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TiO2 films from nanopowders
Applied Surface Science 257 (2011) 4227–4231
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural study of sol–gel Au/TiO2 films from nanopowders Dorel Cris¸an a , Nicolae Dr˘agan a , M˘alina R˘aileanu a , Maria Cris¸an a,∗ , Adelina Ianculescu b , Dumitru Luca c , Andrei N˘astut¸a˘ c , Diana Mardare c a
Romanian Academy, Ilie Murgulescu Institute of Physical Chemistry, 202 Splaiul Independent¸ei, 060021 Bucharest, Romania Department of Oxide Materials Science and Engineering, “Politehnica” University of Bucharest, 1-7 Gh. Polizu, P.O. Box 12-134, 011061 Bucharest, Romania c Al.I. Cuza University, Faculty of Physics, 11 Carol I Blvd., R-700506 Ias¸i, Romania b
a r t i c l e
i n f o
Article history: Received 22 September 2010 Received in revised form 1 December 2010 Accepted 4 December 2010 Available online 13 December 2010 Keywords: Sol–gel process Au-doped TiO2 Nanopowder Film Surface properties Structural studies
a b s t r a c t TiO2 represents one of the most important sol–gel materials, due to its photocatalytic properties, in the case of both powders and coatings. Nanostructured titania has been reported to be used in many applications in different fields ranging from optics to gas sensor via solar energy. Recent researches point out the existence of new procedures used in order to enhance the efficiency of the photocatalytic process. The metal ion doping is such an example. Two types of 2 wt.% Au containing TiO2 powders have been embedded in sol–gel vitreous TiO2 matrices. Au-doped TiO2 films have been prepared from these sols, by dipping procedure using quartz microscopic slides, as substrates. The relationship between the synthesis conditions and the properties of titania nanosized materials, such as thermal stability, phase composition, crystallinity, and the influence of dopant was investigated. The hydrophilic properties of the films were correlated with their structure, composition and surface morphology. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Chemical methods, such as sol–gel process, provide an attractive route for the preparation of nanocomposite thin films, homogeneous and with good adherence to the support. TiO2 represents one of the most important sol–gel materials, due to its photocatalytic properties, in the case of both powders and coatings. Nanostructured titania films possess an immense range of applications, e.g. in the field of optics, electrical insulation, photovoltaic solar cells, electrochromic displays, antibacterial coatings, photocatalytic reactors, high performance anodes in ion batteries, and for gas sensing [1]. They are preferred for environmental protection due to their high ratio surface/volume, taking into account that photocatalytic process is based on the chemical reactions at the surface of the material. In order to enhance the efficiency of the photocatalytic process, the metal ion doping is applied. The study of Au-doped TiO2 nanomaterials for photocatalytic applications became an interesting topic for research [2–9]. Recent investigations on gold titania nanocomposite particles show that metal ion doping extended the response of the photocatalyst into visible region. The catalytic properties of Au based materials depend on the support, on the preparation method and particularly on the shape and size of the Au clusters [4]. The noble metals, such as
∗ Corresponding author. Tel.: +40 213 167 912; fax: +40 213 121 147. E-mail addresses: [email protected], [email protected] (M. Cris¸an). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.024
gold and platinum are capable to produce the highest Schottky barrier among the metals facilitating the electron capture [2,6]. In the present work, Au-doped sol–gel TiO2 nanopowders have been embedded in vitreous TiO2 matrices and the corresponding coatings have been prepared. Their structural study has been completed by hydrophilicity tests. 2. Experimental Sol–gel Au/TiO2 films from nanopowders have been obtained by the dipping procedure, using quartz microscopic slides, as substrates. Two types of Au containing TiO2 powders have been embedded in the TiO2 matrix, after the aging of the sol. They have been prepared using ultrasonically and microwave assisted hydrothermal synthesis (samples U and M, respectively). Both samples contained 2 wt.% dopant proceeding from hexachloroauric acid (HAuCl4 ) and they were prepared as previously described [10]. The TiO2 matrix was directly obtained from tetraethylorthotitanate (Merck), Ti(OC2 H5 )4 , using absolute ethanol (Riedel de Haën), as solvent, water for hydrolysis and nitric acid as catalyst. A very rigorous pH and viscosity control of the TiO2 sol was necessary in order to establish the most propitious moment for including the nanopowder. The preparation of Au-doped TiO2 films from nanopowders has followed the same flowchart as in the case of Pd-doped TiO2 coatings previously prepared [11]. The obtained films have been dried at room temperature and then densified by thermal treatment at 300, 400 and 500 ◦ C for 1 h,
