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Synthesis and characterization of a new organic–inorganic hybrid NiO–chlorophyll-a as optical material
Optical Materials 29 (2007) 927–931 www.elsevier.com/locate/optmat
Synthesis and characterization of a new organic–inorganic hybrid NiO–chlorophyll-a as optical material Z. Mehraban a, F. Farzaneh a
a,*
, A. Shafiekhani
b
Department of Chemistry, University of Alzahra, Vanak, Tehran, 1993891167, Iran Department of Physics, University of Alzahra, Vanak, Tehran, 1993891167, Iran
b
Received 14 August 2005; accepted 10 February 2006 Available online 3 April 2006
Abstract The organic–inorganic hybrid NiO–chlorophyll-a (NiO–Chl-a) as an optical material was synthesized with nickel acetate and saponificated chlorophyll-a (Chl-a) by sol–gel method. It was characterized by FTIR, XRD, TGA and DTA. Its optical properties were examined with UV–Vis reflectance and illumination with a visible light. It was found that NiO–Chl-a emitted a monochromatic fluorescence at 658 nm wavelength with 10 nm width. It shows a high stability against visible light as well. Ó 2006 Elsevier B.V. All rights reserved. Keywords: NiO–chlorophyll-a; Organic–inorganic hybrid; Optical material; Photostability; UV–Vis reflectance
1. Introduction An increasing number of studies on the optical properties of dye-doped porous silica were published in the past decades. One of the advantages of an inorganic matrix for embedding of functional chromophores is attributed to the more rigid stability and strongly increased photostability compared to organic polymer matrix. The development of the room-temperature sol–gel process for the preparation of porous materials opened the possibility of adding organic chromophores to exhibit photophysical and photochemical functionality to the synthetic mixtures. This procedure results in a uniform dispersion of dye in the porous host. In fact, chromophores in inorganic–organic polymers have proved to exhibit various optical properties for potential applications such as sensors, lasers, amplifiers or storage devices as well as solar concentrators [1]. Several methods have been reported for preparations of dye-doped sol–gel silica [2–4]. In general, the dissolved dye is added to the reaction mixture to yield entrapped chro*
Corresponding author. Tel.: +98 21 88258977; fax: +98 21 66404848. E-mail address: [email protected] (F. Farzaneh).
0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.02.007
mophores in the final solid composites. The optical biosensors by using sol–gel techniques are prepared, since enzymes systems in porous glass monoliths like pyridine nucleotide, co-enzymes, sorbitol dehydrogenase/NADH, or lactate dehydrogenase/NADH exhibit strong absorption and fluorescence. It was found that organic fluorescent dyes have a high fluorescence quantum yield (95%) and low excited-state absorption. Therefore, they have strong potential for applications, such as optical materials. Some disadvantages of these organic dyes are low photo and thermal stability. sol–gel derived organic–inorganic hybrid optical materials are considered to be good candidates to assemble a combination of properties required for the development of new optical materials such as good mechanical, thermal and chemical stability with high room temperature emission quantum yields [5,6]. Organic photocatalysts of chlorophyll and porphyrin derivatives act under visible light illumination. They are well known for their biological conduction and photoactive properties and are very useful as catalysts [7–15]. Chl-a absorbs light due to chelation of conjugated double bonds of chlorin ring with Mg (II) [16].
