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Sulfated Fe2O3–TiO2 synthesized from ilmenite ore: A visible light active photocatalyst
Colloids and Surfaces A: Physicochem. Eng. Aspects 367 (2010) 140–147
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Sulfated Fe2 O3 –TiO2 synthesized from ilmenite ore: A visible light active photocatalyst York R. Smith a,b , K. Joseph Antony Raj a , Vaidyanathan (Ravi) Subramanian b , B. Viswanathan a,∗ a b
Department of Chemistry, National Centre for Catalysis Research, Indian Institute of Technology-Madras, Chennai 600036, India Chemical & Materials Engineering Department, University of Nevada, Reno 89557, USA
a r t i c l e
i n f o
Article history: Received 16 March 2010 Received in revised form 2 June 2010 Accepted 2 July 2010 Available online 13 July 2010 Keywords: Ilmenite ore Sulfated Fe2 O3 –TiO2 Photocatalytic activity Phenol oxidation
a b s t r a c t Sulfated Fe2 O3 –TiO2 (SFT) was synthesized by treatment of ilmenite ore with sulfuric acid. The presence of sulfated Fe2 O3 –TiO2 and mixed phases of Fe2 O3 –TiO2 was confirmed by DRIFT spectra and XRD. The dispersion of sulfate displayed thermal stability up to 500 ◦ C. The adsorption–desorption of pyridine investigated by DRIFT spectra revealed the presence of both Brønsted and Lewis acid sites for the samples calcined up to 500 ◦ C. The DRS/UV–vis spectra showed UV and visible light absorbance for samples calcined up to 900 ◦ C. A band gap value of 2.73 eV is obtained for 500 ◦ C calcined sample. The photocatalytic activity was evaluated by the oxidation of 4-chlorophenol (4-CP) in aqueous medium under UV–vis and visible light irradiation. SFT calcined at 500 ◦ C demonstrated the highest photocatalytic activity. When compared with high surface area sulfated titania (275 m2 /g), the photocatalytic activity was greater due to the presence of iron, despite the low surface area of the SFT samples (12–17 m2 /g). © 2010 Elsevier B.V. All rights reserved.
1. Introduction An increasing awareness of the environmental impacts from pollution and stringent standards on emission regulations has prompted the development of catalytic routes for waste management. The development and practical application of systems that are clean and green have shown to be a formidable challenge for scientists and engineers. Photocatalytic technologies have shown practical application in antibacterial and deodorant filters for air purification owing to its property of promoting various chemical reactions such as the degradation of aqueous organic pollutants and sources of offensive odors using light. Titania-based materials have received considerable attention for their potential in environmental catalytic applications such as air purification, water disinfection, hazardous wastewater remediation, and deodorization [1–6]. Recently, the application of titania with different architectures such as nanotubes has been examined [7–9]. Owing to its large band gap (Eg = 3.2 eV) titania, however, can only utilize photons in the UV region (<380 nm), which limits its practical application for sun light irradiation [7,8,10–13]. One of the promising approaches to overcome this disadvantage is coupling titania with other narrow band gap semiconductors capable of promoting charge separation in the visible light spectrum [14,15]. Many studies have reported sensitizer-loaded titania, such as CdS/TiO2
∗ Corresponding author. Tel.: +91 44 22574241; fax: +91 44 22574202. E-mail address: [email protected] (B. Viswanathan). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.07.001
[16,17]. CdSe/TiO2 [18,19], Bi2 O3 /SrTiO3 [20], Bi2 S3 /TiO2 [17,21], ZnMn2 O4 /TiO2 [22], TiO2 /Ti2 O3 [23] under visible light irradiation and have shown efficient visible light photoactivity. In most of these catalysts, the addition of sensitizers reduces the band gap of the material enabling the coupled material to absorb visible light. The conduction band (CB) of the loaded sensitizer has a more negative reduction potential than that of titania enabling visible light photoinduced electrons to be injected into the lower-energy CB of titania. However, the photogenerated holes of the sensitizer remain in the valence band (VB) resulting in an accumulation of holes on the sensitizer leading to photocorrosion of the catalyst. As a result, the stability of the composite photocatalyst becomes less [24]. Furthermore, most currently produced sensitizers are heavy metal chalcogenides (e.g., CdSe, PbS). These may constitute harm to ecological systems and humans as well due to their nanoscale and toxic metal release. A recent study by King-Heiden and coworkers [25] examined the toxicological effects of CdSe/ZnS nanoparticles on the growth of zebrafish embryos and showed Cd toxicity even at very low levels of CdSe nanoparticles. To counter the potential negative environmental problems of using heavy metal sensitizers, iron as a dopant in titania-based systems has been investigated to enhance the photocatalytic efficiency under visible light irradiation [26–30]. Iron is one of the most abundant elements found in the Earth’s crust. Similar to titania, iron and its oxides show promise as an eco-friendly catalyst in many applications. For example, FeTiO3 has a band gap of 2.58–2.9 eV, [31–34] and has been used as a chemical and as a photocatalyst. [32,33] Ye et al. observed that under UV irradiation of TiO2 –Fe3 O4 mixed
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oxide coatings exhibited higher photocatalytic efficiency than titania alone due to the formation of FeTiO3 [33,34], which may form a p–n junction with titania to induce spatial separation of the photogenerated electron/hole pairs. Iron-doped titania has shown to exhibit higher photocatalytic activity in visible light. A report by Choi and coworkers [35] shows that creating a shallow trap in the titania lattice due to its half-filled electronic configuration induces a red shift in the band gap and alters electron/hole pair recombination rates. Although the mechanism of narrowing the band gap and reducing the recombination rates of titania with the aid of iron remains in the realm of debate, [36–39] in general, it is assumed that the photocatalytic behavior and efficiency are greatly influenced by the doping of iron oxides [40]. Hence, further investigations are essential to explore iron as a doping catalyst through appropriate synthesis processes. The objective of this study is to address some of the questions raised in previous related work and provide a photocatalyst which possesses high catalytic activity, non-toxic, inexpensive, and allows for the use of visible light directly to carry out photocatalytic reactions. Herein, we report the synthesis of sulfated Fe2 O3 –TiO2 (SFT) using ilmenite ore and sulfuric acid as the starting material and its effect of photocatalytic activity is evaluated by the oxidation of 4-chlorophenol (4-CP) in water. 2. Experimental 2.1. Synthesis of sulfated Fe2 O3 –TiO2 10 g of ball milled ilmenite ore was homogeneously mixed with 20 g of concentrated H2 SO4 and aged for 2 h at 30 ◦ C. To this mixture, 10 g of water was added while stirring to initiate the reaction and maintaining constant stirring of the reaction mass for about 1 h. Thereafter the reaction mass was treated with 100 g of water to remove any remaining soluble residues. The remaining mass obtained was then dried in air at 100 ◦ C for 12 h. The samples were calcined in air at various temperatures to prepare Fe2 O3 –TiO2 containing different quantities of sulfate. The ilmenite ore was analyzed and found to have the composition of 55 wt.% of TiO2 , 42 wt.% of Fe3 O4 , 2.9 wt.% of SiO2 and traces of alumina, zirconia, vanadia and chromia was used for preparation of the catalyst. 2.2. Characterization Wide-angle XRD patterns for the calcined and as-synthesized materials were obtained using a Rigaku Miniflex II, with CuK␣ irradiation with a scan range of 2 = 0–60◦ with a scan rate of 5◦ /min. The composition of the catalysts was analyzed using an XRF spectrometer. The pyridine adsorption–desorption measurement for the identification of Brønsted and Lewis acidity was determined by diffuse reflectance infrared Fourier transform (DRIFT) spectra and recorded using a Bruker Tensor-27. The thermal analyses of the samples were performed on PerkinElmer TG/DTA with alumina as the reference. The BET-surface area of the samples was determined by the nitrogen adsorption and desorption isotherms at −195.6 ◦ C and measured by a Micromeritics ASAP-2020 analyzer after the samples were degassed in vacuum at 300 ◦ C for 3 h. The UV–vis/DRS studies of the samples were performed on a Thermo Scientific instrument. 2.3. Photocatalytic experiments SFT catalyst samples calcined at various temperatures were studied for the photocatalytic degradation of 4-chlorophenol (4CP) and compared with the activity of commercial Degussa P-25 (P25), as well as sulfated titania (ST) prepared in this laboratory [41]. In a typical experiment, 40 ml (50 ppm) of aqueous solution
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Fig. 1. XRD patterns of sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures. The peaks are indexed with standard JCPDS cards for, A – anatase, R – rutile, F – Fe2 O3 , T – Fe2 TiO5 , # – FeTiO3 , @ – sulfated Fe2 O3 –TiO2 , and * – Fe2 TiO4 .
