- Email: [email protected]
Effect of the mechanoactivation on the structure, sorption and photocatalytic properties of titanium dioxide
Materials Chemistry and Physics 110 (2008) 291–298
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of the mechanoactivation on the structure, sorption and photocatalytic properties of titanium dioxide M. Uzunova-Bujnova a , D. Dimitrov a , D. Radev b , A. Bojinova a , D. Todorovsky a,∗ a b
Department of General and Inorganic Chemistry, Faculty of Chemistry, University of Sofia, 1, James Bourchier Blvd., Sofia 1164, Bulgaria Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. Bonchev Street, Building 11, Sofia 1113, Bulgaria
a r t i c l e
i n f o
Article history: Received 12 September 2007 Received in revised form 27 January 2008 Accepted 13 February 2008 Keywords: Tribology Surface properties Oxides
a b s t r a c t The changes in crystallites size, phase composition, morphology, sorption and photocatalytic properties of TiO2 (Degussa P-25) as a result of mechanoactivation in planetary ball mill Pulverisette 5 are reported. These alterations are investigated as a function of the times of milling and storage before the photochemical test, the nature of triboreactor material (agate or steel) and the activation mode (in air or as methanol suspension). Malachite green and methylene blue solutions are used as model pollutants in the photocatalytic tests. The energy stored in the powder after milling is calculated. Decrease of the size of crystallites and morphological grains, small increase of the anatase relative content (at activation in air), a significant increase of the TiO2 sorption ability (especially after mechanoactivation in steel vessels) without substantial changes in the photocatalytic activity and an increase of the photocatalytic effectiveness after mechanoactivation in agate vessel at certain conditions are observed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction High-energy ball milling has been widely used as a method for synthesis and modifying the properties of materials. Under milling conditions, the surrounding of an atom changes in time, because of two mechanisms acting in parallel: thermally activated jumps of point defects, as under classical thermodynamic equilibrium conditions, and forced processes such as shearing, sticking of powders along freshly formed surfaces, etc. [1]. High-energy ball milling is moreover a way of modifying the conditions in which chemical reactions usually take place either by changing the reactivity of as-milled solids (mechanical activation: increase of the reaction rates, lowering of the reaction temperatures of ground powders, etc.) or by inducing chemical reactions during milling (mechanochemical reactions) [2]. In the last few years, the ball milling is widely used for synthesis of Ti-compounds and Ti-based materials, formation of nanocrystals with high specific surface areas [3], for preparation of doped, co-doped and coupled photocatalysts [4–9], synthesis of tabular crystal microstructures [10], as well as so-called mechanically induced self-propagating reactions [11]. However, although there are published models for milling of just pure materials in a planetary ball mill, the process is not yet completely understood [12]. Semiconductor photocatalysis has attracted the attention of many researchers for the past decade for the light-stimulated
∗ Corresponding author. Tel.: +359 2 8161322; fax: +359 2 9625438. E-mail address: [email protected]fia.bg (D. Todorovsky). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.02.005
degradation of both, aqueous and atmospheric pollutants [13,14]. TiO2 is one of the most effective and most widely used photocatalysts, but the intensive studies aiming to improve its effectiveness continue. Phase transformations induced in TiO2 by ball milling have been reported as early as 1965 [15]. The detailed study of the process ´ was performed in the last decade. In a series of papers Begin-Colin et al. [2,16–21] show the formation of the high-pressure TiO2 -II modification in the course of milling as an intermediate in anatase to rutile transformation. The influence of the milling media nature and the milling time, the kinetics and mechanism of the process are studied. The authors found that the zeta potential curves of TiO2 (containing 18% rutile) shift towards acidic pH after 15–60 min of grinding [2]. They suggested future investigation to characterize the nature of observed surface modifications. Ren et al. [22] found that polymorphic transformation of anatase to srilankite and rutile took place during milling. Furthermore, amorphization of the crystalline phases and crystallization of the amorphous phase occurred at the same time in the course of treatment. In their triboreactor (modified Szegvari attritor with WC balls) the local temperature at the collision site could reach about 350 ◦ C for water-cooling and about 450 ◦ C for air-cooling, which (because of the increased defect density in anatase) may cause phase transformation as well. Raman spectroscopy and TEM studies reveal the diminution of the TiO2 to nanometric size, when milled in WC or agate vials by Pulverisette 7 planetary ball mill [23]. Rietveld’s whole powder profile fitting method is applied to extract microstructure information of TiO2 phases grown during the planetary ball milling of anatase [24,25].
292
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
The most rapid decrease of weight fraction of anatase is observed within the first 16 h of milling, where it is decreased from 100% to about 60%. It is transformed mainly to the high-pressure TiO2 -II phase. In summary, the changes in the crystal structure, crystallite size, specific surface area and some surface properties of the TiO2 as a result of its milling at different conditions is rather well studied. However, to the best of our knowledge, only few papers have dealt with the photocatalytic efficiency of the mechanoactivated TiO2 on the degradation of pollutants even in both, UV and visible ranges. Jifan et al. [26] observed that the catalytic efficiency of mechanoactivated TiO2 on the methylene blue degradation becomes poor with the increasing of milling time. Hidalgo et al. [27] found that the changes generated by low-energy grinding considerably alter the intrinsic physicochemical properties of the non-modified TiO2 samples, inducing a decrease in their photoactivities towards Cr(VI) photoreduction. Billik et al. [28] found, however, twice increased photochemical activity of the mechanochemically produced TiO2 using TiOSO4 ·xH2 O and Na2 CO3 as starting materials. Preparation of nitrogen-doped titania with visible light-induced photocatalytic activity by mechanochemical reaction of titania and hexamethylenetetramine was reported by Yin et al. [29,30]. They investigated in detail the effect of the reaction conditions on the phase composition, particle size, specific surface area, amount of nitrogen-doping and microstructure as well as the photocatalysis of nitrogen monoxide destruction [30]. Accounting for the complicated dependences of the milled materials properties on the milling conditions [31] and, on the other hand, of the pollutant degradation on both, its structure and the surface properties of TiO2 [32], in the present work we focused on the changes in phase composition, crystal structure, morphology, sorption ability and the photocatalytic activity on malachite green and methylene blue degradation by commercially available TiO2 Degussa P-25 caused by the mechanoactivation in a planetary ball mill. 2. Experimental
substrates. The films were heated at 300 ◦ C after the deposition. The details of the coating procedure will be described elsewhere. 2.2. Mechanoactivation The mechanoactivation was performed in a planetary ball mill Pulverisette 5 (Fritsch) in stainless steel (80 cm3 ) or agate (50 cm3 ) vessels and balls of the same material (10 mm in diameter) for 15–300 min at 200 rpm. The TiO2 was treated as a powder in an air atmosphere or as a suspension in methanol (1 g TiO2 /cm3 methanol). In both cases powder to balls weight ratios were R = 1/8 and R = 1/4 in steel and agate vessels, respectively; the difference is due to difference in the density of ball materials. After the treatment the suspension was filtered and the TiO2 was dried for a short time in air. The activation in stainless steel vessels unavoidably leads to some contamination of TiO2 with iron. Its content in 5 h, air-milled material was determined by atomic absorption spectrometry to be 1.2 ± 0.2%. Despite that fact this type of triboreactors are widely used due to the heavier mechanical loading of treated material. 2.3. Analysis The crystal structure, phase composition and crystallite size of the treated materials were determined by X-ray diffractometry (XRD) using a powder diffractometer Siemens D-500 (Germany), with Cu K␣ radiation, 40 kV, 30 mA, 0.01◦ s−1 . SEM images were taken by JEOL JSM 5510 (Japan). The IR spectra were taken by Specord 75 spectrometer (Carl Zeiss, Germany). The surface was determined by krypton adsorption at 77 K. The value of 50 m2 g−1 found for the initial product is the same as one reported by Hidalgo et al. [27]. No special measures were taken for deaglomeration of the materials. 2.4. Photocatalytic tests The sorption and photocatalytic behaviour of the mechanically activated materials were tested in slurry (loading 1 g dm−3 MG solution) or by immersing the above described films in 250 cm3 MB solution. The procedure described in Ref. [33] was followed. The suspension was magnetically agitated for 30 min in dark at a carefully controlled ambient temperature (20 ± 2 ◦ C). After that the system was illuminated with UV lamp (Sylvania 18 W BLB T8, light wavelength 350–400 nm with a maximum around 360 nm) placed at a distance of 9 cm from the top of the suspension. The light intensity on the top of suspension was 5 × 10−5 W cm−2 . Air was bubbled through the solution (1.4 dm3 min−1 ). The change in pollutant concentration was determined spectrophotometrically from the band of MG at 616 nm and of MB at 662 nm. No concentration change was observed (within the measurement error limits) when the dye is treated at the same conditions, but without TiO2 .
