Scientific Bases for the Preparation of Heterogeneous Catalysts E.M. Gaigneaux et al. (Editors) © 2006 Elsevier B.V. All rights reserved.
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Preparation in mild conditions of photocatalytically active nanostructured TiO2 rutile E. Garcia-Lopez, M. Addamo, A. Di Paola, G. Marci,L. Palmisano Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy
Abstract Nanostructured TiO2 rutile samples were prepared in mild conditions by hydrolysis of TiCl4 in different solutions. The powders revealed a fair photoactivity for the photocatalytic degradation of 4-nitrophenol. The influence of preparation procedure, pH and presence of Cl" and N3" anions on the physicochemical features and photocatalytic activity of the powders was investigated. 1. Introduction Heterogeneous photocatalysis is a promising technology for the photo-oxidation of many organic and inorganic pollutants present in water or in air [1]. TiO2 is the most used photocatalyst, due to its photostability and low cost. Tt is a polymorphic solid that crystallizes in three major different structures: rutile, anatase and brookite. In the three polimorphs the basic building block consists of a titanium atom surrounded by six oxygen atoms in a more or less distorted octahedral configuration [2]. Rutile is thermodynamically more stable than anatase and brookite at atmospheric pressure and room temperature whereas the thermodynamic stable polymorph becomes anatase when the primary particle size decreases to ca. 15 nm [3]. Rutile can be obtained with the minimum size of 10-20 nm in the presence of inorganic compounds as HC1, NaCl, NH4C1, SnCl4 or SnO2 [4,5]. Anatase is the most studied [6] and generally the most photoactive polymorph, probably because it shows the better photoadsorption of oxygen and the lower recombination rate of the photoproduced electron-hole
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pairs [7]. Rutile samples, often completely inactive for photocatalytic purposes, usually consist of large particles because they are prepared by calcination of anatase. Rutile presents great interest for photocatalytic applications due to its band gap energy, lower than those of anatase and brookite. The preparation of rutile nanoparticles (primary particle size lower than 100 nm) should improve the activity of the photocatalyst due to the practical absence of band bending in the nanostructured materials which allows an easy access of the photoproduced electrons and holes to the surface of the particles and to the larger surface/volume ratio [8,9]. This paper deals with the preparation and characterization of nanostructured TiO2 rutile samples showing a fair photocatalytic activity. 2. Experimental The photocatalysts were obtained by hydrolysis of titanium tetraehloride in pure water or aqueous solutions at room temperature, with a molar ratio Ti:H2O equal to 1:220 (0.25 M TiCl4 solution). Two different routes were followed: A) TiCl4 (Fluka) was added dropwise to water or to a NaN3 solution (Ti:N molar ratio: 1:4 or 1:40). In all cases a white suspended solid was obtained. The pH of the resulting suspension was 0.7. In some preparations NaOH was added to adjust the pH to 3.0 or 5.5. The suspension was stirred for 12 h and, after centrifugation, the precipitate was washed and dried at 298 K. Table 1 reports the preparation conditions of the various samples. Tablel. Samples prepared by Route A
Al A2 A3 AM AN2 AN3 AN4
Additive
Ti:N
Final pH
NaN3 NaN3 NaN3 NaN3
1:4 1:4 1:4 1:40
0.7 3.0 5.5 0.7 3.0 5.5 0.7
B) TiCl4 was added dropwise to HC1 or NaCl solutions. A white powder immediately formed and dissolved after few minutes of stirring due to the large amount of chloride ions present in solution [10]. A white suspended solid was obtained after a refluxing treatment at 373 K for 72 h. The solid separated after centrifugation was washed and dried at 298 K. Two samples were prepared by following the Route B but hydrolyzing TiCl4 in pure water (Sample B5) or in the presence of HNO3 with a molar Ti:NO3" ratio equal to 1:4 (sample BN). Experimental details are shown in Table 2.
