. This route provides aneasy and inexpensive way to prepare molybdates at lowertemperatures than those employed on the solid state reaction. Themethod implies the co-precipitation of the pure hydroxides and theprecipitation of bismuth with polymerized molybdenum ions.These inorganic intermediates can be easily decomposed by using

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temperatures of synthesis of 300–400 C which are below thetemperature employed in the classical method.

The synthesis of oxides by the co-precipitation method leads tomaterials with higher surface area than those obtained by hightemperature reaction. From the textural properties point of view,this situation implies that the number of active sites on the materialsurface is increased. The existence of these active sites allows theinteraction of the material with molecules of adsorbate on anaqueous dispersion of them, and then the adsorption phenomenatake place. Different models have been employed to describe theadsorption process of an adsorbate on the surface of an adsorbentsuch as the Langmuir and Freundlich isotherms [3,4]. In the past,the adsorption techniques have showed potential for removingorganics from water due to their high efficiency and ability toseparate a wide range of chemical compounds [5–7]. Especially, theelimination of organic dyes by adsorption process is an importanttopic due to approximately 5–10% of the annual global productionis discharged as effluent mainly by paint and textile industries [8].

Recently, Kudo et al. have reported the ability of the molybdatesa-Bi2Mo3O12, b-Bi2Mo2O9 and g-Bi2MoO6 to act as photocatalysts inthe O2-evolution from an AgNO3 solution by using VIS irradiation[9]. This situation has opened a new field of application of themolybdates as photocatalysts on heterogeneous photocatalysisreactions. Nowadays, from the three molybdates a-, b-, and g-, onlythe g-Bi2MoO6 [10–14] and a-Bi2Mo3O12 [15] oxides have beentested as photocatalysts on the degradation of pollutants organics.

A. Martınez-de la Cruz, S. Obregon Alfaro / Solid State Sciences 11 (2009) 829–835830

Following this line of investigation, we will explore by first time thepossibility of a-Bi2Mo3O12 prepared by co-precipitation method aspotential photocatalyst for the degradation of organic dyes.

As a model reaction, the rhodamine B xanthene dye (rhB, C.I.number 45170) has been selected to study its degradation due to itsimportance and use as dye laser, water tracing agent, biologicalstain, and in the pharmaceutical and cosmetic industries [16]. Thiscompound is now excluded from use in foods and cosmeticsbecause it has been found to be potentially toxic and carcinogenic[17,18]. The molecular structure of rhB is shown in Fig. 1.

In this work, the rhB has been removed from aqueous solutionsby the combined action of physical adsorption and heterogeneousphotocatalysis with nanoparticles of a-Bi2Mo3O12. A study of bothprocesses will be presented in the following lines.

2. Experimental

2.1. Samples preparation of a-Bi2Mo3O12

The oxide a-Bi2Mo3O12 was synthesized by the so-calledco-precipitation method. The method implies the preparationof two aqueous solutions at 70 �C. In the first one, 6.484 g ofBi(NO3)3$5H2O [Aldrich, 99.99%] were dissolved on 100 mL ofdiluted HNO3. The second one was prepared by dissolving 3.5399 gof (NH4)6Mo7O24$4H2O [Productos Quımicos Monterrey, 99%þ] in100 mL of diluted NH4OH. The bismuth nitrate solution was addeddrop by drop (w5 mL min�1) to the molybdate solution. Thisprocess was accompanied with a vigorous stirring. When the mixwas reached, the pH of the solution was adjusted at 9 by usingNH4OH. The resulting yellow suspension was maintained at 70 �Cto promote a slow evaporation of the solvent. The yellow powderobtained after this thermal treatment was used as precursor ofa-Bi2Mo3O12. A slow thermal treatment of 5 �C min�1 in air at 250,300, 400 and 480 �C was employed to obtain polycrystallinepowders of a-Bi2Mo3O12. The thermal treatments were carried outuntil the loss mass of the material was negligible. On the otherhand, for comparative purposes, a sample of a-Bi2Mo3O12 was alsosynthesized by melting stoichiometric amounts of Bi2O3 [Aldrich,99.9%þ] and MoO3 [Merck, 99.9%þ] at 700 �C for 24 h. Aftercooling, the formed crystals were ground into powder [19].