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according to DTA/TGA results, using in all cases the same heating rate of 1 ◦ C min−1 . The films were noticed GU and GM, depending on the nature of the embedded powder. The index corresponding to the temperature of thermal treatment was added. Thus, the samples were named as GU3–GU5 and GM3–GM5, respectively, while the un-doped films were labeled as TiO2 -M3–TiO2 -M5. In order to compare their structural behavior with the TiO2 matrix the same thermal schedules have been applied. The thermal and crystallization behaviors of the prepared samples were followed by thermal analysis, using a Mettler Toledo Star System TGA/SDTA851/LF 1600 ◦ C apparatus. The structural evolution was studied by the X-ray diffraction with a Shimadzu XRD 6000 ˚ The diffractometer, using Ni-filtered CuK␣ radiation ( = 1.5418 A). surface morphology of the films has been studied by atomic force microscopy (AFM) in non-contact mode. An OCA 15 EC (Data Physics Instruments) was used in the study of TiO2 film hydrophilicity. The contact angle (CA) measurements between a drop of deionized water (500 nL) and the film surface were performed at room temperature, under 70% environment humidity conditions. A Hamilton motorized syringe (500 L), operated by a computer, allowed the drop volume control. The samples were irradiated for photo-activation using filtered UV light (100 mW/cm2 , photon energy: 3.18–3.65 eV) from a high-pressure mercury lamp (150 W). The CA versus the UV irradiation dose was monitored until the surface photoactivation reached the saturation. Then CA values were measured during the back-reaction regime, with UV irradiation removed, every 9 h, in 7 time steps, keeping the samples in a dark place between measurements.
3. Results and discussion In the experimental conditions mentioned in Section 2, continuous and homogeneous coatings with good adherence to the support were obtained. The thermal processing of the films was determined taking into account the thermal behavior of the gels obtained from the gelation of the sols used for coating preparation. DTA/TGA results of the Au/TiO2 gel coatings (GM and GU) are presented in Fig. 1. DTA curves show an intense endothermic effect at ∼100 ◦ C, due to adsorbed water and alcohol removal associated with mass losses on TG curves. Another endothermic effect at ∼260 ◦ C can be seen for both samples, due to the decomposition of the organic matter. For the GM sample, a supplementary small endothermic effect at 325 ◦ C can be noticed. For the GU sample, two exothermic effects at 192 and 335 ◦ C are observed, the first one correlated with mass loss and the second one without mass loss, probably due to TiO2 anatase crystallization. Structural evolution with temperature of the Au/TiO2 gel coatings obtained from nanopowders, compared with the pure TiO2 one (TiO2 -M), in the same range of temperature (300–500 ◦ C), is presented in Table 1. The experimental data were obtained from computerized analysis of XRD spectra with a proper XRAY5.0 program [12]. The evolution of the normalized microstructural factors, for anatase phase, was represented as histograms (Fig. 2) in which: ıUCV represents the difference between the volumes of the elemental cells, D is the mean value of the crystallite size, and S signifies the average value of the tensile strain. For the un-doped sample, TiO2 -M, with the increase of the temperature, a decrease of the internal strain concomitant with the increase of the crystallite size was noticed. The increase of the rate of crystallite growth with temperature is significantly diminished for the samples belonging to the GM3–GM5 series, compared to the GU3–GU5 one. For the GM3–GM5 samples the variation of UCV follows almost precisely the tensile strain evolution, while in the GU3–GU5 case, a progres-
Fig. 1. Thermal behavior of GM (a) and GU gel coatings (b).
sive decrease of the UCV with the increase of temperature can be observed. The worst case regarding the structural disorder for both series of prepared materials refers to sample GU4. In this case, the profile analysis finds the lowest value of the mean crystallites size, corresponding to the highest average tensile strain. For samples thermally treated at 400 and 500 ◦ C, the dopant presence induces greater internal strain. For all series of samples (TiO2 -M3–TiO2 -M5, GU3–GU5 and GM3–GM5, respectively) the rutile phase has been detected only at 500 ◦ C. Fig. 3 presents the corresponding histograms. An ideal correlation between the tensile strain variation and the crystallite size has been observed: the crystalline order is present at short distances for higher strains. In the same time, the UCV of the rutile phase from the GM5 sample seems to suffer a contraction, compared to sample GU5. The presence of the dopant generates greater internal strain. From Fig. 4 it can be observed that the contact angle decreases with the increase of the UV-irradiation dose, but not all the samples became super-hydrophilic (CA values below 10◦ [13]). In fact, only the films thermally treated at 400 ◦ C, namely GM4 and GU4, reach CA values of 4.0◦ and 6.0◦ respectively. They are followed by GU5 sample, which still presents good hydrophilic properties (CA of 13.0 for a saturated surface photoactivation). For the mentioned samples, it is worth noting that the CA decreases rapidly, the surface photoactivation reaching the saturation for an UV dose of about 12 J/cm2 . When keeping these samples in darkness, the recovery of the initial contact angle values occurs after time intervals larger then 43 h (Fig. 5), depending on the sample structure and surface condition. Of note especially GM4 and GU4 samples which maintain their super-hydrophilicity for 18 h. The same pictures show that the other three samples have worse hydrophilic properties.
