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Effect of Schiff base ligand on the size and the optical properties of TiO2 nanoparticles
Superlattices and Microstructures 62 (2013) 30–38
Contents lists available at SciVerse ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Effect of Schiff base ligand on the size and the optical properties of TiO2 nanoparticles Maryam Masjedi b, Noshin Mir c, Elham Noori b, Tahereh Gholami b, Masoud Salavati-Niasari a,b,⇑ a Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran b Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran c Department of Chemistry, University of Zabol, Zabol, Islamic Republic of Iran
a r t i c l e
i n f o
Article history: Received 30 March 2013 Received in revised form 11 June 2013 Accepted 13 July 2013 Available online 22 July 2013 Keywords: Agglomeration Semiconductors Band gap Sol-Gel
a b s t r a c t The effect of a Schiff-base ligand (N,N0 -ethylenebis(acetylacetone iminato)dianion = acacen) on size and optical properties of TiO2 nanoparticles in a two-step sol–gel method was investigated. Different amounts of Schiff-base ligand were applied and the as-prepared products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectrum, Electron Dispersive X-ray spectroscopy (EDX) and ultraviolet– visible (UV–Vis) spectroscopy. Molecular orbital structure of acacen was calculated by density functional theory (DFT) in order to determine the exact orbital energies and electron transfer pathways. The results demonstrated that applying the appropriate amount of Schiff-base ligand could be effective in particle size control. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Titanium dioxide (TiO2) is a semiconductor material with versatile application due to its optical and electronic properties. Nanostructured TiO2 is widely used in applications such as dye-sensitized solar cells, photocatalysis, lithium ion batteries and gas sensing systems [1–4]. ⇑ Corresponding author at: Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran. Tel.: +98 361 5555 333; fax: +98 361 555 29 30. E-mail address: [email protected] (M. Salavati-Niasari). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.07.003
M. Masjedi et al. / Superlattices and Microstructures 62 (2013) 30–38
31
A variation of possible routes including chemical vapor deposition [5], hydrothermal [6], ultrasonic irradiation [7], sol–gel [8], and solvothermal [9] have been applied to obtain different types of TiO2. Recently, a two-step gel–sol process has been developed by Sugimoto [10] to obtain pure anatase TiO2. The main advantage of this method is production of monodispersed particles with controlled mean size in order to improve different properties of TNPs [11,12]. To develop TiO2 for alternative energy applications, many efforts have been devoted in various fields of science. Previous reports reveal that the most preponderant strategies to achieve desired properties of TiO2 is including particle size control [13,14], shape and morphology control [15,16], and band gap changing by introducing foreign species as dopants [17–19]. In such efforts, to optimize the TiO2 properties, many different materials and reaction conditions have been examined. One of the significant aspects of preparing nanoparticle with specific properties is choosing appropriate ligand shell. The ligand shell is, most simply, a molecular monolayer encapsulating the core. In the case of organic species, ligands that fulfill the task of adsorbent and stabilizer have a functional head group and one or more hydrocarbon tails, and both elements play a role in the control of nucleation and growth [20]. Beside the well-known surfactants, recently our team has focused on the effect of ligands containing hard atoms (N, O) as well as bulky aromatic groups on the properties of obtained products [21–25]. It has been demonstrated that these kind of organic agents have an effective role on controlling size, shape, and optical properties of final products. In this paper, the effect of a Schiff base ligand as a complexing agent on morphology, size, and optical properties of TiO2 nanoparticles was investigated. Acacen, a tetradentate Schiff base ligand containing four hard binding sites (N and O), is capable of stabilizing transition metal centers very well. Acacen could be quite interesting considering the fact that bonding via two oxo groups to the TiO2 surface may lead to more efficient charge transfer between an adsorbed molecule and the oxide surface similar to dicarboxylic acids. These kinds of molecules are of interest in preventing crystal growth of particular surface planes, or limiting the size of crystals in the preparation of nanoparticles [26].
2. Materials and methods 2.1. Materials and characterization Titanium (IV) ethoxide, N,N0 -ethylenebis(acetylacetone iminato)dianion (acacen) and triethanolamine (TEOA) used in our experiments were purchased from Merck. For characterization of the products X-ray diffraction (XRD) patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu Ka radiation. Scanning electron microscopy (SEM) images were obtained on Philips XL-30ESEM. Transmission electron microscopy (TEM) image was obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets. Optical analyses were performed using a V-670 UV–Vis–NIR Spectrophotometer (Jasco).
2.2. Synthetic method The standard procedure for the preparation of TiO2 nanoparticles was conducted according to the literature [10]. Stock solution of Ti4+ (0.5 M) which are stable against hydrolysis at room temperature were prepared by mixing titanium(IV) ethoxide (TEO) and TEOA with a molar ratio of TEO:TEOA = 1:2, followed by the addition of distilled water. The final pH of solutions was 9.6 and was fixed at this pH by mixing with HClO4 or NaOH solution after addition of complexing agents. Acacen ligand with different ratios to the Ti4+ precursor (Table 1) was further added as complexing agents to each solution. The solution was placed in a Teflon-lined autoclave and aged at 100 °C for 24 h for gelation; then, the temperature was increased to 140 °C for 72 h to nucleate and grow TiO2 nanocrystals. The resulting sol was washed with and centrifuged from NaOH (six times), HNO3 (two times), and distilled water (four times) to remove residual organic compounds from the surface of the nanoparticles.
