Understanding the photophysical and surface properties of TiO2–Al2O3 nanocomposites

Materials Letters 107 (2013) 10–13

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Understanding the photophysical and surface properties of TiO2–Al2O3 nanocomposites Soo-Wohn Lee a, F. Paraguay-Delgado b, R.D. Arizabalo c, R. Gómez d, V. Rodríguez-González e,n a

Department of Environmental Engineering, Sun Moon University, Asan, Republic of Korea Departamento de Materiales Nanoestructurados, Centro de Investigación en Materiales Avanzados, Miguel de Cervantes 120 Chihuahua, 31109 Chih., México c Dirección de Investigación y Posgrado, Instituto Mexicano del Petróleo, México, D.F., 07730, México d Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, México, D.F., 09340, México e División de Materiales Avanzados. IPICYT, Instituto Potosino de Investigación Científica y Tecnológica, Camino a la Presa San José 2055, Lomas 4a. sección C. P. 78216, San Luis Potosí, S.L.P., México b

art ic l e i nf o

a b s t r a c t

Article history: Received 15 April 2013 Accepted 15 May 2013 Available online 23 May 2013

TiO2–Al2O3 nanocomposites (2–50 wt% Al2O3) were synthesized by the sol–gel process, simultaneously gelling aluminum and titanium alkoxide. Focusing on surface phenomena and thermal stability of synthesized nanocomposites, this research describes the way by which alumina binds to the TiO2 anatase phase and how the new framework modifies the photophysical and surface properties to achieve complete 2,4 Dichlorophenoxiacetic acid (2,4-D) degradation with t1/2 ¼38.3 min−1. The nanocomposite with 5 wt% of Al2O3 shows optimum textural, adsorption abilities, and photocatalytic properties. & 2013 Elsevier B.V. All rights reserved.

Keywords: TiO2–Al2O3 semiconductors Herbicide photodegradation Fractal dimension Adsorption abilities

1. Introduction Nanocomposite oxides are becoming great importance since semiconductor-high surface area composites frequently exhibit unexpected hybrid properties synergistically derived from the two components [1]. TiO2 is an efficient photocatalytic semiconductor with wide band-gap energy and remarkable photophysical properties, despite its low surface area. Its potential as a photocatalyst is directly related to its structural, physicochemical properties, and rutile impurity [2,3]. A high specific surface area of TiO2–Al2O3 oxides can enhance their photocatalytic activity by improving the surface contact between the TiO2 semiconductor and Al2O3 [3–9]. It has been reported that the presence of nanostructured TiO2 in mixed oxides promotes degradation of organic pollutants mostly because the electron–hole pair separation is diminished [1,5,8,10]. On the other hand, the alumina gamma phase combined with other metallic oxides was widely used in hydrodesulphurization, combustion processes, and reduction of NOx [9–17]. Their activity has been attributed to high specific surface area (300 m2/g), mesoporosity, thermal stability,

n

Corresponding author. Tel.: +52 444 834 2000x7295. E-mail addresses: [email protected], [email protected] (V. Rodríguez-González). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.061

and acid–base properties [11–19]. The use of the sol–gel route to synthesize TiO2-based oxides leads to the formation of structural defects that are evident as mesoporosity, high specific surface area as well as important surface defects of the interfacial alumina– titania moieties [3,6,11–15]. In the present work, the sol–gel synthesis and characterization of titania–alumina oxides are reported. The aim of this work is to evaluate the synergic surface abilities and textural properties of nanostructured TiO2–Al2O3 in the UV-degradation of the herbicide 2,4-D.

2. Experimental The TiO2–Al2O3 oxides were prepared by adding simultaneously titanium (IV) (Aldrich 97%) and aluminum (Aldrich 98%) alkoxides to an aqueous solution of n-butanol. The alkoxide/nbutanol/water molar ratio was 1/3/8 [20]. The co-gelled aluminum alkoxide quantity was calculated to provide 2, 5, 10, 15, and 50 wt% Al2O3 in the TiO2. The sol–gel was maintained under reflux at 70 1C for 48 h. Then, the solvents evaporated in a vacuum at 80 1C. Afterwards, the solids were dried in air at 100 1C for 12 h, and then annealing at 500 1C for 4 h using a heating rate of 2 1C/min. The resulting samples were labeled as TA–X–Y, where X denotes TiO2 content, and Y Al2O3 content (Table 1).