4-CP and 50 mg of catalyst was stirred in the dark at room temperature for 30 min in a jacketed quartz reactor. Subsequently, the solution was irradiated with a high-pressure 500 W mercury lamp (Newport, ORIEL), 35 cm away from the solution while maintaining constant stirring. For visible light experiments, UV irradiation was filtered out using a band pass filter with a cut-off at < 420 nm (HOYA L-42). The decomposition of 4-CP was quantitatively analyzed using a UV–vis spectrophotometer (Jasco V-530) by monitoring the change in absorbance at = 250 nm. The analyzed transparent liquid was segregated from mixture using centrifugation at a speed of ∼10,000 rpm. Identification of intermediate products, reaction network, and kinetic modeling in an otherwise similar system is discussed elsewhere [42]. All experiments were conducted and measured consecutively twice. Blank experiments where performed in the absence of photocatalyst and negligible decomposition of 4-CP was observed. 3. Results and discussion 3.1. X-ray diffraction The XRD patterns of SFT samples calcined at various temperatures are shown in Fig. 1 and have been indexed and compared with standard JCPDS cards. The peaks appeared at 2 values of 27.5, 36.1, and 41.3 are due to the presence of rutile (PDF#: 881172) in the sample. The intensity of peaks at a 2 value of 27.5 and 36.1 was found to increase with temperature of calcination due to the effect of rutilation. As a result, the rutile content [43] was found to increase from 11% to 15% with temperature. The peak obtained at a 2 value of 25.4, which is due to anatase (PDF#: 894203) phase of titania, showed an increase in intensity with temperature. In general, anatase forms initially and is subsequently converted to rutile on heat treatment. Employing wet chemical-based synthesis protocols for preparing coupled oxides based on titania have shown similar results in related work such as preparation of TiO2 –Al2 O3 . [44] In this study, the anatase content was found to increase from 2.9% to 7.2% with temperature. This observation shows the presence of a greater quantity of rutile over anatase in the acid treated ilmenite samples. The reflections at 32.8, 34.9, 48.8, and 56.7 are due to the formation of FeTiO3 (PDF#: 751212). The peak at a 2
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Table 1 BET-surface area, particle size, and crystallite size of the sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
Table 2 Composition of the sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures by XRF method.
S. no.
Sample
BET-surface area (m2 /g)
Particle size (nm) SBET -methoda
Crystallite size (nm) XRD-methodb
S. no.
Sample
Sulfate (wt.%)
SiO2 (wt.%)
TiO2 (wt.%)
Fe2 O3 (wt.%)
1. 2. 3. 4. 5.
SFT-100 SFT-300 SFT-500 SFT-700 SFT-900
17 17 16 12 12
84 84 89 119 119
86 94 103 106 108
1. 2. 3. 4. 5.
SFT-100 SFT-300 SFT-500 SFT-700 SFT-900
8.9 8.3 8.1 1.4 1.2
2.8 2.8 2.9 3.1 3.1
60.5 60.5 61.3 65.5 65.8
27.7 28.3 27.7 30.0 29.9
a Particle size (nm) = 6000/(SBET ) where, SBET is the BET-surface area; is the density. b Calculated using the Scherrer equation of the (1 0 1) peak of anatase and (1 1 0) peak of rutile.
value of 54.5 is due to the presence of Fe2 O3 (PDF#: 890599). Moreover, a peak at 41.3 for FeO–Fe2 O3 (PDF#: 070322) is possible but, rutile phase titania also has the same reflection. The peak at a 2 value of 18.2 could be due to Fe2 TiO5 (PDF#: 761743) and the peak at 39.4 is due to Fe2 TiO4 (PDF#: 050696). The peak at a 2 value of 31.1 is due to sulfated Fe2 O3 –TiO2 (PDF#: 280500) and is observed for all the samples irrespective of the calcination temperature. The peak at a 2 value of 18.2 is obtained only for the sample calcined at 900 ◦ C for 2 h. This peak was not observed for the samples calcined less than 700 ◦ C. Similarly, the peak at a 2 value of 39.4 for Fe2 TiO4 was obtained only for the samples calcined at 700 and 900 ◦ C.
iron oxide in the samples. The samples calcined up to 500 ◦ C showed virtually the same composition with samples caclined at 300 and 100 ◦ C. Nevertheless, the calcination at 700 ◦ C decreased the sulfate content from 8.1% to 1.4%. Negligible variation in the composition of the samples, surface area, and crystal size suggests that the samples are thermally stable up to 500 ◦ C. The SFT-900 sample showed almost a same sulfate content as that of SFT-700. Fig. 2 shows the nitrogen adsorption–desorption isotherm for the SFT samples calcined at various temperatures. The adsorption isotherm is classified as type II for the SFT samples calcined at 300–900 ◦ C. The type II isotherm obtained for the SFT samples classified its grouping as a non-porous material. Almost overlaying adsorption and desorption lines explains the non-porous characteristics with type II isotherm. This is also shown by the low BET-surface area of the samples (Table 1).
3.2. BET-surface area, particle size, and crystallite size 3.3. DRIFT spectra Table 1 shows the value of BET-surface area, particle size and crystallite size of the SFT samples calcined at various temperatures. Although the surface area values remained constant with temperatures up to 500 ◦ C, the crystallite size showed an increase of 16% for SFT-500 in contrast to SFT-100. The SFT-700 and SFT-900 showed a lower surface area than the samples calcined at ≤500 ◦ C, this could be attributed to the agglomeration of the particles on treatment at higher temperatures. The composition of the SFT samples calcined at various temperatures is determined by XRF and shown in Table 2. The composition revealed the presence of sulfate, silica, titania and
The characteristics of sulfate species on the surface of TiO2 and Fe2 O3 during calcination were examined in detail using DRIFT spectra. A DRIFT spectral study of sulfated metal oxides showing high catalytic activity is generally related to strong absorption bands in the region of 800–1200 cm−1 [45]. Fig. 3(a) shows the DRIFT spectra of the SFT samples calcined at various temperatures, the wave number range displayed is between 700 and 1300 cm−1 . A broad absorption band was observed between 700 and 1250 cm−1 for the SFT samples calcined at 300 and 500 ◦ C. A peak was observed at
Fig. 2. Adsorption–desorption isotherm of sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
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Fig. 4. DRIFT spectra of pyridine adsorbed sulfated Fe2 O3 –TiO2 (SFT) samples.