2.1. Materials
3. Results and discussion Commercially available TiO2 Degussa P-25 was used in this study. Malachite green oxalate (MG, supplied by Chroma) and methylene blue (MB, Fluka), representatives of the most important impurities in the textile industry wastewater were used as model pollutants as 10−5 M and 2 × 10−6 aqueous solutions, respectively. Their structures are shown below:
Limited number of experiments was performed by immersing TiO2 film in MB solution. Films of ∼0.6 mg cm−2 in thickness and with a surface area of 5 cm2 from mechanoactivated and not activated TiO2 samples were deposited by the spray method using suspension of the oxide in methanol as started material and O2 as a carrier gas. Carefully cleaned prior the use matted microscopic slides were used as
3.1. Influence of the mechanoactivation on the crystal structure and phase composition of TiO2 X-ray diffractograms of the initial and activated materials are shown in Fig. 1. The data for the interplanar distances and the relative intensity of the reflexes are shown in Table 1. (The reflexes relative intensities IA and IR for anatase and rutile are measured for each phase separately.) It is seen that the mechanoactivation under the described conditions does not change significantly the TiO2 interplanar distances. Taking into account the degree of deformation of the crystal lattice a, the amount of stored energy during the ball milling was estimated as W = 3(a/a)2 E [34], where W is the energy of microdeformation, a is the cell constant and E is the Young’s module of elasticity of titania (230 GPa [35]). The calculated value of stored energy for different milling conditions is presented in Table 2. It was calculated by using the above formula for all interplanar distances for both anatase and rutile and after that taking an average value for certain milling conditions. The lower value of stored energy during the milling in air in a stainless steel vessel than in a agate one is quite surprising—the heavier iron loading should be more effective in increase of the energy of microdistortion. This contradiction could be explained with the effect of iron ions presented in TiO2 milled in a steel triboreactor. Two types of processes for Fe2+ production could be considered: the mechanoactivated electron emission and the elec-
Table 1 ˚ relative intensity, (I,%) and Miller indexes (h k l) of the TiO2 mechanoactivated at different milling conditions Interplanar distances (d, A), Rutile JCPDS 77-0442
Anatase JCPDS 84-1286
Mechanoactivated TiO2 in: Agate vessels
In air
In air
As suspension
As suspension
1h d 3.2557 2.4944
IR
hkl
999 434
d
IA
hkl
d
IA
3.5140
999
101
3.51 3.24 2.488 2.417 2.380
100
110 101 2.4283 2.3755 2.3305
2.3021 2.1932 2.0591
1.6917 1.6278
64 169 60
465 129
62 190 72
103 004 112
200 111 210 1.8911 1.6981
249 158
200 105
1.6652
155
211
211 220
1.4838
63
002
1.4560 1.4272 1.3632
59 4 142
310 221 301
1.3502
73
112
1.3071 1.2770
7 2
311 320
1.4920
27
213
1.4795
118
204
2.298 2.186 2.054 1.892 1.688 1.667 1.626
IR 100 51
8 18 8 31 7 16
d
IA
3.50 3.24 2.481 2.416 2.365 2.326
100
2.181 2.049 1.888 1.694
IR 100 47
6 17 8 24 10 25 16
5h
d
IA
3.51 3.24 2.481 2.430 2.374 2.327
100
2.181 2.049 1.890 1.695
IR 100 53
7 20 8 28 10 25 17
80 16 23
1.481 1.453 1.406
12 7 7
1.664 1.621 1.535
17
1.478 1.449
26
17
16
1.664 1.621
1.481 1.455
13
16
1h
d
IA
IR
3.51 3.24 2.481 2.417 2.368 2.332 2.293 2.186 2.051 1.891
100 100 49 8 17 8 13 28 10 25
21
1.685 1.667 1.623
1.479 1.454
10
10
5h
d
IA
3.51 3.24 2.481 2.430 2.374 2.332
100
2.181 2.051 1.890 1.698
IR
d
IA
3.51 3.24 2.481 2.430 2.374 2.327 2.293 2.186 2.054 1.888 1.697
100
1.664 1.621
17
19
1.479 1.451
13
12
100 53 6 18 7 23 10 23 16
IR 100 44
7 19 9 16 24 10 25 17
79 12
1.667 1.623
15
20
1.479 1.450
11
10
18
2
10
1.3627
51
116
1.361
10
1.361
16
1.359
7
1.359
7
1.360
7
1.360
8
1.3372
55
220
1.337
7
1.336
14
1.339
6
1.341
6
1.338
6
1.337
6
1.2776 1.2635
5 85
107 215
1.263
7
1.263
17
1.263
7
1.263
7
1.263
8
1.263
8
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
Stainless steel vessels (5 h)
293
294
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298 Table 3 Relative content of anatase and crystallites sizes of the mechanoactivated TiO2 Mechanoactivation
Fig. 1. X-ray diffractograms of the initial TiO2 Degussa P-25 (a), mechanoactivated 5 h in: air in stainless steel (b) and agate (d) vessels and as methanol suspension in stainless steel (c) and agate (e) vessels. The Miller indexes are shown: R, rutile; A, anatase; T, TiO2 -II.
Table 2 Calculated value for the stored energy (J cm−3 ) of microdistortion of TiO2 mechanoactivated for 5 h at different milling conditions Vessel Stainless steel In air As suspension Agate In air As suspension
4±5 9±8 10 ± 8 8 ± 10
trochemical process. During the milling in a stainless steel vessel, microscopic pieces of iron are formed and the defects concentration in the iron crystal lattice will rise. This may produce areas with different electrode potentials, i.e. the metal is no longer under standard conditions, and a sort of cell is produced. As the milling is done simply in air, this cell becomes active by using oxygen, water and, probably, an electrolyte produced with the participation of gaseous impurities in the air. Further, the processes leading to the formation of iron ions will be analogous to the processes of iron rusting described in Ref. [36]. The formed Fe2+ -ions (very similar in size to Ti4+ [36]) can incorporate into the free regular titanium positions in TiO2 , as occurs in the general doping of TiO2 with iron [37]. According to the latter, the solubility of iron into TiO2 at atmospheric pressure changes from 1% at 800 ◦ C up to 3% at 1350 ◦ C. The incorporation of Fe2+ into the free positions for titanium ions will lead to decrease of the point defects number.
Time (min)
Vessel
Mode
Content of anatase (%)
0 300 300 60 300 60 300
– Steel Steel Agate Agate Agate Agate
– Air Suspension Air Air Suspension Suspension
85 74 85 85 79 85 85
± ± ± ± ± ± ±
2 2 2 2 2 2 2
Size of crystallites (nm) Anatase
Rutile
45 31 40 38 33 39 38
66 47 53 47 42 48 50
Two possible reasons could be responsible for the higher value of stored energy in the case of milling the material in stainless steel vessels as a suspension than that in air. The reduced particle size, much more caused by the wet milling (Fig. 2), could change the cell constant of the material [34], which will alter in turn the stored energy. The second possible reason is the lower local temperature of the as-suspension-milled material, which will give lesser amount of Fe2+ diffused into the TiO2 crystal lattice. The data for the crystallites sizes determined by the broadening of |1 0 1| and |1 1 0| reflexes for anatase and rutile, respectively, and the phase composition are summarized in Table 3. The weight fraction of anatase f, is calculated from the integrated intensities of |1 0 1| reflection of anatase, IA , and |1 1 0| reflection of rutile, IR , using the equation [38]: f = 1/(1 + 1.26IR /IA ). The data show that the milling in air leads to crystallite size reduction with 31% and 29% for anatase and rutile, respectively, in the case of stainless steel vessel and with 27% and 36%, when the activation is performed in agate vessel. The mechanoactivation of TiO2 as suspension in methanol does not change its phase composition and the decrease of the crystallites size is significantly lower: 12% and 20% for anatase or rutile, respectively, in stainless steel vessels and 15% and 24% in agate vessels. It seems (Table 3) that 1 h activation in suspension in agate vessel is enough to reach the described changes. It is worth mentioning that the changes in crystallite size resulted from the different activation modes are, generally, in accordance with the data for the stored energy (Table 2). However, the phase transformation of anatase to the more stable rutile modification is more pronounced when the activation is performed in air. It is seen that longer activation time and heavier mechanical loading (realized with steel tools) promotes the process, which normally takes place above 550 ◦ C. As can be expected, the size of morphological grains decreases significantly, especially in the case of milling as suspension (Figs. 2 and 3). The particle average diameters decrease from 118 ± 40 nm of the initial TiO2 to 88 ± 30 nm and 56 ± 12 nm after activation for 5 h in agate vessel in air and as suspension, respectively (the standard deviation is shown). The much less decrease in grain size as a result of the activation in air is in agreement with previous works. The crushing is more effective in the case of wet milling, because of the unwedging effect
Fig. 2. SEM images of initial TiO2 (a) and after mechanoactivation for 5 h in agate vessel in air (b) and as suspension in methanol (c), (magnification 50,000×).
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
295
Fig. 3. Size distribution of the titania powder treated by different ways and estimated from Fig. 2.
of liquid into the defects of crystallites as well as the prevention of cracks to stick together. At the same time, this activation mode gives more uniform size distribution of the particles (Fig. 3). The results for surface specific area (Table 4) are in agreement with the above reported results. The results obtained by air milling in agate are little higher than the ones found in [27] with Retsch MM2000 mixing-mill. The result for 30 min-milling in steel vessel is also little higher than the found in [2] but for anatase containing 18% rutile. EPR signal with g = 2.0019 due to the presence of Ti3+ was found in the sample activated for 5 h in air in agate vessel. This signal was not observed in the non-activated sample. The appearance of Ti3+ signal in the activated sample is consistent with the observations made in Ref. [39], where it results from the mechanical treatment into planetary ball mill AGO-2. Table 4 Specific surface area (m2 g−1 ) of the milled products Milling time (min)
Triboreactor type/milling medium Agate in
30 300
Steel in
Air
Suspension
Air
Suspension
53 56
54 58
58 65
60 69
Fig. 4. IR spectra (800–400 cm−1 ) of TiO2 (a) and of the same sample after 5 h of milling in stainless steel vessel as methanol suspension (b) and in air (c).