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Natural chlorophylls extracted from living leaves have rarely been used as catalysts because of their instability against light and heat. In the layered membrane of chloroplasts in intact leaves, chlorophyll molecules bind to proteins to form chlorophyll–protein conjugates in which the chlorophyll–protein interactions play an important role in stabilization and physiological functions of living plant leaves. The adsorbed Chl-a within silicate layers of smectite and modified FSM shows photostability and photocatalytic properties [17–20], which demonstrates the importance of chlorophyll–support interactions. Synthesis of organic–inorganic hybrid optical materials can be carried out by different methods such as incorporation of organic molecules dyes in inorganic networks [21] or sol–gel derived organic–inorganic hybrid materials. In this work, we report the synthesis of a new organic– inorganic hybrid optical material by sol–gel method with nickel acetate and saponificated Chl-a as inorganic and organic component, respectively. They form NiO–Chl-a with a high photo and thermal stability. 2. Experimental 2.1. Material Nickel acetate, potassium hydroxide, n-hexane, ethanol 98% and chlorophyll-a were obtained from Merck Chemical Company and were used without further purification. 2.2. Synthesis NiO–Chl-a optical material The organic–inorganic hybrid NiO–Chl-a material (NiO–Chl-a) was prepared by dropwise addition of 100 ml nickel acetate solution (0.2 molar) to the saturated saponificated solution of Chl-a in ethanol [22]. The resultant dark green gel was aged for 24 h. The gel mixture was filtered, soxhelt extracted with ethanol to remove the excess saponificated Chl-a and then dried at 100 °C. 2.3. Synthesis of NiO blank
from Micromeritics Corporation. Mid-IR spectra of NiO–Chl-a were collected on Bruker instrument using KBr pellet technique in the range of 4000–400 cm 1 (0.005 g sample with 0.1 g KBr). The UV–Vis reflectance experiment was performed with a Stellar–Net detector with a deuterium lamp. The optical fibers are used as carrier light. The reflectance of NiO–Chl-a was compared with NiO as reference. The photostability of NiO–Chl-a was examined by illuminating the Ni–Chl-a in water, as well as free chlorophyll-a in n-hexane, with a 100 W incandescent lamp at a distance of 10 cm. The light intensity on a sample was approximately 460 J m 2 S 1. During the illumination, the sample was shielded from heat by the circulation of cold water. The absorption of samples with the step size of 30 min are detected by Unicam 8700 series UV–Vis spectrometer. 3. Results and discussion The thermal property of the NiO–Chl-a was investigated by TGA and DTA (Fig. 1). In TGA plot, the sample shows two weight losses steps, 8.36% up to 100 °C and 23.31% at 270 °C. DTA shows a mild endothermic and sharp exothermic behavior at 100 °C and 270 °C, respectively, that consistent with TGA data, and shows physically desorption of adsorbed water of the sample at 100 °C and the oxidative desorption of the organic template (chlorine ring of Chl-a and ethanol) at 270 °C. Therefore, the sample has a high thermal stability up to 270 °C. DTA data also confirms the formation of one phase because NiO–Chl-a shows only one exothermic step. Figs. 2 and 3 show the XRD patterns of the both dried and calcined NiO–Chl-a at room temperature and at 270 °C in the region of 2h < 10° and 10–90°, respectively. In fact, the observation of an intense peak at 2h 1° (Fig. 2) might be attributed to the mesoporous structure of this compound [23]. It was also observed that the intensity of peak at 2h 1° has been increased and shifted to the lower angel after calcination (Fig. 2b). This can be the result of pores expansion due to removal of template.
NiO was prepared under the similar conditions of NiO– Chl-a without addition of Chl-a.
20
40 DTA
TGA
0
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Weight loss (%)
Thermogravimetric-differential thermal analysis was carried out in a Rheometric Scientific (STAH) thermo balance. The NiO–Chl-a was loaded and the O2 and Ar flow rate was 50 ml/min. The heating rate was 20 K/min and the final temperatures were 800 °C and 1000 °C in O2 and Ar atmosphere, respectively. Powder XRD patterns of NiO–Chl-a was recorded on a diffractometer type, SEIFERT XRD 3003 PTS, Cu Ka1 radiation (k = 0.1540 nm) in the 2h range of 0.5–10° and 10–90° Surface area, pore volume and pore size distribution were measured by nitrogen adsorption at 76 K with an ASAP-2010 porosimeter
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Heat Flow (mW)
2.4. Characterization
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Temperature (oC) Fig. 1. TGA–DTA diagram of dried NiO–Chl-a at room temperature.
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Fig. 2. X-ray powder diffraction patterns of NiO–Chl-a in the range of 2h = 0.5–3.5° (a) dried at room temperature, (b) calcined at 270 °C.