fate groups from the sample when the calcination temperature is increased beyond 500 ◦ C. Fig. 3(b) shows the DRIFT spectra of the SFT samples obtained in the range of 600–4000 cm−1 . The samples showed a broad peak at 600–1440 cm−1 . Generally, sulfate salts show S O absorptions frequencies at 1100–1200 cm−1 [46]; however, the SFT samples showed absorption up to about 1440 cm−1 which is consistent with covalent sulfates in organic compounds [47]. Thus, the shift in observed DRIFT spectra indicates the possibility of covalently bonded sulfate to Fe2 O3 and/or TiO2 . The sulfates start to become polynuclear complex sulfates such as S2 O7 2− and/or S3 O10 2− type, which are characterized by absorptions between 1400 and 1600 cm−1 [48]. The specific absorption bands were not observed between 1400 and 1600 cm−1 for the SFT samples, however, indicating the absence of polynuclear sulfates. Absorption peaks at about 3400 and 1640 cm−1 observed for SFT samples are attributed to stretching and bending modes of adsorbed water and hydroxyl groups. The SFT samples calcined at 900 ◦ C for 2 h showed significant absorptions at 1880, 2000, 2140 and 2245 cm−1 which could be due to the formation of iron titanate and sulfated Fe2 O3 –TiO2 . These observations are also suggested by the XRD data where reflections of 2 = 18.2 for Fe2 TiO5 and 2 = 31.1 for sulfated Fe2 O3 –TiO2 are observed for SFT samples calcined at 900 ◦ C (vide supra). 3.4. Acidity Fig. 3. (a) DRIFT spectra of the sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures. (b) DRIFT spectra of the sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
1140 cm−1 which is generally attributed to asymmetric stretching characteristic of sulfate vibrations [46]. The other absorption peaks were detected at about 840, 940, 1000, 1050, 1150 and 1240 cm−1 . These bands are assigned to S O and S–O, symmetric and asymmetric stretching frequencies, respectively [46]. The peak at 1140 cm−1 for SFT-100 is significant and its intensity was lowered with increasing calcination temperature. The SFT samples calcined at 700 and 900 ◦ C showed low intensity bands at 840 and 940 cm−1 , with no absorption bands observed in the region of 700–1300 cm−1 . This demonstrates the removal of sul-
The calcined SFT samples were evacuated at 300 ◦ C for 2 h before the adsorption of pyridine. The adsorption was performed at 0.01 bar pressure by exposing the samples to pyridine vapor at 150 ◦ C for 30 min. The desorption of pyridine was carried out at 150 ◦ C for 1 h and thereafter the samples were cooled to room temperature and the DRIFT spectra recorded. Fig. 4 shows the DRIFT spectra of the pyridine adsorbed on SFT samples calcined at various temperatures. Both the pyridinium ion band at 1540 cm−1 and the coordinated pyridine band at 1485 cm−1 were found with the SFT samples calcined at ≤500 ◦ C, indicating the presence of both Brønsted and Lewis acid sites. In addition, the intensities of peaks at 1540 cm−1 for Brønsted acidity is greater than the Lewis acid peaks obtained at 1480 cm−1 . It is noticeable from the intensity of the peaks that no change in acidity was observed for the samples calcined at 100
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Fig. 5. Thermograms of sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
and 300 ◦ C. The decrease in intensity of peaks obtained for SFT-500, when compared to SFT-300 and SFT-100 can be attributed to the removal of sulfate groups when calcined at 500 ◦ C. The adsorption of pyridine on SFT samples calcined at 700 and 900 ◦ C showed no peaks at 1485 and 1540 cm−1 revealing the absence of Brønsted and Lewis acid sites on these samples. The increasing calcination temperature from 500 to 700 ◦ C accounted a removal of 82 wt.% sulfate shows the catalysts can be effectively used at less than 500 ◦ C. Hence, it is apparent from the pyridine adsorption studies that the adsorption of sulfate on the surface of TiO2 and Fe2 O3 causes the generation of acid sites on the samples.
3.6. DRS/UV–vis Fig. 6 shows the diffuse reflectance spectra of the SFT samples calcined at 300–700 ◦ C. The band gap was determined by Tauc plots of the samples. To determine the band-to-band transitions of the samples, the absorption data was fitted for indirect and direct band gap transitions. Plotting (F(R) × h)n vs. h, where F(R) is the absorbance, h is planck’s constant, the frequency and n indicating indirect (n = ½) or direct (n = 2) band gap material, and extrapolating the linear region to the abscissa yields the band gap of the sample [49]. When plotted for indirect band gap transitions, the analysis indicates that band gaps for the samples to be less than
3.5. TG/DTA The thermograms obtained for SFT samples calcined at various temperatures for 2 h are shown in Fig. 5. The samples SFT-100, SFT-300, and SFT-500 showed two weight losses at about 200 and 610 ◦ C. Nevertheless, SFT-100 demonstrated moderately higher weight loss of 5.7% at 200 ◦ C and 8.4% at 610 ◦ C than the SFT-300 and SFT-500 samples. The SFT–300 and SFT-500 showed almost the same weight loss of 4.6% at 200 ◦ C and 7.8% at 610 ◦ C. In all these samples the weight loss up to 200 ◦ C could be attributed to the removal of adsorbed water molecules. The weight loss obtained between 430 and 610 ◦ C for the samples, SFT-100, SFT-300, and SFT-500 could be due to the removal of sulfates groups. This possibility is confirmed by the significant decrease in the sulfate content as deduced by XRF analysis (Table 2). The thermogram of SFT-700 showed a weight loss of 0.13% up to 900 ◦ C indicating the absence of adsorbed water molecules, hydroxyl groups, and sulfates present in the sample. The same weight loss patterns were obtained for the samples calcined up to 500 ◦ C. A plateau beyond 610 ◦ C with no significant weight loss for SFT-700 comprehensively demonstrates that the SFT samples are thermally stable up to 500 ◦ C.
Fig. 6. UV–vis/DRS of (a) SFT-300, (b) SFT-500 and (c) SFT-700.
