Powders of iron(III)-doped titanium dioxide obtained by direct way from a natural ilmenite

Powder Technology 302 (2016) 254–260

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Powders of iron(III)-doped titanium dioxide obtained by direct way from a natural ilmenite Juan A. Torres-Luna, Nancy R. Sanabria, José G. Carriazo ⁎ Estado Sólido y Catálisis Ambiental (ESCA), Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Carrera 30 # 45-03, Bogotá, Colombia

a r t i c l e

i n f o

Article history: Received 20 February 2016 Received in revised form 22 August 2016 Accepted 24 August 2016 Available online 25 August 2016 Keywords: Titanium dioxide Ilmenite Iron-doped titanium dioxide Photocatalyst Synthesis of TiO2

a b s t r a c t Powders of porous titanium dioxide doped with iron (III) ions having several iron contents were synthesized, and an ilmenite as source of iron and titanium was used. The synthesis was directly performed starting from a natural ilmenite under conditions (temperature and acid concentrations) softer than those frequently used throughout the sulfate process. The powders were characterized by X-ray diffraction, FTIR spectroscopy, scanning electron microscopy (SEM), nitrogen adsorption at 77 K and Ultraviolet-visible spectroscopy (UV–vis) in diffuse reflectance mode. A unique crystalline phase (anatase structure) was identified in the new obtained solids. All the synthesized powders showed an important shift for the band gap energy to the visible region. This energy value was decreased by increasing the iron content in the solids. The results obtained confirm that the synthesis pathway was successful to produce iron(III)-doped titanium dioxide with potential properties to be used as a photocatalyst. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Titanium dioxide (TiO2) is a semiconductor widely used in different fields of recent interest such as ceramics glazes, pigments, inorganic membranes, sensors, solar cells, anti-ultraviolet products, and photocatalysts, among others [1–3]. Recently, the importance of this particular oxide has increased exponentially due to its photosensitivity and photocatalytic activity [4]. Anatase, rutile and brookite are known crystal structures (polymorphs) of TiO2, which form a solid with excellent catalytic properties on the degradation of pollutants found in air or aqueous media, becoming an attractive strategy to solving environmental problems [5,6]. Nevertheless, bare anatase has low quantum efficiency due to the electron-hole recombination phenomena [7], leading to a detrimental effect on the photocatalytic performance. Furthermore, anatase is only active under UV radiation (λ b 390 nm) [8] and, thus, it does not allow using the huge potential of (visible photocatalysis) solar energy [7]. Several strategies have been suggested to improve the photocatalytic activity of this semiconductor [2,9], among them the use of doping with transition metal ions [3,10,11] is highly interesting because it not only slows down the electron-hole recombination but also decreases the band gap energy [12,13]. However, the favorable effect of metal ion dopants on the photocatalytic activity of TiO2 is a complex issue, and the understanding of its mechanism is controversial [10]. ⁎ Corresponding author at: Carrera 30 No. 45-03, Ciudad Universitaria, Edificio 451, oficina 109, Bogotá, Colombia. E-mail addresses: [email protected] (J.A. Torres-Luna), [email protected] (N.R. Sanabria), [email protected] (J.G. Carriazo).

http://dx.doi.org/10.1016/j.powtec.2016.08.056 0032-5910/© 2016 Elsevier B.V. All rights reserved.

Photoactivity of doped-TiO2 substantially depends on several factors such as the nature and concentration of ion dopants, thermal treatment, and thus on the preparation method [8,14]. Many scientific works show the study of the effect of different cations used as TiO2 dopants and, besides, the Fe(III)-TiO2 (titanium dioxide doped with iron(III) ions) has shown to be a system with higher potential application as photocatalyst [1,3,15–18] whose photoactivity is improved according to certain optimal content of Fe3+ in the solid (matrix) [1]. The synthesis of Fe(III)-TiO2 can be carried out by several ways, among them the sol-gel method, impregnation, precipitation, hydrothermal, flame spray pyrolysis [1,14], and ultra sound assisted methods [13]. These methods generally accomplish the synthesis of Fe(III)-TiO2 by using iron (III) salts (nitrates or chlorides) and either titanium tetrachloride (TiCl4) or titanium alkoxides (including titanium tetraisopropoxide and tetrabutyl orthotitanate). These titanium precursors are expensive and risky. Thus, it is desirable to use inexpensive, abundant, and simple handling precursors. For that reason, the use of ilmenite as a source of both Ti4+ and Fe3+ species is suggested in the present work. The ilmenite ore is the main sources for production of metallic titanium and titanium containing compounds [19]. Ilmenite (FeTiO3 or FeO·TiO2) is a natural mineral with hexagonal structure, frequently found in igneous rocks and black sands of beaches [20,21]. The ilmenite structure is similar to that of hematite but with some distortion in the oxygen layers. Along the triad axis, pairs of Ti4+ ions alternate with pairs of Fe2+ ions; thus each cation layer is a mixture of Fe2+ and Ti4+ [22]. It is one of the major titanium containing minerals from which titanium dioxide is produced [22,23]. TiO2 contents in ilmenite ores vary according to the geological origin but, in general,

J.A. Torres-Luna et al. / Powder Technology 302 (2016) 254–260

they are lower than 65% [24]. This material supplies about 91% of the world demand for titanium minerals, and its world exploitation in 2009 reached 5.19 millions of metric tons [24,25]. World reserves are higher than 680 millions of metric tons, with the largest reserves located in China, Australia, Norway, India, Brazil, The United States of America, and Canada [25]. Colombia has important deposits of ilmenite at different sites of the national territory [26,27]. High quality TiO2 is commercially obtained from the natural mineral by two ways: the chloride process and the sulfate process [23,28–30]. These processes are different on both the chemical treatment and those requirements of the starting mineral [24]. Although the chloride process has some advantages on the sulfate process, the first procedure requires ores with titanium content between 90 and 95% [24], which makes the sulfate process be the one commercially preferred [31]. Other processes of minor importance use carbothermic reduction of iron into metallic or ferrous state followed by acid or ammoniacal leaching in order to extract TiO2 [4]. Throughout the sulfate process, finely ground ilmenite is subjected to very hard reaction conditions (highly concentrated sulfuric acid and 150 °C) to be digested and produce hydrolyzed titanium species that finally lead to a precipitate which, after purification and calcination, produces TiO2 [28]. Taking into account the concentrated acid used and the quantities of iron sulfate produced in the sulfate process, it is desirable to carry out the process at lower temperature making use of the iron contents. The synthesis of Fe(III)-TiO2 (to be used as photocatalyst) by low cost synthetic ways is a challenge because of its environmental application. Consequently, the use of ilmenite is very interesting because under controlled conditions it can be employed as a simultaneous source of both titanium dioxide and iron(III), and thus this natural mineral may provide a material based on TiO2 directly doped with iron (Fe3 +). Therefore, the aim of this work is to synthesize iron(III)-doped anatase (Fe(III)-TiO2) by a direct pathway starting from a natural mineral (ilmenite), using chemical conditions softer than those traditionally used for the commercial titanium dioxide. This work improves the preparation of iron(III)-doped titania straightforwardly from natural ores. 2. Materials and methods 2.1. Materials Ilmenite (FeTiO3) extracted in Mitú town (Vaupés, Colombia), and supplied by the Colombian Geological Service, was used as the raw material. This natural mineral was purified by magnetic separation in order to discard the sand particles and other non-magnetic solid impurities. Sulfuric acid (Merck, 98%), iron powder (J. T. Baker), and sodium hydroxide (Mallinckrodt, 99.0%) were used as reagents. 2.2. Synthesis of solids A series of solids based on iron(III)-titanium dioxide (Fe(III)-TiO2) was synthesized by direct acidic extraction of iron-titanium species that subsequently were precipitated and separated. Sulfuric acid concentrations and temperature values were lower than those typically used in the sulfate process. Several experiments were performed using a constant mass of ilmenite (18.0 g) and a volume of 32.0 mL sulfuric acid solution with different concentration values (20%, 50%, 80%). Reaction temperature was varied at three different values (50, 70, 110 °C), and the reaction time was fixed at 90 min. The extraction process was carried out in a conventional experimental setup, which is comprised of a 500 mL three-neck round-bottom flask provided with a reflux column connected to a gas trap, allowing reactant addition and temperature control through different necks. The reaction medium was magnetically stirred and its temperature was controlled by a thermocouple, using a stirring hotplate (IKA C-MAG HS 7). Involved reactions in the extraction process generate titanium(IV) oxysulfate (TiOSO4) and iron(II) sulfate (FeSO4) as byproducts [28,29]:

255

FeTiO3 + 2H2SO4 → TiOSO4 + FeSO4 + 2H2O Ferric ions contained in solution were partially reduced into ferrous ions by adding iron powder to excess and, then, the resulting solution was cooled (10 °C) in order to remove iron(II) sulfate heptahydrate (FeSO4·7H2O) crystals [28,29]. Solid residues were removed by filtration/decantation, and the remained solution (containing iron(III) and titanium polyhydroxycationic species, such as [FexTi(8 − x)O8(OH)12 (H2O)m]n+, where n = 4 − x [32]) was adjusted at pH = 1.2 using 1 M sodium hydroxide. The solution was refluxed for 1 h at atmospheric pressure, and the precipitate (solid) was separated, washed with deionized water, dried at 60 °C in an oven and then calcined at 400 °C (with static air atmosphere) for 2 h. Finally, these solids were ground and passed through a 100 ASTM mesh. The new obtained materials were iron-titanium oxidic species, named according to the extraction conditions (temperature and acid concentration). Thus, P50–70 and P50– 110 are those materials synthesized using a 50% sulfuric acid solution and either 70 or 110 °C, respectively. 2.3. Characterization of solids Chemical analysis of solids was carried out by X-ray fluorescence (XRF), using a MagixPro PW-2440 Philips spectrometer with a rhodium tube and a maximum power of 4 kW. X-ray powder diffraction profiles were recorded using a Panalytical X'Pert PRO MPD equipment with copper anticathode (Cu Kα radiation: λ = 1.54056 Å). All diffractograms were taken at room temperature, with 0.02 °2θ step size and 10 s step time. IR analyses were performed with a NICOLET Thermo Scientific iS10 spectrometer, making KBr thin tablets (2 mg of sample in 200 mg of KBr). SEM micrographs were obtained in a microscope FEI Quanta 200, taking several images at different points of the solids. Textural properties of the solids were determined from nitrogen adsorption isotherms taken at 77 K (Micromeritics ASAP 2020) in a relative pressure (P/P0) range between 1 × 10− 5 and 0.99. Samples were previously outgassed at 90 °C for 1 h and then at 350 °C for 4 h. The BET model was used to calculate the surface areas (SBET) in the pressure range 0.05 ≤ P/P0 ≤ 0.35. Total pore volumes were determined by the Gurvitsch method [33], while pore size distributions were calculated with the BJH (Barrett, Joyner and Halenda) model. Band gap energies (Eg) of the solids were determined from diffuse reflectance ultraviolet-visible spectroscopy (Uv-vis/DRS) data. A spectrophotometer VARIAN Cary 5000 with diffuse reflectance accessory and barium sulfate as reference material were used. 3. Results and discussion Chemical composition of ilmenite (starting material) showed an important content of titanium (27.52% = 5.75 × 10−3 mol Ti/g of ilmenite) and iron (34.61% = 6.20 × 10−3 mol Fe/g of ilmenite) with small content of impurities of aluminum and silicon oxides (0.70% of silicon and 0.32% of aluminum). Similar composition has been observed in other ilmenites [19,22]. All the titanium and iron contents of synthesized Table 1 Chemical composition of the synthesized solids (quantities expressed as moles were calculated to 1 g of solid). Solid P80–50 P50–50 P20–50 P80–70 P50–70 P20–70 P80–110 P50–110 P20–110 a

Fe (%) 5.45 2.85 0.37 2.57 0.99 0.30 0.50 0.24 0.71

Ti (%) 41.96 39.47 49.34 44.16 42.49 53.65 57.00 57.67 46.99

S (%) 2.11 1.62 2.00 1.82 1.65 1.33 2.33 2.02 1.09

Fe (mol)

Molar % Fea

Ti (mol) −4

9.77 × 10 5.10 × 10−4 6.67 × 10−5 4.61 × 10−4 1.77 × 10−4 5.29 × 10−5 9.05 × 10−5 4.29 × 10−5 1.28 × 10−4

−3

8.76 × 10 8.24 × 10−3 1.03 × 10−2 9.22 × 10−3 8.87 × 10−3 1.12 × 10−2 1.19 × 10−2 1.20 × 10−2 9.81 × 10−3

Molar percentage was calculated as 100(mol Fe)/(mol Fe + mol Ti).

10.03 5.83 0.64 4.76 1.96 0.47 0.75 0.35 1.29

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materials are compared in Table 1. It is observed that, at constant extraction temperature (50 and 70 °C), the iron content fell down when the concentration of sulfuric acid was decreased. Furthermore, when maintaining constant the concentration of sulfuric acid at 80 and 50%, and varying the extraction temperature, an increase of iron content was observed when temperature was decreased. This indicates that the quantity of iron incorporated in the solids may be adjusted varying both temperature and acid concentration. On the other hand, high quantities of titanium were extracted under all experimental conditions used (Table 1), as consequence of a successful extraction of TiO2 from ilmenite. However, the highest contents of titanium can be observed in those solids synthesized at higher temperature (110 °C) and both 50 and 80% of sulfuric acid. Finally, Table 1 reveals small quantities of sulfur remained in the synthesized solids as result of the sulfate process used here. X-ray diffraction profile of ilmenite (Fig. 1) showed the characteristic peaks of iron titanate (°2θ = 23.79; 32.47; 35.24; 48.70 and 53.93) [34–36]. XRD patterns of the synthesized solids indicated a unique crystalline phase corresponding to anatase structure. In fact, a comparison of XRD profiles of the synthesized solids with that of commercial TiO2 (Evonik P25 used as reference), which has anatase phase accompanied with small quantities of rutile (°2θ = 27.5; 36.2), clearly shows (° 2θ = 25.3; 37.8; 48.1; 54.4) anatase peaks (Fig. 1). Although iron was detected and quantified by X-ray fluorescence, no signal of iron oxide was observed, which may be a result of the effective isomorphic substitution of Fe3+ ions in the anatase structure [7,37]. This isomorphic substitution is possible according to the Hume-Rothery rules [38]. The high similarity between ionic radii (0.74 Å for Ti4 + and 0.78 Å for Fe3 +) makes difficult for any shift of XRD peak positions to be observed [18]. However, the possibility of finding small contents of amorphous iron oxide in the synthesized solids is not fully denied. On the other hand, according to literature data, the broadening of peaks and their intensity decrease (Fig. 1) can be associated with iron (Fe3 +) incorporation in the anatase (TiO2) framework [1,11,13,39]. This incorporation may cause a lattice energy variation of the substituted anatase, leading to a more difficult crystallization of solids [18]. Recently, using XPS (X-ray photoelectron spectroscopy) analysis, the oxidation state of iron has been confirmed when this ion (Fe3+) was incorporated to titanium dioxide by sol-gel method and finally calcined to 400 °C [3]. Particle size of microcrystallites contained in powders were determined by the Scherrer equation ( D ¼ β:0:9λ cosθ ) and using the software X'Pert HighScore Plus. In this equation, D is the average particle size of microcrystallites, λ is the wavelength used (Cu Kα radiation), θ is the Bragg angle, and β is the width of the diffraction peak at half-height expressed in radians. The highest intensity signal (°2θ = 25.4), corresponding to the crystallographic plane 101, was used to calculate these average particle sizes. Table 2 shows a comparison of the particle sizes of the synthesized solids with those obtained by other authors to

Table 2 Particle sizes of the synthesized solids, determined by the Scherrer equation, and their comparison with similar synthesized materials (according to their composition) in the literature. Solid

Molar % Particle size (nm) Fe determined here

Particle size (nm) from literature [reference]

P50–110 P20–70 P20–50 P80–110 P20–110 P50–70 P80–70 P50–50 P80–50 TiO2 Evonik

0.35 0.47 0.64 0.75 1.29 1.96 4.76 5.83 10.03

– 8.7 [13], 10.8 [11], 15.1 [35] – – 10.8 [11], 14.4 [35] 8.9 [11] 8.1 [14], 13.8 [35] – 10.7 [1], 13.2 [35] 21 [40]

