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Does water in synthesized TiO2 have an effect on the photocatalytic activity? Towards a spectacular response
Materials Letters 204 (2017) 188–191
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Does water in synthesized TiO2 have an effect on the photocatalytic activity? Towards a spectacular response L. Elsellami a,b,⇑, F. Dappozze b, A. Houas a,c, C. Guillard b a
Unité de recherche Catalyse et Matériaux pour l’Environnement et les Procédés URCMEP (UR11ES85), Université de Gabes, Tunisia IRCELYON, CNRS UMR 5256/Université Lyon 1, France c Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Department of Chemistry, Riyadh 11623, Saudi Arabia b
a r t i c l e
i n f o
Article history: Received 11 April 2017 Accepted 2 June 2017 Available online 3 June 2017 Keywords: Nanocrystalline materials Adding water Sol-gel preparation FTIR Hydroxyl radical Photocatalysis
a b s t r a c t The influence of hydroxyl radicals on the photocatalytic performance of synthesized TiO2 resulting from the addition of water was studied. Two TiO2 samples were prepared using the sol-gel method followed by calcination at 600 °C. The aim of this work was to study the effect of water on the photocatalytic activity of a model compound – formic acid (FA) – by comparing between the titanium oxide synthesized with water (TiO2-600-w) and without water (TiO2-600). The TiO2-600-w, consisting of anatase and a little rutile, was the most active catalyst, whereas pure anatase (in case of TiO2-600), showed low photocatalytic efficiency. In this study, we confirmed that the filling of the TiO2 surface by hydroxyl radicals could produce better photocatalytic activity by reducing e/h+ recombination. It is notable, however, that with TiO2-600-w, the degradation rate (found under UV) was about 4–5 times greater than that obtained with TiO2-600. This result suggests that the presence of water was very important in the synthesis of TiO2 and brought a dramatic gain in the photocatalytic response as well. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis quantitatively represents an efficient method for producing radicals, and in particular hydroxyl radicals. The hydroxyl radical can be categorically considered as a water molecule fragment. Being a particularly stable molecule, the fragmentation of water necessitates the a priori contribution of an important energy. Titanium dioxide (TiO2) is the most commonly used catalyst in photo-induced reactions, mainly because of its chemical stability and high photocatalytic activity. On its surface, the irradiation of the catalyst causes the decomposition of water into OH radicals that are able to degrade the organic pollutants until complete mineralization [1,2]. The interaction of water with the TiO2 surface is an important research topic since it plays a major role, especially in photocatalysis [3]. Very few authors, however, have been interested in the effect of the adsorption state of the nanocrystal surfaces on their structure [4] and few studies are concerned with the moisture content during the synthesis of TiO2 [5]. Previous studies concluded that the improvement in the photocatalytic activity is due to a bet⇑ Corresponding author at: Unité de recherche Catalyse et Matériaux pour l’Environnement et les Procédés URCMEP (UR11ES85), Université de Gabes, Tunisia. E-mail address: [email protected] (L. Elsellami). http://dx.doi.org/10.1016/j.matlet.2017.06.010 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
ter separation of electron-hole pairs yet they lacked precision [6,7]. Current studies, by contrast, often lack correlation between the properties of water and the photocatalytic performance of TiO2 [8]. This work made it possible to better understand the surface reactivity of TiO2 nanoparticles synthesized with and without water and its photocatalytic role through the degradation of formic acid. The formic acid was chosen here because its photodegradation could be conducted both by direct hole transfer and by photogenerated surface radicals [9]. The photocatalytic activities of both TiO2-600-w and TiO2-600 samples were then studied and compared with the high activity of the reference TiO2 P25.