photocatalytically active nanostructured TiO Preparation in mild conditions of photocatalytically TiO22 rutile
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Table 2. Samples prepared by route B
Bl B2 B3 B4 B5 BN
Additive
Ti:Cl
Final pH
NaCl NaCl HC1 HC1
1:8 1:24 1:8 1:24 1:4 1:4
0.7 0.7 0 0 0.7 0
HNO3
XRD patterns of the powders were collected by a Philips powder diffractometer using the Cu Ka radiation and a 28 scan rate of 2°/min. The specific surface areas (SSA) were determined by the single-point BET method using a Flow Sorb 2300 Micromeritics apparatus. Scanning electron microscopy observations (SEM) were performed with a Philips XL30 ESEM microscope. The degradation of 4-nitrophenol (4-NP) was employed in order to test the photocatalytic activity of the powders. A 50 mL batch photoreactor was irradiated by means of a SOLARBOX apparatus (CO.FO.ME.GRA) equipped with a 1500 W Xe lamp. The high energy UV light was cut-off by using a Pyrex filter. The irradiance reaching the photoreactor was 1.38 mW-cm"2 (measured in the range 300-400 nm). The amount of catalyst used for the experiments was 0.8 gvL"1, and the initial 4-NP concentration was 20 mg-L"1. The pH of the suspension was adjusted to 4 by addition of H2SO4. The quantitative determination of 4-NP was performed by a spectrophotometer Shimadzu UV2401 PC setted at 315 nm. The photoactivity of the various samples was compared to that of two commercial TiO2 samples (Tioxide Huntsman), 100% anatase and 100% rutile, indicated as Tiox A and Tiox R, respectively. 3. Results and Discussion Table 3 reports characterization results of the samples obtained by Route A along with those of the commercial powders. The percentages of anatase and rutile in the samples were estimated through the Spurr and Myers method [11]. The crystallite sizes were calculated from the Scherrer equation [12]. Fig. 1 shows X-ray diffraction patterns of some selected samples. The powders prepared by Route A at pH 0.7 and pH 5.5 (samples Al and A3) consisted of pure rutile. When the pH of the solution was 3.0, ca. 60% of anatase and 40% of rutile were obtained (sample A2). According to Gopal et al. [2], anatase and rutile nucleate in competition. The first stage, during which primary crystallites form, controls the obtained phase. The mechanism for the formation of rutile and anatase using low temperature
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R
I \
I J
R
1
TioxR __
R
i
AN1 AN3
20
25
30
35
40
45 20
50
55
60
65
70
Figure 1. XRD patterns of commercial and home prepared TiO2 samples. synthesis routes [2,13] considers, as first step, the condensation of two titania octahedra joined along an edge. The placement of a third octahedron determines the formation of rutile or anatase. The linear arrangement of the octhaedra, corresponding to rutile, is thermodinamically favoured because the electrostatic repulsion among the cations is minimised. Tf the three octahedra join forming a right angle, the basic structure of anatase is obtained. The anatase formation is statistically favoured because there are many edges where the third octahedron can bond. Table 3. Samples prepared by Route A: percentages of anatase and rutile, size of anatase (OA) and rutile (OR) cystallites, specific surface area values and initial reaction rates
Al A2 A3 AN1 AN2 AN3 AN4 Tiox A TioxR
Anatase (%) _ 60 _ 100 40 100 100 -
Rutile (%) 100 40 100 100 _ 60
OA (nm)
100
-
4 4 4 6
O R (nm) 19 17 17 16 . 23 -
SSA (m2-g"') 63 88 107 77 90 143 63 8 8
ro-108
(M-s"V) 21.5 11.7 11.5 25.5 12.5 19.2 8.20 47.0 negligible
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The formed polymorph depends on the rate of aggregation of the octahedra. A slow aggregation rate causes the formation of rutile whereas a faster one favours the formation of anatase. The charge of the octahedral complexes may be changed by the chemisorption of ions [14]. At pH 0.7, rutile is formed since the aggregation processes are slow, due to the repulsion among the crystallization nuclei consisting of octahedra, caused by the adsorption of H+ ions. At pH 3, the repulsive forces decrease, allowing a faster aggregation and consequently rutile and mainly anatase are formed simultaneously. At pH 5.5, the adsorption of OH" ions inhibits the aggregation, promoting the formation of rutile. Similar results were obtained by Sun et al. who studied the influence of pH on the crystallization of precipitates obtained from TiCU [15]. TiCl4 was hydrolysed in NaN3 solutions with the aim to investigate the role played by N3~ ions both on the crystalline phase and on the size of the particles. As shown in Table 1, rutile was formed at pH 0.7 when the Ti:N molar ratio was 1:4 (sample AN1), whilst anatase was obtained if the ratio was 1:40 (sample AN4). For Ti:N molar ratio equal to 1:4, only anatase was formed at pH 3.0 (sample AN2), whilst a mixture of anatase and rutile was produced at pH 5.5 (sample AN3). These results confirm the mechanism of crystallization above described and indicate that the presence of the azide anion favours the formation of the anatase polymorph while has not a significant effect on the particles size. The crystallite sizes of the rutile particles obtained by route A were in the range of 16-23 nm and the BET specific surface areas ranged between 63 and 143 m2-g"1. Table 4. Samples prepared by Route B: percentages of anatase and rutile, size of anatase (A rutile (OR) cystallites, specific surface area values and initial reaction rates.
Bl B2 B3 B4 B5 BN
Anatase
Rutile
-
100 100 100 100 100 100
OA (nm)
OR (nm)
SSA
ro-10s
13 17 10 17 15 21
76 80 65 60 60 78
29.2 19.5 11.2 16.5 7.00 11.2
Table 4 reports characterization results for the samples obtained by Route B. The solids consisted, in all cases, of pure rutile with crystallites size in the range of 10-21 nm. The presence of chloride anions and of an acid pH favour the formation of the rutile polymorph as already reported in letterature [10]. Rutile was the only phase isolated when nitrate ions were present confirming that such crystalline structure is formed when the acid concentration is high. The specific surface areas of the powders ranged between 60 and 80 m2-g"' and they were not much influenced by the presence of NaCl, HC1 or HNO3.
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Figure 2 shows selected SEM micrographs of two samples, representative of the routes A and B. It can be noted that both Al (Fig. 2 (a) and (c)) and Bl (Fig. 2 (b) and (d)) samples, consisted of spherical primary particles whose average dimensions were ca. 75 nm. Both samples revealed aggregates of primary particles that were larger in the case of the Bl sample. Both morphology and size of the primary particles of the samples prepared by the Routes A and B were very similar. This means that the thermal treatment to which the Route B samples were subjected as well as the presence of azide, chloride or nitrate ions (micrographs not shown for the sake of brevity) did not influence the features of the primary particles, but can affect in some extent the size of the aggregates.
Figure 2. SEM micrographs of the samples Al (a) and (c) and Bl (b) and (d). Magnification: (a) and (b) x 20,000; (c) and (d) x 50,000. Figure 3 shows the results of the photocatalytic degradation of 4-NP in the presence of some representative samples. Straight lines fit the experimental data corresponding to the 4-NP concentration versus time, indicating that during the first two hours of irradiation the photodegradation reaction follows a zero-order kinetics. Tables 3 and 4 report the initial reaction rates values (r0) for runs carried out in the presence of the commercial samples and of the home prepared powders. All r0 values are the average of at least three measurements. All home prepared samples, although less efficient than the commercial anatase, revealed a good photocatalytic activity on respect to that of the rutile Tiox R sample, that was practically inactive. This finding could be related to the
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influence of physicochemical features of the nanostructured powders as, for instance, a larger extent of surface hydroxylation present in the samples. As shown in Table 3, the most efficient samples prepared according to the Route A were obtained at pH 0.7 (Al and AN1). The initial reaction rates decreased by increasing the final pH of the preparation suspension. The presence of azide with a Ti:N molar ratio equal to 1:4 led to an improvement of the photoactivity on respect to that of the samples prepared at the same pH but in the absence of the azide anion. On the contrary, by increasing ten times the amount of azide in the preparation mixture (sample AN4) a noticeable decrease of the photocatalytic efficiency was observed. The increase of photoactivity could be related to a doping effect due to the presence of nitrogen, that becomes detrimental when its content is too high. It should be noted that, unexpectedly, the less active samples contained anatase with the lowest size of crystallites and this finding is probably due to the poor crystallinity of the powders.