S.-W. Lee et al. / Materials Letters 107 (2013) 10–13

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Table 1 Physicochemical characteristics and kinetic parameters of TiO2–Al2O3 oxide semiconductors. Oxide

Cristallite size (nm)

SBET (m2/g)

Pore size (nm)

Dsa

Eg (eV)

Aa (%)

Ca (%)

t1/2 (min)

K (min−1)

TiO2 barea TA-98-2 TA-95-5 TA-90-10 TA-85-15 TA-50-50

24.4 9.6 10.7 7.3 8.2 8.3

65 132 149 238 149 315

13 3.7 3.7 1.8 3.8 3.7

– 2.52 2.53 2.52 2.51 2.47

3.3 3.23 3.19 3.24 3.17 3.24

4 4 0 0 19 450

60 69 92 95 79 –

100 90 44 38 71 –

0.0067 0.0085 0.0144 0.0169 0.0086 –

Valeus taken from Rodriguez-Gonzalez et al. [20], Ds: Surface fractal dimension ; A (%): % adsorption ; and C (%): % Conversion.

The thermal evolution of the gels was followed by TGA–DSC–FTIR under dry air flow at a heating rate of 10 1C/min in a DSC–TG-system (Netzsch–Bruker IFS 66v/S). The specific surface areas were obtained from N2 adsorption isotherms at −196 1C using a Quantachrome 3B sorptometer. The BET method was used to calculate the specific surface area, while the BJH method was used to estimate the pore size distribution. The surface fractal dimension was calculated from the N2 isotherms using the Avnir–Jaroniec method [21].The ultraviolet–visible (UV–vis-RDS) spectra were obtained with a Varian Cary 100 spectrophotometer. The XRD powder patterns were recorded on a Brucker D8 Advance diffractometer using monochromatic CuKα radiation (λ¼1.54 Ǻ1. The oxide nanostructures and morphologies were determined by transmission electron microscopy (TEM) using a JEM2200FS JEOL microscope with spherical aberration correction in a STEM mode working at 200 KeV. The elemental composition was determined by energy dispersive spectroscopy (EDS) Oxford, in which the qualitative elemental analysis was made in the STEM mode. The photocatalytic degradation of 2,4-D (Aldrich, 98%) was performed with a Pen-Ray UV lamp (UVP Products, intensity of 4400 μW/cm2 at 254 nm). The lamp was immersed in a cooled vessel slurry home-made reactor containing 150 mL with 40 ppm of 2,4-D and 150 mg of catalyst at room temperature. Before turning on the light source to assure the adsorption–desorption equilibrium of 2,4-D on TiO2–Al2O3, dry air was bubbled for 30 min (1 mL s−1) into the suspension under magnetic stirring. The photodegradation rate was determined by a UV–visible spectrophotometer at 229 nm as a function of time. Estimated errors did not exceed 3.0%. To know the 2,4-D adsorption on the oxides, the composites were monitored in identical conditions without UV irradiation and adsorption equilibrium time.

3. Results and discussion The XRD patterns of selected TiO2–Al2O3 oxides are shown in Fig. 1, where anatase is the principal TiO2 crystalline phase. At low Al2O3 content (2≤wt% o15), anatase is the only detectable phase. The peak corresponding to the (101) anatase reflection (25.418 of 2θ; JCPDF 070–6026) is slightly broad for the sample with the highest alumina content (50%). The TG and DSC profiles are also shown in Fig. 1. The TG analysis indicates that the decomposition of the precursors takes place in two steps below 500 1C. A total weight loss (27%) is observed in this range. The solids exhibited weight loss from 50 to 200 1C, attributed to solvent evaporation. The combustion of residual organic precursors is indicated by exothermic peaks at 240, 260 and 376 1C for the TA-98-2, TA-5050 and TA-90-10 samples, respectively. The DSC peak intensity indicates indirectly TiO2 crystallization. The crystallite size was calculated employing the full width at half-maximum (FWHM) of the (101) 2θ anatase peak at 25.21, using Scherrer's equation: D¼ kλ/β cos θ, where D is the crystallite size, k is a constant (k ¼0.89), λ is the CuKα X-ray wavelength (1.5406 Å) and β is the FWHM of the 2θ peak; these values are

A: Anatase B: Brokite

Intensity (a. u.)

a

TA-95-5

TA-85-15 TA-50-50

B

10

20

30

40

50

60

70

2 Theta Fig. 1. XRD patterns for the selected sol–gel TiO2–Al2O3 materials. The inset shows TG–DSC profiles of the thermal evolution for dry samples, indicating the temperature of phase transition: (a) TA-98-2, (b) TA-50-50, and c) TA-90-10.

listed in Table 1. The average TiO2 crystallite size ranged from 7.3 to 10.7 nm, where the smallest crystallite sizes correspond to samples with the highest Al2O3 content. Alumina incorporation restricts the TiO2 anatase crystallite growth. Normally, a well crystallized, commercially available TiO2 exhibits a crystallite size of  27 nm. High SBET (132–315 m2/g) for all the solids were obtained, see Table 1. The highest SBET area is observed with the TA-50-50 (315 m2/g) sample. For the TA-90-10 sample (SBET area of 238 m2/g), Al2O3 incorporation provides thermal stability to the TiO2– Al2O3 nanocomposite. The sample TA-90-10 annealed at 600 1C also shows the anatase as the main phase crystalline of TiO2 and SBET area of 217 m2/g. As for the TiO2–Al2O3 oxides prepared by a hybrid the sol–gel route using boehmite as alumina precursor and titanium isopropoxide as titania precursor, high SBET have also been reported (291 m2/g) [11]. The alumina–titania synthesis in an ionic liquid via the sol-gel method with alkoxide produces assynthesized alumina–titania of 486 m2/g [12]. In Fig. 2, selected N2 isotherms are presented. They correspond to type 5 in the BDDT classification with a type E hysteresis loop that agrees with the De Boer classification for mesoporous materials [22]. In addition, the surface fractal dimension Ds (2.45–2.50) calculated from N2 adsorption isotherms shows that these semiconductors have significant roughness. The surface fractal dimension (Ds) describes the irregularity and roughness of the surface oxides in the 2≤Ds o 3 range. The materials synthesized in the present work have rough and irregular surfaces, as shown in Table 1. The band-gap energies (Eg) were determined from the UV–vis spectra of these composites. The values ranging from 3.17 to 3.24 eV. These results suggest that the alumina’s primary role is the TiO2 dispersion. It has been reported that when the TiO2 framework is doped with another oxide (In2O3, NiO), the Eg values are shifted either to the blue UV or visible regions, depending on

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the type of dopant and the crystallite size [9,20,23]. In this case, since Al2O3 is not a semiconductor, then the Eg values are unchanged; however, the crystallite size decreased almost three times, 8 nm on average. The nanocomposite microstructure consisted of TiO2–Al2O3 conglomerates formed from fine-grained crystalline particles, Fig. 3a. The sizes of the conglomerates are shown in Fig. 3b. The sizes are 362 7140 nm for TA-95-5C. To confirm the presence of highly dispersed Al2O3 particles on titania, localized elemental distributions at different positions were analyzed in a STEM mode 500 500 °C a

Volume (cc/g)

400

b

300

200

c d

100

0 0

0.2

0.4

0.6

0.8

1

P/Po Fig. 2. Absorption–desorption N2 isotherms of the TiO2–Al2O3 materials annealed at 500 1C: (a) TA-50-50, (b) TA-90-10, (c) TA-85-15, and (d) TA-98-2.