2. Experiment The precursor used for the synthesis of TiO2 was titanium tetrachloride (TiCl4) (99.99%) purchased from Sigma Aldrich. The solvent, absolute ethanol, (99.99%) was bought from Merck Millipore (Germany). The reagents and the model pollutant (HCOOH) were obtained from Acros Organics and were used without any further purification. Two samples of TiO2 were prepared by the sol-gel method described as follows: an amount of TiCl4 was slowly added to 10 ml of absolute ethanol. During the synthesis, an amount of water (volumetric ratio of water/TiCl4 = 1/3) was only added to one sample of TiO2. In this catalyst (TiO2-600-w),
189
L. Elsellami et al. / Materials Letters 204 (2017) 188–191
(w) indicates the presence of water. Next, the solution obtained with and without water converted into a colorless solution that produced TiO2 nanopowders through the drying process at 85 °C in an oven for 15 h. The obtained TiO2 nanopowders were eventually calcined in an oven for 2 h at a temperature of 600 °C. X-ray diffraction analyses (XRD) were carried out using a X’Pert Pro Panalytical diffractometer with Cu Ka radiation (k = 1.54184 Å). The material to be tested was introduced into a photo-reactor containing 30 ml of the solution of the pollutant whose initial concentration was 50 ppm of formic acid. The solution was stirred in the dark for 60 min to reach the adsorption equilibrium at the catalyst surface. The samples were, then, irradiated and aliquots were taken every 5 min and filtered later through Millipore filters (porosity 0.45 lm). Next, they were analyzed using a high performance liquid chromatography (HPLC).
Table 1 Percentages of the crystalline phase of different samples and degradation rate of TiO2-600-w, TiO2-600, P25. A and R denote anatase and rutile, respectively. Samples
Phase content (%)
r0 (mmol/L/min)
P25 TiO2-600 TiO2-600-w
A: 80; R: 20 A: 100 A: 96; R: 4
26.74 17.16 71.93
This band could be caused by the presence of hydroxyl groups OH adsorbed on the surface which came from the water added during the synthesis and which were gradually released and strongly linked to the TiO2 surface. 3.2. Evaluation of photocatalytic activities of the TiO2 samples 3.2.1. Impact of water on the adsorption of FA Fig. 3a displays the adsorbed amounts of formic acid on the three photocatalysts TiO2-600-w, TiO2-600 and TiO2-P25. If we go through Fig. 3a, we can notice that after 60 min in the dark, a slight adsorption of formic acid on each type of oxide samples was observed: 10% for TiO2-600-w, 6% for TiO2-600 and 8% for TiO2-P25. A large number of parameters could have influenced the adsorption of formic acid on the surface of TiO2 such as surface hydration and adsorption mechanisms [13]. The adsorption of formic acid was due to the hydrogen bond between the oxygen of the carboxylic group on the acid and the hydroxyl groups of TiO2.
3. Results and discussion 3.1. Structural and morphological properties Fig. 1 shows the XRD spectra of pure TiO2 nanopowders with and without water. The obtained results show that the presence of water has influenced the composition of the TiO2 powders and the crystallization phase and that, due to the presence of water during the synthesis, the anatase phase has been modified a bit into a rutile. The phase transition was accompanied by crystal growth. Table 1 summarizes the percentages of anatase and rutile in both samples. The crystallite size of TiO2 nanoparticles was determined based on the Scherrer equation. In fact, anatase TiO2 is considered as the most efficient catalyst [10]. The presence of a small amount of rutile, however, seems to improve the effectiveness of anatase TiO2. Despite the fact that this improvement is still a subject of much debate, most publications have mentioned that the source of their effectiveness could be attributed to an interface between anatase and rutile [11,12]. Measurements in infrared (FT-IR) spectroscopy were carried out so as to determine the surface reactivity and, more precisely, the surface charge which could immediately influence the adsorption properties. In Fig. 2, we can observe clear changes between the two spectra of TiO2 with and without water in the band intensity, especially a broad band observed between 3100 cm1 and 3400 cm1 in the spectrum of TiO2-600-w and which disappeared in the spectrum of TiO2-600.
3.2.2. Impact of water on the disappearance of FA Fig. 3b shows the kinetics of formic acid in the presence of TiO2-600-w, TiO2-600 and TiO2-P25 under exposure to UV-A radiation. The comparison between titanium oxide with and without water shows a beneficial influence of water and a distinct increase in the photocatalytic response. The presence of a larger amount of water in the synthesis of TiO2-600-w than that of TiO2-600 was able to overload the catalyst surface with active radicals. This, then, could explain the systematic activity of the greater degradation of TiO2-600-w. The results shown in Table 1 clearly demonstrate the important photocatalytic role of water. The degradation rate with TiO2-600-w is about 4 or 5 times greater than with TiO2-600. The improvement in velocity is also explained by the production of OH which very often came with the appearance of other chemical
Intensity (a.u.)