The similarity of X-ray powder diffraction patterns between dried NiO–Chl-a at room temperature (Fig. 2a) and calcined sample at 270 °C (Fig. 2b) in the range of 2h = 10–90°, confirms the existence of nickel oxide hydroxide phase [24]. The surface area and BJH pore volume of calcined NiO– Chl-a were 250 m2/g and 0.32 cm3/g, respectively. The pore size distributions calculated from desorption branch using the BJH model is quite narrow with a maximum peak at 3.55 nm. These results indicate that NiO–Chl-a is a mesopore compound after calcination. The molar ratios of the calcinated NiO–Chl-a and the number of atoms ratio at 270 °C were determined for Ni:K:Mg as 0.878:0.024:0.0016 and Ni:K:Mg = 563:15:1, respectively. The FTIR spectra of NiO (blank) and dried NiO–Chl-a either dried at room temperature or calcined at 270 °C and 800 °C are shown in Fig. 4a–d, respectively. For NiO (blank) and NiO–Chl-a (Fig. 4a and b) the intense peak at 3644 cm 1 is attributed to free hydroxyl groups. A broad peak at 3400 cm 1 belongs to OH stretching of water. The peaks at 2916 and 2848 cm 1 are assigned to the C–H
Fig. 4. FTIR spectra of (a) NiO (blank), (b) NiO–Chl-a dried at room temperature, (c) calcined at 270 °C, and (d) calcined at 800 °C.
symmetric and asymmetric vibrations of –CH2 of remained acetate groups. The weak and broad bands at 1632 and 1475 cm 1 are also assigned to bending of adsorbed species such as water. The intensity of these peaks were reduced after calcination [25–27] of the sample at 270 °C and approximately disappeared at 800 °C. As seen in the FTIR spectrum of pure Chl-a (Fig. 5) the C–H, C@C, C@O and C–N stretching bands of chlorine ring have been overlapped with acetate groups vibrations bands present at terminals groups of NiO network. In all spectra, the intense peak at 410 cm 1 is due to the stretching mode of NiO [28]. The UV–Vis reflectances of NiO blank, NiO–Chl-a and Chl-a are shown in Fig. 6. Compared to Chl-a in methanol which shows reflectance at 680 nm (Fig. 6c) [29], NiO–Chla exhibits a sharp and narrow reflection at 658.5 nm with 10 nm width attributed to the red monochromatic reflectance of Chl-a group (Fig. 6b). The observation of a blue shift in NiO–Chl-a spectrum might be due to the existence
Fig. 3. X-ray powder diffraction patterns of NiO–Chl-a in the range of 2h = 10–90° (a) dried at room temperature, (b) calcined at 270 °C.
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NiO-Chl-a
Chl-a
100
Absorption %
80
Fig. 5. FTIR spectrum of pure Chl-a.
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Time (min) Fig. 7. Photostability of NiO–Chl-a and pure Chl-a in n-hexane against illumination with visible light.
Fig. 6. UV–Vis reflectance spectra of (a) NiO (blank), (b) NiO–Chl-a, and (c) Chl-a.
of the material distributed molecular rather than being aggregated within the NiO support. This property makes NiO–Chl-a unique in optical applications especially for dye lasers. Photostability was examined by illumination with visible light (Fig. 7). The relative absorbance value decreased to 30% since free chlorophyll a in n-hexane, with an absorption maximum at 665 nm (curve a) was obviously discolored by the illumination after 360 min. On the other hand, NiO–Chl-a, with an absorption maximum at 660 nm, was much more photostable in water in comparison with free chlorophyll. A relative absorbance value of 97% is retained at the absorption maximum in the red region after 360 min of illuminating (curve b). This stability could be arising from incorporation of Chl-a as organic dye in inorganic of nickel oxide hydroxide network.