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M. Masjedi et al. / Superlattices and Microstructures 62 (2013) 30–38 Table 1 Different samples prepared in this work. Sample abbreviation
Ti4+:L
Ti4+:TEOA
L0 L0.5 L1 L3 L5 L1T0
1:0 1:0.5 1:1 1:3 1:5 1:1
1:2
1:0
2.3. Calculations The molecular structure of Acacen in the ground state were optimized on the basis of density function theory (DFT) at the Becke3–Lee–Yang–Parr (B3LYP) (with 6-31G basis) and by means of visual inspection using the GAUSSVIEW program (Version 5.0). All the calculations were performed using the GAUSSIAN 09 software package. 3. Results and discussion 3.1. Characterization of two-step sol–gel process-based TiO2 nanoparticles
3.1.1. X-ray diffraction patterns The X-ray diffraction patterns of as-prepared product obtained from the two-step reaction are shown in Fig. 1. Fig. 1a–c show the XRD patterns of samples L1, L0 and L1T0 which are prepared in presence of both ligand and TEOA, in absence of ligand, and in absence of TEOA, respectively. All diffraction peaks can be well indexed to pure anatase structural titanium dioxide. All the peaks are labeled and can be indexed to JCPDS Card No. 71-1167, 04-0477, and 03-0380 for sample L1, L0 and L1T0, respectively. The XRD patterns of samples show different line broadening. Generally, the crystalline domain size decreases with increasing line broadening. The line broadening of the peak of the (1 0 1) index is related to the size of the crystalline phase. The data for the full width at half maximum (FWHM) of sample L1, L0 and L1T0 at 2h = 25.47, 25.38, and 25.50 was estimated to be 0.3542, 0.4133, and 1.0627, respectively. The crystallite size diameter (D) of the TiO2 products has been calculated by Debye– Scherrer equation, D = 0.9k/b cos h [27]. Calculated crystalline domain sizes have been found to be 22.7, 19.3, and 7.6 nm for sample L1, L0 and L1T0, respectively. It can be concluded that the employed Schiff base ligand has a significant influence on crystalline size of the product so that in the presence of only the ligand (sample L1T0) the calculated crystalline domain size decreased down to lower that 10 nm. It seems that applying both the ligand and TEOA (L1) has a different effect so that the particles size of L0 sample is smaller than L1. Preparation of TiO2 nanoparticles with tuneable size is of great interest for different applications. Here, it is shown that different conditions lead to different crystalline size from ca. 7.5–22.5. 3.1.2. TEM images The shapes and structure of TiO2 nanoparticle prepared in presence of the Schiff base ligand was elucidated by TEM and HRTEM images as well as SAED pattern. Fig. 2a shows the TEM image of sample L1 which contains ellipsoidal nanoparticles with the average length of 35 and width of 20 nm. Fig. 2b shows representative HRTEM image of the specimen. The SAED pattern (inset in Fig. 2b) from a TiO2 particle oriented with its [1 0 0] axis parallel to the microscope electron beam indicates that the ellipsoids are monocrystalline with the rotation axis in accord with the c-axis of the tetragonal system [28].
M. Masjedi et al. / Superlattices and Microstructures 62 (2013) 30–38
Fig. 1. XRD patterns of the TiO2 samples: (a) L1T0, (b) L0 and (c) L1.
Fig. 2. (a) TEM image, (b) HRTEM image (the inset shows the corresponding SAED pattern) of L1 sample.
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3.1.3. FT-IR and EDX analysis The surface conditioning of as-prepared TiO2 product was recorded using FT-IR spectrum in order to detect the residual organic compounds. Fig. 3a shows FT-IR spectrum of L1 sample. The absorption from 3000 to 3600 cm1 can be assigned to the stretching vibration of the hydrogen-bonded OH groups of the adsorbed water. The absorption around 1628 cm1 is due to the bending vibration of water molecules. The broad band below 950 cm1 in the FT-IR spectrum belongs to the characteristic vibrations of the inorganic TiAOATi network [29]. The bands at around 2920 and 2850 cm1 are assigned to the antisymmetric and symmetric CAH stretching vibrations of hydrocarbon moiety [30]. From the FT-IR result, it can be concluded that probably a small amount of ligand is attached to the surface of nanoparticles which may have an effective role in preventing nanoparticles agglomeration. EDX further confirmed that the nanocrystals are composed of TiO2. The EDX of the L1 sample is shown in Fig. 3b. Peaks associated with Ti and O are clearly observed and provide strong evidence that the nanocrystals are composed of only TiO2. The peaks associated with sodium in the EDX spectrum come from impurities associated with the TEM specimen holder. The carbon content with atomic percentage of 4.53 ± 0.4% may be referred to the trace organic residue which is already evidenced by FT-IR spectrum. After evidencing the purity and crystallinity of the product, the effect of ligand concentration on the properties of TiO2 nanoparticles is of interest which will be discussed. 3.1.4. SEM image The effect of ligand concentration on surface morphologies of the obtained TiO2 products is shown in Fig. 4. Fig. 4a–e show TiO2 samples L1T0, L0, L0.5, L3, and L5 respectively. The average particle size of L1T0 sample is estimated to be from 10 to 18 nm (Fig. 4a) which is in agreement with XRD results. For other four samples, the average particle size is estimated to be 20–35, 30–50, 40–60, and 20–30 nm for L0, L0.5, L3, and L5, respectively. It is observed that with increasing ligand concentration
Fig. 3. (a) FT-IR spectrum and (b) EDX of L1 samples.