A
A
A
0
10
20
A
A
R
30
A
40
A
50
A TiO -600 2
ATiO2-600-w
60
2 (degree) Fig. 1. XRD patterns of TiO2-600-w and TiO2-600 nanomaterials. (A: anatase, R: rutile).
70
L. Elsellami et al. / Materials Letters 204 (2017) 188–191
Transmitance (%)
190
TiO2-600 TiO2-600-w
3100-3400 -OH 0
500
1000
1500
2000
2500
3000
3500
4000
Wavenumbers (Cm-1) Fig. 2. FTIR spectra of TiO2, with water (TiO2-600-w), and without water (TiO2-600) reduced at 600 °C.
5
substances capable of accepting or donating electrons [14]. These were oxidation-reduction reactions which were useful for environmental remediation [15]. Among the adsorbed electron donors was water. The electric field created by the flow of electrons through water led to its ionization. This ionization could be considered as an inelastic collision between the incident electrons and those of the water molecule and represented here a prominent transfer mode of energy to water (Eq. (2)). Nevertheless, it is proposed that they most often underwent dissociation to lead to the formation of H and OH [16] (Eq. (3)), but they were also capable of leading to the formation of molecular dihydrogen and singlet oxygen 1D (Eq. (4)). The latter then reacted quickly with water to form either two radicals OH or hydrogen peroxide H2O2 (Eq. (5)). The H2O+ ion was not stable and reacted rapidly with a water molecule to produce the OH radical and a hydronium ion H3O+ (Eq. (6)). This process produced the greatest contribution to the yield of hydroxyl radical. Eventually, by dissociative attachment to a water molecule, the electron could bring about the formation of a hydroxyl radical and a hydride H (Eq. (7)). The latter was not stable and reacted with a water molecule to form molecular dihydrogen as well as a hydroxide anion OH (Eq. (8)). All these proposed mechanisms contain the surface regeneration step of TiO2 which has never been mentioned in the literature.
(A)
4,5 4 3,5
Q (mg/g)
3 2,5
2 1,5
1 0,5 0 TiO2-600-w
TiO2-P25
TiO2-600
1200
(B) TiO2-600
1000
P25 TiO2-600-w
C (µmol/L)
800
600
200
-40
-20
0
ð1Þ
H2 O!H2 Oþ þ e
ð2Þ
H2 O !H þ OH
ð3Þ
H2 O!H2 þ Oð1 DÞ
ð4Þ
Oð1 DÞ þ H2 O!2 OHðor H2 O2 Þ
ð5Þ
H2 Oþ þ H2 O! OH þ H3 Oþ
ð6Þ
e þ H2 O! OH þ H
ð7Þ
OH þ H þ H2 O! OH þ H2 þ HO
ð8Þ
þ
400
-60
TiO2 þ ðhm > 3; 2 eVÞ!eBC þ hBV
0
20
40
60
80
100
t(min) Fig. 3. (A) Amounts of FA adsorbed per gram of TiO2 as a function of different catalysts (TiO2-600-w, TiO2-600 and TiO2-P25) and (B) Evolution with irradiation time of the concentration of formic acid in solution using TiO2-600-w and TiO2-600 samples. Comparison is provided with the P25 reference.