These observations can be interpreted based on the fact that at the down boundary of homogeneity area (range) of TiO2 the interstitial ions of titanium are the main type of defects [37]. Such a state is realized at high temperature and low oxygen pressure meaning that in that case increasing the Ti4+ vibration amplitude is caused by changing the surroundings of the ion, because of increasing the number of oxygen vacancies and, consequently, weaker bonding of the Ti4+ -ions. The high-energy milling causes the Ti4+ ions to vibrate with higher amplitude and they can thus leave their equilibrium positions going to interstitial states. At the same time the so-formed interstitial Ti4+ -ion can participate in the reaction e− + Tii 4+ → Tii 3+ [39] producing interstitial Ti3+ -ion. The IR spectrum (800–400 cm−1 ) of the initial TiO2 (Fig. 4a) shows bands at 640 cm−1 and 450 cm−1 , which are ascribed to anatase according to Refs. [26,40]. After milling for 5 h in both cases (the suspension and air) the characteristic band of anatase at 640 cm−1 is “blue”-shifted and combined with that at 450 cm−1 (Fig. 4b, c). The pattern formed after 5 h is characteristic for TiO2 -II [40]. The latter authors also found disappearance of the 450 cm−1 band as a result of prolonged milling. The presence of this polymorph in our sample is confirmed also by XRD analysis (Fig. 1). The amount of TiO2 -II at the experimental conditions applied in
296
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
Fig. 5. Sorption and degradation of MG under the action of TiO2 () and TiO2 , mechanoactivated in agate vessel for 1 h as suspension in methanol (). In the inset the experimental data (points) and the theoretical fits of the data (solid lines) are shown (see the text, also). Correlation coefficients for the linear approximation of the semi-logarithmic plot are 0.9949 for the fresh and 0.9862 for the milled materials, respectively.
the present work is rather low and it is hardly to believe that its influence on the mechanoactivated TiO2 behaviour will be significant.
(ii)
3.2. Sorption and photocatalytic properties of the mechanoactivated TiO2 Data for the sorption of the pollutant on the TiO2 surface during the equilibrium 30 min “dark” period in the course of the photocatalytic tests are shown in Table 5. The kinetics of the degradation process can be followed by fitting the experimental data according to the equation Ct /Cd = exp[−k(t–t0 )] or ln (Cd /Ct ) = kt − a0 , where Ct , concentration in the moment t; t, illumination time, Cd , concentration at the end of the “dark” period (corresponding to time t0 after loading of the system); k, rate constant (the slope of the straight lines in the figure); a0 , intercept of the lines. The standard data fitting is performed first on the semilogarithmic plots ln (Cd /Ct ) (Fig. 5). The time constant t0 is determined from the relationship a0 = kt0 . Then the curves Ct /Cd (the inset to the figure) are drawn by the least square method using the predetermined k and t0 at one adjustable parameter Cd along with the experimental points. The calculated rate constants are summarized in Table 5.
(iii)
(iv)
3.2.1. Sorption Generally, the sorption of the dye on the mechanoactivated TiO2 is equal to or greater than the one on the untreated product. The decrease of the pollutant concentration, found at the end of the equilibrium period (Table 5), depends on: (i) The milling vessel material. The adsorbed amount of dye on TiO2 activated for 5 h in the stainless steel vessel is five times greater than that on the inactivated sample, comparing with approximately 150–200% increase resulting from the mechanoactivation in agate vessel using both activation modes. The difference looks quite surprising, because the energy, stored in TiO2 as a result of milling in steel vessel (Table 2) is less, which means less number of defects and, consequently, less number of defects that could play a role of sorption centres will exist. For explanation of this fact, the role of the iron introduced by ball milling should be considered. It is known [41] that iron doping induces a structural transformation from anatase
(v)
to rutile and the electrical measurements indicate that iron acts as an acceptor impurity. A transition from n-type to ptype conduction at an iron concentration of 0.13 mol% is shown. In the studied system, the iron concentration of 1.2 ± 0.2% will lead clearly to p-type conductance of TiO2 even if only a part of the iron is incorporated as Fe2+ in the TiO2 . The increasing of the p-type impurity concentration in TiO2 will shift the Fermi level down relative to the top of valence band as is well known from the semiconductors physics fundamentals. According to Ref. [42] when the Fermi level is moving down, the adsorption ability of the semiconductor surface relatively to gases that obey donor properties increases. The lower is the Fermi level in the band gap of TiO2 , the higher is the adsorption ability for particles that obey donor properties. Because of high concentration of Fe2+ , we expect the Fermi level to shift down enough for noticeable promotion of the adsorption of molecules having donor’s properties. Consequently, when these donor species are chemisorbed at a semiconductor’s surface, they will be able to localize the free hole from the vicinity of semiconductor’s surface. Because of the high electron density of the benzene ring, it can be expected the molecule of MG to follow such pattern, leading to the above reported observations. The activation mode (in air or as suspension). The effects of the milling in air and as suspension at equal activation times are similar (Table 5), suggesting that the specific surface area (which is bigger after milling in suspension) is not the only factor determining the adsorption and that the mechanically induced changes of surface properties should be considered as well. The time of mechanoactivation. The activation in agate as methanol suspension for less than one hour has no significant effect on the TiO2 sorption ability (Table 5). The activation for 1 h leads to an increase of the adsorption ability approximately by twofold. It seems that such an activation time is sufficient to reach a “saturation” of the effect of activation on the adsorption ability. Activation in air for 5 h leads to a decrease of the sorption when compared to 1 h activation at the same conditions. This fact could be explained with the agglomeration, taking place at longer activation times. The agglomeration is less expressed for activation in a suspension and, accordingly, no significant difference between samples activated for 1 h and 5 h is observed. The time between the mechanoactivation and the photocatalytic test. The changes of materials sorption properties as a result of storage after mechanoactivation strongly depend on the activation mode (Table 5, Fig. 6). The sorption ability of TiO2 milled in agate vessel as suspension is not altered after 90 days of storage. The efficiency of the oxide milled also as suspension, but in steel triboreactor, decreases twice. However, TiO2 milled in the same vessel, but, in air, shows twice as increased efficiency after the storage. No such radical changes have been observed after storage of the activated material for the same time, but in liquid N2 . These facts show that the increased dispersity is not the only and not the main factor responsible for the changed adsorption ability. To our opinion, the reported observations strongly suggest that crystal defects introduced in the course of milling and their relaxations in time are of significant importance for the sorption behaviour of the mechanoactivated TiO2 . The pollutant chemical nature. The sorption ability of nonactivated TiO2 to MB is higher than that to MG, but its increase as a result of air-activation in stainless steel vessels is 2.5 times lower than the respective increase in the case of MG sorption. Obviously, a complicated interrelation between the pollutant
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
297
Table 5 Sorption and photocatalytic degradation of malachite green and methylene blue on mechanoactivated TiO2 a Mechanoactivation
Storage after activation (days)
Time (min)
Vessel
Medium
0 60 300 15 30 60 180 300 300 300 300 300 300 0 300
– Agate Agate Agate Agate Agate Agate Agate Agate Steel Steel Steel Steel – Steel
– Air Air Methanol Methanol Methanol Methanol Methanol Methanol Air Methanol Methanol Air – Air
a
–
90
90 90
Pollutant (photocatalytic test mode)
(C0 − Cd )/C0 (%)
k (min−1 )
MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MB (film) MB (film)
9 20 13 8 7 19 17 18 18 45 50 27 92 14 28
0.0140 0.0140 0.0155 0.0123 0.0116 0.0243 0.0144 0.0155 0.0132 0.0029 0.0033
0.00219 0.00174
C0 , dye initial concentration; Cd , concentration at the end of the “dark” period; k, photodegradation rate constant.
Few effects have to be considered as possible factors determining the behaviour of mechanoactivated TiO2 :
Fig. 6. Sorption and degradation of MG under the action of TiO2 mechanoactivated for 5 h in stainless steel vessel in air () and as suspension in methanol () 3 months after the activation.
nature and the mechanically induced changes in the TiO2 does exist. 3.2.2. Photochemical degradation The data for photochemical degradation rate constants (Table 5) could be interpreted as follows: (i) The photocatalytic activity of TiO2 , milled in agate vessel as methanol suspension, has a maximum at 1 h of milling time. The rate constant at these conditions is with 25% higher than the one of non-activated oxide. Longer activation times lead to a slight decrease in the reaction rate. (ii) The activity of material treated in the same triboreactor but in air increases slowly with the activation time increase reaching the rate constant value, 20% higher than of the initial TiO2 . (iii) The activity of the TiO2 , air-milled in steel vessel is significantly lower then that of the non-treated titania—with more than 75% for MG and with ∼20% for MB degradation. The latter result is, generally, in agreement with the results reported in Ref. [26]. Obviously the chemical nature of the pollutant is of significant importance for the photocatalytic performance of the mechanically treated material.