2.2. Sample characterization

Structural characterization was carried out by powder X-raydiffraction using a Bruker D8 Advanced diffractometer with CuKa

radiation doted with a Vantec high speed detector. X-ray diffractiondata of samples were collected in the 2q range of 10�–70� witha step scan rate of 0.05� 0.05 s�1. The particle size of the sampleswas observed by Transmission Electron Microscopy. For this

Fig. 1. Molecular structure of rhodamine B.

purpose a JEOL 2010 instrument with an accelerating voltage of200 kV was used.

Thermogravimetric and Differential Thermal Analyses (TGA/DTA) of the precursor were performed on a TA Instruments ModSDT Q600 thermal analyzer. Measurements were carried out undera nitrogen atmosphere with a flux of 100 mL min�1 and witha heating rate of 5 �C min�1. The surface area (BET) and the poresize diameter (BJH) of the photocatalysts were determined by N2

adsorption–desorption measurements by using a Surface Area &Pore Size analyzer Micromeritics ASAP-2000. The isotherms ofadsorption–desorption were evaluated at �196 �C after a pretreat-ment of the sample at 100 �C for 2 h. UV-diffuse reflectancespectrums of the oxides were measured by using an UV–vis spec-trophotometer equipped with a integration sphere (Perkin ElmerLambda 35).

2.3. Adsorption study

To determine the adsorption constant for the samples calcinatedat different temperatures, concentrations of rhB of 5, 7.5, 10, and15 mg L�1 were employed. In a glass beaker, 50 mL of rhB solutioncontaining 50 mg of photocatalyst were put in an ultrasonic bathfor 10 min to eliminate aggregates. After this time, the suspensionwas maintained at 25� 0.1 �C with continuous stirring during theexperiment. All experiments were carried out at the natural pH ofsolution (pH¼ 6.4). Samples of 6 mL were taken at given timeintervals and then separated through double centrifugation(4000 rpm, 20 min). The supernatant solution was decanted andthe concentration of rhB was determined by a spectrophotometricmethod (see further).

The amount of dye uptaken by the adsorbent, q (mg g�1), wascalculated by applying the formula:

q ¼ Co � Ce

mV (1)

where Co is the initial concentration of rhB put in contact with theadsorbent (mg L�1), Ce is the concentration of rhB (mg L�1) onequilibrium, m is the mass of adsorbent (g) and V is the volume ofdye solution put in contact with the adsorbent.

2.4. Photocatalytic reactions

The photochemical reactor employed in this work consisted ofa glass borosilicate beaker surrounded with a water-jacket tomaintain the reaction temperature at 25�1 �C. A Xe lamp of10,000 K with a luminous flux of 2100 lm was used as source ofvisible light. The emission spectrum of the Xe lamp was measuredon an UV–vis spectrophotometer. A negligible contribution of UVradiation was observed (l< 390 nm), moreover, this was filteredby the borosilicate container. The photocatalytical activity ofa-Bi2Mo3O12 was evaluated by the reaction of degradation of rhB inwater. In a glass beaker, 220 mL of rhB solution [5 mg L�1]containing 220 mg of photocatalyst were put in an ultrasonic bathfor 10 min to eliminate aggregates. In order to assure that theequilibrium adsorption–desorption of dye on the catalyst surfacewas reached, the solution was remained in the darkness for 1 h.After this time, the light source was turned on. During the reaction,samples of 8 mL were taken at given time intervals and thenseparated through double centrifugation (4000 rpm, 20 min). Thesupernatant solution was decanted and the concentration ofrhB was determined through its absorption band maximum(554 nm) by using an UV–vis spectrophotometer (Perkin ElmerLambda 35).

10 20 30 40 50 60 702θ

I (u

.a.)

JCPDSNo. 21-0103

Melting oxides method700 °C

480 °C

70°C

250 °C

300 °C

400 °C

Fig. 3. XRD patterns of powders obtained under different calcination temperatures ofthe precursor and by the reference method.