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Table 1 Microstructural factors calculated from the computerized profile analysis of the XRD spectra and root mean square roughness (Rrms ) of the studied samples. Sample
TiO2 -M3 TiO2 -M4 TiO2 -M5 GM3 GM4 GM5 GU3 GU4 GU5
Identified phases
A A A – 70% R – 30% A A A > 90% R < 10% A A A > 95% R < 5%
Rrms
(nm)
1.2 7.25 19.9 2.4 23.9 33.6 17.6 17.8 33.6
Microstructural factors a [Å]
c [Å]
UCV [Å3 ]
D [Å]
S × 103
3.7770 (12) 3.7835 (19) 3.7756 (16) 4.5886 (38) 3.7784 (21) 3.7669 (20) 3.7787 (12) 4.5852 (58) 3.7825 (24) 3.7806 (22) 3.7714 (20) 4.6100 (179)
9.4561 (42) 9.4883 (67) 9.4740 (57) 2.9561 (30) 9.4672 (73) 9.4849 (73) 9.5018 (42) 2.9550 (35) 9.4982 (84) 9.4789 (79) 9.4832 (71) 2.9452 (99)
134.90 (14) 135.83 (23) 135.05 (19) 62.24 (17) 136.16 (25) 134.58 (25) 135.67 (14) 62.13 (23) 135.90 (29) 135.48 (27) 134.88 (24) 62.59 (69)
127 (25) 161 (26) 443 (92) 685 (193) 191 (52) 280 (105) 289 (44) 286 (66) 164 (40) 151 (46) 455 (170) 531 (205)
3.7 (1.1) 1.3 (7) 0.5 (4) 1.1 (4) 1.77 (82) 0.74 (83) 1.33 (32) 2.51 (61) 1.39 (85) 2.2 (1.1) 1.23 (79) 1.23 (64)
A – anatase; R – rutile; a, c – lattice parameters; UCV – unit cell volume; D – crystallite size; S – internal strain.
The un-doped films TiO2 -M4 and TiO2 -M5 tend to become superhydrophilic too (CA range between 8.0◦ and 9.8◦ ), but they reach surface photoactivation saturation only under UV doses higher than 15 J/cm2 . The initial CA values (for the non-irradiated samples) are correlated with the root mean square roughness (Rrms ) values, estimated by AFM, for each sample. A lower surface roughness favors the increase in CA, if the initial value of CA is smaller than 90◦ [14]. From Table 1, one can observe that there is a good agreement between the surface roughness (Rrms ranging from 2.44 nm for GM3 sample and 33.6 nm for GU5 sample) and the initial values of the CA (ranging between 76.0◦ and 37.0◦ respectively). From the XRD mea-
surements it can be seen that, except the samples thermally treated at 500 ◦ C, all samples present only the anatase phase, which has a higher oxidizing power than rutile. Figs. 6 and 7 show the 3D-AFM images of the doped samples GU5 (with the highest roughness value) and GM3 (with the lowest roughness value). According to our observations, the best hydrophilic performances among the doped samples can be attributed, in order, to samples GM4, GU4 and GU5. The first two are the roughest samples among those which contain 100% anatase. On the other hand,
Fig. 2. The histograms of lattice strain S, crystallite size D and unit cell volume (UCV) variations for thermally treated TiO2 -M, GM and GU gel coatings, for anatase ˚ Dmin = 127 A; ˚ UCV expressed phase; Smax = 3.7 × 10−3 ; Smin = 0.5 × 10−3 ; Dmax = 455 A; as ı difference (Vmax = 135.9 A˚ 3 ; Vmin = 134.6 A˚ 3 ); a–c: samples TiO2 -M; d–f: samples GM3–GM5; g–i: samples GU3–GU5.
Fig. 3. The histograms of lattice strain S, crystallite size D and unit cell volume (UCV) variations for TiO2 -M5, GM5 and GU5 gel coatings for rutile phase; ˚ Dmin = 286 A; ˚ UCV expressed as ı Smax = 2.5 × 10−3 ; Smin = 1.1 × 10−3 ; Dmax = 685 A; difference (Vmax = 62.6 A˚ 3 ; Vmin = 62.1 A˚ 3 ); a: sample TiO2 -M5; b: sample GM5; c: sample GU5.