(i) The increased dispersity will enhance the sorption and photocatalytic process, taking place on the TiO2 surface. However, rather similar results for the rate constant from samples, activated in air or as suspension (the latter leading to higher specific surface area) supports the affirmation that this factor is not of a primary importance for determination of the activated TiO2 behaviour. (ii) Particles with different size are expected to posses different numbers of point defects, which in the case of a nonstoichiometric oxide as TiO2 play the role of charge carriers. As known [14,43,44], the latter affect strongly the photocatalytic properties of TiO2 . In addition to that, for small colloidal particles, there is nearly no band bending and the existing electric field is usually small. So that high doping levels are needed to produce a significant potential difference (permanent electric field) between the surface and the centre of particles to efficiently separate the photoinduced electron–hole pair [44]. (iii) The higher photoreactivity of anatase is due to its slightly higher Fermi level, lower capacity to adsorb oxygen and higher degree of hydroxylation (i.e. number of hydroxy groups on the surface) [14]. The decrease of anatase relative content as a result of mechanoactivation is decreasing the catalytic activity of TiO2 as well. The result reported in Table 3 shows that this process takes place only during the activation in air in steel vessel and could be considered as one of the factors leading to the decreased photocatalytic activity of air-milled TiO2 . Accounting for this effect, it can be supposed that the mechanoactivation could be more promising if performed with anatase. It is known that small amount of rutile in the photocatalyst (which will be produced for not very long activation times) will trap electrons from the anatase conduction band thus reducing the recombination rate of the photoinduced electron–hole pairs. (iv) In some cases [45], the mechanoactivation in air may cause dehydration of the milled substance. The decrease of the relative band intensity around 3200 cm−1 in the IR spectrum of the air-milled TiO2 confirms that such a process takes place with as well. The effect of dehydroxylation on the photocatalytic activity was already mentioned. Few consequences of it are potentially important for explanation of the reported observations. The decrease of surface TiIV -OH− species con-
298
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
centration will decrease the production of oxidizing agent, • OH , thus suppressing the dye degradation. Contrary, the holes formed as a result of dehydroxylation will enhance the reac• tion h+ + OH− → OH thus forming primary oxidizing agent. However, it is difficult to predict whether these holes will be energetically suitable electron traps to ensure the progressing of reaction. In the same time, if traps for holes, more effective than OH− , are formed as a result of milling, the production • of OH will decrease. The results obtained for the rate constant for air-milled TiO2 suggest that the negative effect of the dehydroxylation dominates. (v) The data in Table 5 reveal a clear dependence between the TiO2 sorption ability and the photocatalytic activity. The materials milled as suspension in agate vessel show relatively weak sorption properties and relatively higher photodegradation efficiency increasing simbatically with the sorption. Heavily milled product (in steel vessels and, to some extent, in agate vessel, but in air) posses a rather strong adsorption. It could be supposed that the latter impedes the degradation of pollutant. In some cases (Fig. 6), the effect of photodesorption (phenomenon, which quite often appears on semiconductor’s surfaces [42]) is observed leading to the increase of the dye concentration in the solution. However, it is quite probable that some crystal defects (for example hole traps, mentioned above) formed as a result of milling can be the main factor for the degradation rate decrease. It is well known that the milling in air resulted in higher concentration of the crystal defects (than the treatment at the same conditions but in suspension) thus leading to the observed in negative effect of the air milling on the TiO2 photocatalytic ability. (vi) As we mentioned above, some Ti3+ is formed as a results of milling of TiO2 in agate vessel, because of the reduction of interstitial Ti4+ . It acts like a general p-type dopant that works as a charge-carrier recombination centre, advancing the recombination of electron–hole pairs, and then photoactivity may decrease [46]. 4. Conclusion The reported results reveal some new data on the influence of mechanoactivation conditions on the values of stored energy, crystal structure, morphology and phase composition of the most widely used photocatalyst Degussa P-25 TiO2 . The sorption ability of the material significantly increases as a result of mechanoactivation due not only to the increased dispersity, but also and, probably, mainly to the changes of the surface properties, phase composition and crystal defect concentration. However, the rather strong sorption impedes the photocatalytic degradation of the dye and, in order to benefit from the mechanoactivation, a suitable conjunction of the conditions of its performance should be found. The obtained results suggest that a relatively moderate mechanical loading get by milling of TiO2 as suspension in agate vessel increases the photodegradation rate constant with ∼50%. It seems that the effect of the mechanoactivation depends on the pollutant structure as well.
Acknowledgements The study is performed with the financial support of NATO “Science for Peace” Program (Contract SfP 977986). The authors are
indebted to Dr. C. Dushkin (Faculty of Chemistry, University of Sofia) for the help in data analysis and valuable discussion, Dr. A. Apostolov (Faculty of Chemistry, University of Sofia) for the X-ray diffractograms recording and to Mrs. R. Todorovska (Institute of Electronics, Bulgarian Academy of Sciences) for the films preparation. References [1] Y. Chen, M. Bibole, R. Le Hazif, G. Martin, Phys. Rev. B 48 (1993) 14. ´ ¨ F. Villieras, X. Devaux, M.O. Simonnot, T. Girot, P. [2] S. Begin-Colin, G. Le Caer, Weisbecker, J. Metastable Nanocryst. Mater. 12 (2002) 27. [3] J. Wang, S. Yin, T. Sato, Mater. Sci. Eng. B 126 (2006) 53. [4] J. Wang, S. Yin, Q. Zhang, F. Saito, T. Sato, Solid State Ionics 172 (2004) 191. [5] J. Wang, S. Yin, M. Komatsu, T. Sato, J. Eur. Ceram. Soc. 25 (2005) 3207. [6] G. Liu, F. Li, Z. Chen, G.Q. Lu, H.-M. Cheng, J. Solid State Chem. 179 (2006) 331. [7] C. Shifu, C. Lei, G. Shen, C. Gengyu, Mater. Chem. Phys. 98 (2006) 116. [8] C. Shifu, C. Lei, G. Shen, C. Gengyu, Powder Technol. 160 (2005) 198. [9] S. Yin, H. Yamaki, Q. Zhang, M. Komatsu, J. Wang, Q. Tang, F. Saito, T. Sato, Solid State Ionics 172 (2004) 205. [10] H. Liu, X. Sun, Q. Zhao, J. Xiao, S. Ouyang, Solid State Electron. 47 (2003) 2295. [11] L. Takacs, Progr. Mater. Sci. 47 (2002) 355. ´ [12] J. Alkebro, S. Begin-Colin, A. Mocellin, R. Warren, J. Solid State Chem. 164 (2002) 88. [13] K.T. Ranjit, I. Willner, S.H. Bossmann, A.M. Braun, Environ. Sci. Technol. 35 (2001) 1544. [14] O. Carp, C.L. Huisman, A. Reller, Progr. Solid State Chem. 32 (2004) 33. [15] Y. Tekiz, C. Legrand, C.R. Acad. Sci., Paris 261 (1965) 3619. ´ ¨ M. Zandona, E. Bouzy, B. Malaman, J. Alloys Compd. [16] S. Begin-Colin, G. Le Caer, 227 (1995) 157. ´ ¨ A. Mocellin, M. Zandona, Phil. Mag. Lett. 69 (1994) 1. [17] S. Begin-Colin, G. Le Caer, ´ ¨ A. Mocellin, J. Mater. Synth. Process [18] T. Girot, S. Begin-Colin, X. Devaux, G. Le Caer, 8 (2000) 139. ´ ¨ A. Mocellin, J. Solid State Chem. 149 (2000) [19] S. Begin-Colin, T. Girot, G. Le Caer, 41. ´ [20] T. Girot, X. Devaux, S. Begin-Colin, G. Le Caer, A. Mocellin, Phil. Mag. A 81 (2001) 489. [21] S. Coste, G. Bertrand, Codoler, E. Gaffet, H. Hahn, Sieger H., J. Alloys Compd. 434–435 (2007) 489. [22] R. Ren, Z. Yang, L.L. Shaw, J. Mater. Sci. 35 (2000) 6015. ´ M. Stubicar, M. Ivanda, K. Furic, J. Mol. Struct. 563–564 (2001) 315. [23] A. Gajovic, [24] H. Dutta, P. Sahu, S.K. Pradhan, M. De, Mater. Chem. Phys. 77 (2002) 153. [25] P. Bose, S.K. Pradham, S. Sen, Mater. Chem. Phys. 80 (2003) 73. [26] H. Jifan, Q. Hongwei, S. Zhengui, L.H. Liang, Rare Met. Mater. Eng. 32 (2003) 5. ´ J.A. Nav´ıo, J. Photochem. Photobiol. A 148 (2002) 341. [27] M.C. Hidalgo, G. Colon, ´ J. Phys. Chem. [28] P. Billik, G. Plesch, V. Brezova, L. Kuchta, M. Valko, M. Mazur, Solids 68 (2007) 1112. [29] S. Yin, Q. Zhang, F. Saito, T. Sato, Chem. Lett. 32 (2003) 358. [30] S. Yin, H. Yamaki, M. Komatsu, Q. Zhang, J. Wang, Q. Tang, F. Saito, T. Sato, J. Mater. Chem. 13 (2003) 2996. [31] G.B. Schaffer, J.S. Forrester, J. Mater. Sci. 32 (1997) 3157. [32] S. Matsuo, N. Sakaguchi, K. Yamada, T. Matsuo, H. Wakita, Appl. Surf. Sci. 228 (2004) 233. [33] M. Kostadinov, P. Georgiev, A. Bojinova, C. Dushkin, in: E. Balabanova, I. Dragieva (Eds.), Nanoscience and Nanotechnology, 3, Heron Press, Sofia, 2003, pp. 207–209. [34] A.A. Polyakov, Technology of Ceramic’s Radio-electronics Materials, Radio I Ssvyaz, Moscow, 1989, p. 52, 54 (in Russ.). [35] http://www.azom.com/details.asp?ArticleID=1179# Key Properties. [36] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Butterworth/Heinemann/Pergamon Press, Oxford, 1998, p. 963, 1076. [37] P. Kofstad, Nonstoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, Wiley-Interscience, New York, 1972, p. 153, 159 (Russian edition). [38] R. Spurr, H. Myers, Anal. Chem. 29 (1957) 760. [39] V.V. Zyryanov, Neorganisheskie Materiali 41 (1) (2005) (in Russ.). ´ K. Furic, ´ N. Tomaˇsic, ´ S. Popovic, ´ Zˇ . Skoko, S. Music, ´ J. Alloys Compd. [40] A. Gajovic, 398 (2005) 188. ´ [41] A.R. Bally, E.N. Korobeinikova, P.E. Schmid, F. Levy, F. Bussy, J. Phys. D: Appl. Phys. 31 (1998) 1149. [42] F.F. Wolkenstein, Electronic Processes at the Surface of Semiconductors During the Chemisorptions, Nauka, Moscow, 1987, p. 118, pp. 285–286 (in Russ.). [43] M.A. Fox, M.T. Dulay, Chem. Rev. (1993) 431. ¨ [44] A. Hagfeldt, M. Gratzel, Chem. Rev. 95 (1995) 49. [45] D. Todorovsky, A. Terziev, M. Milanova, Hydrometallurgy 45 (1997) 13. [46] F.B. Li, X.Z. Li, M.F. Hou, Appl. Catal. B: Environ. 48 (2004) 185.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of the mechanoactivation on the structure, sorption and photocatalytic properties of titanium dioxide M. Uzunova-Bujnova a , D. Dimitrov a , D. Radev b , A. Bojinova a , D. Todorovsky a,∗ a b