A. Martınez-de la Cruz, S. Obregon Alfaro / Solid State Sciences 11 (2009) 829–835 831

3. Results and discussions

3.1. Sample characterization

A yellow powder was obtained as product of the co-precipita-tion process. This material will be identified hereafter as precursor.The decomposition of the precursor to form a-Bi2Mo3O12 wasdetermined by TGA/DTA simultaneous analysis, as shown in Fig. 2.Two endothermic peaks without weight loss were observed near138 �C and 168 �C. Due to its complexity, the nature of theprecursor was not elucidated. Previously Reilly et al. [20] havereported similar endothermic peaks during the formation ofg-Bi2MoO6 when it was synthesized by co-precipitation startingfrom the same salts. The authors attributed the origin of the peaksto a rearrangement of the H bonds in the system. From the TGA plot,nearly 66% of weight loss is observed in the temperature range of192–250 �C due to the elimination of NO2, NH3, O2 and H2O. Thisprocess was identified through a broad endothermic peak on theDTA curve. Above 250 �C there is no evidence of loss of weight. Theendothermic peak observed at 658 �C corresponds to the meltingpoint of a-Bi2Mo3O12.

Different works related the formation by co-precipitation of a-Bi2Mo3O12 at 480 �C [2,21,22]. In order to explore the possibility ofgetting this phase at lower temperatures with higher surface area,additional thermal treatments of the precursor were carried out at250, 300, and 400 �C. The thermal treatments were applied to theprecursor until constant weight is reached. Fig. 3 shows the X-raydiffraction patterns of the powders obtained after each thermaltreatment. As can be appreciated, the crystalline formation of a-Bi2Mo3O12 takes place after 250 �C (JCPDS Card No. 21-0103). In thetemperature range of 250–480 �C there was no evidence of theformation of b- and g-molybdates phases. This is a very interestingresult because this implies the synthesis of pure a-Bi2Mo3O12 withdifferent morphological characteristics that could harness itsapplication as catalyst. A different relation of intensities on thethree most important diffraction lines of a-Bi2Mo3O12 wasobserved for the sample obtained at 700 �C by the referencemethod with respect to the samples obtained at lower tempera-tures. Due to the melting point of a-Bi2Mo3O12 is 658 �C, thesynthesis of the oxide at 700 �C implies its obtaining from a meltmedium [19].

The surface area BET was measured for these samples. Theobtained values for the different thermal treatments are shown inTable 1. The surface area obtained for the sample calcinated at480 �C, 2.56 m2/g, is in agreement with some published data usingthe co-precipitation method at similar temperature [23,24]. Theformation of the phase a-Bi2Mo3O12 since 250 �C leads tomolybdates with higher surface area than those reported previously

melting point

-66 %

20

30

40

50

60

70

80

90

100

110

0 100 200 300 400 500 600 700 800

Weig

ht lo

ss %

-0.4

-0.3

-0.2

-0.1

0

0.1

Tem

peratu

re d

ifferen

ce (°C

/m

g)

Temperature (ºC)

0.2Weight (%)

Temperature Difference(°C/mg)

Fig. 2. TGA and DTA of the precursor used to prepare a-Bi2Mo3O12 nanoparticles.

by the same method. The values observed in the interval of 250–400 �C were similar among them, w7 m2/g. On the basis of theseresults, a possible sinterization mechanism should take placebetween 400 and 480 �C. Taking into account the application ofa-Bi2Mo3O12 as catalysts, lower calcination temperatures are pref-erable because they produce materials with higher specific surfacearea.

Diffuse reflectance of the calcinated samples at differenttemperatures was analyzed by UV–vis spectroscopy. A blue-shift ofthe absorption edges spectra was observed for the samples calci-nated at lower temperatures as is observed in Fig. 4. Thisphenomenon can be associated with the presence of an amount ofamorphous material on the samples synthesized [25]. From thesedata the Eg value of each sample was calculated and the valuesobtained are reported in Table 1.