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Fig. 4. SEM images of the TiO2 samples: (a) L1T0, (b) L0, (c) L0.5, (d) L3 and (e) L5.
up to 3 times to metal (L3), an increase in particle size occurs. However, with addition of five times of ligand (L5) a considerable size reduction is distinguished. This trend is possibly due to the presence of TEOA. In absence of TEOA, acacen could readily adsorb on the crystal facets of TiO2 without any repulsion or attraction by its neighboring molecules. Having N and O groups, acacen could facially form hydrogen bonds to TEOA molecules having intact OH groups in pH = 9.6. Increasing the concentration of acacen ligand will enhance the percentage of free acacen ligands which could easily adsorb on the surface of TiO2 facets and control the particles growth. 3.1.5. Possible growth mechanism Generally, effective shape controllers have relatively high stability constants as strong adsorbents to inhibit the growth of specific facets. Since the rate-determining step for the growth of TiO2 particles is not the dissolution process of Ti(OH)4 gel but the deposition process of the solute onto the growing TiO2 particles [31], the adsorption of ligands on the embryos of TiO2 would decrease the growth rate. Adsorption of organic molecules on TiO2 single crystal surface has been widely studied in various
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experimental and theoretical works [26,32,33]. Among the various studied adsorption of organic ligands, dicarboxylic acids and amino acids have the most similar structure to acacen. Therefore, the possible obtained adsorption mechanism of mentioned ligand could be extended to this work. Two of the possible adsorption modes are depicted in Scheme 1. According to DFT calculations of glycine adsorption on the anatase (1 0 1) surface done by Szieberth et al. [34], the highest adsorption energy was obtained for a model where the carbonyl oxygen of glycine bonds to a Ti5c site, the hydroxyl group forms a hydrogen bond to a twofold coordinated oxygen ion and the amine group is bonded to Ti5c via the nitrogen lone pair. Considering the anionic nature of oxygen groups in acacen, OATi5c is the main anticipated interaction (Scheme 1a), however the nitrogen sites could possibly coordinate through their lone pairs to the Ti neighbors (Scheme 1b). In the case of glycine, the authors point out that the energy difference between the best models and other less important ones such as adsorption solely via the amine group, is very small thus it is possible all modes of adsorption may be present. In acacen case, other interactions such as adsorption of oxygen groups and one nitrogen group should be considered, as well. Here, it is proposed that strong adsorption of acacen on the embryos of TiO2 effectively decreases the growth rate and controls the particle size. 3.1.6. Optical studies The absorption spectra of TiO2 nanoparticles are shown in Fig. 5. It is shown that with increasing the ligand ratio from 0.5 to 1, the absorption edge was shifted to the higher-energy region. The fundamental absorption edge in most semiconductors follows the exponential law. Using the absorption data, the band gap was estimated by Tauc’s relationship:
a ¼ a0 ðhm Eg Þn =hm
ð1Þ
where a is absorption coefficient, hm is the photon energy, a0 and h are the constants, Eg is the optical band gap of the material, and n depends on the type of electronic transition and can be any value between ½ and 3 [35]. The energy gaps of the samples have been determined by extrapolating the linear portion of the plots of (aht)1/2 against ht to the energy axis (inset in Fig. 5) [36–38]. The Eg values are calculated 3.22, 3.26, and 3.32 eV for the L0.5, L3, and L1 samples, respectively. For the TiO2 anatase phase, values from 2.86 to 3.34 eV have been reported in the literature and the differences have been attributed to variations in the stoichiometric of the synthesis, the impurities content, the crystalline size and the type of electronic transition [39,40]. Fig. 6a shows the electron distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of acacen. It can be observed that the electron cloud all delocalized over the AC@CA and AC@NA bands. Due to the long band length of the bridging ACH2ACH2A, no electron transfer from HOMO to LUMO could occur in acacen. Therefore all the electron transfer would be either ‘‘from’’ or ‘‘to’’ only half of the ligand. By bonding acacen to TiO2 surface, narrow HOMO–LUMO energy gaps similar to that shown in Fig. 6b would be obtained. The narrow energy gap may absorb long wavelengths and slightly change the band gap of TiO2 nanoparticles. From
Scheme 1. Two possible interactions of acacen with (1 0 1) TiO2 facet.
M. Masjedi et al. / Superlattices and Microstructures 62 (2013) 30–38
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Fig. 5. UV–Vis diffuse absorption spectra of the TiO2 samples: (a) L0.5, (b) L1, (c) L3 (the inset shows corresponding linear portion of the plots of (aht)1/2 against ht).
Fig. 6. (a) Molecular orbital structure of HOMO and LUMO of acacen and (b) energy level of acacen frontier orbitals.
calculated band gaps of UV–Vis spectroscopy, it is assumed that addition of Schiff base ligand would slightly change the band gap value, so that with increasing the ligand concentration, a considerable increase in band gap value is observed.
