- Email: [email protected]
silica composite fibers
EUROPEAN POLYMER JOURNAL
European Polymer Journal 43 (2007) 3679–3682
www.elsevier.com/locate/europolj
Short communication
Preparation of dendritic and network PANI/silica composite fibers Xiaocong Wang a
a,*
, Xuefeng Feng b, Yan Zhao a, Ruzhen Zhang a, Donglan Sun
a
College of Science, Tianjin University of Science and Technology, Tianjin 300022, PR China b East China Institute of Technology, JiangXi 344000, PR China Received 24 March 2007; received in revised form 13 May 2007; accepted 21 May 2007 Available online 8 June 2007
Abstract Dendritic and network PANI fibers with controlled diameters from nanosize to sub-micrometer-size were prepared at room temperature. Conducting polyaniline (PANI)/silica composite fibers were synthesized via a sol–gel progress thereafter. Structural characterization was performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). FT-IR and UV–vis were used to verify the incorporation of the silica. 2007 Elsevier Ltd. All rights reserved. Keywords: Conducting polymer; PANI; Composites; Dendritic; Nanofibers
1. Introduction Conducting polymer/inorganic have attracted more and more attention since they can combine the performances of each component usually with synergistic effect, have interesting physical properties and many potential applications [1]. Conducting PANI, as one of the most important conducting polymers, has been intensively studied in recent years. This is mainly due to its high conductivity, ease of preparation, good environmental stability, and a large variety of applications especially in light-emitting and electronic devices [2], chemical sensors [3], separation membranes [4], and antistatic
*
Corresponding author. Fax: +86 22 60600808. E-mail address: [email protected] (X. Wang).
coatings [5]. Many PANI/inorganic nanocomposites such as nanofibers and nanotubes [6–8], nanoparticles [9] and nanosheets [10] have been prepared recently. Following the extensive interest in low dimensional materials, dendritic nanomaterial has attracted another increasing attention [11,12]. Due to their unusual structure, superior performances are arisen creating new opportunities for their potential applications in many fields such as nanoscale devices, photovoltaic cells, and chemical sensors [11–13]. One of the promising properties is their extremely low threshold value on forming network, which is rather important in condensed matter physics and materials science. Herein, in this paper, we describe a very simple method to synthesize dendritic and network PANI fibers with controlled diameters from nanosize to
0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.05.039
3680
X. Wang et al. / European Polymer Journal 43 (2007) 3679–3682
sub-micrometer-size at room temperature. The as-prepared PANI thereafter used as template for preparing dendritic and network PANI/silica composite fibers via a self-assembly sol–gel process. The incorporation of silica could give the PANI new functionality, which could result in enhancing its dispersion and improving its processability [14]. This self-assembly sol–gel method employed here is a simple and inexpensive route to synthesize multifunctional fibers and could be extended to prepare other PANI/inorganic composites. 2. Experimental part 2.1. Materials
2.3. Preparation of PANI/silica fibers In a typical experiment to prepare PANI/silica composites, dried dendritic PANI (20 mg) with diameters 80–150 nm was dispersed in ethanol/ water (20 mL, V/V = 7/3) solution, shaking with hand for several minutes, then added tetraethoxysilane (TEOS, 20 mg) in the solution, stirred very slowly with magnetic for 24 h. Then, stopped stirring, and held for several hours till the composites deposited. The resulting precipitate was filtered and washed with deionized water and ethanol for several times to remove the residual TEOS and left to dry at room temperature. 2.4. Characterization
Aniline (An, Beijing Chem. Co.) was distilled under reduced pressure before using. Ammonium Persulfate ((NH4)2S2O8, APS, A.R., Beijing 3rd Chemical Reagents Factory). Salicylic acid (SA) and other reagents were purchased from Beijing Chem. Co. 2.2. Preparation of PANI fibers The typical procedure to prepare dendritic and network polyaniline fibers is as follows: salicylic acid (SA, 0.0268 g) was dissolved in water (19 mL) and stirred for 30 min. Aniline (An, 0.182 g) was added in the solution and stirred for another 30 min. Then, APS (1 mL, 1 mol/L) was added, shaking for several minutes, the reaction was continued for 24 h. During the polymerization process, the color changed to dark green. The resulting PANI precipitate was washed with water and ethanol for several times.
The morphologies of PANI and PANI/silica fibers were investigated with a Hitachi S-4300 field emission scanning electron microscope (SEM) and a JEOL JEM-100CX transmission electron microscope (TEM). The infrared spectras were recorded by a BRUKER EQUINOX 55 FT-IR using KBr disks. The fibers were compressed into pellets to measure the electrical conductivity. It was measured by a stand four-probe method, using a Keithley 196 System DMM digital multimeter. 3. Results and discussion Fig. 1a shows typical SEM image of dendritic and network PANI fibers synthesized at the condition of 0.01 mol/L SA and 0.1 mol/L An. It was found that the diameter of the branches is 80– 150 nm. The lengths of the branches could be several micrometers. TEM image (Fig. 1b) showed that
Fig. 1. Morphologies of dendritic and network PANI fibers: (a) SEM and (b) TEM.
X. Wang et al. / European Polymer Journal 43 (2007) 3679–3682
3681
Fig. 2. Morphologies of dendritic and network PANI/silica fibers: (a) SEM and (b) TEM.
Intensity
514
1236 1297
456
a
1093 816
b
500
1000
1500
2000
Wavenumbers (cm-1) Fig. 3. FT-IR spectra of: (a) the PANI and (b) the PANI/silica composite fibers.
PANI PANI/silica
Absorbance (a.u.)
most of these PANI fibers are solid and inter-connected to form dendritic and network structure, rather than isolated fibers. The diameters of the dendritic PANI fibers could be controlled easily by varying the concentration of An and SA. As Figure S1 (Support information) shows PANI nanofibers with diameters between 30 nm and 60 nm were synthesized when the reactants’ concentration decreased 10 times (using 0.01 mol/L An and 0.001 mol/L SA). The easiness of controlling the diameter of the PANI fibers provides us facility to synthesize PANI/silica composite fibers with different diameters according to the requisition. Fig. 2a is a typical SEM image of PANI/silica composite fibers, it shows many protuberances formed on the dendritic composite nanofibers, and the fibers formed a continuous 3D interconnected network. TEM image (Fig. 2b) indicated all the dendritic composite fibers are solid rather than hollow. The roughly dark structure corresponds to the overlap of several fibers. The little dark spots inside of the fibers are silica nanoparticles. FT-IR spectra are showed in Fig. 3. As one can see, the FT-IR spectrum of dendritic PANI/silica is similar to that of dendritic PANI, besides the characteristic mode of silica at 1093 cm 1 attributed to Si–O–Si stretching vibration. The weak bands at 456 and 816 cm 1 are correspondingly assigned to the vibration of Si–O. The bands of PANI at 514 cm 1 ascribed to the C–N–C torsion, at 1236 cm 1 related to the protonated C–N group and at 1297 cm 1 corresponded to the C–N stretching mode are a bit of different from that of PANI/ silica, which indicates the doped state is changed. The fact that the doped state changed was confirmed by UV–vis spectra too, since the PANI/silica has no absorption at 320 nm which are attributed
a b 300
400
500
600
700
800
Wavelength (nm) Fig. 4. UV–vis spectra of: (a) the PANI and (b) the PANI/silica composite fibers.
to p–p* transitions (Fig. 4), and the absorption peak of the p-polaron transition is broader and shifts to a higher wavelength in the composite fibers. This
3682
X. Wang et al. / European Polymer Journal 43 (2007) 3679–3682
result indicates that the doping level is higher, and the prepared PANI is more delocalized in the PANI/silica composite [8]. The doping of PANI could involve the introduction of holes (p-doped) into their conjugated chains. The inorganic components preferentially interact with the positively charged ammonium and condensed into a solid, continuous framework. The above proposal suggests that the PANI and silica nanoparticles cowork to fabricate composite through a self-assembly sol– gel process [15]. The sol–gel reaction involves the hydrolysis of Si–OR groups produced Si–OH groups that formed covalent bond with organic components. Those previously resultant containing silicate will act as templates in the formation of subsequent PANI/silica composite. The production of composites combined the microstructure and intrinsic properties of the two components. The inorganic molecules will in general contribute their chemical activity and will need the structure support of organic polymer to become solid composite leading to a material with properties superior to the sum of the properties of its components [16]. The PANI content in dendritic PANI/silica composite was ca. 53-wt%, as determined by means of thermogravimetric analysis (TGA). The room conductivity of the dendritic composites is 4.6 · 10 2 S cm 1, much higher than that of PANI (1.3 · 10 2 S cm 1). The conductivity increasing might be caused by the composite have a higher density than PANI alone and the presence of silica tend to condense the network. When PANI/silica is compressed into a disk at higher pressure, the conductivity will increase. In conclusion, dendritic composite fibers of PANI/silica were prepared through a sol–gel process. The diameters of the composite fibers were adjustable in that the diameters of the PANI fibers could be controlled easily by changing reaction condition. We demonstrate a very easy approach to synthesize dendritic or network PANI composites fibers. This approach is applicable to synthesize composites of PANI and other inorganic which easily self-assembled with PANI. More dendritic and network composite fibers such as combined with TiO2, Fe3O4 and Ag are being fabricated by this method in our lab. Acknowledgements This work was supported by Natural Science Foundation of Tianjin Education Committee (No.
20060514) and Natural Science Foundation of Tianjin University of Science and Technology (No. 0218152). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.eurpolymj.2007.05.039. References [1] