Synthesis, characterization and photocatalytic properties of lanthanum oxy-carbonate, lanthanum oxide and lanthanum hydroxide nanoparticles

Superlattices and Microstructures 77 (2015) 295–304

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Synthesis, characterization and photocatalytic properties of lanthanum oxy-carbonate, lanthanum oxide and lanthanum hydroxide nanoparticles Mahnaz Ghiasi, Azim Malekzadeh ⇑ School of Chemistry, Damghan University, P.O. Box 36715/364, Damghan, Islamic Republic of Iran

a r t i c l e

i n f o

Article history: Received 2 May 2014 Received in revised form 11 September 2014 Accepted 14 September 2014 Available online 6 October 2014 Keywords: Nanoparticles La2O2CO3 La2O3 Photocatalyst Citric acid Salicylaldehyde

a b s t r a c t A simple thermal decomposition route has been developed to prepare La2O3 and La2O2CO3 nanoparticles. Sonication of La2O3 nanoparticles in water at room temperature is accompanied to the formation of La(OH)3 nanoparticles. The effect of addition of citric acid, as disperser, was also investigated on the phase formation and particle size distribution of the products. It is observed that citric acid has no effect on the particle size of the samples. The prepared nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM) analyses. Photocatalytic activity of the products was examined for degradation of methyl orange, a common reactive dye, as a pollutant under ultraviolet irradiation in the wastewater. The results show that La2O2CO3 nanoparticles are promising materials in this photocatalytic degradation with no significant loss of activity even after four cycles of successive uses. A pseudo-first-order kinetic is obtained for the photocatalytic degradation of methyl orange over La2O2CO3 nanoparticles according to the Langmuir–Hinshelwood analysis. Ó 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel./fax: +98 23 35235431. E-mail addresses: [email protected], [email protected] (A. Malekzadeh). http://dx.doi.org/10.1016/j.spmi.2014.09.027 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.

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1. Introduction Particles of metal oxides that are in the nanometer size regime have attracted significant interests because of their atom-like size dependent properties [1]. Many improved pathways for the synthesis of such nanomaterials with tunable properties have been reported [2,3]. Recently, synthesis of metal oxide nanoparticles with new inorganic precursor has been interested, taking profit of the tools of organometallic chemistry [4–7]. A major interest at the moment is in the development of organometallic or inorganic compounds for preparation of nanoparticles [8, 9]. Using of the novel compound can be useful and open a new way for preparing nanomaterials to control nanocrystal size, shape and distribution size. A reduction in particle size to nanometer scale results in various interesting properties compared with the bulk properties. Having a large surface area, metal oxide and hydroxide nanomaterials show great advantages over conventional materials in many applications. For example, lanthanum oxide has different applications such as synthesis of ferroelectric and optical materials [10]. It has the lowest lattice energy of the rare earth oxides, with very high dielectric constant of 27 [11]. It is widely used in industrial applications and research projects. It shows a p-type semi-conducting property. Its resistivity at ambient temperature is equal to 10 kXcm [12]. Lanthanum oxide is used to make optical glasses, which increases their density, refractive index, and hardness. In combination with oxides of tungsten, tantalum, and thorium, La2O3 improves the resistance of the glass against alkali compounds and is known as one of the ingredients for production of piezoelectric and thermoelectric materials. It is also used as a catalyst for the oxidative coupling of methane [13,14]. Owing to its excellent physical and chemical properties, La(OH)3 has been extensively used as high-potential oxide ceramic, hydrogen storage materials, superconductive materials and, etc [15]. Until quite recently, the catalytic and sorbet properties of La(OH)3 have been concerned intensively for their potential applications [16]. Different metal oxides, hydroxides and carbonates were examined as photocatalysts for wastewater treatment [17–19]. The effluents of textile and dye industries are the main pollutants in wastewater. This causes serious environmental problems such as increase of toxicity of environment, chemical oxygen demand (COD), biochemical oxygen demand (BOD), bad smell, and color of the wastewater [20]. The colored organic dyes are heavily polluted the water system [18]. The complete remediation of these dyes into less harmful chemicals is required to overcome these problems [21]. Among various dye remediation process, the heterogeneous photocatalytic process is well known method for the decomposition of hazardous waste materials especially organic compounds into less harmful chemicals [22]. In general, the semiconducting materials are required to facilitate the heterogeneous photocatalytic reaction. So far, many semiconductor materials such as TiO2, ZnO, Fe2O3, CdS, and ZnS are effectively used as photocatalysts [18,19,23]. The aim of the present work is to prepare and characterize nanocrystals of La2O2CO3, La2O3 and La(OH)3 using an easily obtained precursor; [tris(salicylaldehydeato)Lanthanum(III)]; La(sal)3. This is the first report on the synthesis of La2O3 nanoparticles from La(sal)3. The photocatalytic activities were also evaluated using methyl orange degradation as a model of the organic pollutant in the wastewater under ultraviolet irradiation.