using EDS, Fig. 3c. The three elements present in the material from inner-shell ionization edges are Al (K4), O (M16), and Ti (L23). The values are shown in Table S1, where there is uniformly dispersed alumina. The elemental analysis of the TA-95-5 composite shows that Al2O3 is highly dispersed on titania. A HRTEM image of the TA-95-5 composite is shown in Fig. 3d, where TiO2 nanoparticles of  11 nm are seen. The crystalline structure can also be observed (Fig. 3d). The lattice fringes for the TiO2 anatase phase with the adjacent fringe spacing of about 0.35 nm (101) can be observed. The particle size distributions are 11 72.4 nm and 10 71.9 nm for the TA-95-5 and TA-85-15 compounds, respectively. A slight decrease in the particle size when the alumina content is increased can be observed, which correlates well with the specific surface area values reported in Table 1. It is possible that alumina be linked to the TiO2 surface and inhibits the size of the anatase crystallites. In the case of the formation of brookite, it is also directly related with the small crystalline size and confirms the alumina inhibition of anatase phase in the samples with more that 15 wt% of alumina incorporation [24]. The adsorption tests of the 2,4-D herbicide on the TiO2–Al2O3 oxides showed that SBET is an important factor to promote the 2,4-D adsorption during the reaction. A SBET greater than 150 m2/g favors the herbicide adsorption. When the alumina content increases to 10 wt%, adsorption reaches almost 20% of 2,4-D adsorbed on the oxide surface. In the decomposition of 2,4-D, all the catalysts followed pseudo-firstorder kinetics and the apparent rate constant was calculated by plotting ln(C0/C) as a function of time. The corresponding activities are reported as t1/2 in Table 1. The photocatalytic decomposition of 2,4D using TA-98-2, TA-95-5 and TA-90-10 nanocomposites is obtained

Fig. 3. (a) STEM images of the TA-95 material from conglomerate of particles, (b) particle size distribution measured from micrographs (c) qualitative elemental analysis positions by EDS. The obtained values are reported in Table S1. (d) STEM micrograph obtained by HAADF detector clearly showing lattice spaces.

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after 180-min irradiation with photodecomposition percentages of 92, 95 and 79, respectively. The velocity constant is reported in Table 1; the low-Al2O3-content samples show the best velocities. At contents 45 wt%, adsorption governs, due to the coexistence of the phases anatase, brookite and gamma alumina that diminishes the crystallinity and diversifies the surface phenomena resulting in a high adsorption of 2,4-D. The generation of vacancies and the wide bang-gap of brookite also may favor the recombination of charges during the photocatalytic process. The graphical abstract shows the half time life as a function of the SBET of the materials; on the graph left side, we find undoped materials that have the lowest SBET, and the reaction t1/2 diminishes until finding a minimum at 150 m2/g, which corresponds to 30 min (TA-95-5); then the photodecomposition t1/2 increases as the SBET increases, showing a dominant adsorption rather than degradation. Synergic effects between Al2O3 and TiO2 occur at alumina contents from 2 to 5 wt% range. 4. Conclusion The TiO2–Al2O3 nanocomposites improve surface phenomena showing high specific surface areas ≥100 m2/g, which may promote and stabilize the crystallization of the TiO2 anatase phase, and provide solids with evident surface roughness. The mesoporosity in the new oxide diversify the surface active sites and defects without changing the band-gap. The electron is trapped by generating hole–Al3+ and surface adsorbed O2 to diminish recombination and generate various reactive oxidative species having better photocatalytic performance in then UV 2,4-D decomposition, and enabling to know the optimal surface area required to limit the organic molecule adsorption. The nanocomposite with 5 wt% Al2O3 shows optimum textural, adsorption abilities, and photocatalytic properties.

supported by the GRL Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (grant number 2010-00339).

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.05.061.

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Acknowledgments The authors thank both the National Institute for Nanotechnology at CIMAV—Chihuahua Mexico for TEM support provided in the study, and C. Ornelas for his help. This research was partially

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