species that were of great interest as well, increasing afterwards the kinetics of degradation of the pollutants. This led to a Separation of the photogenerated charges and a limitation of the recombination of electron-hole pairs. When TiO2 was irradiated with an energy larger than 3.2 eV (energy gap) (Eq.1), the produced charges migrated to the TiO2 surface and reacted with the adsorbed
The results obtained in this study make it possible to propose a mechanism explaining the performances of TiO2/water and formic acid under UV illumination. The photogenerated electrons of the conduction band of TiO2 oxidized water (Eq. (9)) leaving holes
L. Elsellami et al. / Materials Letters 204 (2017) 188–191
available for the oxidation reactions by increasing the lifetime of the photogenerated carriers. The holes (h+) could then react directly with the formic acid adsorbed by the photo-Kolbe mechanism (Eq. (10)) or by the OH radical attack (Eq. (11))
TiO2 ðeBC Þ þ H2 Oads !TiO2 þ Hþ þ OHads þ
HCOO þ h !HðadsÞ þ CO2
HCOO þ OH!H2 O þ
CO2
ð9Þ ð10Þ ð11Þ
The presence of water, however, could explain the additional characterizations specific to surface properties, charge-carrier dynamics, the reactivity and all of the photocatalytic results. 4. Conclusion We have demonstrated, in this paper, that the addition of water positively influenced the intrinsic performance of TiO2 by the photodegradation of formic acid and that the big difference between the two oxides concerned mostly the presence of rutile phase and the formation of the OH radical derived from water. During TiO2 synthesis, water was used to improve photocatalytic activity by limiting the recombination rate of photogenerated electronhole pairs. Besides, the presence of water simultaneously improved
191
its electron affinity and its electrical conductivity leading to an easier transfer of the photogenerated electrons of the conduction band of TiO2 and thus to a successful charge separation. The water had a large gauge of effective materials during the photocatalysis in order to synthesize a photocatalytically-suitable oxide for a judicious degradation of the pollutant. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
M. Adachi, Y. Murata, M. Harada, S. Yoshikawa, Chem. Lett. 29 (2000) 942–943. A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1–21. C.S. Turchi, D.F. Ollis, J. Catal. 122 (1990) 178–192. M.W. Peterson, J.A. Tuner, A.J. Nozik, J. Phys. Chem. 95 (1991) 221–225. C.C. Wang, J.Y. Ying, Chemistry of Materials 11 (1999) 3113–3120. K. Tanaka, M. Capule, T. Hisanaga, Chem. Phys. Lett. 187 (1991) 73–76. S. Yamazaki, S. Matsunaga, K. Hori, Water Res. 35 (2001) 1022–1028. J. Zhang, Y. Nosaka, J. Phys. Chem. C 118 (2014) 10824–10832. A. Turki, H. Kochkar, C. Guillard, G. Berhault, A. Ghorbel, Appl. Catal. B 138-139 (2013) 401–415. X. Xin, T. Xu, J. Yin, L. Wang, C. Wang, Appl. Catal. B 176–177 (2015) 354–362. M.A. Henderson, Surface Sci. Rep. 66 (2011) 185–297. Q. Sun, Y. Xu, J. Phys. Chem., C 114 (2010) 18911–18918. A. Turki, C. Guillard, F. Dappozze, G. Berhault, Z. Ksibi, H. Kochkar, J. Photochem. Photobiol. A: Chemistry 279 (2014) 8–16. S.C. Ameta, R. Ameta, J. Vardia, Z. Ali, J. Ind. Chem. Soc. 76 (1999) 281–287. D.F. Ollis, Cata. Technol. 2 (1998) 149–156. M.G. Gonzalez, E. Oliveros, M. Worner, A.M.J. Braun, Photochem. Photobiol. C Photochem. Rev. 5 (2004) 225–246.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Does water in synthesized TiO2 have an effect on the photocatalytic activity? Towards a spectacular response L. Elsellami a,b,⇑, F. Dappozze b, A. Houas a,c, C. Guillard b a
Unité de recherche Catalyse et Matériaux pour l’Environnement et les Procédés URCMEP (UR11ES85), Université de Gabes, Tunisia IRCELYON, CNRS UMR 5256/Université Lyon 1, France c Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Department of Chemistry, Riyadh 11623, Saudi Arabia b
a r t i c l e
i n f o
Article history: Received 11 April 2017 Accepted 2 June 2017 Available online 3 June 2017 Keywords: Nanocrystalline materials Adding water Sol-gel preparation FTIR Hydroxyl radical Photocatalysis