4. Conclusions Shortly, the effect of a Schiff-base ligand on the growth of TiO2 nanoparticles via a two-step sol–gel method was investigated. It was shown that using acacen Schiff base ligand could prevent the particles from agglomeration. Moreover, optical studies indicated that increasing the amount of acacen
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ligand change the band gap of final TiO2 nanoparticles. This may open a new pathway in further band gap narrowing studies of different semiconductors. Acknowledgements Authors are grateful to the council of University of Kashan for supporting this work by Grant No (159271/79). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]
B. O’Regan, M. Gräatzel, J. Nat. 353 (1991) 737–740. J. Fu, J. Mater. Lett. 68 (2012) 419–422. L.L. Wang, X.M. Wu, S.C. Zhang, J. Appl. Mech. Mater. 130–134 (2011) 1281–1285. M.H. Seo, M. Yuasa, T. Kida, J.S. Huh, K. Shimanoe, N. Yamazoe, J. Sens. Actuator B – Chem. 137 (2009) 513–520. B.H. Kim, J.Y. Lee, Y.H. Choa, M. Higuchi, N. Mizutani, J. Mater. Sci. Eng. B 107 (2004) 289–294. P.-T. Hsiao, M.-D. Lu, Y.-L. Tung, H. Teng, J. Phys. Chem. C 114 (2010) 15625–15632. F. Peng, L. Cai, L. Huang, H. Yu, H. Wang, J. Phys. Chem. Solids. 69 (2008) 1657–1664. H.S. Jung, H. Shin, J.R. Kim, J.Y. Kim, K.S. Hong, J.K. Lee, Langmuir 20 (2004) 11732–11737. L.D. Marco, M. Manca, R. Giannuzzi, F. Malara, G. Melcarne, G. Ciccarella, I. Zama, R. Cingolani, G. Gigli, J. Phys. Chem. C 114 (2010) 4228–4236. T. Sugimoto, K. Okada, H. Itoh, J. Colloid Interface Sci. 193 (1997) 140–143. S. Lee, I.S. Cho, J.H. Lee, D.H. Kim, D.W. Kim, J.Y. Kim, H. Shin, J.K. Lee, H.S. Jung, N.G. Park, K. Kim, M.J. Ko, K.S. Hong, J. Chem. Mater. 22 (2010) 1958–1965. C.H. Liao, W.T. Shih, C.C. Chen, Y.L. Lee, P.L. Kuo, J. Thin Solid Films 519 (2011) 5552–5557. Z. Zhang, C.C. Wang, R. Zakaria, J.Y. Ying, J. Phys. Chem. B. 102 (1998) 10871–10878. S. Nakade, Y. Saito, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, J. Phys. Chem. B. 107 (2003) 8607–8611. O. Durupthy, J. Bill, F. Aldinger, J. Cryst. Growth Des. 7 (2007) 2696–2704. Y.W. Jun, M.F. Casula, J.H. Sim, S.Y. Kim, J. Cheon, A.P. Alivisatos, J. Am. Chem. Soc. 125 (2003) 15981–15985. W. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669–13679. A. Zaleska, Recent Patents Eng. 2 (2008) 157–164. W. Zhu, X. Qiu, V. Iancu, X.Q. Chen, H. Pan, W. Wang, N.M. Dimitrijevic, T. Rajh, H.M. Meyer, M.P. Paranthaman, G.M. Stocks, H.H. Weitering, B. Gu, G. Eres, Z. Zhang, J. Phys. Rev. Lett. 103 (2009) 226401–226404. Z. Hens, I. Moreels, B. Fritzinger, J.C. Martins, J. Comprehens. Nanosci. Technol. 5 (2011) 21–49. M. Dadkhah, M. Salavati-Niasaria, F. Davar, J. Adv. Powder Technol. 24 (2013) 14–20. M. Salavati-Niasaria, D. Ghanbari, J. Particuol. 10 (2012) 628–633. N. Mir, M. Salavati-Niasaria, J. Mater. Res. Bull. 48 (2013) 1660–1668. S.M. Hosseinpour-Mashkani, F. Mohandes, M. Salavati-Niasari, K. Venkateswara-Rao, J. Mater. Res. Bull. 47 (2012) 3148– 3159. N. Mir, M. Salavati-Niasari, Sol. Energy 86 (2012) 3397–3404. A.G. Thomas, K.L. Syres, Chem. Soc. Rev. 41 (2012) 4207–4217. H.P. Klug, L.F. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, second ed., John Wiley and Sons Press, 1954. K. Kanie, T. Sugimoto, Chem. Commun. (2004) 1584–1585. P.D. Cozzoli, A. Kornowski, H. Weller, J. Am. Chem. Soc. 125 (2003) 14539–14548. J. Liu, Y. Wang, D. Sun, J. Renew. Energy 38 (2012) 214–218. T. Sugimoto, X. Zhou, A. Muramatzu, J. Colloid Interface Sci. 259 (2003) 43–52. U. Diebold, Surf. Sci. Rep. 48 (2003) 53–229. L. Pang, R. Lindsay, G. Thornton, Chem. Soc. Rev. 37 (2008) 2328–2353. D. Szieberth, A. Maria Ferrari, X. Dong, Phys. Chem. Chem. Phys. 12 (2010) 11033–11040. J. Tauc, R. Grigorovici, A. Vancu, J. Phys. Status Solidi B. 15 (1966) 627–637. M. Abaker, A. Umar, S. Baskoutas, G.N. Dar, S.A. Zaidi, S.A. Al-Sayari, A. Al-Hajry, S.H. Kim, S.W. Hwang, J. Phys. D: Appl. Phys. 44 (2011) 425401–425407. M. Abaker, A. Umar, S. Baskoutas, S.H. Kim, S.W. Hwang, J. Phys. D: Appl. Phys. 44 (2011) 155405–155411. A. Umar, M. Abaker, M. Faisal, S.W. Hwang, S. Baskoutas, S.A. Al-Sayari, J. Nanosci. Nanotechnol. 11 (2011) 3474–3480. M. Hidalgo, M. Aguilar, M. Maicu, J. Navio, G. Colon, J. Catal. Today 129 (2007) 50–58. F. Hossain, L. Sheppard, J. Nowotny, G. Murch, J. Phys. Chem. Solids 69 (2008) 1820–1828.