2. Experimental 2.1. Materials and physical measurements All the chemicals and solvents were purchased from Merck and used as received without further purification. The FT-IR spectra of samples were recorded in a Perkin–Elmer FT-IR spectrometer. DR UV–Vis spectra were recorded by an Analytikjena UV–Vis spectrometer. Elemental analyses for C, H and N were performed on a LECO 600 CHN elemental analyzer. The Inductively coupled plasma (ICP) analysis was implemented for La content, using an INTEGRA model of GBC Company. Thermogravimetric-differential thermal analysis (TG-DTA) was carried out using a thermal gravimetric analysis instrument (TG/DTA6300 Japan) with a heating rate of 10 °C/min in the air atmosphere from ambient temperature to 750 °C. A single wave ultrasonic generator (Parsonic 2600s), operating at 28 ± 5% kHz with a maximum power output of 50 W, was used for the ultrasonic irradiation. The ultrasonic generator automatically adjusted the power level. X-ray diffraction patterns of the freshly calcined samples

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were recorded in a Bruker AXS diffractometer D8 ADVANCE with Cu–Ka radiation filtered by a nickel monochromator and operated at 40 kV and 30 mA. Transmission electron microscopy (TEM) image was obtained on a Philips XL30 transmission electron microscope with an accelerating voltage of 100 kV. AFM measurements were performed using an AFM Explorer microscope (DME Model Igloo, Germany). 2.2. Synthesis of precursor complex The precursor complex, i.e. La(sal)3, was prepared according to the procedure described previously (Scheme 1) [6]. The resulting precipitate was collected by filtration and washed with sufficient amount of hot methanol for several times. The final product was sonicated in acetone for 30 min at room temperature. The sample was dried at 50 °C overnight and then under vacuum at 40 °C for 6 h. Anal. Calc. for La(sal)3: C, 50.21; H, 3.01; N, 0; La, 27.66. Found: C, 48.51; H, 3.36; N, 0; La, 25.24. IR (KBr): t = 582 (La@O), 1668 (C@O in salicylaldehyde) and 1652 (C@O in complex). The shift in the frequency of C@O stretching from 1668 to 1652 cm1 confirms the chelation of salicylaldehyde to lanthanum. DR UV–Vis: kmax = 320–460 nm (ligand n?p⁄). The synthesized precursor is stable in air at room temperature and starts to decompose at 278 °C without melting. 2.3. Preparation of La2O2CO3, La2O3 and La(OH)3 nanoparticles The as-prepared complex was completely powdered and calcined stepwise at 400, 700 and 800 °C for 5 h by a heating rate of 10 °C/min from room temperature. The role of citric acid as emulsifier on the phase formation, morphology and particle size of products, was also investigated. Thus, a solution of La(sal)3 in methanol was added to a methanolic solution of citric acid. Mole number of citric acid was selected to be equal of the mole number of precursor complex (Table 1). The sample was sonicated, dried and calcined as before. Lanthanum hydroxide was prepared by sonication of the prepared lanthanum oxide in water for 25 min at room temperature. 2.4. Photocatalysis studies Methyl orange (MO) was used as a model compound to examine the photocatalytic activity of the samples that are prepared in this study. In a typical experiment, 30 mL of a MO aqueous solution (16 ppm) and 12 mg of catalyst were mixed and exposed to photoirradiation. A 15 W Hg arc lamp was used as the UV light source. Any MO was not photolysed in a blank run and in the absence of prepared samples. Prior to irradiation, the reaction suspensions were equilibrated for 30 min in darkness to achieve adsorption–desorption equilibrium of the reactant solution and catalyst particles.