In some photoinduced hydrogen production systems with visible light colloidal platinum acting as hydrogenproducing catalysts, water-soluble zinc porphyrins have been widely used as effective photosensitizers [29–31]. Since they have an absorption band in the visible light region (380–600 nm). However, the molar absorption coefficient of zinc porphyrins in the visible light region (500– 600 nm) was lower than that in the near ultravisible light region (380–400 nm). Alternatively, Mg Chlorophyll-a, which acts as an effective photosensitizer in photosynthesis, has an absorption maximum at 670 nm. Thus, Mg Chl-a is attractive as a visible region photosensitizer. Photoinduced hydrogen production systems with chemically modified chlorophyll and hydrogenase [19,32], and chlorophyll derivatives [33–35], were reported previously. Photoinduced hydrogen production systems must consist of an electron donor, photosensitizer, electron relay, and catalyst, the photoexcited photosensitizer reacts with an electron relay to form a reduced electron relay, hydrogen evolves by proton reduction with the catalyst, and then the oxidized photosensitizer is then reduced by an electron-donating reagent [36,37]. Therefore, compared to free chlorophyll-a and NiO–chlorophyll-a organic–inorganic hybrid with high photostability against visible light and an absorption maximum at 660 nm can act as a very effective photosensitizer in photoinduced hydrogen production systems. 4. Conclusion Synthesis of NiO–Chl-a was carried out by sol–gel method in ambient conditions. Based on DTA–TGA results, photostability and UV–Vis reflectance experiments, this organic–inorganic hybrid optical material has a high photo and thermal stability with a red monochromatic
Z. Mehraban et al. / Optical Materials 29 (2007) 927–931
reflectance. Therefore, it can be used as a good photosensitizer, photosensor, dye lasers and photocatalyst. Acknowledgement The authors gratefully acknowledge the University of Alzahra for financial support. References [1] G. Schulz-Ekloffa, D. Wo¨hrle, B. van Duffel, R.A. Schoonheydtc, Micropor. Mesopor. Mater. 51 (2002) 91. [2] R. Reisfeld, C.K. Jørgensen, Chemistry, spectroscopy and applications of sol–gel glassesStructure and Bonding, vol. 77, Springer, Berlin, 1992. [3] D. Levy, D. Avnir, J. Phys. Chem. 92 (1988) 4734. [4] D. Levy, D. Avnir, J. Photochem. Photobiol. A: Chem. 57 (1991) 41. [5] C. Sanchez, B. Lebeau, Hybrid organic–inorganic materials, in: D.A. Loy (Ed.), Mater. Res. Soc. Bull., vol. 26, 2001, p. 377. [6] C. Sanchez, F. Ribot, R. Lebeau, J. Mater. Chem. 9 (1999) 35. [7] I. Willner, D. Mandler, Enzyme Microb. Technol. 11 (1989) 467. [8] I. Willner, D. Mandler, J. Am. Chem. Soc. 106 (1984) 5352. [9] I. Willner, D. Mandler, J. Chem. Soc., Chem. Commun. (1986) 851. [10] I. Okura, N. Kaji, S. Aono, T. Kita, A. Yamada, Inorg. Chem. 24 (1985) 451. [11] I. Okura, S. Kusunoki, S. Aono, Bull. Chem. Soc. Jpn. 57 (1984) 1184. [12] H. Noriko, A. Yutaka, Energy Fuels 17 (2003) 1641. [13] S. Yoshinobu, A. Yutaka, Bioconjug. Chem. 13 (2002) 898. [14] Sh. Kashimura, H. Yamashita, Y. Murai, Y. Kera, N. Yamashita, H. Murase, M. Ishifune, Electrochim. Acta 48 (2002) 7. [15] M. Ogawa, K. Kuroda, Chem. Rev. 95 (1995) 399. [16] H. Scheer, The Chlorophylls, CRC Press, Inc., Boca Raton, FL, 1991, p. 3.