Department of General and Inorganic Chemistry, Faculty of Chemistry, University of Sofia, 1, James Bourchier Blvd., Sofia 1164, Bulgaria Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. Bonchev Street, Building 11, Sofia 1113, Bulgaria
a r t i c l e
i n f o
Article history: Received 12 September 2007 Received in revised form 27 January 2008 Accepted 13 February 2008 Keywords: Tribology Surface properties Oxides
a b s t r a c t The changes in crystallites size, phase composition, morphology, sorption and photocatalytic properties of TiO2 (Degussa P-25) as a result of mechanoactivation in planetary ball mill Pulverisette 5 are reported. These alterations are investigated as a function of the times of milling and storage before the photochemical test, the nature of triboreactor material (agate or steel) and the activation mode (in air or as methanol suspension). Malachite green and methylene blue solutions are used as model pollutants in the photocatalytic tests. The energy stored in the powder after milling is calculated. Decrease of the size of crystallites and morphological grains, small increase of the anatase relative content (at activation in air), a significant increase of the TiO2 sorption ability (especially after mechanoactivation in steel vessels) without substantial changes in the photocatalytic activity and an increase of the photocatalytic effectiveness after mechanoactivation in agate vessel at certain conditions are observed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction High-energy ball milling has been widely used as a method for synthesis and modifying the properties of materials. Under milling conditions, the surrounding of an atom changes in time, because of two mechanisms acting in parallel: thermally activated jumps of point defects, as under classical thermodynamic equilibrium conditions, and forced processes such as shearing, sticking of powders along freshly formed surfaces, etc. [1]. High-energy ball milling is moreover a way of modifying the conditions in which chemical reactions usually take place either by changing the reactivity of as-milled solids (mechanical activation: increase of the reaction rates, lowering of the reaction temperatures of ground powders, etc.) or by inducing chemical reactions during milling (mechanochemical reactions) [2]. In the last few years, the ball milling is widely used for synthesis of Ti-compounds and Ti-based materials, formation of nanocrystals with high specific surface areas [3], for preparation of doped, co-doped and coupled photocatalysts [4–9], synthesis of tabular crystal microstructures [10], as well as so-called mechanically induced self-propagating reactions [11]. However, although there are published models for milling of just pure materials in a planetary ball mill, the process is not yet completely understood [12]. Semiconductor photocatalysis has attracted the attention of many researchers for the past decade for the light-stimulated
∗ Corresponding author. Tel.: +359 2 8161322; fax: +359 2 9625438. E-mail address: [email protected]fia.bg (D. Todorovsky). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.02.005
degradation of both, aqueous and atmospheric pollutants [13,14]. TiO2 is one of the most effective and most widely used photocatalysts, but the intensive studies aiming to improve its effectiveness continue. Phase transformations induced in TiO2 by ball milling have been reported as early as 1965 [15]. The detailed study of the process ´ was performed in the last decade. In a series of papers Begin-Colin et al. [2,16–21] show the formation of the high-pressure TiO2 -II modification in the course of milling as an intermediate in anatase to rutile transformation. The influence of the milling media nature and the milling time, the kinetics and mechanism of the process are studied. The authors found that the zeta potential curves of TiO2 (containing 18% rutile) shift towards acidic pH after 15–60 min of grinding [2]. They suggested future investigation to characterize the nature of observed surface modifications. Ren et al. [22] found that polymorphic transformation of anatase to srilankite and rutile took place during milling. Furthermore, amorphization of the crystalline phases and crystallization of the amorphous phase occurred at the same time in the course of treatment. In their triboreactor (modified Szegvari attritor with WC balls) the local temperature at the collision site could reach about 350 ◦ C for water-cooling and about 450 ◦ C for air-cooling, which (because of the increased defect density in anatase) may cause phase transformation as well. Raman spectroscopy and TEM studies reveal the diminution of the TiO2 to nanometric size, when milled in WC or agate vials by Pulverisette 7 planetary ball mill [23]. Rietveld’s whole powder profile fitting method is applied to extract microstructure information of TiO2 phases grown during the planetary ball milling of anatase [24,25].
292
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
The most rapid decrease of weight fraction of anatase is observed within the first 16 h of milling, where it is decreased from 100% to about 60%. It is transformed mainly to the high-pressure TiO2 -II phase. In summary, the changes in the crystal structure, crystallite size, specific surface area and some surface properties of the TiO2 as a result of its milling at different conditions is rather well studied. However, to the best of our knowledge, only few papers have dealt with the photocatalytic efficiency of the mechanoactivated TiO2 on the degradation of pollutants even in both, UV and visible ranges. Jifan et al. [26] observed that the catalytic efficiency of mechanoactivated TiO2 on the methylene blue degradation becomes poor with the increasing of milling time. Hidalgo et al. [27] found that the changes generated by low-energy grinding considerably alter the intrinsic physicochemical properties of the non-modified TiO2 samples, inducing a decrease in their photoactivities towards Cr(VI) photoreduction. Billik et al. [28] found, however, twice increased photochemical activity of the mechanochemically produced TiO2 using TiOSO4 ·xH2 O and Na2 CO3 as starting materials. Preparation of nitrogen-doped titania with visible light-induced photocatalytic activity by mechanochemical reaction of titania and hexamethylenetetramine was reported by Yin et al. [29,30]. They investigated in detail the effect of the reaction conditions on the phase composition, particle size, specific surface area, amount of nitrogen-doping and microstructure as well as the photocatalysis of nitrogen monoxide destruction [30]. Accounting for the complicated dependences of the milled materials properties on the milling conditions [31] and, on the other hand, of the pollutant degradation on both, its structure and the surface properties of TiO2 [32], in the present work we focused on the changes in phase composition, crystal structure, morphology, sorption ability and the photocatalytic activity on malachite green and methylene blue degradation by commercially available TiO2 Degussa P-25 caused by the mechanoactivation in a planetary ball mill. 2. Experimental
substrates. The films were heated at 300 ◦ C after the deposition. The details of the coating procedure will be described elsewhere. 2.2. Mechanoactivation The mechanoactivation was performed in a planetary ball mill Pulverisette 5 (Fritsch) in stainless steel (80 cm3 ) or agate (50 cm3 ) vessels and balls of the same material (10 mm in diameter) for 15–300 min at 200 rpm. The TiO2 was treated as a powder in an air atmosphere or as a suspension in methanol (1 g TiO2 /cm3 methanol). In both cases powder to balls weight ratios were R = 1/8 and R = 1/4 in steel and agate vessels, respectively; the difference is due to difference in the density of ball materials. After the treatment the suspension was filtered and the TiO2 was dried for a short time in air. The activation in stainless steel vessels unavoidably leads to some contamination of TiO2 with iron. Its content in 5 h, air-milled material was determined by atomic absorption spectrometry to be 1.2 ± 0.2%. Despite that fact this type of triboreactors are widely used due to the heavier mechanical loading of treated material. 2.3. Analysis The crystal structure, phase composition and crystallite size of the treated materials were determined by X-ray diffractometry (XRD) using a powder diffractometer Siemens D-500 (Germany), with Cu K␣ radiation, 40 kV, 30 mA, 0.01◦ s−1 . SEM images were taken by JEOL JSM 5510 (Japan). The IR spectra were taken by Specord 75 spectrometer (Carl Zeiss, Germany). The surface was determined by krypton adsorption at 77 K. The value of 50 m2 g−1 found for the initial product is the same as one reported by Hidalgo et al. [27]. No special measures were taken for deaglomeration of the materials. 2.4. Photocatalytic tests The sorption and photocatalytic behaviour of the mechanically activated materials were tested in slurry (loading 1 g dm−3 MG solution) or by immersing the above described films in 250 cm3 MB solution. The procedure described in Ref. [33] was followed. The suspension was magnetically agitated for 30 min in dark at a carefully controlled ambient temperature (20 ± 2 ◦ C). After that the system was illuminated with UV lamp (Sylvania 18 W BLB T8, light wavelength 350–400 nm with a maximum around 360 nm) placed at a distance of 9 cm from the top of the suspension. The light intensity on the top of suspension was 5 × 10−5 W cm−2 . Air was bubbled through the solution (1.4 dm3 min−1 ). The change in pollutant concentration was determined spectrophotometrically from the band of MG at 616 nm and of MB at 662 nm. No concentration change was observed (within the measurement error limits) when the dye is treated at the same conditions, but without TiO2 .
2.1. Materials
3. Results and discussion Commercially available TiO2 Degussa P-25 was used in this study. Malachite green oxalate (MG, supplied by Chroma) and methylene blue (MB, Fluka), representatives of the most important impurities in the textile industry wastewater were used as model pollutants as 10−5 M and 2 × 10−6 aqueous solutions, respectively. Their structures are shown below:
Limited number of experiments was performed by immersing TiO2 film in MB solution. Films of ∼0.6 mg cm−2 in thickness and with a surface area of 5 cm2 from mechanoactivated and not activated TiO2 samples were deposited by the spray method using suspension of the oxide in methanol as started material and O2 as a carrier gas. Carefully cleaned prior the use matted microscopic slides were used as
3.1. Influence of the mechanoactivation on the crystal structure and phase composition of TiO2 X-ray diffractograms of the initial and activated materials are shown in Fig. 1. The data for the interplanar distances and the relative intensity of the reflexes are shown in Table 1. (The reflexes relative intensities IA and IR for anatase and rutile are measured for each phase separately.) It is seen that the mechanoactivation under the described conditions does not change significantly the TiO2 interplanar distances. Taking into account the degree of deformation of the crystal lattice a, the amount of stored energy during the ball milling was estimated as W = 3(a/a)2 E [34], where W is the energy of microdeformation, a is the cell constant and E is the Young’s module of elasticity of titania (230 GPa [35]). The calculated value of stored energy for different milling conditions is presented in Table 2. It was calculated by using the above formula for all interplanar distances for both anatase and rutile and after that taking an average value for certain milling conditions. The lower value of stored energy during the milling in air in a stainless steel vessel than in a agate one is quite surprising—the heavier iron loading should be more effective in increase of the energy of microdistortion. This contradiction could be explained with the effect of iron ions presented in TiO2 milled in a steel triboreactor. Two types of processes for Fe2+ production could be considered: the mechanoactivated electron emission and the elec-
Table 1 ˚ relative intensity, (I,%) and Miller indexes (h k l) of the TiO2 mechanoactivated at different milling conditions Interplanar distances (d, A), Rutile JCPDS 77-0442
Anatase JCPDS 84-1286
Mechanoactivated TiO2 in: Agate vessels
In air
In air
As suspension
As suspension
1h d 3.2557 2.4944
IR
hkl
999 434
d
IA
hkl
d
IA
3.5140
999
101
3.51 3.24 2.488 2.417 2.380
100
110 101 2.4283 2.3755 2.3305
2.3021 2.1932 2.0591
1.6917 1.6278
64 169 60
465 129
62 190 72
103 004 112
200 111 210 1.8911 1.6981
249 158
200 105
1.6652
155
211
211 220
1.4838
63
002
1.4560 1.4272 1.3632
59 4 142
310 221 301
1.3502
73
112
1.3071 1.2770
7 2
311 320
1.4920
27
213
1.4795
118
204
2.298 2.186 2.054 1.892 1.688 1.667 1.626
IR 100 51
8 18 8 31 7 16
d
IA
3.50 3.24 2.481 2.416 2.365 2.326
100
2.181 2.049 1.888 1.694
IR 100 47
6 17 8 24 10 25 16
5h
d
IA
3.51 3.24 2.481 2.430 2.374 2.327
100
2.181 2.049 1.890 1.695
IR 100 53
7 20 8 28 10 25 17
80 16 23
1.481 1.453 1.406
12 7 7
1.664 1.621 1.535
17
1.478 1.449
26
17
16
1.664 1.621
1.481 1.455
13
16
1h
d
IA
IR
3.51 3.24 2.481 2.417 2.368 2.332 2.293 2.186 2.051 1.891
100 100 49 8 17 8 13 28 10 25
21
1.685 1.667 1.623
1.479 1.454
10
10
5h
d
IA
3.51 3.24 2.481 2.430 2.374 2.332
100
2.181 2.051 1.890 1.698
IR
d
IA
3.51 3.24 2.481 2.430 2.374 2.327 2.293 2.186 2.054 1.888 1.697
100
1.664 1.621
17
19
1.479 1.451
13
12
100 53 6 18 7 23 10 23 16
IR 100 44
7 19 9 16 24 10 25 17
79 12
1.667 1.623
15
20
1.479 1.450
11
10
18
2
10
1.3627
51
116
1.361
10
1.361
16
1.359
7
1.359
7
1.360
7
1.360
8
1.3372
55
220
1.337
7
1.336
14
1.339
6
1.341
6
1.338
6
1.337
6
1.2776 1.2635
5 85
107 215
1.263
7
1.263
17
1.263
7
1.263
7
1.263
8
1.263
8
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
Stainless steel vessels (5 h)
293
294
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298 Table 3 Relative content of anatase and crystallites sizes of the mechanoactivated TiO2 Mechanoactivation
Fig. 1. X-ray diffractograms of the initial TiO2 Degussa P-25 (a), mechanoactivated 5 h in: air in stainless steel (b) and agate (d) vessels and as methanol suspension in stainless steel (c) and agate (e) vessels. The Miller indexes are shown: R, rutile; A, anatase; T, TiO2 -II.
Table 2 Calculated value for the stored energy (J cm−3 ) of microdistortion of TiO2 mechanoactivated for 5 h at different milling conditions Vessel Stainless steel In air As suspension Agate In air As suspension
4±5 9±8 10 ± 8 8 ± 10
trochemical process. During the milling in a stainless steel vessel, microscopic pieces of iron are formed and the defects concentration in the iron crystal lattice will rise. This may produce areas with different electrode potentials, i.e. the metal is no longer under standard conditions, and a sort of cell is produced. As the milling is done simply in air, this cell becomes active by using oxygen, water and, probably, an electrolyte produced with the participation of gaseous impurities in the air. Further, the processes leading to the formation of iron ions will be analogous to the processes of iron rusting described in Ref. [36]. The formed Fe2+ -ions (very similar in size to Ti4+ [36]) can incorporate into the free regular titanium positions in TiO2 , as occurs in the general doping of TiO2 with iron [37]. According to the latter, the solubility of iron into TiO2 at atmospheric pressure changes from 1% at 800 ◦ C up to 3% at 1350 ◦ C. The incorporation of Fe2+ into the free positions for titanium ions will lead to decrease of the point defects number.
Time (min)
Vessel
Mode
Content of anatase (%)
0 300 300 60 300 60 300
– Steel Steel Agate Agate Agate Agate
– Air Suspension Air Air Suspension Suspension
85 74 85 85 79 85 85
± ± ± ± ± ± ±
2 2 2 2 2 2 2
Size of crystallites (nm) Anatase
Rutile
45 31 40 38 33 39 38
66 47 53 47 42 48 50
Two possible reasons could be responsible for the higher value of stored energy in the case of milling the material in stainless steel vessels as a suspension than that in air. The reduced particle size, much more caused by the wet milling (Fig. 2), could change the cell constant of the material [34], which will alter in turn the stored energy. The second possible reason is the lower local temperature of the as-suspension-milled material, which will give lesser amount of Fe2+ diffused into the TiO2 crystal lattice. The data for the crystallites sizes determined by the broadening of |1 0 1| and |1 1 0| reflexes for anatase and rutile, respectively, and the phase composition are summarized in Table 3. The weight fraction of anatase f, is calculated from the integrated intensities of |1 0 1| reflection of anatase, IA , and |1 1 0| reflection of rutile, IR , using the equation [38]: f = 1/(1 + 1.26IR /IA ). The data show that the milling in air leads to crystallite size reduction with 31% and 29% for anatase and rutile, respectively, in the case of stainless steel vessel and with 27% and 36%, when the activation is performed in agate vessel. The mechanoactivation of TiO2 as suspension in methanol does not change its phase composition and the decrease of the crystallites size is significantly lower: 12% and 20% for anatase or rutile, respectively, in stainless steel vessels and 15% and 24% in agate vessels. It seems (Table 3) that 1 h activation in suspension in agate vessel is enough to reach the described changes. It is worth mentioning that the changes in crystallite size resulted from the different activation modes are, generally, in accordance with the data for the stored energy (Table 2). However, the phase transformation of anatase to the more stable rutile modification is more pronounced when the activation is performed in air. It is seen that longer activation time and heavier mechanical loading (realized with steel tools) promotes the process, which normally takes place above 550 ◦ C. As can be expected, the size of morphological grains decreases significantly, especially in the case of milling as suspension (Figs. 2 and 3). The particle average diameters decrease from 118 ± 40 nm of the initial TiO2 to 88 ± 30 nm and 56 ± 12 nm after activation for 5 h in agate vessel in air and as suspension, respectively (the standard deviation is shown). The much less decrease in grain size as a result of the activation in air is in agreement with previous works. The crushing is more effective in the case of wet milling, because of the unwedging effect
Fig. 2. SEM images of initial TiO2 (a) and after mechanoactivation for 5 h in agate vessel in air (b) and as suspension in methanol (c), (magnification 50,000×).
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
295
Fig. 3. Size distribution of the titania powder treated by different ways and estimated from Fig. 2.
of liquid into the defects of crystallites as well as the prevention of cracks to stick together. At the same time, this activation mode gives more uniform size distribution of the particles (Fig. 3). The results for surface specific area (Table 4) are in agreement with the above reported results. The results obtained by air milling in agate are little higher than the ones found in [27] with Retsch MM2000 mixing-mill. The result for 30 min-milling in steel vessel is also little higher than the found in [2] but for anatase containing 18% rutile. EPR signal with g = 2.0019 due to the presence of Ti3+ was found in the sample activated for 5 h in air in agate vessel. This signal was not observed in the non-activated sample. The appearance of Ti3+ signal in the activated sample is consistent with the observations made in Ref. [39], where it results from the mechanical treatment into planetary ball mill AGO-2. Table 4 Specific surface area (m2 g−1 ) of the milled products Milling time (min)
Triboreactor type/milling medium Agate in
30 300
Steel in
Air
Suspension
Air
Suspension
53 56
54 58
58 65
60 69
Fig. 4. IR spectra (800–400 cm−1 ) of TiO2 (a) and of the same sample after 5 h of milling in stainless steel vessel as methanol suspension (b) and in air (c).