a b s t r a c t The influence of hydroxyl radicals on the photocatalytic performance of synthesized TiO2 resulting from the addition of water was studied. Two TiO2 samples were prepared using the sol-gel method followed by calcination at 600 °C. The aim of this work was to study the effect of water on the photocatalytic activity of a model compound – formic acid (FA) – by comparing between the titanium oxide synthesized with water (TiO2-600-w) and without water (TiO2-600). The TiO2-600-w, consisting of anatase and a little rutile, was the most active catalyst, whereas pure anatase (in case of TiO2-600), showed low photocatalytic efficiency. In this study, we confirmed that the filling of the TiO2 surface by hydroxyl radicals could produce better photocatalytic activity by reducing e/h+ recombination. It is notable, however, that with TiO2-600-w, the degradation rate (found under UV) was about 4–5 times greater than that obtained with TiO2-600. This result suggests that the presence of water was very important in the synthesis of TiO2 and brought a dramatic gain in the photocatalytic response as well. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis quantitatively represents an efficient method for producing radicals, and in particular hydroxyl radicals. The hydroxyl radical can be categorically considered as a water molecule fragment. Being a particularly stable molecule, the fragmentation of water necessitates the a priori contribution of an important energy. Titanium dioxide (TiO2) is the most commonly used catalyst in photo-induced reactions, mainly because of its chemical stability and high photocatalytic activity. On its surface, the irradiation of the catalyst causes the decomposition of water into OH radicals that are able to degrade the organic pollutants until complete mineralization [1,2]. The interaction of water with the TiO2 surface is an important research topic since it plays a major role, especially in photocatalysis [3]. Very few authors, however, have been interested in the effect of the adsorption state of the nanocrystal surfaces on their structure [4] and few studies are concerned with the moisture content during the synthesis of TiO2 [5]. Previous studies concluded that the improvement in the photocatalytic activity is due to a bet⇑ Corresponding author at: Unité de recherche Catalyse et Matériaux pour l’Environnement et les Procédés URCMEP (UR11ES85), Université de Gabes, Tunisia. E-mail address: [email protected] (L. Elsellami). http://dx.doi.org/10.1016/j.matlet.2017.06.010 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
ter separation of electron-hole pairs yet they lacked precision [6,7]. Current studies, by contrast, often lack correlation between the properties of water and the photocatalytic performance of TiO2 [8]. This work made it possible to better understand the surface reactivity of TiO2 nanoparticles synthesized with and without water and its photocatalytic role through the degradation of formic acid. The formic acid was chosen here because its photodegradation could be conducted both by direct hole transfer and by photogenerated surface radicals [9]. The photocatalytic activities of both TiO2-600-w and TiO2-600 samples were then studied and compared with the high activity of the reference TiO2 P25.
2. Experiment The precursor used for the synthesis of TiO2 was titanium tetrachloride (TiCl4) (99.99%) purchased from Sigma Aldrich. The solvent, absolute ethanol, (99.99%) was bought from Merck Millipore (Germany). The reagents and the model pollutant (HCOOH) were obtained from Acros Organics and were used without any further purification. Two samples of TiO2 were prepared by the sol-gel method described as follows: an amount of TiCl4 was slowly added to 10 ml of absolute ethanol. During the synthesis, an amount of water (volumetric ratio of water/TiCl4 = 1/3) was only added to one sample of TiO2. In this catalyst (TiO2-600-w),
189
L. Elsellami et al. / Materials Letters 204 (2017) 188–191
(w) indicates the presence of water. Next, the solution obtained with and without water converted into a colorless solution that produced TiO2 nanopowders through the drying process at 85 °C in an oven for 15 h. The obtained TiO2 nanopowders were eventually calcined in an oven for 2 h at a temperature of 600 °C. X-ray diffraction analyses (XRD) were carried out using a X’Pert Pro Panalytical diffractometer with Cu Ka radiation (k = 1.54184 Å). The material to be tested was introduced into a photo-reactor containing 30 ml of the solution of the pollutant whose initial concentration was 50 ppm of formic acid. The solution was stirred in the dark for 60 min to reach the adsorption equilibrium at the catalyst surface. The samples were, then, irradiated and aliquots were taken every 5 min and filtered later through Millipore filters (porosity 0.45 lm). Next, they were analyzed using a high performance liquid chromatography (HPLC).