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Fig. 7. Photocatalytic decomposition of 4-CP under UV–vis light irradiation for 60 min with Degussa P-25 (P25), sulfated titania (ST) and sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
1.5 eV, which does not seem realistic since band gap values for ␣Fe2 O3 are reported to be 1.9–2.3 eV [50,51]. Hence, the band gap energy of the samples should be greater than 2.3 eV and less than 3.2 eV (titania) due to the mixed phase of Fe2 O3 –TiO2 . When the data is fit for direct band gap transitions, a better fit is noted than the corresponding indirect band gap fit. For direct band gap transitions the SFT-300, SFT-500 and SFT-700 samples showed band gaps of 2.74, 2.73 and 2.63 eV, respectively. The absorbance of ST and P25 were also measured (not included) and the band gaps were found to be 3.2 and 3.1 eV, respectively. SFT-500 showed the highest light absorbance of all the samples. SFT-700 showed higher UV absorption than SFT-300 up to 550 nm and thereafter showed lower visible light absorption than SFT-300. The results show that a decrease in sulfate content correlates with a decrease in band gap. The sulfate content between SFT-300 and SFT500 differ slightly (0.2 wt.%, Table 2), as do the band gaps. Moreover, the difference in sulfate content between SFT-500 and SFT-700 is greater (by 6.7 wt.%, Table 2) as well as the difference in band gap. The lowering of band gap is due to the result of lower sulfate content and increased Fe2 O3 (Table 2). Colon et al. [52] observed an increase in band gap for sulfated titania and subsequent decrease in band gap with increasing calcination temperature as a result of removal of sulfate. Although SFT-900 showed lower band gap than SFT-500, SFT-500 showed greater photocatalytic activity than SFT-900 due to the overall increase in light absorbance. 4. Photocatalytic activity The photocatalytic activity of SFT samples was determined for the model compound 4-CP under UV–vis and visible light illumination. SFT samples calcined at 300–900 ◦ C were compared with P25 and ST. The results of UV–vis illumination after 60 min are shown in Fig. 7. All samples containing iron exhibited higher photocatalytic activity than P25 and ST. The role of iron content can be vital as Fe3+ can serve as not only a mediator of interfacial charge transfer, but also as a recombination center through quantum tunneling. [53] If the iron content is too high, the Fe3+ ions can steadily become recombination centers and the photocatalytic activity can decrease as a result of the following reactions [54]: Fe3+ + e− → Fe2+
(1)
+
(2)
2+
Fe
+ h → Fe
3+
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Fig. 8. Photocatalytic decomposition of 4-CP under visible light irradiation for 120 min with Degussa P-25 (P25), sulfated titania (ST), and sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
However, this attribute is not evident in the SFT samples as the Fe/Ti ratio does not change significantly for the samples calcined at various temperatures (0.45–0.47, calculated from Table 2). An increase in photocatalytic activity with increasing calcination temperature is observed for SFT samples calcined at 300–500 ◦ C; however, the reverse trend was noted for samples calcined above 500 ◦ C. This behavior can be attributed to the acidity of the photocatalysts. SFT samples calcined from 300 to 500 ◦ C show Brønsted and Lewis acid sites, whereas SFT samples calcined from 700 to 900 ◦ C exhibit no acidity (Fig. 4). Increased photocatalytic activity of SFT-300 and SFT-500 can be ascribed by the activation of 4-CP with strong Brønsted acid sites interaction near the active photocatalytic sites of Fe2 O3 –TiO2 . Since photocatalysis is a surface phenomenon, the photocatalytic performance can be attributed to the charge on the surface of the photocatalyst and the reactant. The acidity of the photocatalyst favors the adsorption of 4-CP and any dissolved oxygen in solution. Lewis acid sites can react with water and convert to Brønsted acid sites leading to the activation of water. This conversion promotes the formation of hydroxyl radicals on the photocatalyst surface, which is a highly reactive oxidizing species and may account for the increased photocatalytic activity of SFT-300 and SFT-500 samples. Similar trends for increased photocatalytic activity of sulfated photocatalysts over non-sulfated photocatalyst have been reported [55,56]. Although SFT-300 exhibited more acidity than SFT-500 and the band gaps of the two samples are nearly the same (vide supra), there appears to be a synergistic effect of calcination temperature and photocatalytic activity. A study by Pal et al. [57] showed the effect of calcination temperature for the highest photocatalytic activity of Fe2 O3 –TiO2 . Furthermore, when comparing the results of ST which possess acidity and greater surface area (275 m2 /g) [41] than SFT-700 and SFT-900 (Table 2), ST showed lower photocatalytic activity than SFT-700 and SFT-900 samples. These results demonstrate the significant role of iron for increased photocatalytic activity by decreasing the band gap and increasing the intensity of visible light absorbance. The visible light ( > 420 nm) photocatalytic activity of SFT-300, SFT-500, SFT-700, ST, and P25 was also examined. The results, as shown in Fig. 8, demonstrate P25 and ST to have the lowest visible light photocatalytic activity while SFT samples demonstrate higher activity. SFT-500 shows the highest photocatalytic activity followed by SFT-700 and SFT-300 showing nearly the same degradation of 4-CP after 120 min. These results are complimented by the DRS/UV–vis study. P25 and ST do not absorb light in the visible spectrum, thus the energy supplied by wavelengths greater
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than 420 nm is not sufficient to promote electron/hole separation as indicated by their band gaps of 3.1 and 3.2 eV, respectively. It should be mentioned that although photoinduced electron/hole pair separation is not possible for P25 and ST under visible light irradiation, photocatalytic oxidation is still achievable, the mechanism on the other hand, differs from the UV irradiation pathway [58,59]. As for the SFT-300, SFT-700, and SFT-500 samples, the band gaps are sufficient for visible light photoinduced electron/hole pair separation as indicated by the DRS/UV–vis study. The photoactivity of the iron containing photocatalysts under visible light can be attributed to the electron transfer from the Fe2 O3 . The conduction band edge of Fe2 O3 is greater than that of titania. With Fe2 O3 having a lower band gap energy, it acts as a photosensitizer by facilitating an interfacial electron transfer from Fe2 O3 to titania. Under visible light irradiation, only Fe2 O3 is activated. Photogenerated electrons transfer from the conduction band of Fe2 O3 into titania and accumulate at the conduction band of titania, while holes accumulate at the valence band of Fe2 O3 . The photogenerated electrons are then scavenged by oxygen in water and finally form hydroxyl radicals to degrade 4-CP. The photocatalytic degradation is also complimented when the photocatalyst exhibits acidic properties (vide supra). The visible light study results are interesting as it showed a reverse trend of photocatalytic activity to the UV-light study results for SFT-300 and SFT-700. These results may be due to the Fe2 O3 content. It is possible that an increase in Fe2 O3 content would increase degradation rates by generating more oxidizing species under visible light illumination as SFT-700 has a higher Fe2 O3 content (Table 2) than SFT-300. It should be noted, however, that an increase in the Fe/Ti ratio can hinder photocatalytic activity by an unfavorable charge transfer process to adsorbed substrates during irradiation where excess accumulation of electrons and holes undergo recombination within a short time without taking part in the photocatalytic degradation of reactants at the surface of the photocatalyst. SFT-500 exhibited higher photoactivity although it has lower Fe2 O3 than SFT-300 and SFT-700. Moreover, SFT-300 has acidic properties while SFT-700 does not. From these results it is unclear to determine the controlling factors of the photocatalytic activity as non-optical factors appear to play a significant role and there is a synergistic effect of calcination temperature, acidity and photocatalytic activity. The purpose of the work presented here is to demonstrate a possible new visible light photocatalyst that can be easily synthesized. 5. Conclusion Sulfuric acid treatment of ilmenite ore has demonstrated to yield an effective photocatalyst for the photocatalytic decomposition of 4-chlorophenol in water. The sulfated iron titania (SFT) exhibits visible light photocatalytic activity and is thermally stable up to 500 ◦ C. The presence of FeTiO3 , Fe2 O3 , and sulfated Fe2 O3 –TiO2 was confirmed by XRD and DRIFT spectra. The adsorption–desorption of pyridine coupled with DRIFT spectra revealed the presence of both Brønsted and Lewis acid sites for samples calcined up to 500 ◦ C. The DRS/UV–vis spectra showed the SFT-500 sample to have a band gap of 2.73 eV with increased absorbance in both the UV and visible light region. When compared to high surface area sulfated titania (ST), SFT samples calcined at 700 and 900 ◦ C, exhibit no acidity and low surface area and this showed greater photocatalytic activity under UV–vis and visible light illumination. These observations revealed the significance of the presence of iron in titania for photocatalytic activity. Acknowledgements The authors acknowledge the Department of Science and Technology, Government of India for funding the National Centre for
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Sulfated Fe2 O3 –TiO2 synthesized from ilmenite ore: A visible light active photocatalyst York R. Smith a,b , K. Joseph Antony Raj a , Vaidyanathan (Ravi) Subramanian b , B. Viswanathan a,∗ a b
Department of Chemistry, National Centre for Catalysis Research, Indian Institute of Technology-Madras, Chennai 600036, India Chemical & Materials Engineering Department, University of Nevada, Reno 89557, USA
a r t i c l e
i n f o
Article history: Received 16 March 2010 Received in revised form 2 June 2010 Accepted 2 July 2010 Available online 13 July 2010 Keywords: Ilmenite ore Sulfated Fe2 O3 –TiO2 Photocatalytic activity Phenol oxidation
a b s t r a c t Sulfated Fe2 O3 –TiO2 (SFT) was synthesized by treatment of ilmenite ore with sulfuric acid. The presence of sulfated Fe2 O3 –TiO2 and mixed phases of Fe2 O3 –TiO2 was confirmed by DRIFT spectra and XRD. The dispersion of sulfate displayed thermal stability up to 500 ◦ C. The adsorption–desorption of pyridine investigated by DRIFT spectra revealed the presence of both Brønsted and Lewis acid sites for the samples calcined up to 500 ◦ C. The DRS/UV–vis spectra showed UV and visible light absorbance for samples calcined up to 900 ◦ C. A band gap value of 2.73 eV is obtained for 500 ◦ C calcined sample. The photocatalytic activity was evaluated by the oxidation of 4-chlorophenol (4-CP) in aqueous medium under UV–vis and visible light irradiation. SFT calcined at 500 ◦ C demonstrated the highest photocatalytic activity. When compared with high surface area sulfated titania (275 m2 /g), the photocatalytic activity was greater due to the presence of iron, despite the low surface area of the SFT samples (12–17 m2 /g). © 2010 Elsevier B.V. All rights reserved.