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80
GM3 GM4 GM5 GU3 GU4 GU5 TiO2-M3
Contact angle (deg.)
70 60 50
TiO2-M4
40
TiO2-M5
30 20 10 0 0
10
20
30
40
Fig. 7. AFM image of GM3 sample.
UV radiation dose (J/cm2 ) Fig. 4. Contact angle versus UV irradiation dose dependences for the studied films.
cation of the wetting [14]. This might explain the photoenhanced hydrophilicity of the prepared samples. 4. Conclusions
100
Contact angle (deg.)
80
60
GM3 GM4 GM5 GU3 GU4 GU5 TiO2-M3 TiO2-M4 TiO2-M5
40
20
0 -10
0
10
20
30
40
50
60
70
Time in darkness (hours) Fig. 5. Time dependence of the contact angle, measured under surface de-activation conditions for the studied films.
the highest roughness which can be observed belongs to the GU5 sample which also contains the rutile phase (see Table 1). For hydrophilic cases (as those of our samples, which present CA values less than 90◦ ), the surface roughness has an effect on the amplifi-
Au-doped sol–gel TiO2 nanopowders have been embedded in vitreous TiO2 matrices and the corresponding films coated on quartz microscopic slides, as substrates were performed. Continuous and homogeneous coatings with good adherence to the support were obtained. The relationship between the synthesis conditions, the influence of the dopant and the TiO2 crystallization tendency were investigated. Structural evolution (lattice parameters, crystallite sizes, internal strains) with temperature for the Au/TiO2 gel coatings obtained from nanopowders, compared with the pure TiO2 one was studied. A correlation between the tensile strain variation and the crystallite size with the temperature has been observed. The presence of the dopant in the TiO2 lattice induces greater internal strains and smaller crystallite sizes. Among the studied samples, the best hydrophilic performance is attributed to the Au-doped sample prepared using microwave assisted hydrothermal synthesis and thermally treated at 400 ◦ C (GM4), for which the CA reaches 4.0◦ quite rapidly, for an UV dose of about 12 J/cm2 . It is followed by another Au-doped sample, thermally treated at 400 ◦ C and prepared using ultrasonically assisted hydrothermal synthesis (GU4), for which the CA reaches 6.0◦ for the same UV dose. Concerning the other studied samples, only the un-doped ones reached super-hydrophilicity (CA between 8.0◦ and 9.8◦ ), but under UV doses higher than 15 J/cm2 . The superhydrophilic properties of the investigated films are explained by the presence of highly active anatase phase and the high surface roughness. Acknowledgements This work was supported by the Romanian Excellence grants CEEX-NATIOD-SBM and the projects PCCE-ID 76 and PN-2MAMAINCOPAE. References
Fig. 6. AFM image of GU5 sample.
[1] W. Chen, J. Zhang, Q. Fang, S. Li, J. Wu, F. Li, K. Jiang, Sol–gel preparation of thick titania coatings aided by organic binder materials, Sens. Actuators B 100 (2004) 195–199. [2] F.B. Li, X.Z. Li, Photocatalytic properties of gold/gold ion-modified titanium dioxide for wastewater treatment, Appl. Catal. A: Gen. 228 (2002) 15–27. [3] M.A. Debeila, M.C. Raphulu, E. Mokoena, M. Avalos, V. Petranovskii, N.J. Coville, M.S. Scurrell, The effect of gold on the phase transitions of titania, Mater. Sci. Eng. A 396 (2005) 61–69.
D. Cris¸an et al. / Applied Surface Science 257 (2011) 4227–4231 [4] R.S. Sonawane, M.K. Dongare, Sol–gel synthesis of Au/TiO2 thin films for photocatalytic degradation of phenol in sunlight, J. Mol. Catal. A: Chem. 243 (2006) 68–76. [5] E.V. Milsom, J. Novak, M. Oyama, F. Marken, Electrocatalytic oxidation of nitric oxide at TiO2 -Au nanocomposite film electrodes, Electrochem. Commun. 9 (2007) 436–442. [6] Z. Du, C. Feng, Q. Li, Y. Zhao, X. Tai, Photodegradation of NPE-10 surfactant by Au-doped nano-TiO2 , Colloid Surface. A: Physicochem. Eng. Aspects 315 (2008) 254–258. [7] V. Rodriguez-Gonzalez, R. Zanella, G. del Angel, R. Gomez, MTBE visible-light photocatalytic decomposition over Au/TiO2 and Au/TiO2 -Al2 O3 sol–gel prepared catalysts, J. Mol. Catal. A: Chem. 281 (2008) 93–98. [8] J. Hernandez-Fernandez, A. Aguilar-Elguezabal, S. Castillo, B. Ceron-Ceron, Oxidation of NO in gase phase by Au-TiO2 photocatalysts prepared by the sol–gel method, Catal. Today 148 (2009) 115–118.