Contents lists available at SciVerse ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Effect of Schiff base ligand on the size and the optical properties of TiO2 nanoparticles Maryam Masjedi b, Noshin Mir c, Elham Noori b, Tahereh Gholami b, Masoud Salavati-Niasari a,b,⇑ a Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran b Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran c Department of Chemistry, University of Zabol, Zabol, Islamic Republic of Iran
a r t i c l e
i n f o
Article history: Received 30 March 2013 Received in revised form 11 June 2013 Accepted 13 July 2013 Available online 22 July 2013 Keywords: Agglomeration Semiconductors Band gap Sol-Gel
a b s t r a c t The effect of a Schiff-base ligand (N,N0 -ethylenebis(acetylacetone iminato)dianion = acacen) on size and optical properties of TiO2 nanoparticles in a two-step sol–gel method was investigated. Different amounts of Schiff-base ligand were applied and the as-prepared products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectrum, Electron Dispersive X-ray spectroscopy (EDX) and ultraviolet– visible (UV–Vis) spectroscopy. Molecular orbital structure of acacen was calculated by density functional theory (DFT) in order to determine the exact orbital energies and electron transfer pathways. The results demonstrated that applying the appropriate amount of Schiff-base ligand could be effective in particle size control. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Titanium dioxide (TiO2) is a semiconductor material with versatile application due to its optical and electronic properties. Nanostructured TiO2 is widely used in applications such as dye-sensitized solar cells, photocatalysis, lithium ion batteries and gas sensing systems [1–4]. ⇑ Corresponding author at: Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran. Tel.: +98 361 5555 333; fax: +98 361 555 29 30. E-mail address: [email protected] (M. Salavati-Niasari). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.07.003
M. Masjedi et al. / Superlattices and Microstructures 62 (2013) 30–38
31
A variation of possible routes including chemical vapor deposition [5], hydrothermal [6], ultrasonic irradiation [7], sol–gel [8], and solvothermal [9] have been applied to obtain different types of TiO2. Recently, a two-step gel–sol process has been developed by Sugimoto [10] to obtain pure anatase TiO2. The main advantage of this method is production of monodispersed particles with controlled mean size in order to improve different properties of TNPs [11,12]. To develop TiO2 for alternative energy applications, many efforts have been devoted in various fields of science. Previous reports reveal that the most preponderant strategies to achieve desired properties of TiO2 is including particle size control [13,14], shape and morphology control [15,16], and band gap changing by introducing foreign species as dopants [17–19]. In such efforts, to optimize the TiO2 properties, many different materials and reaction conditions have been examined. One of the significant aspects of preparing nanoparticle with specific properties is choosing appropriate ligand shell. The ligand shell is, most simply, a molecular monolayer encapsulating the core. In the case of organic species, ligands that fulfill the task of adsorbent and stabilizer have a functional head group and one or more hydrocarbon tails, and both elements play a role in the control of nucleation and growth [20]. Beside the well-known surfactants, recently our team has focused on the effect of ligands containing hard atoms (N, O) as well as bulky aromatic groups on the properties of obtained products [21–25]. It has been demonstrated that these kind of organic agents have an effective role on controlling size, shape, and optical properties of final products. In this paper, the effect of a Schiff base ligand as a complexing agent on morphology, size, and optical properties of TiO2 nanoparticles was investigated. Acacen, a tetradentate Schiff base ligand containing four hard binding sites (N and O), is capable of stabilizing transition metal centers very well. Acacen could be quite interesting considering the fact that bonding via two oxo groups to the TiO2 surface may lead to more efficient charge transfer between an adsorbed molecule and the oxide surface similar to dicarboxylic acids. These kinds of molecules are of interest in preventing crystal growth of particular surface planes, or limiting the size of crystals in the preparation of nanoparticles [26].
2. Materials and methods 2.1. Materials and characterization Titanium (IV) ethoxide, N,N0 -ethylenebis(acetylacetone iminato)dianion (acacen) and triethanolamine (TEOA) used in our experiments were purchased from Merck. For characterization of the products X-ray diffraction (XRD) patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu Ka radiation. Scanning electron microscopy (SEM) images were obtained on Philips XL-30ESEM. Transmission electron microscopy (TEM) image was obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets. Optical analyses were performed using a V-670 UV–Vis–NIR Spectrophotometer (Jasco).
2.2. Synthetic method The standard procedure for the preparation of TiO2 nanoparticles was conducted according to the literature [10]. Stock solution of Ti4+ (0.5 M) which are stable against hydrolysis at room temperature were prepared by mixing titanium(IV) ethoxide (TEO) and TEOA with a molar ratio of TEO:TEOA = 1:2, followed by the addition of distilled water. The final pH of solutions was 9.6 and was fixed at this pH by mixing with HClO4 or NaOH solution after addition of complexing agents. Acacen ligand with different ratios to the Ti4+ precursor (Table 1) was further added as complexing agents to each solution. The solution was placed in a Teflon-lined autoclave and aged at 100 °C for 24 h for gelation; then, the temperature was increased to 140 °C for 72 h to nucleate and grow TiO2 nanocrystals. The resulting sol was washed with and centrifuged from NaOH (six times), HNO3 (two times), and distilled water (four times) to remove residual organic compounds from the surface of the nanoparticles.
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M. Masjedi et al. / Superlattices and Microstructures 62 (2013) 30–38 Table 1 Different samples prepared in this work. Sample abbreviation
Ti4+:L
Ti4+:TEOA
L0 L0.5 L1 L3 L5 L1T0
1:0 1:0.5 1:1 1:3 1:5 1:1
1:2
1:0
2.3. Calculations The molecular structure of Acacen in the ground state were optimized on the basis of density function theory (DFT) at the Becke3–Lee–Yang–Parr (B3LYP) (with 6-31G basis) and by means of visual inspection using the GAUSSVIEW program (Version 5.0). All the calculations were performed using the GAUSSIAN 09 software package. 3. Results and discussion 3.1. Characterization of two-step sol–gel process-based TiO2 nanoparticles
3.1.1. X-ray diffraction patterns The X-ray diffraction patterns of as-prepared product obtained from the two-step reaction are shown in Fig. 1. Fig. 1a–c show the XRD patterns of samples L1, L0 and L1T0 which are prepared in presence of both ligand and TEOA, in absence of ligand, and in absence of TEOA, respectively. All diffraction peaks can be well indexed to pure anatase structural titanium dioxide. All the peaks are labeled and can be indexed to JCPDS Card No. 71-1167, 04-0477, and 03-0380 for sample L1, L0 and L1T0, respectively. The XRD patterns of samples show different line broadening. Generally, the crystalline domain size decreases with increasing line broadening. The line broadening of the peak of the (1 0 1) index is related to the size of the crystalline phase. The data for the full width at half maximum (FWHM) of sample L1, L0 and L1T0 at 2h = 25.47, 25.38, and 25.50 was estimated to be 0.3542, 0.4133, and 1.0627, respectively. The crystallite size diameter (D) of the TiO2 products has been calculated by Debye– Scherrer equation, D = 0.9k/b cos h [27]. Calculated crystalline domain sizes have been found to be 22.7, 19.3, and 7.6 nm for sample L1, L0 and L1T0, respectively. It can be concluded that the employed Schiff base ligand has a significant influence on crystalline size of the product so that in the presence of only the ligand (sample L1T0) the calculated crystalline domain size decreased down to lower that 10 nm. It seems that applying both the ligand and TEOA (L1) has a different effect so that the particles size of L0 sample is smaller than L1. Preparation of TiO2 nanoparticles with tuneable size is of great interest for different applications. Here, it is shown that different conditions lead to different crystalline size from ca. 7.5–22.5. 3.1.2. TEM images The shapes and structure of TiO2 nanoparticle prepared in presence of the Schiff base ligand was elucidated by TEM and HRTEM images as well as SAED pattern. Fig. 2a shows the TEM image of sample L1 which contains ellipsoidal nanoparticles with the average length of 35 and width of 20 nm. Fig. 2b shows representative HRTEM image of the specimen. The SAED pattern (inset in Fig. 2b) from a TiO2 particle oriented with its [1 0 0] axis parallel to the microscope electron beam indicates that the ellipsoids are monocrystalline with the rotation axis in accord with the c-axis of the tetragonal system [28].
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Fig. 1. XRD patterns of the TiO2 samples: (a) L1T0, (b) L0 and (c) L1.
Fig. 2. (a) TEM image, (b) HRTEM image (the inset shows the corresponding SAED pattern) of L1 sample.
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3.1.3. FT-IR and EDX analysis The surface conditioning of as-prepared TiO2 product was recorded using FT-IR spectrum in order to detect the residual organic compounds. Fig. 3a shows FT-IR spectrum of L1 sample. The absorption from 3000 to 3600 cm1 can be assigned to the stretching vibration of the hydrogen-bonded OH groups of the adsorbed water. The absorption around 1628 cm1 is due to the bending vibration of water molecules. The broad band below 950 cm1 in the FT-IR spectrum belongs to the characteristic vibrations of the inorganic TiAOATi network [29]. The bands at around 2920 and 2850 cm1 are assigned to the antisymmetric and symmetric CAH stretching vibrations of hydrocarbon moiety [30]. From the FT-IR result, it can be concluded that probably a small amount of ligand is attached to the surface of nanoparticles which may have an effective role in preventing nanoparticles agglomeration. EDX further confirmed that the nanocrystals are composed of TiO2. The EDX of the L1 sample is shown in Fig. 3b. Peaks associated with Ti and O are clearly observed and provide strong evidence that the nanocrystals are composed of only TiO2. The peaks associated with sodium in the EDX spectrum come from impurities associated with the TEM specimen holder. The carbon content with atomic percentage of 4.53 ± 0.4% may be referred to the trace organic residue which is already evidenced by FT-IR spectrum. After evidencing the purity and crystallinity of the product, the effect of ligand concentration on the properties of TiO2 nanoparticles is of interest which will be discussed. 3.1.4. SEM image The effect of ligand concentration on surface morphologies of the obtained TiO2 products is shown in Fig. 4. Fig. 4a–e show TiO2 samples L1T0, L0, L0.5, L3, and L5 respectively. The average particle size of L1T0 sample is estimated to be from 10 to 18 nm (Fig. 4a) which is in agreement with XRD results. For other four samples, the average particle size is estimated to be 20–35, 30–50, 40–60, and 20–30 nm for L0, L0.5, L3, and L5, respectively. It is observed that with increasing ligand concentration
Fig. 3. (a) FT-IR spectrum and (b) EDX of L1 samples.