1. Introduction An increasing awareness of the environmental impacts from pollution and stringent standards on emission regulations has prompted the development of catalytic routes for waste management. The development and practical application of systems that are clean and green have shown to be a formidable challenge for scientists and engineers. Photocatalytic technologies have shown practical application in antibacterial and deodorant filters for air purification owing to its property of promoting various chemical reactions such as the degradation of aqueous organic pollutants and sources of offensive odors using light. Titania-based materials have received considerable attention for their potential in environmental catalytic applications such as air purification, water disinfection, hazardous wastewater remediation, and deodorization [1–6]. Recently, the application of titania with different architectures such as nanotubes has been examined [7–9]. Owing to its large band gap (Eg = 3.2 eV) titania, however, can only utilize photons in the UV region (<380 nm), which limits its practical application for sun light irradiation [7,8,10–13]. One of the promising approaches to overcome this disadvantage is coupling titania with other narrow band gap semiconductors capable of promoting charge separation in the visible light spectrum [14,15]. Many studies have reported sensitizer-loaded titania, such as CdS/TiO2
∗ Corresponding author. Tel.: +91 44 22574241; fax: +91 44 22574202. E-mail address: [email protected] (B. Viswanathan). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.07.001
[16,17]. CdSe/TiO2 [18,19], Bi2 O3 /SrTiO3 [20], Bi2 S3 /TiO2 [17,21], ZnMn2 O4 /TiO2 [22], TiO2 /Ti2 O3 [23] under visible light irradiation and have shown efficient visible light photoactivity. In most of these catalysts, the addition of sensitizers reduces the band gap of the material enabling the coupled material to absorb visible light. The conduction band (CB) of the loaded sensitizer has a more negative reduction potential than that of titania enabling visible light photoinduced electrons to be injected into the lower-energy CB of titania. However, the photogenerated holes of the sensitizer remain in the valence band (VB) resulting in an accumulation of holes on the sensitizer leading to photocorrosion of the catalyst. As a result, the stability of the composite photocatalyst becomes less [24]. Furthermore, most currently produced sensitizers are heavy metal chalcogenides (e.g., CdSe, PbS). These may constitute harm to ecological systems and humans as well due to their nanoscale and toxic metal release. A recent study by King-Heiden and coworkers [25] examined the toxicological effects of CdSe/ZnS nanoparticles on the growth of zebrafish embryos and showed Cd toxicity even at very low levels of CdSe nanoparticles. To counter the potential negative environmental problems of using heavy metal sensitizers, iron as a dopant in titania-based systems has been investigated to enhance the photocatalytic efficiency under visible light irradiation [26–30]. Iron is one of the most abundant elements found in the Earth’s crust. Similar to titania, iron and its oxides show promise as an eco-friendly catalyst in many applications. For example, FeTiO3 has a band gap of 2.58–2.9 eV, [31–34] and has been used as a chemical and as a photocatalyst. [32,33] Ye et al. observed that under UV irradiation of TiO2 –Fe3 O4 mixed
Y.R. Smith et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 367 (2010) 140–147
oxide coatings exhibited higher photocatalytic efficiency than titania alone due to the formation of FeTiO3 [33,34], which may form a p–n junction with titania to induce spatial separation of the photogenerated electron/hole pairs. Iron-doped titania has shown to exhibit higher photocatalytic activity in visible light. A report by Choi and coworkers [35] shows that creating a shallow trap in the titania lattice due to its half-filled electronic configuration induces a red shift in the band gap and alters electron/hole pair recombination rates. Although the mechanism of narrowing the band gap and reducing the recombination rates of titania with the aid of iron remains in the realm of debate, [36–39] in general, it is assumed that the photocatalytic behavior and efficiency are greatly influenced by the doping of iron oxides [40]. Hence, further investigations are essential to explore iron as a doping catalyst through appropriate synthesis processes. The objective of this study is to address some of the questions raised in previous related work and provide a photocatalyst which possesses high catalytic activity, non-toxic, inexpensive, and allows for the use of visible light directly to carry out photocatalytic reactions. Herein, we report the synthesis of sulfated Fe2 O3 –TiO2 (SFT) using ilmenite ore and sulfuric acid as the starting material and its effect of photocatalytic activity is evaluated by the oxidation of 4-chlorophenol (4-CP) in water. 2. Experimental 2.1. Synthesis of sulfated Fe2 O3 –TiO2 10 g of ball milled ilmenite ore was homogeneously mixed with 20 g of concentrated H2 SO4 and aged for 2 h at 30 ◦ C. To this mixture, 10 g of water was added while stirring to initiate the reaction and maintaining constant stirring of the reaction mass for about 1 h. Thereafter the reaction mass was treated with 100 g of water to remove any remaining soluble residues. The remaining mass obtained was then dried in air at 100 ◦ C for 12 h. The samples were calcined in air at various temperatures to prepare Fe2 O3 –TiO2 containing different quantities of sulfate. The ilmenite ore was analyzed and found to have the composition of 55 wt.% of TiO2 , 42 wt.% of Fe3 O4 , 2.9 wt.% of SiO2 and traces of alumina, zirconia, vanadia and chromia was used for preparation of the catalyst. 2.2. Characterization Wide-angle XRD patterns for the calcined and as-synthesized materials were obtained using a Rigaku Miniflex II, with CuK␣ irradiation with a scan range of 2 = 0–60◦ with a scan rate of 5◦ /min. The composition of the catalysts was analyzed using an XRF spectrometer. The pyridine adsorption–desorption measurement for the identification of Brønsted and Lewis acidity was determined by diffuse reflectance infrared Fourier transform (DRIFT) spectra and recorded using a Bruker Tensor-27. The thermal analyses of the samples were performed on PerkinElmer TG/DTA with alumina as the reference. The BET-surface area of the samples was determined by the nitrogen adsorption and desorption isotherms at −195.6 ◦ C and measured by a Micromeritics ASAP-2020 analyzer after the samples were degassed in vacuum at 300 ◦ C for 3 h. The UV–vis/DRS studies of the samples were performed on a Thermo Scientific instrument. 2.3. Photocatalytic experiments SFT catalyst samples calcined at various temperatures were studied for the photocatalytic degradation of 4-chlorophenol (4CP) and compared with the activity of commercial Degussa P-25 (P25), as well as sulfated titania (ST) prepared in this laboratory [41]. In a typical experiment, 40 ml (50 ppm) of aqueous solution
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Fig. 1. XRD patterns of sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures. The peaks are indexed with standard JCPDS cards for, A – anatase, R – rutile, F – Fe2 O3 , T – Fe2 TiO5 , # – FeTiO3 , @ – sulfated Fe2 O3 –TiO2 , and * – Fe2 TiO4 .
4-CP and 50 mg of catalyst was stirred in the dark at room temperature for 30 min in a jacketed quartz reactor. Subsequently, the solution was irradiated with a high-pressure 500 W mercury lamp (Newport, ORIEL), 35 cm away from the solution while maintaining constant stirring. For visible light experiments, UV irradiation was filtered out using a band pass filter with a cut-off at < 420 nm (HOYA L-42). The decomposition of 4-CP was quantitatively analyzed using a UV–vis spectrophotometer (Jasco V-530) by monitoring the change in absorbance at = 250 nm. The analyzed transparent liquid was segregated from mixture using centrifugation at a speed of ∼10,000 rpm. Identification of intermediate products, reaction network, and kinetic modeling in an otherwise similar system is discussed elsewhere [42]. All experiments were conducted and measured consecutively twice. Blank experiments where performed in the absence of photocatalyst and negligible decomposition of 4-CP was observed. 3. Results and discussion 3.1. X-ray diffraction The XRD patterns of SFT samples calcined at various temperatures are shown in Fig. 1 and have been indexed and compared with standard JCPDS cards. The peaks appeared at 2 values of 27.5, 36.1, and 41.3 are due to the presence of rutile (PDF#: 881172) in the sample. The intensity of peaks at a 2 value of 27.5 and 36.1 was found to increase with temperature of calcination due to the effect of rutilation. As a result, the rutile content [43] was found to increase from 11% to 15% with temperature. The peak obtained at a 2 value of 25.4, which is due to anatase (PDF#: 894203) phase of titania, showed an increase in intensity with temperature. In general, anatase forms initially and is subsequently converted to rutile on heat treatment. Employing wet chemical-based synthesis protocols for preparing coupled oxides based on titania have shown similar results in related work such as preparation of TiO2 –Al2 O3 . [44] In this study, the anatase content was found to increase from 2.9% to 7.2% with temperature. This observation shows the presence of a greater quantity of rutile over anatase in the acid treated ilmenite samples. The reflections at 32.8, 34.9, 48.8, and 56.7 are due to the formation of FeTiO3 (PDF#: 751212). The peak at a 2
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Table 1 BET-surface area, particle size, and crystallite size of the sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
Table 2 Composition of the sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures by XRF method.
S. no.
Sample
BET-surface area (m2 /g)
Particle size (nm) SBET -methoda
Crystallite size (nm) XRD-methodb
S. no.
Sample
Sulfate (wt.%)
SiO2 (wt.%)
TiO2 (wt.%)
Fe2 O3 (wt.%)
1. 2. 3. 4. 5.
SFT-100 SFT-300 SFT-500 SFT-700 SFT-900
17 17 16 12 12
84 84 89 119 119
86 94 103 106 108
1. 2. 3. 4. 5.
SFT-100 SFT-300 SFT-500 SFT-700 SFT-900
8.9 8.3 8.1 1.4 1.2
2.8 2.8 2.9 3.1 3.1
60.5 60.5 61.3 65.5 65.8
27.7 28.3 27.7 30.0 29.9
a Particle size (nm) = 6000/(SBET ) where, SBET is the BET-surface area; is the density. b Calculated using the Scherrer equation of the (1 0 1) peak of anatase and (1 1 0) peak of rutile.
value of 54.5 is due to the presence of Fe2 O3 (PDF#: 890599). Moreover, a peak at 41.3 for FeO–Fe2 O3 (PDF#: 070322) is possible but, rutile phase titania also has the same reflection. The peak at a 2 value of 18.2 could be due to Fe2 TiO5 (PDF#: 761743) and the peak at 39.4 is due to Fe2 TiO4 (PDF#: 050696). The peak at a 2 value of 31.1 is due to sulfated Fe2 O3 –TiO2 (PDF#: 280500) and is observed for all the samples irrespective of the calcination temperature. The peak at a 2 value of 18.2 is obtained only for the sample calcined at 900 ◦ C for 2 h. This peak was not observed for the samples calcined less than 700 ◦ C. Similarly, the peak at a 2 value of 39.4 for Fe2 TiO4 was obtained only for the samples calcined at 700 and 900 ◦ C.
iron oxide in the samples. The samples calcined up to 500 ◦ C showed virtually the same composition with samples caclined at 300 and 100 ◦ C. Nevertheless, the calcination at 700 ◦ C decreased the sulfate content from 8.1% to 1.4%. Negligible variation in the composition of the samples, surface area, and crystal size suggests that the samples are thermally stable up to 500 ◦ C. The SFT-900 sample showed almost a same sulfate content as that of SFT-700. Fig. 2 shows the nitrogen adsorption–desorption isotherm for the SFT samples calcined at various temperatures. The adsorption isotherm is classified as type II for the SFT samples calcined at 300–900 ◦ C. The type II isotherm obtained for the SFT samples classified its grouping as a non-porous material. Almost overlaying adsorption and desorption lines explains the non-porous characteristics with type II isotherm. This is also shown by the low BET-surface area of the samples (Table 1).
3.2. BET-surface area, particle size, and crystallite size 3.3. DRIFT spectra Table 1 shows the value of BET-surface area, particle size and crystallite size of the SFT samples calcined at various temperatures. Although the surface area values remained constant with temperatures up to 500 ◦ C, the crystallite size showed an increase of 16% for SFT-500 in contrast to SFT-100. The SFT-700 and SFT-900 showed a lower surface area than the samples calcined at ≤500 ◦ C, this could be attributed to the agglomeration of the particles on treatment at higher temperatures. The composition of the SFT samples calcined at various temperatures is determined by XRF and shown in Table 2. The composition revealed the presence of sulfate, silica, titania and
The characteristics of sulfate species on the surface of TiO2 and Fe2 O3 during calcination were examined in detail using DRIFT spectra. A DRIFT spectral study of sulfated metal oxides showing high catalytic activity is generally related to strong absorption bands in the region of 800–1200 cm−1 [45]. Fig. 3(a) shows the DRIFT spectra of the SFT samples calcined at various temperatures, the wave number range displayed is between 700 and 1300 cm−1 . A broad absorption band was observed between 700 and 1250 cm−1 for the SFT samples calcined at 300 and 500 ◦ C. A peak was observed at
Fig. 2. Adsorption–desorption isotherm of sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
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Fig. 4. DRIFT spectra of pyridine adsorbed sulfated Fe2 O3 –TiO2 (SFT) samples.