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[9] A. Kafizas, S. Kellici, J.A. Darr, I.P. Parkin, Titanium dioxide and composite metal/metal oxide titania thin films on glass: A comparative study of photocatalytic activity, J. Photochem. Photobiol. A: Chem. 204 (2009) 183–190. [10] C. Laz˘au, L. Mocanu, I. Miron, P. Sfirloag˘a, G. T˘an˘asie, C. Tatu, A. Gruia, I. Grozescu, Consideration regarding the use of TiO2 doped nanoparticles in medicine, Digest J. Nanomater. Biostruct. 2 (2007) 257–263. [11] M. Cris¸an, A. Br˘aileanu, D. Cris¸an, M. R˘aileanu, N. Dr˘agan, D. Mardare, V. Teodorescu, A. Ianculescu, R. Bîrjega, M. Dumitru, Thermal behaviour study of some sol–gel TiO2 based materials, J. Therm. Anal. Cal. 92 (2008) 7–13. [12] N. Dr˘agan, D. Cris¸an, C. Lep˘adatu, Fast analysis of the XRD profile with the X-RAY3.0 program, Rom. J. Mater. 33 (2003) 133–148. [13] D. Luca, D. Mardare, F. Iacomi, C.M. Teodorescu, Increasing surface hydrophilicity of titania thin films by doping, Appl. Surf. Sci. 252 (2006) 6122–6126. [14] R.N. Wenzel, Surface roughness and contact angle, J. Phys. Colloid Chem. 53 (1949) 1466–1467.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural study of sol–gel Au/TiO2 films from nanopowders Dorel Cris¸an a , Nicolae Dr˘agan a , M˘alina R˘aileanu a , Maria Cris¸an a,∗ , Adelina Ianculescu b , Dumitru Luca c , Andrei N˘astut¸a˘ c , Diana Mardare c a
Romanian Academy, Ilie Murgulescu Institute of Physical Chemistry, 202 Splaiul Independent¸ei, 060021 Bucharest, Romania Department of Oxide Materials Science and Engineering, “Politehnica” University of Bucharest, 1-7 Gh. Polizu, P.O. Box 12-134, 011061 Bucharest, Romania c Al.I. Cuza University, Faculty of Physics, 11 Carol I Blvd., R-700506 Ias¸i, Romania b
a r t i c l e
i n f o
Article history: Received 22 September 2010 Received in revised form 1 December 2010 Accepted 4 December 2010 Available online 13 December 2010 Keywords: Sol–gel process Au-doped TiO2 Nanopowder Film Surface properties Structural studies
a b s t r a c t TiO2 represents one of the most important sol–gel materials, due to its photocatalytic properties, in the case of both powders and coatings. Nanostructured titania has been reported to be used in many applications in different fields ranging from optics to gas sensor via solar energy. Recent researches point out the existence of new procedures used in order to enhance the efficiency of the photocatalytic process. The metal ion doping is such an example. Two types of 2 wt.% Au containing TiO2 powders have been embedded in sol–gel vitreous TiO2 matrices. Au-doped TiO2 films have been prepared from these sols, by dipping procedure using quartz microscopic slides, as substrates. The relationship between the synthesis conditions and the properties of titania nanosized materials, such as thermal stability, phase composition, crystallinity, and the influence of dopant was investigated. The hydrophilic properties of the films were correlated with their structure, composition and surface morphology. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Chemical methods, such as sol–gel process, provide an attractive route for the preparation of nanocomposite thin films, homogeneous and with good adherence to the support. TiO2 represents one of the most important sol–gel materials, due to its photocatalytic properties, in the case of both powders and coatings. Nanostructured titania films possess an immense range of applications, e.g. in the field of optics, electrical insulation, photovoltaic solar cells, electrochromic displays, antibacterial coatings, photocatalytic reactors, high performance anodes in ion batteries, and for gas sensing [1]. They are preferred for environmental protection due to their high ratio surface/volume, taking into account that photocatalytic process is based on the chemical reactions at the surface of the material. In order to enhance the efficiency of the photocatalytic process, the metal ion doping is applied. The study of Au-doped TiO2 nanomaterials for photocatalytic applications became an interesting topic for research [2–9]. Recent investigations on gold titania nanocomposite particles show that metal ion doping extended the response of the photocatalyst into visible region. The catalytic properties of Au based materials depend on the support, on the preparation method and particularly on the shape and size of the Au clusters [4]. The noble metals, such as
∗ Corresponding author. Tel.: +40 213 167 912; fax: +40 213 121 147. E-mail addresses: [email protected], [email protected] (M. Cris¸an). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.024