L + La(NO3)3·6H 2O

1 700°C

N(C2H5)3 Methanol Room temperature

La2O 2(CO 3) 800°C

LaL3

(1)

(L = salicylaldehyde)

La2O 3 Sonication water

La(OH )3

Scheme 1. Preparation procedure for the precursor complex and synthesized nanoparticles.

Table 1 Experimental conditions for preparation of the samples.a,b,c

a

Sample No.

1

2

3

4b

5b

6b

7c

Calcination temperature

400

700

800

400

700

800

–

3 g of precursor complex (0.00353 mol) was used for all synthesis. Samples 4, 5 and 6 were prepared in the presence of citric acid as emulsifier. Mole of citric acid was equal of precursor complex, i.e. 0.00353 mol. c It was prepared by the sonication of sample 3 or 6 in water for 25 min at room temperature. b

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3. Results and discussion Thermal properties of the prepared complex in the presence and absence of citric acid were examined by TG analysis coupled with DTA up to 750 °C in air atmosphere (Fig. 1). The overall weight losses are almost the same in both cases. Thus different products are obtained over calcination at 750 °C. In the presence of citric acid, the process of decomposition of La(sal)3 has been divided into seven stages in which the last stage is endothermic. The final product at 750 °C in the presence of citric acid is possibly La2O2CO3 + 0.12 La2(CO3)3. A most complicated degradation of the organic–inorganic fragments is observed in the presence of citric acid. Degradation of organic parts is taken place at lower temperatures in the presence of citric acid. The process of decomposition of La(sal)3 in the absence of citric acid is carried on during five exothermic stages with an overall mass loss of 73.1% (calculated mass loss 71.4%). The final product, La2O3, may have formed above 750 °C. The formation of La2O3 at 800 °C was confirmed by the XRD and FT-IR studies (Figs. 2 and 3). Shown in Fig. 2 is the wide-angle XRD pattern of samples that are reported in Table 1. The XRD data were analyzed using a commercial Xpert package. An amorphous solid is formed upon heat treating of La(sal)3 at 400 °C. All reflection peaks of XRD patterns of samples that are obtained by thermal decomposition at 700 °C could be readily indexed to crystalline phase of La2O2CO3, with lattice parameters that are shown in Table 2. XRD patterns reveal that samples Nos. 2 and 5 are lanthanum oxy-carbonate with mixed monoclinic and hexagonal phases. However, stronger intensities of crystal faces of (1 1 0) (2h = 22.8°) and (130) (2h = 29.5°) show the dominance of monoclinic phase. All reflection peaks of

750

80

500

80

475

70

650

60

550

50

350

40

25

170

315

460

605

Temperature (°C)

100

90 70

450

B

525

60

450

50

425

30

400

20 750

375

90

TG (%)

DTA (μV)

850

550

100

DTA (μV)

A

TG (%)

950

40 30 25

170

315

460

605

20 750

Temperature (°C)

Fig. 1. TG/DTA curves of La(sal)3 complex. (A) in the absence of citric acid, (B) in the presence of citric acid.

Fig. 2. XRD patterns of samples of Table 1. (A) in the absence of citric acid, (B) in the presence of citric acid.

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Fig. 3. FT-IR spectra of samples of Table 1.