fate groups from the sample when the calcination temperature is increased beyond 500 ◦ C. Fig. 3(b) shows the DRIFT spectra of the SFT samples obtained in the range of 600–4000 cm−1 . The samples showed a broad peak at 600–1440 cm−1 . Generally, sulfate salts show S O absorptions frequencies at 1100–1200 cm−1 [46]; however, the SFT samples showed absorption up to about 1440 cm−1 which is consistent with covalent sulfates in organic compounds [47]. Thus, the shift in observed DRIFT spectra indicates the possibility of covalently bonded sulfate to Fe2 O3 and/or TiO2 . The sulfates start to become polynuclear complex sulfates such as S2 O7 2− and/or S3 O10 2− type, which are characterized by absorptions between 1400 and 1600 cm−1 [48]. The specific absorption bands were not observed between 1400 and 1600 cm−1 for the SFT samples, however, indicating the absence of polynuclear sulfates. Absorption peaks at about 3400 and 1640 cm−1 observed for SFT samples are attributed to stretching and bending modes of adsorbed water and hydroxyl groups. The SFT samples calcined at 900 ◦ C for 2 h showed significant absorptions at 1880, 2000, 2140 and 2245 cm−1 which could be due to the formation of iron titanate and sulfated Fe2 O3 –TiO2 . These observations are also suggested by the XRD data where reflections of 2 = 18.2 for Fe2 TiO5 and 2 = 31.1 for sulfated Fe2 O3 –TiO2 are observed for SFT samples calcined at 900 ◦ C (vide supra). 3.4. Acidity Fig. 3. (a) DRIFT spectra of the sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures. (b) DRIFT spectra of the sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
1140 cm−1 which is generally attributed to asymmetric stretching characteristic of sulfate vibrations [46]. The other absorption peaks were detected at about 840, 940, 1000, 1050, 1150 and 1240 cm−1 . These bands are assigned to S O and S–O, symmetric and asymmetric stretching frequencies, respectively [46]. The peak at 1140 cm−1 for SFT-100 is significant and its intensity was lowered with increasing calcination temperature. The SFT samples calcined at 700 and 900 ◦ C showed low intensity bands at 840 and 940 cm−1 , with no absorption bands observed in the region of 700–1300 cm−1 . This demonstrates the removal of sul-
The calcined SFT samples were evacuated at 300 ◦ C for 2 h before the adsorption of pyridine. The adsorption was performed at 0.01 bar pressure by exposing the samples to pyridine vapor at 150 ◦ C for 30 min. The desorption of pyridine was carried out at 150 ◦ C for 1 h and thereafter the samples were cooled to room temperature and the DRIFT spectra recorded. Fig. 4 shows the DRIFT spectra of the pyridine adsorbed on SFT samples calcined at various temperatures. Both the pyridinium ion band at 1540 cm−1 and the coordinated pyridine band at 1485 cm−1 were found with the SFT samples calcined at ≤500 ◦ C, indicating the presence of both Brønsted and Lewis acid sites. In addition, the intensities of peaks at 1540 cm−1 for Brønsted acidity is greater than the Lewis acid peaks obtained at 1480 cm−1 . It is noticeable from the intensity of the peaks that no change in acidity was observed for the samples calcined at 100
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Fig. 5. Thermograms of sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
and 300 ◦ C. The decrease in intensity of peaks obtained for SFT-500, when compared to SFT-300 and SFT-100 can be attributed to the removal of sulfate groups when calcined at 500 ◦ C. The adsorption of pyridine on SFT samples calcined at 700 and 900 ◦ C showed no peaks at 1485 and 1540 cm−1 revealing the absence of Brønsted and Lewis acid sites on these samples. The increasing calcination temperature from 500 to 700 ◦ C accounted a removal of 82 wt.% sulfate shows the catalysts can be effectively used at less than 500 ◦ C. Hence, it is apparent from the pyridine adsorption studies that the adsorption of sulfate on the surface of TiO2 and Fe2 O3 causes the generation of acid sites on the samples.
3.6. DRS/UV–vis Fig. 6 shows the diffuse reflectance spectra of the SFT samples calcined at 300–700 ◦ C. The band gap was determined by Tauc plots of the samples. To determine the band-to-band transitions of the samples, the absorption data was fitted for indirect and direct band gap transitions. Plotting (F(R) × h)n vs. h, where F(R) is the absorbance, h is planck’s constant, the frequency and n indicating indirect (n = ½) or direct (n = 2) band gap material, and extrapolating the linear region to the abscissa yields the band gap of the sample [49]. When plotted for indirect band gap transitions, the analysis indicates that band gaps for the samples to be less than
3.5. TG/DTA The thermograms obtained for SFT samples calcined at various temperatures for 2 h are shown in Fig. 5. The samples SFT-100, SFT-300, and SFT-500 showed two weight losses at about 200 and 610 ◦ C. Nevertheless, SFT-100 demonstrated moderately higher weight loss of 5.7% at 200 ◦ C and 8.4% at 610 ◦ C than the SFT-300 and SFT-500 samples. The SFT–300 and SFT-500 showed almost the same weight loss of 4.6% at 200 ◦ C and 7.8% at 610 ◦ C. In all these samples the weight loss up to 200 ◦ C could be attributed to the removal of adsorbed water molecules. The weight loss obtained between 430 and 610 ◦ C for the samples, SFT-100, SFT-300, and SFT-500 could be due to the removal of sulfates groups. This possibility is confirmed by the significant decrease in the sulfate content as deduced by XRF analysis (Table 2). The thermogram of SFT-700 showed a weight loss of 0.13% up to 900 ◦ C indicating the absence of adsorbed water molecules, hydroxyl groups, and sulfates present in the sample. The same weight loss patterns were obtained for the samples calcined up to 500 ◦ C. A plateau beyond 610 ◦ C with no significant weight loss for SFT-700 comprehensively demonstrates that the SFT samples are thermally stable up to 500 ◦ C.
Fig. 6. UV–vis/DRS of (a) SFT-300, (b) SFT-500 and (c) SFT-700.
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Fig. 7. Photocatalytic decomposition of 4-CP under UV–vis light irradiation for 60 min with Degussa P-25 (P25), sulfated titania (ST) and sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
1.5 eV, which does not seem realistic since band gap values for ␣Fe2 O3 are reported to be 1.9–2.3 eV [50,51]. Hence, the band gap energy of the samples should be greater than 2.3 eV and less than 3.2 eV (titania) due to the mixed phase of Fe2 O3 –TiO2 . When the data is fit for direct band gap transitions, a better fit is noted than the corresponding indirect band gap fit. For direct band gap transitions the SFT-300, SFT-500 and SFT-700 samples showed band gaps of 2.74, 2.73 and 2.63 eV, respectively. The absorbance of ST and P25 were also measured (not included) and the band gaps were found to be 3.2 and 3.1 eV, respectively. SFT-500 showed the highest light absorbance of all the samples. SFT-700 showed higher UV absorption than SFT-300 up to 550 nm and thereafter showed lower visible light absorption than SFT-300. The results show that a decrease in sulfate content correlates with a decrease in band gap. The sulfate content between SFT-300 and SFT500 differ slightly (0.2 wt.%, Table 2), as do the band gaps. Moreover, the difference in sulfate content between SFT-500 and SFT-700 is greater (by 6.7 wt.%, Table 2) as well as the difference in band gap. The lowering of band gap is due to the result of lower sulfate content and increased Fe2 O3 (Table 2). Colon et al. [52] observed an increase in band gap for sulfated titania and subsequent decrease in band gap with increasing calcination temperature as a result of removal of sulfate. Although SFT-900 showed lower band gap than SFT-500, SFT-500 showed greater photocatalytic activity than SFT-900 due to the overall increase in light absorbance. 4. Photocatalytic activity The photocatalytic activity of SFT samples was determined for the model compound 4-CP under UV–vis and visible light illumination. SFT samples calcined at 300–900 ◦ C were compared with P25 and ST. The results of UV–vis illumination after 60 min are shown in Fig. 7. All samples containing iron exhibited higher photocatalytic activity than P25 and ST. The role of iron content can be vital as Fe3+ can serve as not only a mediator of interfacial charge transfer, but also as a recombination center through quantum tunneling. [53] If the iron content is too high, the Fe3+ ions can steadily become recombination centers and the photocatalytic activity can decrease as a result of the following reactions [54]: Fe3+ + e− → Fe2+
(1)
+
(2)
2+
Fe
+ h → Fe
3+
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Fig. 8. Photocatalytic decomposition of 4-CP under visible light irradiation for 120 min with Degussa P-25 (P25), sulfated titania (ST), and sulfated Fe2 O3 –TiO2 (SFT) samples calcined at various temperatures.