gold and platinum are capable to produce the highest Schottky barrier among the metals facilitating the electron capture [2,6]. In the present work, Au-doped sol–gel TiO2 nanopowders have been embedded in vitreous TiO2 matrices and the corresponding coatings have been prepared. Their structural study has been completed by hydrophilicity tests. 2. Experimental Sol–gel Au/TiO2 films from nanopowders have been obtained by the dipping procedure, using quartz microscopic slides, as substrates. Two types of Au containing TiO2 powders have been embedded in the TiO2 matrix, after the aging of the sol. They have been prepared using ultrasonically and microwave assisted hydrothermal synthesis (samples U and M, respectively). Both samples contained 2 wt.% dopant proceeding from hexachloroauric acid (HAuCl4 ) and they were prepared as previously described [10]. The TiO2 matrix was directly obtained from tetraethylorthotitanate (Merck), Ti(OC2 H5 )4 , using absolute ethanol (Riedel de Haën), as solvent, water for hydrolysis and nitric acid as catalyst. A very rigorous pH and viscosity control of the TiO2 sol was necessary in order to establish the most propitious moment for including the nanopowder. The preparation of Au-doped TiO2 films from nanopowders has followed the same flowchart as in the case of Pd-doped TiO2 coatings previously prepared [11]. The obtained films have been dried at room temperature and then densified by thermal treatment at 300, 400 and 500 ◦ C for 1 h,
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D. Cris¸an et al. / Applied Surface Science 257 (2011) 4227–4231
according to DTA/TGA results, using in all cases the same heating rate of 1 ◦ C min−1 . The films were noticed GU and GM, depending on the nature of the embedded powder. The index corresponding to the temperature of thermal treatment was added. Thus, the samples were named as GU3–GU5 and GM3–GM5, respectively, while the un-doped films were labeled as TiO2 -M3–TiO2 -M5. In order to compare their structural behavior with the TiO2 matrix the same thermal schedules have been applied. The thermal and crystallization behaviors of the prepared samples were followed by thermal analysis, using a Mettler Toledo Star System TGA/SDTA851/LF 1600 ◦ C apparatus. The structural evolution was studied by the X-ray diffraction with a Shimadzu XRD 6000 ˚ The diffractometer, using Ni-filtered CuK␣ radiation ( = 1.5418 A). surface morphology of the films has been studied by atomic force microscopy (AFM) in non-contact mode. An OCA 15 EC (Data Physics Instruments) was used in the study of TiO2 film hydrophilicity. The contact angle (CA) measurements between a drop of deionized water (500 nL) and the film surface were performed at room temperature, under 70% environment humidity conditions. A Hamilton motorized syringe (500 L), operated by a computer, allowed the drop volume control. The samples were irradiated for photo-activation using filtered UV light (100 mW/cm2 , photon energy: 3.18–3.65 eV) from a high-pressure mercury lamp (150 W). The CA versus the UV irradiation dose was monitored until the surface photoactivation reached the saturation. Then CA values were measured during the back-reaction regime, with UV irradiation removed, every 9 h, in 7 time steps, keeping the samples in a dark place between measurements.
3. Results and discussion In the experimental conditions mentioned in Section 2, continuous and homogeneous coatings with good adherence to the support were obtained. The thermal processing of the films was determined taking into account the thermal behavior of the gels obtained from the gelation of the sols used for coating preparation. DTA/TGA results of the Au/TiO2 gel coatings (GM and GU) are presented in Fig. 1. DTA curves show an intense endothermic effect at ∼100 ◦ C, due to adsorbed water and alcohol removal associated with mass losses on TG curves. Another endothermic effect at ∼260 ◦ C can be seen for both samples, due to the decomposition of the organic matter. For the GM sample, a supplementary small endothermic effect at 325 ◦ C can be noticed. For the GU sample, two exothermic effects at 192 and 335 ◦ C are observed, the first one correlated with mass loss and the second one without mass loss, probably due to TiO2 anatase crystallization. Structural evolution with temperature of the Au/TiO2 gel coatings obtained from nanopowders, compared with the pure TiO2 one (TiO2 -M), in the same range of temperature (300–500 ◦ C), is presented in Table 1. The experimental data were obtained from computerized analysis of XRD spectra with a proper XRAY5.0 program [12]. The evolution of the normalized microstructural factors, for anatase phase, was represented as histograms (Fig. 2) in which: ıUCV represents the difference between the volumes of the elemental cells, D is the mean value of the crystallite size, and S signifies the average value of the tensile strain. For the un-doped sample, TiO2 -M, with the increase of the temperature, a decrease of the internal strain concomitant with the increase of the crystallite size was noticed. The increase of the rate of crystallite growth with temperature is significantly diminished for the samples belonging to the GM3–GM5 series, compared to the GU3–GU5 one. For the GM3–GM5 samples the variation of UCV follows almost precisely the tensile strain evolution, while in the GU3–GU5 case, a progres-
Fig. 1. Thermal behavior of GM (a) and GU gel coatings (b).