Table 2 Crystallographic parameters of La2O2CO3, La2O3 and La(OH)3 nanoparticles.a Sample #

Composition

Crystal system

Space group

a (Å)

b (Å)

c (Å)

c (°)

2, 5 2, 5 3, 6 7

La2O2CO3 La2O2CO3 La2O3 La(OH)3

Hexagonal Monoclinic Hexagonal Hexagonal

P63/mmc – P321 P63/m

4.0755 4.0803 3.9300 6.5286

4.0755 13.5090 3.9300 6.5286

15.9570 4.0720 6.1200 3.8588

120 90 120 120

a Diffraction peaks of X-ray match well with that of the reported X-pert high score PDF code: 00-048-1113 (monoclinic) and 01-084-1963 (hexagonal) for La2O2CO3, 01-074-1144 for La2O3 and 00-036-1481 for La(OH)3.

samples 3 and 6 can be readily indexed to the pure hexagonal phase of La2O3 with space group P321. Any secondary phase is not detected in XRD patterns of samples Nos. 3 and 6, which ensured the phase purity of the final product. The XRD pattern of sample No. 7 confirm the formation of La(OH)3 during sonication of La2O3 in water at room temperature (Fig. 2). From XRD data, the crystallite size of as-prepared samples was calculated using the Scherrer (Eq. (1)) and Williamson–Hall (Eq. (2)) equations (Table 3) [24]. The Williamson–Hall equation also leads to the information about the microstrain (e) in the structure.

D ¼ Kk=b cos h

ð1Þ

b2h cos h 0:94 4e sin h ¼  k D k

ð2Þ

where b is the breadth of the observed diffraction line at its half-intensity maximum, K the so-called shape factor, which usually takes a value of about 0.9, k the wavelength of X-ray source used in XRD, and e the lattice strain Detected phases are observed to be in nanosized scale. The crystallite sizes that are estimated from XRD line profile by Williamson–Hall equation are somewhat larger than ones calculated by the Scherrer formula, which is related to the some strain in the structure of samples [24]. Results show negligible effect of citric acid as disperser on particle size. This can be from the fact that La ions are capped completely with salicylaldehyde ligands, so that the La ions are protected against agglomeration. Thus, the presence of citric acid does not have a characteristic effect on the particles size of the samples that were prepared via the method of this study.

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Nanoparticle

S

W–H

2 3 5 6 7

La2O2CO3 La2O3 La2O2CO3 La2O3 La(OH)3

35 39 34 38 33

50 49 52 47 45

a Calculations were done using Scherrer (S) and Williamson–Hall (W–H) equations. Calculations were done using peaks positioned at 2h° = 13, 23, 31, 34, 40, 41, 45, 46, 48, 50, 52, 54, 61, 75, 79 for La2O2CO3, 2h° = 26, 29, 30, 40, 46, 50, 52, 54, 60, 62, 67, 72, 74, 75, 79 for La2O3 and 2h° = 16, 25, 27, 28, 32, 40, 42, 47, 49, 50, 55, 56, 58, 59, 64, 65, 71, 76, 78 for La(OH)3.