These observations can be interpreted based on the fact that at the down boundary of homogeneity area (range) of TiO2 the interstitial ions of titanium are the main type of defects [37]. Such a state is realized at high temperature and low oxygen pressure meaning that in that case increasing the Ti4+ vibration amplitude is caused by changing the surroundings of the ion, because of increasing the number of oxygen vacancies and, consequently, weaker bonding of the Ti4+ -ions. The high-energy milling causes the Ti4+ ions to vibrate with higher amplitude and they can thus leave their equilibrium positions going to interstitial states. At the same time the so-formed interstitial Ti4+ -ion can participate in the reaction e− + Tii 4+ → Tii 3+ [39] producing interstitial Ti3+ -ion. The IR spectrum (800–400 cm−1 ) of the initial TiO2 (Fig. 4a) shows bands at 640 cm−1 and 450 cm−1 , which are ascribed to anatase according to Refs. [26,40]. After milling for 5 h in both cases (the suspension and air) the characteristic band of anatase at 640 cm−1 is “blue”-shifted and combined with that at 450 cm−1 (Fig. 4b, c). The pattern formed after 5 h is characteristic for TiO2 -II [40]. The latter authors also found disappearance of the 450 cm−1 band as a result of prolonged milling. The presence of this polymorph in our sample is confirmed also by XRD analysis (Fig. 1). The amount of TiO2 -II at the experimental conditions applied in
296
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
Fig. 5. Sorption and degradation of MG under the action of TiO2 () and TiO2 , mechanoactivated in agate vessel for 1 h as suspension in methanol (). In the inset the experimental data (points) and the theoretical fits of the data (solid lines) are shown (see the text, also). Correlation coefficients for the linear approximation of the semi-logarithmic plot are 0.9949 for the fresh and 0.9862 for the milled materials, respectively.
the present work is rather low and it is hardly to believe that its influence on the mechanoactivated TiO2 behaviour will be significant.
(ii)
3.2. Sorption and photocatalytic properties of the mechanoactivated TiO2 Data for the sorption of the pollutant on the TiO2 surface during the equilibrium 30 min “dark” period in the course of the photocatalytic tests are shown in Table 5. The kinetics of the degradation process can be followed by fitting the experimental data according to the equation Ct /Cd = exp[−k(t–t0 )] or ln (Cd /Ct ) = kt − a0 , where Ct , concentration in the moment t; t, illumination time, Cd , concentration at the end of the “dark” period (corresponding to time t0 after loading of the system); k, rate constant (the slope of the straight lines in the figure); a0 , intercept of the lines. The standard data fitting is performed first on the semilogarithmic plots ln (Cd /Ct ) (Fig. 5). The time constant t0 is determined from the relationship a0 = kt0 . Then the curves Ct /Cd (the inset to the figure) are drawn by the least square method using the predetermined k and t0 at one adjustable parameter Cd along with the experimental points. The calculated rate constants are summarized in Table 5.
(iii)
(iv)
3.2.1. Sorption Generally, the sorption of the dye on the mechanoactivated TiO2 is equal to or greater than the one on the untreated product. The decrease of the pollutant concentration, found at the end of the equilibrium period (Table 5), depends on: (i) The milling vessel material. The adsorbed amount of dye on TiO2 activated for 5 h in the stainless steel vessel is five times greater than that on the inactivated sample, comparing with approximately 150–200% increase resulting from the mechanoactivation in agate vessel using both activation modes. The difference looks quite surprising, because the energy, stored in TiO2 as a result of milling in steel vessel (Table 2) is less, which means less number of defects and, consequently, less number of defects that could play a role of sorption centres will exist. For explanation of this fact, the role of the iron introduced by ball milling should be considered. It is known [41] that iron doping induces a structural transformation from anatase
(v)
to rutile and the electrical measurements indicate that iron acts as an acceptor impurity. A transition from n-type to ptype conduction at an iron concentration of 0.13 mol% is shown. In the studied system, the iron concentration of 1.2 ± 0.2% will lead clearly to p-type conductance of TiO2 even if only a part of the iron is incorporated as Fe2+ in the TiO2 . The increasing of the p-type impurity concentration in TiO2 will shift the Fermi level down relative to the top of valence band as is well known from the semiconductors physics fundamentals. According to Ref. [42] when the Fermi level is moving down, the adsorption ability of the semiconductor surface relatively to gases that obey donor properties increases. The lower is the Fermi level in the band gap of TiO2 , the higher is the adsorption ability for particles that obey donor properties. Because of high concentration of Fe2+ , we expect the Fermi level to shift down enough for noticeable promotion of the adsorption of molecules having donor’s properties. Consequently, when these donor species are chemisorbed at a semiconductor’s surface, they will be able to localize the free hole from the vicinity of semiconductor’s surface. Because of the high electron density of the benzene ring, it can be expected the molecule of MG to follow such pattern, leading to the above reported observations. The activation mode (in air or as suspension). The effects of the milling in air and as suspension at equal activation times are similar (Table 5), suggesting that the specific surface area (which is bigger after milling in suspension) is not the only factor determining the adsorption and that the mechanically induced changes of surface properties should be considered as well. The time of mechanoactivation. The activation in agate as methanol suspension for less than one hour has no significant effect on the TiO2 sorption ability (Table 5). The activation for 1 h leads to an increase of the adsorption ability approximately by twofold. It seems that such an activation time is sufficient to reach a “saturation” of the effect of activation on the adsorption ability. Activation in air for 5 h leads to a decrease of the sorption when compared to 1 h activation at the same conditions. This fact could be explained with the agglomeration, taking place at longer activation times. The agglomeration is less expressed for activation in a suspension and, accordingly, no significant difference between samples activated for 1 h and 5 h is observed. The time between the mechanoactivation and the photocatalytic test. The changes of materials sorption properties as a result of storage after mechanoactivation strongly depend on the activation mode (Table 5, Fig. 6). The sorption ability of TiO2 milled in agate vessel as suspension is not altered after 90 days of storage. The efficiency of the oxide milled also as suspension, but in steel triboreactor, decreases twice. However, TiO2 milled in the same vessel, but, in air, shows twice as increased efficiency after the storage. No such radical changes have been observed after storage of the activated material for the same time, but in liquid N2 . These facts show that the increased dispersity is not the only and not the main factor responsible for the changed adsorption ability. To our opinion, the reported observations strongly suggest that crystal defects introduced in the course of milling and their relaxations in time are of significant importance for the sorption behaviour of the mechanoactivated TiO2 . The pollutant chemical nature. The sorption ability of nonactivated TiO2 to MB is higher than that to MG, but its increase as a result of air-activation in stainless steel vessels is 2.5 times lower than the respective increase in the case of MG sorption. Obviously, a complicated interrelation between the pollutant
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
297
Table 5 Sorption and photocatalytic degradation of malachite green and methylene blue on mechanoactivated TiO2 a Mechanoactivation
Storage after activation (days)
Time (min)
Vessel
Medium
0 60 300 15 30 60 180 300 300 300 300 300 300 0 300
– Agate Agate Agate Agate Agate Agate Agate Agate Steel Steel Steel Steel – Steel
– Air Air Methanol Methanol Methanol Methanol Methanol Methanol Air Methanol Methanol Air – Air
a
–
90
90 90
Pollutant (photocatalytic test mode)
(C0 − Cd )/C0 (%)
k (min−1 )
MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MG (slurry) MB (film) MB (film)
9 20 13 8 7 19 17 18 18 45 50 27 92 14 28
0.0140 0.0140 0.0155 0.0123 0.0116 0.0243 0.0144 0.0155 0.0132 0.0029 0.0033
0.00219 0.00174
C0 , dye initial concentration; Cd , concentration at the end of the “dark” period; k, photodegradation rate constant.
Few effects have to be considered as possible factors determining the behaviour of mechanoactivated TiO2 :
Fig. 6. Sorption and degradation of MG under the action of TiO2 mechanoactivated for 5 h in stainless steel vessel in air () and as suspension in methanol () 3 months after the activation.
nature and the mechanically induced changes in the TiO2 does exist. 3.2.2. Photochemical degradation The data for photochemical degradation rate constants (Table 5) could be interpreted as follows: (i) The photocatalytic activity of TiO2 , milled in agate vessel as methanol suspension, has a maximum at 1 h of milling time. The rate constant at these conditions is with 25% higher than the one of non-activated oxide. Longer activation times lead to a slight decrease in the reaction rate. (ii) The activity of material treated in the same triboreactor but in air increases slowly with the activation time increase reaching the rate constant value, 20% higher than of the initial TiO2 . (iii) The activity of the TiO2 , air-milled in steel vessel is significantly lower then that of the non-treated titania—with more than 75% for MG and with ∼20% for MB degradation. The latter result is, generally, in agreement with the results reported in Ref. [26]. Obviously the chemical nature of the pollutant is of significant importance for the photocatalytic performance of the mechanically treated material.