sive decrease of the UCV with the increase of temperature can be observed. The worst case regarding the structural disorder for both series of prepared materials refers to sample GU4. In this case, the profile analysis finds the lowest value of the mean crystallites size, corresponding to the highest average tensile strain. For samples thermally treated at 400 and 500 ◦ C, the dopant presence induces greater internal strain. For all series of samples (TiO2 -M3–TiO2 -M5, GU3–GU5 and GM3–GM5, respectively) the rutile phase has been detected only at 500 ◦ C. Fig. 3 presents the corresponding histograms. An ideal correlation between the tensile strain variation and the crystallite size has been observed: the crystalline order is present at short distances for higher strains. In the same time, the UCV of the rutile phase from the GM5 sample seems to suffer a contraction, compared to sample GU5. The presence of the dopant generates greater internal strain. From Fig. 4 it can be observed that the contact angle decreases with the increase of the UV-irradiation dose, but not all the samples became super-hydrophilic (CA values below 10◦ [13]). In fact, only the films thermally treated at 400 ◦ C, namely GM4 and GU4, reach CA values of 4.0◦ and 6.0◦ respectively. They are followed by GU5 sample, which still presents good hydrophilic properties (CA of 13.0 for a saturated surface photoactivation). For the mentioned samples, it is worth noting that the CA decreases rapidly, the surface photoactivation reaching the saturation for an UV dose of about 12 J/cm2 . When keeping these samples in darkness, the recovery of the initial contact angle values occurs after time intervals larger then 43 h (Fig. 5), depending on the sample structure and surface condition. Of note especially GM4 and GU4 samples which maintain their super-hydrophilicity for 18 h. The same pictures show that the other three samples have worse hydrophilic properties.
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Table 1 Microstructural factors calculated from the computerized profile analysis of the XRD spectra and root mean square roughness (Rrms ) of the studied samples. Sample
TiO2 -M3 TiO2 -M4 TiO2 -M5 GM3 GM4 GM5 GU3 GU4 GU5
Identified phases
A A A – 70% R – 30% A A A > 90% R < 10% A A A > 95% R < 5%
Rrms
(nm)
1.2 7.25 19.9 2.4 23.9 33.6 17.6 17.8 33.6
Microstructural factors a [Å]
c [Å]
UCV [Å3 ]
D [Å]
S × 103
3.7770 (12) 3.7835 (19) 3.7756 (16) 4.5886 (38) 3.7784 (21) 3.7669 (20) 3.7787 (12) 4.5852 (58) 3.7825 (24) 3.7806 (22) 3.7714 (20) 4.6100 (179)
9.4561 (42) 9.4883 (67) 9.4740 (57) 2.9561 (30) 9.4672 (73) 9.4849 (73) 9.5018 (42) 2.9550 (35) 9.4982 (84) 9.4789 (79) 9.4832 (71) 2.9452 (99)
134.90 (14) 135.83 (23) 135.05 (19) 62.24 (17) 136.16 (25) 134.58 (25) 135.67 (14) 62.13 (23) 135.90 (29) 135.48 (27) 134.88 (24) 62.59 (69)
127 (25) 161 (26) 443 (92) 685 (193) 191 (52) 280 (105) 289 (44) 286 (66) 164 (40) 151 (46) 455 (170) 531 (205)
3.7 (1.1) 1.3 (7) 0.5 (4) 1.1 (4) 1.77 (82) 0.74 (83) 1.33 (32) 2.51 (61) 1.39 (85) 2.2 (1.1) 1.23 (79) 1.23 (64)
A – anatase; R – rutile; a, c – lattice parameters; UCV – unit cell volume; D – crystallite size; S – internal strain.