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[17] Y. Kodera, H. Kageyama, H. Sekine, Y. Inada, Biotechnol. Lett. 14 (1992) 119. [18] A. Ishii, T. Itoh, H. Kageyama, T. Mizoguchi, Y. Kodera, A. Matsushima, K. Torii, Y. Inada, Dyes Pigments 28 (1995) 77. [19] T. Itoh, A. Ishii, Y. Kodera, A. Matsushima, M. Hiroto, H. Nishimura, T. Tsuzuki, T. Kamachi, I. Okura, Y. Inada, Bioconjug. Chem. 9 (1998) 409. [20] T. Itoh, K. Yano, Y. Inada, Y. Fukushima, J. Am. Chem. Soc. 124 (2002) 13437. [21] D. Li, J. Zhang, M. Anpo, Opt. Mater. 27 (2005) 671. [22] T.W. GoodwingChemistry and Biochemistry of Plant Pigments, vol. 2, Academic Press, London, 1976, p. 18. [23] W. Deng, P. Bodart, M. Pruski, B.H. Shanks, Micropor. Mesopor. Mater. 52 (2002) 69. [24] Powder Diffraction File, Card no. 40-1179. JCPDS, Swarthmore, PA. [25] D.N. Srivastava, V.G. Pol, O. Palchik, L. Zhang, J.C. Yu, A. Gedanken, Ultrason. Sonochem. 12 (2005) 205. [26] W. Xing, F. Li, Z.-f. Yan, G.Q. Lua, J. Power Sources 134 (2004) 324. [27] F. Porta, S. Recchia, C. Bianchi, F. Confalonieri, G. Scari, Colloid Surf. A 155 (1999) 395. [28] D.N. Srivastava, N. Perkas, G.A. Seisenbaeva, Y. Koltypin, V.G. Kessler, A. Gedanken, Ultrason. Sonochem. 10 (2003) 1. [29] Y. Takeuchi, Y. Amao, Bioconjug. Chem. 14 (2003) 268. [30] I. Okura, Coord. Chem. Rev. 68 (1985) 53. [31] I. Okura, Biochimie 68 (1986) 189. [32] T. Itoh, H. Asada, K. Tobioka, Y. Kodera, A. Matsushima, M. Hiroto, H. Nishimura, T. Kamachi, I. Okura, Y. Inada, Bioconjug. Chem. 11 (2000) 8. [33] T. Koiso, M. Okuyama, T. Sakata, T. Kawai, Bull. Chem. Soc. Jpn. 55 (1982) 2659. [34] A.A. Krasnovski, Chan-van-Ni, V.V. Nikandrov, G.P. Brin, Mol. Biol. 14 (1980) 287. [35] J. Kiwi, M. Gra¨etzel, J. Phys. Chem. 84 (1980) 1503. [36] Y. Saiki, Y. Amao, Bioconjug. Chem. 13 (2002) 898. [37] N. Himeshima, Y. Amao, Energy Fuels 17 (2003) 1641.
Synthesis and characterization of a new organic–inorganic hybrid NiO–chlorophyll-a as optical material Z. Mehraban a, F. Farzaneh a
a,*
, A. Shafiekhani
b
Department of Chemistry, University of Alzahra, Vanak, Tehran, 1993891167, Iran Department of Physics, University of Alzahra, Vanak, Tehran, 1993891167, Iran
b
Received 14 August 2005; accepted 10 February 2006 Available online 3 April 2006
Abstract The organic–inorganic hybrid NiO–chlorophyll-a (NiO–Chl-a) as an optical material was synthesized with nickel acetate and saponificated chlorophyll-a (Chl-a) by sol–gel method. It was characterized by FTIR, XRD, TGA and DTA. Its optical properties were examined with UV–Vis reflectance and illumination with a visible light. It was found that NiO–Chl-a emitted a monochromatic fluorescence at 658 nm wavelength with 10 nm width. It shows a high stability against visible light as well. Ó 2006 Elsevier B.V. All rights reserved. Keywords: NiO–chlorophyll-a; Organic–inorganic hybrid; Optical material; Photostability; UV–Vis reflectance
1. Introduction An increasing number of studies on the optical properties of dye-doped porous silica were published in the past decades. One of the advantages of an inorganic matrix for embedding of functional chromophores is attributed to the more rigid stability and strongly increased photostability compared to organic polymer matrix. The development of the room-temperature sol–gel process for the preparation of porous materials opened the possibility of adding organic chromophores to exhibit photophysical and photochemical functionality to the synthetic mixtures. This procedure results in a uniform dispersion of dye in the porous host. In fact, chromophores in inorganic–organic polymers have proved to exhibit various optical properties for potential applications such as sensors, lasers, amplifiers or storage devices as well as solar concentrators [1]. Several methods have been reported for preparations of dye-doped sol–gel silica [2–4]. In general, the dissolved dye is added to the reaction mixture to yield entrapped chro*
Corresponding author. Tel.: +98 21 88258977; fax: +98 21 66404848. E-mail address: [email protected] (F. Farzaneh).
0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.02.007
mophores in the final solid composites. The optical biosensors by using sol–gel techniques are prepared, since enzymes systems in porous glass monoliths like pyridine nucleotide, co-enzymes, sorbitol dehydrogenase/NADH, or lactate dehydrogenase/NADH exhibit strong absorption and fluorescence. It was found that organic fluorescent dyes have a high fluorescence quantum yield (95%) and low excited-state absorption. Therefore, they have strong potential for applications, such as optical materials. Some disadvantages of these organic dyes are low photo and thermal stability. sol–gel derived organic–inorganic hybrid optical materials are considered to be good candidates to assemble a combination of properties required for the development of new optical materials such as good mechanical, thermal and chemical stability with high room temperature emission quantum yields [5,6]. Organic photocatalysts of chlorophyll and porphyrin derivatives act under visible light illumination. They are well known for their biological conduction and photoactive properties and are very useful as catalysts [7–15]. Chl-a absorbs light due to chelation of conjugated double bonds of chlorin ring with Mg (II) [16].
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Natural chlorophylls extracted from living leaves have rarely been used as catalysts because of their instability against light and heat. In the layered membrane of chloroplasts in intact leaves, chlorophyll molecules bind to proteins to form chlorophyll–protein conjugates in which the chlorophyll–protein interactions play an important role in stabilization and physiological functions of living plant leaves. The adsorbed Chl-a within silicate layers of smectite and modified FSM shows photostability and photocatalytic properties [17–20], which demonstrates the importance of chlorophyll–support interactions. Synthesis of organic–inorganic hybrid optical materials can be carried out by different methods such as incorporation of organic molecules dyes in inorganic networks [21] or sol–gel derived organic–inorganic hybrid materials. In this work, we report the synthesis of a new organic– inorganic hybrid optical material by sol–gel method with nickel acetate and saponificated Chl-a as inorganic and organic component, respectively. They form NiO–Chl-a with a high photo and thermal stability. 2. Experimental 2.1. Material Nickel acetate, potassium hydroxide, n-hexane, ethanol 98% and chlorophyll-a were obtained from Merck Chemical Company and were used without further purification. 2.2. Synthesis NiO–Chl-a optical material The organic–inorganic hybrid NiO–Chl-a material (NiO–Chl-a) was prepared by dropwise addition of 100 ml nickel acetate solution (0.2 molar) to the saturated saponificated solution of Chl-a in ethanol [22]. The resultant dark green gel was aged for 24 h. The gel mixture was filtered, soxhelt extracted with ethanol to remove the excess saponificated Chl-a and then dried at 100 °C. 2.3. Synthesis of NiO blank
from Micromeritics Corporation. Mid-IR spectra of NiO–Chl-a were collected on Bruker instrument using KBr pellet technique in the range of 4000–400 cm 1 (0.005 g sample with 0.1 g KBr). The UV–Vis reflectance experiment was performed with a Stellar–Net detector with a deuterium lamp. The optical fibers are used as carrier light. The reflectance of NiO–Chl-a was compared with NiO as reference. The photostability of NiO–Chl-a was examined by illuminating the Ni–Chl-a in water, as well as free chlorophyll-a in n-hexane, with a 100 W incandescent lamp at a distance of 10 cm. The light intensity on a sample was approximately 460 J m 2 S 1. During the illumination, the sample was shielded from heat by the circulation of cold water. The absorption of samples with the step size of 30 min are detected by Unicam 8700 series UV–Vis spectrometer. 