(i) The increased dispersity will enhance the sorption and photocatalytic process, taking place on the TiO2 surface. However, rather similar results for the rate constant from samples, activated in air or as suspension (the latter leading to higher specific surface area) supports the affirmation that this factor is not of a primary importance for determination of the activated TiO2 behaviour. (ii) Particles with different size are expected to posses different numbers of point defects, which in the case of a nonstoichiometric oxide as TiO2 play the role of charge carriers. As known [14,43,44], the latter affect strongly the photocatalytic properties of TiO2 . In addition to that, for small colloidal particles, there is nearly no band bending and the existing electric field is usually small. So that high doping levels are needed to produce a significant potential difference (permanent electric field) between the surface and the centre of particles to efficiently separate the photoinduced electron–hole pair [44]. (iii) The higher photoreactivity of anatase is due to its slightly higher Fermi level, lower capacity to adsorb oxygen and higher degree of hydroxylation (i.e. number of hydroxy groups on the surface) [14]. The decrease of anatase relative content as a result of mechanoactivation is decreasing the catalytic activity of TiO2 as well. The result reported in Table 3 shows that this process takes place only during the activation in air in steel vessel and could be considered as one of the factors leading to the decreased photocatalytic activity of air-milled TiO2 . Accounting for this effect, it can be supposed that the mechanoactivation could be more promising if performed with anatase. It is known that small amount of rutile in the photocatalyst (which will be produced for not very long activation times) will trap electrons from the anatase conduction band thus reducing the recombination rate of the photoinduced electron–hole pairs. (iv) In some cases [45], the mechanoactivation in air may cause dehydration of the milled substance. The decrease of the relative band intensity around 3200 cm−1 in the IR spectrum of the air-milled TiO2 confirms that such a process takes place with as well. The effect of dehydroxylation on the photocatalytic activity was already mentioned. Few consequences of it are potentially important for explanation of the reported observations. The decrease of surface TiIV -OH− species con-
298
M. Uzunova-Bujnova et al. / Materials Chemistry and Physics 110 (2008) 291–298
centration will decrease the production of oxidizing agent, • OH , thus suppressing the dye degradation. Contrary, the holes formed as a result of dehydroxylation will enhance the reac• tion h+ + OH− → OH thus forming primary oxidizing agent. However, it is difficult to predict whether these holes will be energetically suitable electron traps to ensure the progressing of reaction. In the same time, if traps for holes, more effective than OH− , are formed as a result of milling, the production • of OH will decrease. The results obtained for the rate constant for air-milled TiO2 suggest that the negative effect of the dehydroxylation dominates. (v) The data in Table 5 reveal a clear dependence between the TiO2 sorption ability and the photocatalytic activity. The materials milled as suspension in agate vessel show relatively weak sorption properties and relatively higher photodegradation efficiency increasing simbatically with the sorption. Heavily milled product (in steel vessels and, to some extent, in agate vessel, but in air) posses a rather strong adsorption. It could be supposed that the latter impedes the degradation of pollutant. In some cases (Fig. 6), the effect of photodesorption (phenomenon, which quite often appears on semiconductor’s surfaces [42]) is observed leading to the increase of the dye concentration in the solution. However, it is quite probable that some crystal defects (for example hole traps, mentioned above) formed as a result of milling can be the main factor for the degradation rate decrease. It is well known that the milling in air resulted in higher concentration of the crystal defects (than the treatment at the same conditions but in suspension) thus leading to the observed in negative effect of the air milling on the TiO2 photocatalytic ability. (vi) As we mentioned above, some Ti3+ is formed as a results of milling of TiO2 in agate vessel, because of the reduction of interstitial Ti4+ . It acts like a general p-type dopant that works as a charge-carrier recombination centre, advancing the recombination of electron–hole pairs, and then photoactivity may decrease [46]. 4. Conclusion The reported results reveal some new data on the influence of mechanoactivation conditions on the values of stored energy, crystal structure, morphology and phase composition of the most widely used photocatalyst Degussa P-25 TiO2 . The sorption ability of the material significantly increases as a result of mechanoactivation due not only to the increased dispersity, but also and, probably, mainly to the changes of the surface properties, phase composition and crystal defect concentration. However, the rather strong sorption impedes the photocatalytic degradation of the dye and, in order to benefit from the mechanoactivation, a suitable conjunction of the conditions of its performance should be found. The obtained results suggest that a relatively moderate mechanical loading get by milling of TiO2 as suspension in agate vessel increases the photodegradation rate constant with ∼50%. It seems that the effect of the mechanoactivation depends on the pollutant structure as well.
Acknowledgements The study is performed with the financial support of NATO “Science for Peace” Program (Contract SfP 977986). The authors are
indebted to Dr. C. Dushkin (Faculty of Chemistry, University of Sofia) for the help in data analysis and valuable discussion, Dr. A. Apostolov (Faculty of Chemistry, University of Sofia) for the X-ray diffractograms recording and to Mrs. R. Todorovska (Institute of Electronics, Bulgarian Academy of Sciences) for the films preparation. References [1] Y. Chen, M. Bibole, R. Le Hazif, G. Martin, Phys. Rev. B 48 (1993) 14. ´ ¨ F. Villieras, X. Devaux, M.O. Simonnot, T. Girot, P. [2] S. Begin-Colin, G. Le Caer, Weisbecker, J. Metastable Nanocryst. Mater. 12 (2002) 27. [3] J. Wang, S. Yin, T. Sato, Mater. Sci. Eng. B 126 (2006) 53. [4] J. Wang, S. Yin, Q. Zhang, F. Saito, T. Sato, Solid State Ionics 172 (2004) 191. [5] J. Wang, S. Yin, M. Komatsu, T. Sato, J. Eur. Ceram. Soc. 25 (2005) 3207. [6] G. Liu, F. Li, Z. Chen, G.Q. Lu, H.-M. Cheng, J. Solid State Chem. 179 (2006) 331. [7] C. Shifu, C. Lei, G. Shen, C. Gengyu, Mater. Chem. Phys. 98 (2006) 116. [8] C. Shifu, C. Lei, G. Shen, C. Gengyu, Powder Technol. 160 (2005) 198. [9] S. Yin, H. Yamaki, Q. Zhang, M. Komatsu, J. Wang, Q. Tang, F. Saito, T. Sato, Solid State Ionics 172 (2004) 205. [10] H. Liu, X. Sun, Q. Zhao, J. Xiao, S. Ouyang, Solid State Electron. 47 (2003) 2295. [11] L. Takacs, Progr. Mater. Sci. 47 (2002) 355. ´ [12] J. Alkebro, S. Begin-Colin, A. Mocellin, R. Warren, J. Solid State Chem. 164 (2002) 88. [13] K.T. Ranjit, I. Willner, S.H. Bossmann, A.M. Braun, Environ. Sci. Technol. 35 (2001) 1544. [14] O. Carp, C.L. Huisman, A. Reller, Progr. Solid State Chem. 32 (2004) 33. [15] Y. Tekiz, C. Legrand, C.R. Acad. Sci., Paris 261 (1965) 3619. ´ ¨ M. Zandona, E. Bouzy, B. Malaman, J. Alloys Compd. [16] S. Begin-Colin, G. Le Caer, 227 (1995) 157. ´ ¨ A. Mocellin, M. Zandona, Phil. Mag. Lett. 69 (1994) 1. [17] S. Begin-Colin, G. Le Caer, ´ ¨ A. Mocellin, J. Mater. Synth. Process [18] T. Girot, S. Begin-Colin, X. Devaux, G. Le Caer, 8 (2000) 139. ´ ¨ A. Mocellin, J. Solid State Chem. 149 (2000) [19] S. Begin-Colin, T. Girot, G. Le Caer, 41. ´ [20] T. Girot, X. Devaux, S. Begin-Colin, G. Le Caer, A. Mocellin, Phil. Mag. A 81 (2001) 489. [21] S. Coste, G. Bertrand, Codoler, E. Gaffet, H. Hahn, Sieger H., J. Alloys Compd. 434–435 (2007) 489. [22] R. Ren, Z. Yang, L.L. Shaw, J. Mater. Sci. 35 (2000) 6015. ´ M. Stubicar, M. Ivanda, K. Furic, J. Mol. Struct. 563–564 (2001) 315. [23] A. Gajovic, [24] H. Dutta, P. Sahu, S.K. Pradhan, M. De, Mater. Chem. Phys. 77 (2002) 153. [25] P. Bose, S.K. Pradham, S. Sen, Mater. Chem. Phys. 80 (2003) 73. [26] H. Jifan, Q. Hongwei, S. Zhengui, L.H. Liang, Rare Met. Mater. Eng. 32 (2003) 5. ´ J.A. Nav´ıo, J. Photochem. Photobiol. A 148 (2002) 341. [27] M.C. Hidalgo, G. Colon, ´ J. Phys. Chem. [28] P. Billik, G. Plesch, V. Brezova, L. Kuchta, M. Valko, M. Mazur, Solids 68 (2007) 1112. [29] S. Yin, Q. Zhang, F. Saito, T. Sato, Chem. Lett. 32 (2003) 358. [30] S. Yin, H. Yamaki, M. Komatsu, Q. Zhang, J. Wang, Q. Tang, F. Saito, T. Sato, J. Mater. Chem. 13 (2003) 2996. [31] G.B. Schaffer, J.S. Forrester, J. Mater. Sci. 32 (1997) 3157. [32] S. Matsuo, N. Sakaguchi, K. Yamada, T. Matsuo, H. Wakita, Appl. Surf. Sci. 228 (2004) 233. [33] M. Kostadinov, P. Georgiev, A. Bojinova, C. Dushkin, in: E. Balabanova, I. Dragieva (Eds.), Nanoscience and Nanotechnology, 3, Heron Press, Sofia, 2003, pp. 207–209. [34] A.A. Polyakov, Technology of Ceramic’s Radio-electronics Materials, Radio I Ssvyaz, Moscow, 1989, p. 52, 54 (in Russ.). [35] http://www.azom.com/details.asp?ArticleID=1179# Key Properties. [36] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Butterworth/Heinemann/Pergamon Press, Oxford, 1998, p. 963, 1076. [37] P. Kofstad, Nonstoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, Wiley-Interscience, New York, 1972, p. 153, 159 (Russian edition). [38] R. Spurr, H. Myers, Anal. Chem. 29 (1957) 760. [39] V.V. Zyryanov, Neorganisheskie Materiali 41 (1) (2005) (in Russ.). ´ K. Furic, ´ N. Tomaˇsic, ´ S. Popovic, ´ Zˇ . Skoko, S. Music, ´ J. Alloys Compd. [40] A. Gajovic, 398 (2005) 188. ´ [41] A.R. Bally, E.N. Korobeinikova, P.E. Schmid, F. Levy, F. Bussy, J. Phys. D: Appl. Phys. 31 (1998) 1149. [42] F.F. Wolkenstein, Electronic Processes at the Surface of Semiconductors During the Chemisorptions, Nauka, Moscow, 1987, p. 118, pp. 285–286 (in Russ.). [43] M.A. Fox, M.T. Dulay, Chem. Rev. (1993) 431. ¨ [44] A. Hagfeldt, M. Gratzel, Chem. Rev. 95 (1995) 49. [45] D. Todorovsky, A. Terziev, M. Milanova, Hydrometallurgy 45 (1997) 13. [46] F.B. Li, X.Z. Li, M.F. Hou, Appl. Catal. B: Environ. 48 (2004) 185.