The un-doped films TiO2 -M4 and TiO2 -M5 tend to become superhydrophilic too (CA range between 8.0◦ and 9.8◦ ), but they reach surface photoactivation saturation only under UV doses higher than 15 J/cm2 . The initial CA values (for the non-irradiated samples) are correlated with the root mean square roughness (Rrms ) values, estimated by AFM, for each sample. A lower surface roughness favors the increase in CA, if the initial value of CA is smaller than 90◦ [14]. From Table 1, one can observe that there is a good agreement between the surface roughness (Rrms ranging from 2.44 nm for GM3 sample and 33.6 nm for GU5 sample) and the initial values of the CA (ranging between 76.0◦ and 37.0◦ respectively). From the XRD mea-
surements it can be seen that, except the samples thermally treated at 500 ◦ C, all samples present only the anatase phase, which has a higher oxidizing power than rutile. Figs. 6 and 7 show the 3D-AFM images of the doped samples GU5 (with the highest roughness value) and GM3 (with the lowest roughness value). According to our observations, the best hydrophilic performances among the doped samples can be attributed, in order, to samples GM4, GU4 and GU5. The first two are the roughest samples among those which contain 100% anatase. On the other hand,
Fig. 2. The histograms of lattice strain S, crystallite size D and unit cell volume (UCV) variations for thermally treated TiO2 -M, GM and GU gel coatings, for anatase ˚ Dmin = 127 A; ˚ UCV expressed phase; Smax = 3.7 × 10−3 ; Smin = 0.5 × 10−3 ; Dmax = 455 A; as ı difference (Vmax = 135.9 A˚ 3 ; Vmin = 134.6 A˚ 3 ); a–c: samples TiO2 -M; d–f: samples GM3–GM5; g–i: samples GU3–GU5.
Fig. 3. The histograms of lattice strain S, crystallite size D and unit cell volume (UCV) variations for TiO2 -M5, GM5 and GU5 gel coatings for rutile phase; ˚ Dmin = 286 A; ˚ UCV expressed as ı Smax = 2.5 × 10−3 ; Smin = 1.1 × 10−3 ; Dmax = 685 A; difference (Vmax = 62.6 A˚ 3 ; Vmin = 62.1 A˚ 3 ); a: sample TiO2 -M5; b: sample GM5; c: sample GU5.
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80
GM3 GM4 GM5 GU3 GU4 GU5 TiO2-M3
Contact angle (deg.)
70 60 50
TiO2-M4
40
TiO2-M5
30 20 10 0 0
10
20
30
40
Fig. 7. AFM image of GM3 sample.
UV radiation dose (J/cm2 ) Fig. 4. Contact angle versus UV irradiation dose dependences for the studied films.
cation of the wetting [14]. This might explain the photoenhanced hydrophilicity of the prepared samples. 4. Conclusions
100
Contact angle (deg.)
80
60
GM3 GM4 GM5 GU3 GU4 GU5 TiO2-M3 TiO2-M4 TiO2-M5
40
20
0 -10
0
10
20
30
40
50
60
70
Time in darkness (hours) Fig. 5. Time dependence of the contact angle, measured under surface de-activation conditions for the studied films.
the highest roughness which can be observed belongs to the GU5 sample which also contains the rutile phase (see Table 1). For hydrophilic cases (as those of our samples, which present CA values less than 90◦ ), the surface roughness has an effect on the amplifi-
Au-doped sol–gel TiO2 nanopowders have been embedded in vitreous TiO2 matrices and the corresponding films coated on quartz microscopic slides, as substrates were performed. Continuous and homogeneous coatings with good adherence to the support were obtained. The relationship between the synthesis conditions, the influence of the dopant and the TiO2 crystallization tendency were investigated. Structural evolution (lattice parameters, crystallite sizes, internal strains) with temperature for the Au/TiO2 gel coatings obtained from nanopowders, compared with the pure TiO2 one was studied. A correlation between the tensile strain variation and the crystallite size with the temperature has been observed. The presence of the dopant in the TiO2 lattice induces greater internal strains and smaller crystallite sizes. Among the studied samples, the best hydrophilic performance is attributed to the Au-doped sample prepared using microwave assisted hydrothermal synthesis and thermally treated at 400 ◦ C (GM4), for which the CA reaches 4.0◦ quite rapidly, for an UV dose of about 12 J/cm2 . It is followed by another Au-doped sample, thermally treated at 400 ◦ C and prepared using ultrasonically assisted hydrothermal synthesis (GU4), for which the CA reaches 6.0◦ for the same UV dose. Concerning the other studied samples, only the un-doped ones reached super-hydrophilicity (CA between 8.0◦ and 9.8◦ ), but under UV doses higher than 15 J/cm2 . The superhydrophilic properties of the investigated films are explained by the presence of highly active anatase phase and the high surface roughness. Acknowledgements This work was supported by the Romanian Excellence grants CEEX-NATIOD-SBM and the projects PCCE-ID 76 and PN-2MAMAINCOPAE. References
Fig. 6. AFM image of GU5 sample.
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