3. Results and discussion The thermal property of the NiO–Chl-a was investigated by TGA and DTA (Fig. 1). In TGA plot, the sample shows two weight losses steps, 8.36% up to 100 °C and 23.31% at 270 °C. DTA shows a mild endothermic and sharp exothermic behavior at 100 °C and 270 °C, respectively, that consistent with TGA data, and shows physically desorption of adsorbed water of the sample at 100 °C and the oxidative desorption of the organic template (chlorine ring of Chl-a and ethanol) at 270 °C. Therefore, the sample has a high thermal stability up to 270 °C. DTA data also confirms the formation of one phase because NiO–Chl-a shows only one exothermic step. Figs. 2 and 3 show the XRD patterns of the both dried and calcined NiO–Chl-a at room temperature and at 270 °C in the region of 2h < 10° and 10–90°, respectively. In fact, the observation of an intense peak at 2h 1° (Fig. 2) might be attributed to the mesoporous structure of this compound [23]. It was also observed that the intensity of peak at 2h 1° has been increased and shifted to the lower angel after calcination (Fig. 2b). This can be the result of pores expansion due to removal of template.
NiO was prepared under the similar conditions of NiO– Chl-a without addition of Chl-a.
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40 DTA
TGA
0
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Weight loss (%)
Thermogravimetric-differential thermal analysis was carried out in a Rheometric Scientific (STAH) thermo balance. The NiO–Chl-a was loaded and the O2 and Ar flow rate was 50 ml/min. The heating rate was 20 K/min and the final temperatures were 800 °C and 1000 °C in O2 and Ar atmosphere, respectively. Powder XRD patterns of NiO–Chl-a was recorded on a diffractometer type, SEIFERT XRD 3003 PTS, Cu Ka1 radiation (k = 0.1540 nm) in the 2h range of 0.5–10° and 10–90° Surface area, pore volume and pore size distribution were measured by nitrogen adsorption at 76 K with an ASAP-2010 porosimeter
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2.4. Characterization
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Temperature (oC) Fig. 1. TGA–DTA diagram of dried NiO–Chl-a at room temperature.
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Fig. 2. X-ray powder diffraction patterns of NiO–Chl-a in the range of 2h = 0.5–3.5° (a) dried at room temperature, (b) calcined at 270 °C.
The similarity of X-ray powder diffraction patterns between dried NiO–Chl-a at room temperature (Fig. 2a) and calcined sample at 270 °C (Fig. 2b) in the range of 2h = 10–90°, confirms the existence of nickel oxide hydroxide phase [24]. The surface area and BJH pore volume of calcined NiO– Chl-a were 250 m2/g and 0.32 cm3/g, respectively. The pore size distributions calculated from desorption branch using the BJH model is quite narrow with a maximum peak at 3.55 nm. These results indicate that NiO–Chl-a is a mesopore compound after calcination. The molar ratios of the calcinated NiO–Chl-a and the number of atoms ratio at 270 °C were determined for Ni:K:Mg as 0.878:0.024:0.0016 and Ni:K:Mg = 563:15:1, respectively. The FTIR spectra of NiO (blank) and dried NiO–Chl-a either dried at room temperature or calcined at 270 °C and 800 °C are shown in Fig. 4a–d, respectively. For NiO (blank) and NiO–Chl-a (Fig. 4a and b) the intense peak at 3644 cm 1 is attributed to free hydroxyl groups. A broad peak at 3400 cm 1 belongs to OH stretching of water. The peaks at 2916 and 2848 cm 1 are assigned to the C–H
Fig. 4. FTIR spectra of (a) NiO (blank), (b) NiO–Chl-a dried at room temperature, (c) calcined at 270 °C, and (d) calcined at 800 °C.