5.9 6.3 5.2 5.7 5.5 5.6 5.6 5.6 5.0 19.3

similar Fe-TiO2 solids. In general, particle size is maintained between 5 and 7 nm in all of the cases. These values are slightly smaller than the particle sizes reported by compared literature [1,11,13,14,35]. However, the particle size of TiO2 Evonik-P25 (used as reference) determined in this work was highly similar to the value reported in the literature [40]. Smaller particle sizes may have an advantage in the chemical reactivity of powders. IR analysis for ilmenite (Fig. 2) showed three characteristic vibration bands (440, 531 and 680 cm−1), which have been previously reported for crystalline FeTiO3 [36,41]. But IR spectra for the synthesized solids revealed those typical signals of titanium dioxide used as reference (Evonik P25). A weak vibration band at 1130 cm−1 corresponding to sulfate groups [42,43] was attributed to the residual ions from acidic treatment. Remaining sulfur was also detected by chemical analysis (Table 1). A small band about 1200 cm−1 as result of Ti\\O\\Ti vibrations [35] was also observed. Signals at 3400 cm− 1 and 1636 cm− 1 are assigned to O\\H stretching and bending vibrations of adsorbed water [35]. All differences revealed between IR spectrum of ilmenite regarding those of the synthesized solids, and the similarity of such spectra with that of commercial TiO2 Evonik P25, once more indicated that the synthesis pathway used here has been successful to obtain the anatase structure from ilmenite. Similar IR spectra have been reported in literature for iron-doped titanium dioxide synthesized by different ways [43,44]. A comparison of micrographs obtained by SEM analysis reveals an effective change in the morphology of ilmenite grains (aggregates) when this mineral was transformed into Fe(III)-TiO2 solids (Fig. 3). A highly irregular morphology was observed for ilmenite (Fig. 3a and b), in contrast with the morphologies observed for the synthesized solids. SEM images of some Fe(III)-TiO2 synthesized solids are shown in Fig. 3(c–h). For this morphological analysis, six solids were chosen to

1200 1626

1130 990

% Transmittance (a.u)

3400

531 680 440 Ilmenite P80-50 P50-50 P20-50 P80-70 P50-70 P20-70 P80-110 P50-110 P20-110 TiO2 P25

4000 3600 3200 2800 2400 2000 1600 1200 800

400

Wavenumber (cm-1) Fig. 1. X-ray diffraction patterns of the starting mineral (ilmenite), the synthesized solids and the reference TiO2 (● ilmenite, ◊ anatase, ♦ rutile, reference TiO2 is Evonik P25).

Fig. 2. IR spectra of ilmenite, the synthesized solids and the commercial TiO2 used as reference (Evonik P25).

J.A. Torres-Luna et al. / Powder Technology 302 (2016) 254–260

257

ILMENITE

ILMENITE

(a)

200.0 μm

(b)

(c)

(d)

(e)

(f)

(g)

(h)

50.0 μm

Fig. 3. (a–b) SEM images obtained from ilmenite mineral at different amplification. (c–h) SEM images obtained from some synthesized solids: c) P80–50, d) P80–110, e) P50–50, f) P50– 110, g) P20–70 and h) P20–110.

258

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160

TiO2 Evonik P25 TiO2

140

P80-110

120

P50-110

100

P20-110

80 60

40 20 0

Table 3 Surface area values and pore sizes for the synthesized solids.

c)

160 140 120

TiO2 Evonik P25 TiO2 P50-50 P20-50

100

80 60 40 20

Total pore volume (cm3 g−1)

Mean pore size (radius) (Å)

TiO2 Evonik P80–50 P50–50 P80–70 P50–70 P20–110 P80–110 P20–50 P20–70 P50–110

0.00

55

–

–

10.03 5.83 4.76 1.96 1.29 0.75 0.64 0.47 0.35

213 192 176 – 186 187 200 – 192

0.1835 0.1897 0.1824 – 0.2244 0.2048 0.1579 – 0.2225

34.1 34.4 34.4 – 40.6 33.9 34.0 – 34.2

Molar percentage was calculated as 100(mol Fe)/(mol Fe + mol Ti).

FðRÞ ¼

ð1−R Þ2 α ¼ 2R S

Where R is the diffuse reflectance of sample divided by the reflectance of the reference material (BaSO4), and S is a dispersion factor,

160

TiO2 Evonik P25 TiO2 P80-70 P80-50

140 120

100 80

60 40 20 0

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

Relative pressure (P/P 0)

b) 0,0070

P80-110 P80-70 P50-110 P20-110 P50-50 P80-50 P20-50

0,0060 ΔVp /Δr p (cm3/g-Å)

Volume adsorbed (cm3 /g), STP

a)