Table 1 Percentages of the crystalline phase of different samples and degradation rate of TiO2-600-w, TiO2-600, P25. A and R denote anatase and rutile, respectively. Samples
Phase content (%)
r0 (mmol/L/min)
P25 TiO2-600 TiO2-600-w
A: 80; R: 20 A: 100 A: 96; R: 4
26.74 17.16 71.93
This band could be caused by the presence of hydroxyl groups OH adsorbed on the surface which came from the water added during the synthesis and which were gradually released and strongly linked to the TiO2 surface. 3.2. Evaluation of photocatalytic activities of the TiO2 samples 3.2.1. Impact of water on the adsorption of FA Fig. 3a displays the adsorbed amounts of formic acid on the three photocatalysts TiO2-600-w, TiO2-600 and TiO2-P25. If we go through Fig. 3a, we can notice that after 60 min in the dark, a slight adsorption of formic acid on each type of oxide samples was observed: 10% for TiO2-600-w, 6% for TiO2-600 and 8% for TiO2-P25. A large number of parameters could have influenced the adsorption of formic acid on the surface of TiO2 such as surface hydration and adsorption mechanisms [13]. The adsorption of formic acid was due to the hydrogen bond between the oxygen of the carboxylic group on the acid and the hydroxyl groups of TiO2.
3. Results and discussion 3.1. Structural and morphological properties Fig. 1 shows the XRD spectra of pure TiO2 nanopowders with and without water. The obtained results show that the presence of water has influenced the composition of the TiO2 powders and the crystallization phase and that, due to the presence of water during the synthesis, the anatase phase has been modified a bit into a rutile. The phase transition was accompanied by crystal growth. Table 1 summarizes the percentages of anatase and rutile in both samples. The crystallite size of TiO2 nanoparticles was determined based on the Scherrer equation. In fact, anatase TiO2 is considered as the most efficient catalyst [10]. The presence of a small amount of rutile, however, seems to improve the effectiveness of anatase TiO2. Despite the fact that this improvement is still a subject of much debate, most publications have mentioned that the source of their effectiveness could be attributed to an interface between anatase and rutile [11,12]. Measurements in infrared (FT-IR) spectroscopy were carried out so as to determine the surface reactivity and, more precisely, the surface charge which could immediately influence the adsorption properties. In Fig. 2, we can observe clear changes between the two spectra of TiO2 with and without water in the band intensity, especially a broad band observed between 3100 cm1 and 3400 cm1 in the spectrum of TiO2-600-w and which disappeared in the spectrum of TiO2-600.
3.2.2. Impact of water on the disappearance of FA Fig. 3b shows the kinetics of formic acid in the presence of TiO2-600-w, TiO2-600 and TiO2-P25 under exposure to UV-A radiation. The comparison between titanium oxide with and without water shows a beneficial influence of water and a distinct increase in the photocatalytic response. The presence of a larger amount of water in the synthesis of TiO2-600-w than that of TiO2-600 was able to overload the catalyst surface with active radicals. This, then, could explain the systematic activity of the greater degradation of TiO2-600-w. The results shown in Table 1 clearly demonstrate the important photocatalytic role of water. The degradation rate with TiO2-600-w is about 4 or 5 times greater than with TiO2-600. The improvement in velocity is also explained by the production of OH which very often came with the appearance of other chemical
Intensity (a.u.)
A
A
A
0
10
20
A
A
R
30
A
40
A
50
A TiO -600 2
ATiO2-600-w
60
2 (degree) Fig. 1. XRD patterns of TiO2-600-w and TiO2-600 nanomaterials. (A: anatase, R: rutile).
70
L. Elsellami et al. / Materials Letters 204 (2017) 188–191
Transmitance (%)
190
TiO2-600 TiO2-600-w
3100-3400 -OH 0
500
1000
1500
2000
2500
3000
3500
4000
Wavenumbers (Cm-1) Fig. 2. FTIR spectra of TiO2, with water (TiO2-600-w), and without water (TiO2-600) reduced at 600 °C.