FT-IR spectra were recorded to show the functional groups for every product at each step of the synthesis. The infrared spectra of crystalline samples are shown in Fig. 3. In the spectrum of La2O2CO3, sample No. 5, the absorption bands at 1125 and 1465 cm1 are attributed to the t1 and t3 mode of carbonate anion, respectively [25,26]. Threefold splitting of t1 and t3 indicates the monoclinic type of La2O2CO3 [27]. The XRD analysis confirms the formation of hexagonal and monoclinic phases of La2O2CO3. In the spectrum of La2O2CO3, sample No. 2, the absorption bands at 1125 cm1 are not sharp enough. This could be due to the fact that in La2O2CO3, sample No. 5, which prepared in the presence of citric acid, La2(CO3)3 also exists. This fact confirms the results of TG/DTA analysis. In the spectrum of La2O3 and La(OH)3, samples Nos. 3 and 7, weak and broad absorption bands of carbonate anion are related to the absorbed water and CO2 on the surface of lanthanum oxide and hydroxide. Basic character of lanthanides causes absorption of atmosphere water and carbon dioxide. It has been reported that La2O3 and La(OH)3 are very sensitive to atmospheric conditions [25]. Carbonation occurs during contact to atmospheric carbon dioxide under ambient conditions, leading to the formation of surface carbonates or hydroxyl carbonates. For this reason storing of these compounds away from atmospheric condition is necessary [25,28]. A sharp peak at 3610 cm1 is assigned to the stretching mode of OH (La(OH)3, sample No. 7 in Fig. 3). The sharp peaks at about 460 and 660 cm1 can be attributed to the stretching vibration of the LaAO bond [12,25]. Lanthanum content of the prepared samples was investigated by ICP method (Table 4). In consistence with FT-IR and XRD analyses, results confirm the formation of La2O3, La2O2CO3 and La(OH)3 for samples Nos. 5, 6 and 7, respectively. The TEM photograph of the La2O3 product (sample No. 3) is given in Fig. 4. Histogram of particles size distribution is also included. The average particle size is calculated to be 44 nm. Result is different from the calculated crystallite sizes from XRD line profile (see Table 3) that reveals the irregular shape of nano-particles, observed in TEM micrographs [29,30]. The surface analysis of La2O3 nanoparticles (sample No. 3) was investigated by AFM technique in order to confirm the formation of nanoparticles. AFM measurements were performed in air and at room temperature, with the tip radius less than 10 nm. The processing was conducted using the SPMLab software. Results are related to a dispersed sample in ethanol. Fig. 5A–D show the 2D, 3D, line profile and image of height distribution from AFM image of La2O3 nanoparticles, respectively. In Fig. 5A and B single as well as accumulated particles are clearly visible. From Fig. 5B different growth of nanoparticles along the Z-axis can be concluded. According to line profile, the particle size of La2O3, sample No. 3, is 43 nm (Fig. 5C). From Fig. 5D, the height of the particles, which is a good estimate of the Table 4 Lanthanum content of the prepared samples by ICP method. Sample #

Sample name

Calc.

Exp.

5 6 7

La2O3 La2O2CO3 La(OH)3

85.27 75.12 73.14

83.67 73.21 70.78

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% Counts

     















Particle size (nm) Fig. 4. TEM micrograph and size distribution histogram of La2O3 nanoparticles, sample No. 3.

Fig. 5. AFM topography images, A (2D), B (3D), C (line profile), and D (image of height distribution) of La2O3 nanoparticles, sample No. 3.

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particle size, can be determined. The calculated size of La2O3, sample No. 3, obtained from the image of height distribution is 55 nm. Photoactivity of prepared materials, except La2O3 which converts into La(OH)3 in water solution, for the photodegradation of aqueous solution of Methyl orange (MO) under UV irradiation was evaluated (Fig. 6). In the absence of catalyst no MO degradation is obtained (data not shown). It is observed that La(OH)3 nanoparticles and La(sal)3 complex are not suitable materials for MO degradation under the way that was carried on in this study. Fig. 7A shows the time-resolved absorbance profiles in the wavelength range of 300–600 nm for a UV irradiated MO solution in the presence of La2O2CO3 nanoparticles. More and more MO is photolysed over La2O2CO3 catalyst by time and absorption peak with a kmax at 470 nm is disappeared after 36 h. This is a considerable photocatalytic activity under the weak UV lamp (15 W) that is used in this study. The photocatalytic degradation of MO over La2O2CO3 catalyst, obeys the pseudo-first-order kinetics in terms of modified Langmuir–Hinshelwood (L–H) model [31].