symmetric and asymmetric vibrations of –CH2 of remained acetate groups. The weak and broad bands at 1632 and 1475 cm 1 are also assigned to bending of adsorbed species such as water. The intensity of these peaks were reduced after calcination [25–27] of the sample at 270 °C and approximately disappeared at 800 °C. As seen in the FTIR spectrum of pure Chl-a (Fig. 5) the C–H, C@C, C@O and C–N stretching bands of chlorine ring have been overlapped with acetate groups vibrations bands present at terminals groups of NiO network. In all spectra, the intense peak at 410 cm 1 is due to the stretching mode of NiO [28]. The UV–Vis reflectances of NiO blank, NiO–Chl-a and Chl-a are shown in Fig. 6. Compared to Chl-a in methanol which shows reflectance at 680 nm (Fig. 6c) [29], NiO–Chla exhibits a sharp and narrow reflection at 658.5 nm with 10 nm width attributed to the red monochromatic reflectance of Chl-a group (Fig. 6b). The observation of a blue shift in NiO–Chl-a spectrum might be due to the existence
Fig. 3. X-ray powder diffraction patterns of NiO–Chl-a in the range of 2h = 10–90° (a) dried at room temperature, (b) calcined at 270 °C.
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NiO-Chl-a
Chl-a
100
Absorption %
80
Fig. 5. FTIR spectrum of pure Chl-a.
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Time (min) Fig. 7. Photostability of NiO–Chl-a and pure Chl-a in n-hexane against illumination with visible light.
Fig. 6. UV–Vis reflectance spectra of (a) NiO (blank), (b) NiO–Chl-a, and (c) Chl-a.
of the material distributed molecular rather than being aggregated within the NiO support. This property makes NiO–Chl-a unique in optical applications especially for dye lasers. Photostability was examined by illumination with visible light (Fig. 7). The relative absorbance value decreased to 30% since free chlorophyll a in n-hexane, with an absorption maximum at 665 nm (curve a) was obviously discolored by the illumination after 360 min. On the other hand, NiO–Chl-a, with an absorption maximum at 660 nm, was much more photostable in water in comparison with free chlorophyll. A relative absorbance value of 97% is retained at the absorption maximum in the red region after 360 min of illuminating (curve b). This stability could be arising from incorporation of Chl-a as organic dye in inorganic of nickel oxide hydroxide network.
In some photoinduced hydrogen production systems with visible light colloidal platinum acting as hydrogenproducing catalysts, water-soluble zinc porphyrins have been widely used as effective photosensitizers [29–31]. Since they have an absorption band in the visible light region (380–600 nm). However, the molar absorption coefficient of zinc porphyrins in the visible light region (500– 600 nm) was lower than that in the near ultravisible light region (380–400 nm). Alternatively, Mg Chlorophyll-a, which acts as an effective photosensitizer in photosynthesis, has an absorption maximum at 670 nm. Thus, Mg Chl-a is attractive as a visible region photosensitizer. Photoinduced hydrogen production systems with chemically modified chlorophyll and hydrogenase [19,32], and chlorophyll derivatives [33–35], were reported previously. Photoinduced hydrogen production systems must consist of an electron donor, photosensitizer, electron relay, and catalyst, the photoexcited photosensitizer reacts with an electron relay to form a reduced electron relay, hydrogen evolves by proton reduction with the catalyst, and then the oxidized photosensitizer is then reduced by an electron-donating reagent [36,37]. Therefore, compared to free chlorophyll-a and NiO–chlorophyll-a organic–inorganic hybrid with high photostability against visible light and an absorption maximum at 660 nm can act as a very effective photosensitizer in photoinduced hydrogen production systems. 4. Conclusion Synthesis of NiO–Chl-a was carried out by sol–gel method in ambient conditions. Based on DTA–TGA results, photostability and UV–Vis reflectance experiments, this organic–inorganic hybrid optical material has a high photo and thermal stability with a red monochromatic
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