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Fig. 4. SEM images of the TiO2 samples: (a) L1T0, (b) L0, (c) L0.5, (d) L3 and (e) L5.
up to 3 times to metal (L3), an increase in particle size occurs. However, with addition of five times of ligand (L5) a considerable size reduction is distinguished. This trend is possibly due to the presence of TEOA. In absence of TEOA, acacen could readily adsorb on the crystal facets of TiO2 without any repulsion or attraction by its neighboring molecules. Having N and O groups, acacen could facially form hydrogen bonds to TEOA molecules having intact OH groups in pH = 9.6. Increasing the concentration of acacen ligand will enhance the percentage of free acacen ligands which could easily adsorb on the surface of TiO2 facets and control the particles growth. 3.1.5. Possible growth mechanism Generally, effective shape controllers have relatively high stability constants as strong adsorbents to inhibit the growth of specific facets. Since the rate-determining step for the growth of TiO2 particles is not the dissolution process of Ti(OH)4 gel but the deposition process of the solute onto the growing TiO2 particles [31], the adsorption of ligands on the embryos of TiO2 would decrease the growth rate. Adsorption of organic molecules on TiO2 single crystal surface has been widely studied in various
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experimental and theoretical works [26,32,33]. Among the various studied adsorption of organic ligands, dicarboxylic acids and amino acids have the most similar structure to acacen. Therefore, the possible obtained adsorption mechanism of mentioned ligand could be extended to this work. Two of the possible adsorption modes are depicted in Scheme 1. According to DFT calculations of glycine adsorption on the anatase (1 0 1) surface done by Szieberth et al. [34], the highest adsorption energy was obtained for a model where the carbonyl oxygen of glycine bonds to a Ti5c site, the hydroxyl group forms a hydrogen bond to a twofold coordinated oxygen ion and the amine group is bonded to Ti5c via the nitrogen lone pair. Considering the anionic nature of oxygen groups in acacen, OATi5c is the main anticipated interaction (Scheme 1a), however the nitrogen sites could possibly coordinate through their lone pairs to the Ti neighbors (Scheme 1b). In the case of glycine, the authors point out that the energy difference between the best models and other less important ones such as adsorption solely via the amine group, is very small thus it is possible all modes of adsorption may be present. In acacen case, other interactions such as adsorption of oxygen groups and one nitrogen group should be considered, as well. Here, it is proposed that strong adsorption of acacen on the embryos of TiO2 effectively decreases the growth rate and controls the particle size. 3.1.6. Optical studies The absorption spectra of TiO2 nanoparticles are shown in Fig. 5. It is shown that with increasing the ligand ratio from 0.5 to 1, the absorption edge was shifted to the higher-energy region. The fundamental absorption edge in most semiconductors follows the exponential law. Using the absorption data, the band gap was estimated by Tauc’s relationship:
a ¼ a0 ðhm Eg Þn =hm
ð1Þ
where a is absorption coefficient, hm is the photon energy, a0 and h are the constants, Eg is the optical band gap of the material, and n depends on the type of electronic transition and can be any value between ½ and 3 [35]. The energy gaps of the samples have been determined by extrapolating the linear portion of the plots of (aht)1/2 against ht to the energy axis (inset in Fig. 5) [36–38]. The Eg values are calculated 3.22, 3.26, and 3.32 eV for the L0.5, L3, and L1 samples, respectively. For the TiO2 anatase phase, values from 2.86 to 3.34 eV have been reported in the literature and the differences have been attributed to variations in the stoichiometric of the synthesis, the impurities content, the crystalline size and the type of electronic transition [39,40]. Fig. 6a shows the electron distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of acacen. It can be observed that the electron cloud all delocalized over the AC@CA and AC@NA bands. Due to the long band length of the bridging ACH2ACH2A, no electron transfer from HOMO to LUMO could occur in acacen. Therefore all the electron transfer would be either ‘‘from’’ or ‘‘to’’ only half of the ligand. By bonding acacen to TiO2 surface, narrow HOMO–LUMO energy gaps similar to that shown in Fig. 6b would be obtained. The narrow energy gap may absorb long wavelengths and slightly change the band gap of TiO2 nanoparticles. From
Scheme 1. Two possible interactions of acacen with (1 0 1) TiO2 facet.
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Fig. 5. UV–Vis diffuse absorption spectra of the TiO2 samples: (a) L0.5, (b) L1, (c) L3 (the inset shows corresponding linear portion of the plots of (aht)1/2 against ht).
Fig. 6. (a) Molecular orbital structure of HOMO and LUMO of acacen and (b) energy level of acacen frontier orbitals.
calculated band gaps of UV–Vis spectroscopy, it is assumed that addition of Schiff base ligand would slightly change the band gap value, so that with increasing the ligand concentration, a considerable increase in band gap value is observed.