5
substances capable of accepting or donating electrons [14]. These were oxidation-reduction reactions which were useful for environmental remediation [15]. Among the adsorbed electron donors was water. The electric field created by the flow of electrons through water led to its ionization. This ionization could be considered as an inelastic collision between the incident electrons and those of the water molecule and represented here a prominent transfer mode of energy to water (Eq. (2)). Nevertheless, it is proposed that they most often underwent dissociation to lead to the formation of H and OH [16] (Eq. (3)), but they were also capable of leading to the formation of molecular dihydrogen and singlet oxygen 1D (Eq. (4)). The latter then reacted quickly with water to form either two radicals OH or hydrogen peroxide H2O2 (Eq. (5)). The H2O+ ion was not stable and reacted rapidly with a water molecule to produce the OH radical and a hydronium ion H3O+ (Eq. (6)). This process produced the greatest contribution to the yield of hydroxyl radical. Eventually, by dissociative attachment to a water molecule, the electron could bring about the formation of a hydroxyl radical and a hydride H (Eq. (7)). The latter was not stable and reacted with a water molecule to form molecular dihydrogen as well as a hydroxide anion OH (Eq. (8)). All these proposed mechanisms contain the surface regeneration step of TiO2 which has never been mentioned in the literature.
(A)
4,5 4 3,5
Q (mg/g)
3 2,5
2 1,5
1 0,5 0 TiO2-600-w
TiO2-P25
TiO2-600
1200
(B) TiO2-600
1000
P25 TiO2-600-w
C (µmol/L)
800
600
200
-40
-20
0
ð1Þ
H2 O!H2 Oþ þ e
ð2Þ
H2 O !H þ OH
ð3Þ
H2 O!H2 þ Oð1 DÞ
ð4Þ
Oð1 DÞ þ H2 O!2 OHðor H2 O2 Þ
ð5Þ
H2 Oþ þ H2 O! OH þ H3 Oþ
ð6Þ
e þ H2 O! OH þ H
ð7Þ
OH þ H þ H2 O! OH þ H2 þ HO
ð8Þ
þ
400
-60
TiO2 þ ðhm > 3; 2 eVÞ!eBC þ hBV
0
20
40
60
80
100
t(min) Fig. 3. (A) Amounts of FA adsorbed per gram of TiO2 as a function of different catalysts (TiO2-600-w, TiO2-600 and TiO2-P25) and (B) Evolution with irradiation time of the concentration of formic acid in solution using TiO2-600-w and TiO2-600 samples. Comparison is provided with the P25 reference.
species that were of great interest as well, increasing afterwards the kinetics of degradation of the pollutants. This led to a Separation of the photogenerated charges and a limitation of the recombination of electron-hole pairs. When TiO2 was irradiated with an energy larger than 3.2 eV (energy gap) (Eq.1), the produced charges migrated to the TiO2 surface and reacted with the adsorbed
The results obtained in this study make it possible to propose a mechanism explaining the performances of TiO2/water and formic acid under UV illumination. The photogenerated electrons of the conduction band of TiO2 oxidized water (Eq. (9)) leaving holes
L. Elsellami et al. / Materials Letters 204 (2017) 188–191
available for the oxidation reactions by increasing the lifetime of the photogenerated carriers. The holes (h+) could then react directly with the formic acid adsorbed by the photo-Kolbe mechanism (Eq. (10)) or by the OH radical attack (Eq. (11))
TiO2 ðeBC Þ þ H2 Oads !TiO2 þ Hþ þ OHads þ
HCOO þ h !HðadsÞ þ CO2
HCOO þ OH!H2 O þ
CO2
ð9Þ ð10Þ ð11Þ
The presence of water, however, could explain the additional characterizations specific to surface properties, charge-carrier dynamics, the reactivity and all of the photocatalytic results. 4. Conclusion We have demonstrated, in this paper, that the addition of water positively influenced the intrinsic performance of TiO2 by the photodegradation of formic acid and that the big difference between the two oxides concerned mostly the presence of rutile phase and the formation of the OH radical derived from water. During TiO2 synthesis, water was used to improve photocatalytic activity by limiting the recombination rate of photogenerated electronhole pairs. Besides, the presence of water simultaneously improved
191
its electron affinity and its electrical conductivity leading to an easier transfer of the photogenerated electrons of the conduction band of TiO2 and thus to a successful charge separation. The water had a large gauge of effective materials during the photocatalysis in order to synthesize a photocatalytically-suitable oxide for a judicious degradation of the pollutant. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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