However, this attribute is not evident in the SFT samples as the Fe/Ti ratio does not change significantly for the samples calcined at various temperatures (0.45–0.47, calculated from Table 2). An increase in photocatalytic activity with increasing calcination temperature is observed for SFT samples calcined at 300–500 ◦ C; however, the reverse trend was noted for samples calcined above 500 ◦ C. This behavior can be attributed to the acidity of the photocatalysts. SFT samples calcined from 300 to 500 ◦ C show Brønsted and Lewis acid sites, whereas SFT samples calcined from 700 to 900 ◦ C exhibit no acidity (Fig. 4). Increased photocatalytic activity of SFT-300 and SFT-500 can be ascribed by the activation of 4-CP with strong Brønsted acid sites interaction near the active photocatalytic sites of Fe2 O3 –TiO2 . Since photocatalysis is a surface phenomenon, the photocatalytic performance can be attributed to the charge on the surface of the photocatalyst and the reactant. The acidity of the photocatalyst favors the adsorption of 4-CP and any dissolved oxygen in solution. Lewis acid sites can react with water and convert to Brønsted acid sites leading to the activation of water. This conversion promotes the formation of hydroxyl radicals on the photocatalyst surface, which is a highly reactive oxidizing species and may account for the increased photocatalytic activity of SFT-300 and SFT-500 samples. Similar trends for increased photocatalytic activity of sulfated photocatalysts over non-sulfated photocatalyst have been reported [55,56]. Although SFT-300 exhibited more acidity than SFT-500 and the band gaps of the two samples are nearly the same (vide supra), there appears to be a synergistic effect of calcination temperature and photocatalytic activity. A study by Pal et al. [57] showed the effect of calcination temperature for the highest photocatalytic activity of Fe2 O3 –TiO2 . Furthermore, when comparing the results of ST which possess acidity and greater surface area (275 m2 /g) [41] than SFT-700 and SFT-900 (Table 2), ST showed lower photocatalytic activity than SFT-700 and SFT-900 samples. These results demonstrate the significant role of iron for increased photocatalytic activity by decreasing the band gap and increasing the intensity of visible light absorbance. The visible light ( > 420 nm) photocatalytic activity of SFT-300, SFT-500, SFT-700, ST, and P25 was also examined. The results, as shown in Fig. 8, demonstrate P25 and ST to have the lowest visible light photocatalytic activity while SFT samples demonstrate higher activity. SFT-500 shows the highest photocatalytic activity followed by SFT-700 and SFT-300 showing nearly the same degradation of 4-CP after 120 min. These results are complimented by the DRS/UV–vis study. P25 and ST do not absorb light in the visible spectrum, thus the energy supplied by wavelengths greater
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than 420 nm is not sufficient to promote electron/hole separation as indicated by their band gaps of 3.1 and 3.2 eV, respectively. It should be mentioned that although photoinduced electron/hole pair separation is not possible for P25 and ST under visible light irradiation, photocatalytic oxidation is still achievable, the mechanism on the other hand, differs from the UV irradiation pathway [58,59]. As for the SFT-300, SFT-700, and SFT-500 samples, the band gaps are sufficient for visible light photoinduced electron/hole pair separation as indicated by the DRS/UV–vis study. The photoactivity of the iron containing photocatalysts under visible light can be attributed to the electron transfer from the Fe2 O3 . The conduction band edge of Fe2 O3 is greater than that of titania. With Fe2 O3 having a lower band gap energy, it acts as a photosensitizer by facilitating an interfacial electron transfer from Fe2 O3 to titania. Under visible light irradiation, only Fe2 O3 is activated. Photogenerated electrons transfer from the conduction band of Fe2 O3 into titania and accumulate at the conduction band of titania, while holes accumulate at the valence band of Fe2 O3 . The photogenerated electrons are then scavenged by oxygen in water and finally form hydroxyl radicals to degrade 4-CP. The photocatalytic degradation is also complimented when the photocatalyst exhibits acidic properties (vide supra). The visible light study results are interesting as it showed a reverse trend of photocatalytic activity to the UV-light study results for SFT-300 and SFT-700. These results may be due to the Fe2 O3 content. It is possible that an increase in Fe2 O3 content would increase degradation rates by generating more oxidizing species under visible light illumination as SFT-700 has a higher Fe2 O3 content (Table 2) than SFT-300. It should be noted, however, that an increase in the Fe/Ti ratio can hinder photocatalytic activity by an unfavorable charge transfer process to adsorbed substrates during irradiation where excess accumulation of electrons and holes undergo recombination within a short time without taking part in the photocatalytic degradation of reactants at the surface of the photocatalyst. SFT-500 exhibited higher photoactivity although it has lower Fe2 O3 than SFT-300 and SFT-700. Moreover, SFT-300 has acidic properties while SFT-700 does not. From these results it is unclear to determine the controlling factors of the photocatalytic activity as non-optical factors appear to play a significant role and there is a synergistic effect of calcination temperature, acidity and photocatalytic activity. The purpose of the work presented here is to demonstrate a possible new visible light photocatalyst that can be easily synthesized. 5. Conclusion Sulfuric acid treatment of ilmenite ore has demonstrated to yield an effective photocatalyst for the photocatalytic decomposition of 4-chlorophenol in water. The sulfated iron titania (SFT) exhibits visible light photocatalytic activity and is thermally stable up to 500 ◦ C. The presence of FeTiO3 , Fe2 O3 , and sulfated Fe2 O3 –TiO2 was confirmed by XRD and DRIFT spectra. The adsorption–desorption of pyridine coupled with DRIFT spectra revealed the presence of both Brønsted and Lewis acid sites for samples calcined up to 500 ◦ C. The DRS/UV–vis spectra showed the SFT-500 sample to have a band gap of 2.73 eV with increased absorbance in both the UV and visible light region. When compared to high surface area sulfated titania (ST), SFT samples calcined at 700 and 900 ◦ C, exhibit no acidity and low surface area and this showed greater photocatalytic activity under UV–vis and visible light illumination. These observations revealed the significance of the presence of iron in titania for photocatalytic activity. Acknowledgements The authors acknowledge the Department of Science and Technology, Government of India for funding the National Centre for
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