4. Conclusions Shortly, the effect of a Schiff-base ligand on the growth of TiO2 nanoparticles via a two-step sol–gel method was investigated. It was shown that using acacen Schiff base ligand could prevent the particles from agglomeration. Moreover, optical studies indicated that increasing the amount of acacen
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ligand change the band gap of final TiO2 nanoparticles. This may open a new pathway in further band gap narrowing studies of different semiconductors. Acknowledgements Authors are grateful to the council of University of Kashan for supporting this work by Grant No (159271/79). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]
B. O’Regan, M. Gräatzel, J. Nat. 353 (1991) 737–740. J. Fu, J. Mater. Lett. 68 (2012) 419–422. L.L. Wang, X.M. Wu, S.C. Zhang, J. Appl. Mech. Mater. 130–134 (2011) 1281–1285. M.H. Seo, M. Yuasa, T. Kida, J.S. Huh, K. Shimanoe, N. Yamazoe, J. Sens. Actuator B – Chem. 137 (2009) 513–520. B.H. Kim, J.Y. Lee, Y.H. Choa, M. Higuchi, N. Mizutani, J. Mater. Sci. Eng. B 107 (2004) 289–294. P.-T. Hsiao, M.-D. Lu, Y.-L. Tung, H. Teng, J. Phys. Chem. C 114 (2010) 15625–15632. F. Peng, L. Cai, L. Huang, H. Yu, H. Wang, J. Phys. Chem. Solids. 69 (2008) 1657–1664. H.S. Jung, H. Shin, J.R. Kim, J.Y. Kim, K.S. Hong, J.K. Lee, Langmuir 20 (2004) 11732–11737. L.D. Marco, M. Manca, R. Giannuzzi, F. Malara, G. Melcarne, G. Ciccarella, I. Zama, R. Cingolani, G. Gigli, J. Phys. Chem. C 114 (2010) 4228–4236. T. Sugimoto, K. Okada, H. Itoh, J. Colloid Interface Sci. 193 (1997) 140–143. S. Lee, I.S. Cho, J.H. Lee, D.H. Kim, D.W. Kim, J.Y. Kim, H. Shin, J.K. Lee, H.S. Jung, N.G. Park, K. Kim, M.J. Ko, K.S. Hong, J. Chem. Mater. 22 (2010) 1958–1965. C.H. Liao, W.T. Shih, C.C. Chen, Y.L. Lee, P.L. Kuo, J. Thin Solid Films 519 (2011) 5552–5557. Z. Zhang, C.C. Wang, R. Zakaria, J.Y. Ying, J. Phys. Chem. B. 102 (1998) 10871–10878. S. Nakade, Y. Saito, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, J. Phys. Chem. B. 107 (2003) 8607–8611. O. Durupthy, J. Bill, F. Aldinger, J. Cryst. Growth Des. 7 (2007) 2696–2704. Y.W. Jun, M.F. Casula, J.H. Sim, S.Y. Kim, J. Cheon, A.P. Alivisatos, J. Am. Chem. Soc. 125 (2003) 15981–15985. W. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669–13679. A. Zaleska, Recent Patents Eng. 2 (2008) 157–164. W. Zhu, X. Qiu, V. Iancu, X.Q. Chen, H. Pan, W. Wang, N.M. Dimitrijevic, T. Rajh, H.M. Meyer, M.P. Paranthaman, G.M. Stocks, H.H. Weitering, B. Gu, G. Eres, Z. Zhang, J. Phys. Rev. Lett. 103 (2009) 226401–226404. Z. Hens, I. Moreels, B. Fritzinger, J.C. Martins, J. Comprehens. Nanosci. Technol. 5 (2011) 21–49. M. Dadkhah, M. Salavati-Niasaria, F. Davar, J. Adv. Powder Technol. 24 (2013) 14–20. M. Salavati-Niasaria, D. Ghanbari, J. Particuol. 10 (2012) 628–633. N. Mir, M. Salavati-Niasaria, J. Mater. Res. Bull. 48 (2013) 1660–1668. S.M. Hosseinpour-Mashkani, F. Mohandes, M. Salavati-Niasari, K. Venkateswara-Rao, J. Mater. Res. Bull. 47 (2012) 3148– 3159. N. Mir, M. Salavati-Niasari, Sol. Energy 86 (2012) 3397–3404. A.G. Thomas, K.L. Syres, Chem. Soc. Rev. 41 (2012) 4207–4217. H.P. Klug, L.F. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, second ed., John Wiley and Sons Press, 1954. K. Kanie, T. Sugimoto, Chem. Commun. (2004) 1584–1585. P.D. Cozzoli, A. Kornowski, H. Weller, J. Am. Chem. Soc. 125 (2003) 14539–14548. J. Liu, Y. Wang, D. Sun, J. Renew. Energy 38 (2012) 214–218. T. Sugimoto, X. Zhou, A. Muramatzu, J. Colloid Interface Sci. 259 (2003) 43–52. U. Diebold, Surf. Sci. Rep. 48 (2003) 53–229. L. Pang, R. Lindsay, G. Thornton, Chem. Soc. Rev. 37 (2008) 2328–2353. D. Szieberth, A. Maria Ferrari, X. Dong, Phys. Chem. Chem. Phys. 12 (2010) 11033–11040. J. Tauc, R. Grigorovici, A. Vancu, J. Phys. Status Solidi B. 15 (1966) 627–637. M. Abaker, A. Umar, S. Baskoutas, G.N. Dar, S.A. Zaidi, S.A. Al-Sayari, A. Al-Hajry, S.H. Kim, S.W. Hwang, J. Phys. D: Appl. Phys. 44 (2011) 425401–425407. M. Abaker, A. Umar, S. Baskoutas, S.H. Kim, S.W. Hwang, J. Phys. D: Appl. Phys. 44 (2011) 155405–155411. A. Umar, M. Abaker, M. Faisal, S.W. Hwang, S. Baskoutas, S.A. Al-Sayari, J. Nanosci. Nanotechnol. 11 (2011) 3474–3480. M. Hidalgo, M. Aguilar, M. Maicu, J. Navio, G. Colon, J. Catal. Today 129 (2007) 50–58. F. Hossain, L. Sheppard, J. Nowotny, G. Murch, J. Phys. Chem. Solids 69 (2008) 1820–1828.