Molar % Surface area (BET) (m2/g) Fea

attributed to the agglomeration of small crystals of TiO2 or Fe(III)-TiO2 [2], which agrees with the hysteresis observed for each isotherm. These porosity results show that the synthesized solids have a huge potential to be used as catalytic materials or catalytic supports. Diffuse reflectance UV–vis spectra for the solids are showed in Fig. 5a. The Kubelka-Munk function was plotted against wavelength. All of the solids reveal an abrupt drop of signal about 400 nm, indicating an increase on radiation absorption below this wavelength. This event is associated with the electron transition from the valence band to the conduction band in the anatase structure [48]. For spectra obtained from the synthesized solids, the drop of that signal was shifted toward wavelength values in the visible light region (Fig. 5a), which is correlated with a decrease of the band-gap energy. On the other hand, diffuse reflectance UV–vis data were used to calculate the band-gap energy for each synthesized solid. The KubelkaMunk function, F(R), is given by the equation

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 Relative pressure (P/P 0)

Solid

a

Volume adsorbed (cm3/g), STP

Volume adsorbed (cm3/g), STP

observe the particle shape after the synthesis procedure. Although the aggregates show heterogeneous sizes, edges of regular morphology with right angles can be observed. It is important to take into account that the solids were ground and sifted through a mesh before SEM analysis, thus the original regular morphology of grains has been modified by breaking. However, a similar morphology was obtained by Cernea and coworkers [45] when a similar material was synthesized by hydrolysis of titanium butoxide with iron acetylacetonate at pH = 2 and subsequent annealing at 450 °C. This observation allows speculating that the hydrolysis of titanium cations at acidic pH promotes the growth of anatase grains. Fig. 4(a–c) shows the N2 adsorption-desorption isotherms for different powdered solids. All the solids revealed type IV isotherms [2,33]. Hysteresis can be classified as type H2, which is associated to pores having channel forms with non-uniform size and shape. These pores can be formed as a consequence of agglomeration of particles [33]. The N2 adsorption-desorption isotherm for TiO2 Evonik P25 shows lower adsorption levels and reveals a hysteresis type H1, which can be attributed to pores with uniform shape and size. These results clearly indicate that the synthesized solids have higher adsorption capacity than that for the reference TiO2. Pore size distributions obtained by the BJH (Barrett, Joyner, and Halenda) model (Fig. 4d) show monomodal curves (maximum population of pores) with pore radii between 30 and 50 Å (Table 3). As it can be seen in Table 3, for the Fe(III)-TiO2 synthesized solids, large values of surface area (up to 213 m2/g), higher than those for TiO2 Evonik P25 (55 m2/g), were obtained. Recently, other authors [46] have extracted (by reductive hydrochloric acid leaching) nanosized synthetic rutile (TiO2) from ilmenite to be used in photocatalytic experiments, reaching 53 m2/g for the solids synthesized. Also, Giri and Das [44] synthesized iron-doped titanium dioxide powders with only 54 and 59 m2/g. An increase of surface areas for our solids perhaps could be a result of the iron incorporation in the TiO2 structure, such as reported by Wang and coworkers [47] and Zhou and coworkers [13]. These results clearly indicate that mesoporous iron(III)-doped titania solids with high surface area were synthesized by the way proposed here, without using molecular templates, intermediate surfactants, ultrasound assistance or another complicated method of synthesis. According to the literature, the formation of mesoporous structure in this type of material is

0,0050 0,0040 0,0030 0,0020 0,0010

0,0000

0

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 Relative pressure (P/P 0)

20

d)

30

40 50 Pore Size (Å)

60

Fig. 4. Nitrogen adsorption isotherms of synthesized solids compared to TiO2 Evonik P25 used as reference (a–c) and the BJH pore size distributions (d).

J.A. Torres-Luna et al. / Powder Technology 302 (2016) 254–260

P80-110

2,5

TiO2 (P25 Evonik)

P50-110

2,0 1,5

P20-110

1,0 0,5 0,0 2,0

250

350 450 550 wavelength (nm)

650

2,5

750

3,0 E (eV)

(d) 4,0

4,0 P80-70

3,0 ]1/2

3,0

TiO2 P25 Evonik)

3,5

[F(R)

3,5 ]1/2

P20-70 P50-70 P20-110 P50-110 50-50 P80-110 P20-50 P80-50

3,5

4,0

P80-50 P50-50

3,5 3,0

P50-70

2,5

TiO2 (P25 Evonik)

2,0

P20-70

1,5

]1/2

(c)

(b) 4,0 3,9 3,5 3,1 2,7 2,3 1,9 1,5 1,1 0,7 0,3 -0,1

[F(R)

F(R)

(a)

259

2,5 TiO2 (P25 Evonik)

1,5

1,0

1,0

0,5

0,5

0,0

P20-50

2,0

0,0 2,0

2,5

3,0 E (eV)

3,5

4,0

2,0

2,5

3,0 E (eV)

3,5

4,0

Fig. 5. (a) Comparison of (UV–vis) diffuse reflectance spectra for the synthesized solids and the reference TiO2 (Evonik P25). (b, c, d) Modified spectra for determination of indirect band 1 1 gap (n = 2) according to the equation described above (½FðRÞ∙hvn ¼ ðA S Þn ðhv−Eg Þ).

which depends on the particle size [14,35,48,49]. Because of the Kubelka-Munk function is proportional to the absorption coefficient (α) of material, a mathematical expression equivalent to that known as Tauc's relation [(α ∙ hv) = A(hv − Eg)n] can be obtained. In that expression, data of diffuse reflectance are associated with both energy of incident photon (hv) and the band-gap energy, Eg, according to the following equation. FðRÞ∙hv ¼

n A
hv−Eg S

Where A is a constant which depends on the properties of studied material, and n is a constant that has different values according to electron transition. The value of n is 12 when the band-gap is direct, but n is 2 when the band-gap is indirect [35,48,49]. Thus, from that equation one can obtain the following relationship where F(R) is a function of hv.  . n1
 hv−Eg ½ FðRÞ∙hv ¼ A 1 n

S

Therefore, the estimation of band-gap energy for the solids regarding an indirect electron transition [50] was obtained from intersection 1

of the linear region of that plot corresponding to ½ FðRÞ∙hv2 against hv (Fig. 5b to d) with the X-axis [F(R) = 0] [14,35,48], as it is observed in Fig. 5b to d. Table 4 Experimental band gap energy values for the synthesized solids and TiO2 Evonik P25. Solids

Band gap, Eg (eV)

λ (nm)

Molar % Fe*

TiO2 P80–110 P50–110 P20–110 P80–70 P50–70 P20–70 P80–50 P50–50 P20–50

3.13 2.73 2.85 2.91 2.40 2.73 2.96 2.30 2.51 2.75

396 454 435 426 516 454 419 539 494 451

– 0.75 0.35 1.29 4.76 1.96 0.47 10.03 5.83 0.64

⁎ Molar percentage was calculated as 100(mol Fe)/(mol Fe + mol Ti).

The band-gap energy data showed in Table 4 reveal that Eg to TiO2 (Evonik P25) is 3.13 eV, an experimental value close to that (3.2 eV) reported in the literature [37]. All the Eg values for the synthesized solids are below those obtained for the reference TiO2, indicating that the band-gap energy for these new solids has a spectral shift toward the visible region. This event is attributed to a charge transition (iron “d” electrons) from the valence band to the conduction band of TiO2, indicating that iron(III) ions are substitutional dopants into the crystalline lattice of TiO2 [50]. This fact is correlated with those observed by both X-ray diffraction and X-ray fluorescence analysis, and becomes additional evidence of successful doping of titanium dioxide with iron ions. These results show that these new solids, Fe(III)-TiO2, have interesting properties to be used as photocatalysts under visible radiation with wavelength near to the ultraviolet region. 4. Conclusions A set of new solid materials based on titanium dioxide doped with iron ions were synthesized from a natural ilmenite treated with different concentrations of sulfuric acid at different temperatures. According to the X-ray fluorescence analyses, different iron contents were incorporated in the solids and, in all cases, X-ray diffraction only showed the anatase structure. This indicates a possible inclusion of iron by isomorphic substitution in the anatase structure. Besides, it was observed that quantities of iron that remained in the anatase structure can be controlled by adjusting both temperature and acid concentration. UV– vis spectroscopy analysis shows a shift of the band gap energy toward a visible region for all the synthesized solids, confirming the incorporation of iron into anatase structure, and revealing the potential of these solids as photocatalysts under solar light. Finally, the new solids have attractive catalytic properties and also show mesoporosity and high surface areas with a defined morphology. Acknowledgements The authors are grateful for the financial support offered by the project DIB-UNAL (Code 19065) and the COLCIENCIAS program “Jovenes Investigadores e Innovadores 2012 - Virginia Gutiérrez de Pineda”.

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