0
30
60
90
120
150
Figure 3. Photodegradation of 4-Nitrophenol on TiO2 catalysts: Tiox R (A); Al As far as the rutile powders prepared by Route B are concerned, Table 4 shows that the highest value of r0 was observed with the sample Bl, obtained at pH 0.7 in a solution containing NaCl with a Ti:Cl" molar ratio equal to 1:8. The sample B3, prepared with the same Ti:Cl" ratio but in the presence of HC1, was noticeably less active. The photoactivity decreased when the Ti:Cl" ratio was 1:24, by using NaCl as chloride ion supplier (sample B2). On the contrary, the reactivity increased when this high Ti:Cl" molar ratio was obtained by using HC1 (sample B4). The activity of the samples was higher when they were prepared in the presence of NaCl rather than HC1. This result can be ascribable to the different pH values of the solutions because the reactivity of TiO2 increases with pH [16]. The sample B5 prepared without addition of chloride ions revealed the lowest r0 value, indicating that the presence of an enhanced
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amount of chloride ions increases the photocatalytic activity of the powders. The presence of nitrate ions, with a Ti:NO3" molar ratio equal to 1:4, did not change significantly the activity of the sample BN. It is worth noting that the activity of the most efficient sample obtained by Route B was higher than that of the best samples obtained by Route A with or without addition of azide. There seems to be no relationship between SSA or crystallite size and activity of the various samples, probably because the surface of the individual crystallites is not accessible for light, being scattered outside of the 75 nm particles. The rather constant size of the primary particles observed by SEM (aggregates of crystallites) could explain the absence of an effect of the different specific surface areas. 4. Conclusion Photocatalytically active rutile samples can be prepared by hydrolysis of TiCl4. The presence of chloride ions and a refluxing treatment promote the formation of the rutile polymorph and increase the activity of the samples. Azide anions favour the formation of anatase and, in moderate amounts, enhances the photoreactivity of rutile. The fair photoactivity of the samples is scarcely related to the specific surface area and crystallite size of the powders and depends probably on the particular physico-chemical features of the surface. References 1. A. Fujishima, K. Hashimoto, T. Watanabe, TiO2 Photocatalysis. Fundamentals and Applications, Bkc Inc., Tokyo, 1999. 2. M. Gopal, W. Moberly, L.C. De Jonghe, J. Mater. Sci. 32 (1997) 6001. 3. H. Zhang, J. F. Banfield, J. Phys. Chem. 104 (2000) 3481. 4. K.N.P. Kumar, K. Keizer, AJ. Burggraaf, J. Mater. Sci. Lett. 13 (1994) 59. 5. H. Cheng, J. Ma, Z. Zhao, L. Qi, Chem. Mater. 7 (1995) 663. 6. M. Addamo, V. Augugliaro, A. Di Paola, E. Garcia-Lopez, V. Loddo, G. Marci, R. Molinari, L. Palmisano, M. Schiavello, J. Phys. Chem. B 118 (2004) 3303. 7. A. Linsebigler, G. Lu, J. Yates Jr., Chem. Rev. 95 (1995) 735. 8. A. Hagfeldt, M. Gratzel, Chem. Rev. 95 (1995) 49. 9. M. Fernandez-Garcia, A. Martinez-Arias, J.C. Hanson, J.A. Rodriguez, Chem. Rev. 104 (2004)4063. 10. A. Pottier, C. Chaneac, E. Tronc, L. Mazerolles, J.P. Jolivet, J. Mater. Chem. 11 (2001) 1116. 11. R.A. SpuiT, H. Myers, Anal. Chem. 29 (1957) 760. 12. A.R. West, Solid State Chemistry and its Applications, John Wiley & Sons, Chichester, 1984. 13. S. Watson, D. Beydoun, J. Scott, R. Amal, J. Nanoparticle Res. 6 (2004) 193. 14. D. Bahnemann, A. Henglein, L. Spanhel, Faraday Discuss. Chem. Soc. 78 (1984) 151. 15. J. Sun, L. Gao, J. Am. Ceram. Soc. 85 (2002) 2382. 16. M. Addamo, V. Augugliaro, A. Di Paola, E. Garcia-Lopez, V. Loddo, G. Marci, L. Palmisano, Colloids Surf. A, 265 (2005) 23.