r¼

dC kapp C ¼ dt 1 þ KC

ð3Þ

where ‘‘r’’ is the photodegradation rate, ‘‘kapp’’ is the reaction rate constant, ‘‘K’’ is the equilibrium adsorption coefficient of the reactant, ‘‘t’’ is the irradiation time and ‘‘C’’ is the concentration of the reactant at time ‘‘t’’. When ‘‘C’’ is very small, Eq. (3) can be written as follow

r¼

dC ¼ kapp C dt

ð4Þ

where ‘‘kapp’’ is the pseudo-first-order rate constant. During the photocatalytic process, the MO adsorbs over La2O2CO3 surface and an equilibrium of adsorption–desorption is reached after 30 min. Thus, an equilibrium concentration of the MO solution was used as the initial dye concentration for the kinetic analysis (C0). Integration of Eq. (4) with the limit of C = C0 at t = 0 and C = Ct at time ‘‘t’’ gives:

ln

C0 ¼ kapp t C

ð5Þ

According to Eq. (5), the plot of Ln(C0/C) versus ‘‘t’’ is linear. Thus, the value of kapp can be obtained directly via its slope (Fig. 7B). In this case kapp for La2O2CO3 photocatalyst is estimated to be

Fig. 6. Time resolved absorbance profile of a UV irradiated MO solution in the presence of La2O2CO3 nanoparticles.

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303

Fig. 7. (A) Time-resolved absorbance profiles of an irradiated MO solution in the presence of La2O2CO3 nanoparticles. (B) Kinetic of the photodegradation over La2O2CO3 nanoparticles.

1.12  103 min1. The reaction rate constant is not high due to the low power of the UV light. Some other experiments were carried out without pre-equilibrium in dark in which the catalyst was directly mixed with the MO solution and irradiated under the UV light in different time intervals. The results are similar with and without pre-equilibrium conditions (data not shown). It can be related to the long time of the photocatalytic reaction. Estimating the reusability of the catalyst is a necessity for evaluation of its practical applications. Thus, further set of experiments were carried out to examine the reusability of the La2O2CO3 catalyst. The catalyst was separated after each run by filtration. After separation, the catalyst was washed with acetone followed by diethyl ether and dried at 100 °C for 2 h before using for the next cycle. The results showed no significant loss of activity after four cycles.

4. Conclusion Nanoparticles of La2O2CO3, La2O3 and La(OH)3 were synthesized by means of a novel and simple method. The effect of citric acid as emulsifier on the particle size and thermal properties of the obtained products was negligible, confirming emulsifier free advantage. Small amounts of La2(CO3)3 are also formed in the presence of citric acid. Thermal decomposition of precursor at 700 °C for 5 h leads to the formation of lanthanum oxy-carbonate (La2O2CO3) with average size of 35 nm. Lanthanum oxide nanoparticles were prepared through a facile thermal treatment of La2O2CO3. A simple path was also introduced for preparation of La(OH)3 nanoparticles; sonication of La2O3 in water for short period of time. Furthermore, it suggests as a general principle that other inorganic decomposition precursors may be very good choices for the synthesis of nano metal oxides. The photocatalytic behavior of synthesized samples was evaluated using the degradation of a methyl orange aqueous solution under ultraviolet light irradiation. The results show that lanthanum oxy-carbonate nanoparticle is a promising material in photocatalytic applications such as wastewater purification. As photocatalysis makes use of sunlight or UV radiation, the technology is inexpensive, environmentally friendly and can be applied worldwide. References [1] H. Bojari, A. Malekzadeh, M. Ghiasi, Facile synthesis and characterization of monocrystalline cubic ZrO2.12 nanoparticles, J. Clust. Sci. 25 (2014) 387–395. [2] J.A. Villoria, M.C. Alvarez-Galvana, S.M. Al-Zahrani, P. Palmisano, S. Specchia, V. Specchia, Oxidative reforming of diesel fuel over LaCoO3 perovskite derived catalysts: Influence of perovskite synthesis method on catalyst properties and performance, Appl. Catal. B 105 (2011) 276–288.

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