The morphology of a-Bi2Mo3O12 was determined by TEManalysis. Fig. 5 shows some selected TEM micrographs of samplescalcinated at different temperatures. In general, two differentmorphologies were observed. For the sample synthesized at lowesttemperature, i.e. at 250 �C, the common feature was the presence ofrectangular plates of different sizes. The dimension of these was

Table 1Some physical properties of samples a-Bi2Mo3O12 obtained at different tempera-tures of calcination.

Temperature of calcination (�C) Surface area, BET (m2/g) Band gap, Eg (eV)

250 7.04 2.74300 6.92 2.82400 7.08 2.79480 2.56 2.77700a 0.70 2.45

a Synthesized by melting stoichiometric amounts of Bi2O3 and MoO3.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

340 360 380 400 420 440 460 480 500 520 540 560Wavelenght (nm)

Ab

so

rb

an

ce (arb

itrary u

nits) 700 °C

480 °C400 °C300 °C250 °C

(a)

(e)

(d)

(b)(c)

Fig. 4. Diffuse reflectance spectra of Bi2Mo3O12 calcinated at different temperatures: (a)700 �C – high temperature reaction, (b) 480 �C, (c) 400 �C, (d) 300 �C, and (e) 250 �C.

A. Martınez-de la Cruz, S. Obregon Alfaro / Solid State Sciences 11 (2009) 829–835832

around 50 nm of width and 180–200 nm of long. At 300 �C, themorphology of the particles begins to change to form irregularshapes. The presence of this type of particles predominates on thesample obtained at 400 �C, and rectangular plates were notobserved. A detailed analysis of the particles synthesized at 250,300, and 400 �C revealed the existence of very small particlesincrusted over its surface. Their approximate range size was around2–3 nm and the analysis by EDS determined that its chemicalcomposition was Bi2Mo3O12. When the calcination temperatureemployed was the reported on the literature, i.e. 480 �C, a signifi-cant change of the morphology was observed. In first instance, theparticles became thick due to the natural sintering process. Isolatedparticles with size around 500–700 nm were observed, indicatingthat a critical syntheresis process took place between 400 and480 �C. Thinking on the possible application of this material asphotocatalyst, the formation of nanoparticles of a-Bi2Mo3O12 canplay an important role in the photocatalysis process from the pointof view of surface area. The analysis by TEM technique showeda good agreement with the results obtained by BET technique.

50 nm

50 nm

200 nm175 nm

50 nm

a

b

c

d

Fig. 5. Morphologies of temperature series a-Bi2Mo3O12 samples analyzed

3.2. Adsorption of rhodamine B on a-Bi2Mo3O12

To evaluate the extent of adsorption of rhB on a-Bi2Mo3O12,solutions with different concentrations of rhB were put in contactwith the adsorbent in the darkness. The concentration of rhB wasdetermined at different intervals of time to establish the equilib-rium of adsorption–desorption process on the surface of photo-catalyst. The final concentration of rhB on the surface ofphotocatalyst was very different in each case and dependent on thecalcination temperature, see Fig. 6. Whereas for the sampleobtained at 480 �C the percentage of dye adsorbed on the surface ofthe photocatalyst ranged between w9 and 15%, the correspondingvalues for the sample synthesized at 250 �C were about w20–46%for initial concentrations of dye from 5 to 15 mg L�1. In general twotypes of profile of curve were observed. For samples calcinated at250 and 300 �C, the percentage of dye adsorbed was stronglydepended on the initial concentration of dye. A different behaviorwas observed for the samples obtained at 400, 480 and 700 �C,where the initial concentration of dye has a minor influence on thepercentage of dye adsorbed on the surface of photocatalyst. Theseresults can be explained on the basis of the amount of active siteson the surface of each photocatalyst. For the samples calcinated athigher temperatures, a small concentration of dye is enough tosaturate the active sites over the surface of photocatalyst. Thesaturation is near being reached at the initial concentration of dyeof 5 mg L�1 and no important differences on the percentage of dyeadsorbed were observed with respect to the maximum initialconcentration used, 15 mg L�1. As the amount of active sites onthe surface of adsorbent is increased, a greater concentration of dyeis necessary to reach the saturation by dye molecules. According tothe reasons mentioned above, for the samples obtained at 250 and300 �C an initial concentration of dye of 5 mg L�1 is far tobe necessary to saturate the large amount of active sites on thesurface of the photocatalyst. For these samples, it is possible thatsaturation of active sites is reached at greater concentrations than15 mg L�1.

50 nm 50 nm

50 nm50 nm

f

e

by TEM: (a and b) 250 �C, (c and d) 300 �C, (e) 400 �C, and (f) 480 �C.

0

10

20

30

40

50

0 5 10 15 20Initial concentration of rhB (mg L

-1)

% o

f rh

B ad

so

rb

ed

250 °C 300 °C 400 °C 480 °C 700 °C

Fig. 6. Percentage of rhB adsorbed at different initial concentrations of dye ona-Bi2Mo3O12 synthesized at different temperatures.

A. Martınez-de la Cruz, S. Obregon Alfaro / Solid State Sciences 11 (2009) 829–835 833

Note that the analysis by BET technique revealed that thesamples calcinated at 250, 300 and 400 �C have almost the samevalue of surface area (w7 m2/g). Nevertheless, the capacity ofadsorption of each sample was different, especially for the samplecalcinated at 400 �C. This discrepancy can be explained taking into

0

0.4

0.8

1.2

1.6

2

2.4

2.8

0 5 10

a

c

e

15 20 25 30 35 40 45 50 55 60

q (m

g R

hB

/g

catalyst)

0

0.4

0.8

1.2

1.6

2

q (m

g R

hB

/g

catalyst)

1.2

1

0

0.4

0.6

0.2

0.8

q (m

g R

hB

/g

catalyst)

15 mg/L 10 mg/L 7.5 mg/L 5 mg/L

15 mg/L 10 mg/L 7.5 mg/L 5 mg/L

250 °C

400 °C

Time (min)

0 5 10 15 20 25 30 35 40 45 50 55 60Time (min)

0 5 10 15 20 25 3Time

Fig. 7. Kinetics of adsorption of rhB in the darkness when samples of a-Bi2Mo3O12 synthesize

account the enormous difference on size of the molecule of N2

employed on BET technique and the size of the molecule of rhB.Whereas the BET analysis revealed a true surface area, the adsorp-tion of rhB can be affected by steric effects of the dye molecule onthe interaction with the surface of photocatalyst. In a pioneer work,Graham described the adsorption process of methylene blue onactivated carbons on the basis of the differences between the totalsurface area of the adsorbents and that accessible to methylene blue[26]. In this sense, a pore diameter limit of 13 Å was determined topermit the pass of molecules of methylene blue inside of the pores.The formation of a-Bi2Mo3O12 from the precursor obtained in theco-precipitation method is accompanied with the evolution of gasessuch as NO2, NH3, O2, and H2O. This situation promotes the forma-tion of pores within the material. The pores formed tend to close asthe calcination temperature is increased by a natural sinteringprocess, making difficult the entrance of the big molecules of dye. Adetailed study of the porosity of the samples of a-Bi2Mo3O12

obtained at different temperatures of calcinations is now in prog-ress, but a preliminary study revealed that the pore size diameter ofthe samples synthesized was ranged between 35 and 45 Å.

The kinetics of RhB adsorption for different initial concentra-tions of dye is represented in Fig. 7. In general, a fast equilibriumwas established after 15 min for the samples analyzed(250–480 �C) at all initial concentrations of dye (5–15 mg L�1). The

b

d

0

0.4

0.8

1.2

1.6

2

2.4

q (m

g R

hB

/g

catalyst)

1.4

1.2

1

0

0.4

0.6

0.2

0.8

q (m

g R

hB

/g

catalyst)

15 mg/L 10 mg/L 7.5 mg/L 5 mg/L

15 mg/L 10 mg/L 7.5 mg/L 5 mg/L

15 mg/L 10 mg/L 7.5 mg/L 5 mg/L

700 °C

480 °C

300 °C

0 5 10 15 20 25 30 35 40 45 50 55 60Time (min)

0 5 10 15 20 25 30 35 40 45 50 55 60Time (min)

0 35 40 45 50 55 60 (min)

d at (a) 250 �C (b) 300 �C (c) 400 �C, (d) 480 �C, and (e) 700 �C were used as adsorbent.

R2 = 0.9943

R2 = 0.9963

R2 = 0.9888

R2 = 0.9849

R2 = 0.9811

0

0.4

0.8

1.2

1.6

2

2.4

0 0.1 0.2 0.3 0.4 0.5

1/Ce (L·mg-1

)

1/q

e (g

·m

g-1)

700 °C480 °C400 °C300 °C250 °C

Fig. 8. Transformation of Langmuir isotherm: reciprocal of the quantity adsorbed asa function of reciprocal of equilibrium concentrations.

0

0.2

0.4

0.6

0.8

1

200 40 60 80 100 120 140 160 180 220200 240Time irradiation (min)

C/C

0 Reference method (700 °C)Co-precipitation (480 °C)Co-precipitation (400 °C)Co-precipitation (300 °C)Co-precipitation (250 °C)

Adso

rptio

n-d

esor

ptio

neq

uilib

rium

Ultr

asou

nd

Fig. 9. Variation of rhB concentration versus time when samples of a-Bi2Mo3O12

obtained at different temperatures of calcinations were used as photocatalyst.(Co¼ 5 mg L�1).

0

0.2

0.4

0.6

0.8

1

250 350 450 550 650

Ab

so

rb

an

ce

RhB 5 mg/L without photocatalyst0 min20 min40 min80 min120 min180 min240 min

rhB, 554 nm

250°C

λ (nm)

N,N´-dy-ethylatedrh, 522 nm

N,N,N´-try-ethylated rh,539 nm

rh, 498 nm

N´-ethylated rh,510 nm

rb

an

ce

300°C

0.6

0.8

1 RhB 5 mg/L without photocatalyst0 min20 min40 min80 min120 min180 min240 min

rhB, 554 nmN,N,N´-try-ethylated rh,539 nm

A. Martınez-de la Cruz, S. Obregon Alfaro / Solid State Sciences 11 (2009) 829–835834

profile of the curves is of the Langmuir type. In previous works, theLangmuir model has been successfully employed to determine theadsorption constant of dyes on different types of adsorbents[8,17,27]. Following this model, the coverage q varies as:

q ¼ qqmax

¼ KCe

1þ KCe(2)

the linear transformation of Equation (2) can be expressed as:

1q¼ 1

qmaxþ�

1qmaxK

�1Ce

(3)

where qmax is the maximum amount of dye adsorbed (mg/g), Ce isthe concentration of dye on equilibrium (mg L�1) and K is theadsorption constant. The linear transformation of Equation (3) isgiven in Fig. 8 for the samples analyzed, where the ordinate at theorigin is equal to the reciprocal of qmax, whereas K can be deducedfrom the slope of the curve (1/qmaxK). From these data, the maximaladsorption quantity and the Langmuir adsorption constant for eachsample were calculated, see Table 2.

The adsorption of dye on the surface of the photocatalyst can playan important role in the photodegradation rate. The photocatalysisprocess can be improved efficiently by collecting the dye in thesolution to the surface of photocatalyst [28]. Due to the lifetime ofthe hydroxyl radicals formed during the photodegradation processis very short, they can only react at or very near where they wereformed. In the same way, in the interface of photocatalyst–dye,excited electrons from the p bonds of dye can be injected to theconduction band of the photocatalyst under an appropriate irradi-ation (photosensitization process). In summary, a major concen-tration of active sites leads to a major adsorption of dye and, inconsequence, is possible an increase on the photodegradation rate.

3.3. Photocatalytic activity of a-Bi2Mo3O12

Fig. 9 shows the temporal evolution of the rhB concentra-tion when samples of a-Bi2Mo3O12 synthesized at different

Table 2Adsorption constants (K) and maximum amount of dye adsorbed (qmax) following themodel of Langmuir for a-Bi2Mo3O12 obtained at different temperatures of calcination.

Adsorbent K (L mol�1) qmax (mg g�1)

a-Bi2Mo3O12, 250 �C 6.96� 105 2.73a-Bi2Mo3O12, 300 �C 5.16� 105 2.46a-Bi2Mo3O12, 400 �C 8.10� 104 2.33a-Bi2Mo3O12, 480 �C 5.94� 104 2.17a-Bi2Mo3O12, 700 �Ca 1.18� 104 2.02

a Synthesized by melting stoichiometric amounts of Bi2O3 and MoO3.

temperatures were used as photocatalysts. According to theexperimental conditions the curve can be separated in threestages: 1) dispersion of photocatalyst on dye solution with the aidof ultrasound, 2) the establishment of the adsorption–desorptionequilibrium in the darkness and, 3) irradiation of dispersion withvisible light. As was observed on the adsorption–desorption study,a fast equilibrium was reached after 15 min, and then, during thefollowing hour the concentration of dye was maintained constant.When the lamp turns on the concentration of rhB undergoesdepletion. In general all samples were able to degrade rhB,however, the bleaching of dye solution was reached more quicklyfor the sample calcinated at 250 �C. In this sense, whereas forthe sample calcinated at 480 �C was required more than 180 min oflight irradiation to bleach the dye solution, for the sample

λ (nm)

250 350 450 550 650

Ab

so

0.2

0.4

0

rh, 498 nm

N´-ethylated rh,510 nm

Fig. 10. Absorption changes of rhB solution under photocatalytic process whensamples of a-Bi2Mo3O12 obtained at 250 and 300 �C were used as photocatalysts.

A. Martınez-de la Cruz, S. Obregon Alfaro / Solid State Sciences 11 (2009) 829–835 835

calcinated at lowest temperature the bleaching of solution wasreached at 80 min.

This behavior can be associated with the amount of dye adsorbedon the surface of each sample. The adsorption of dye for the series ofsamples calcinated followed the order 250> 300> 400> 480 �C. Inthe same order the samples have showed to be able to bleach thedye solution.

The temporal evolution of spectral changes during the photo-degradation process of rhB by a-Bi2Mo3O12, which was synthesizedat 250 and 300 �C (the best photocatalysts), is displayed in Fig. 10.According to the spectrum evolution, the depletion of the absor-bance happened with a shift of the absorption maximum to blueregion. The continuous color change of the aqueous solution andthe displacement observed in the strong absorption band can bejustified as follows: according to previous works, the rhB photo-degradation follows a mechanism that implies the formation ofsuccessive de-ethylated intermediates which respective maximumabsorption bands are: N,N,N0-triethylated rhodamine, 539 nm;N,N0-diethylated rhodamine, 522 nm; N-ethylated rhodamine,510 nm; and rhodamine, 498 nm [29]. Having these data in mind, itis possible to conclude that after 180 min the four ethylated groupswere removed from the organic dye.

4. Conclusions

Nanoparticles of a-Bi2Mo3O12 were prepared successfully by theconventional co-precipitation method. This route provides thepossibility to obtain a-Bi2Mo3O12 in pure form from 250 �C, i.e.230 �C below to the temperature previously reported for thesynthesis of a-Bi2Mo3O12 by the same route. This situation leads tothe preparation of materials with higher surface area and smallparticle size than those obtained by high temperature reaction.These factors contribute to consider a-Bi2Mo3O12 compound asphotocatalyst in reactions of dye-organic degradation with visiblelight irradiation. The Langmuir adsorption constant was evaluatedfor the samples obtained at 250, 300, 400 and 480 �C and a corre-lation with the temperature of calcination was established. In thesame way, the photocatalytical activity of the materials to thedegradation of rhB was correlated with the amount of dye adsor-bed. In general, all samples synthesized were able to bleach thesolution of rhB through the combination of adsorption andphotocatalysis processes.

Acknowledgements

We wish to thank to Universidad Autonoma de Nuevo Leon(UANL) for its support through the project PAICYT-2007/2008 andto CONACYT for supports the project 81546. We also thank toDepartamento de Ecomateriales y Energıa of Facultad de IngenierıaCivil (UANL) for its assistance on the materials characterization.

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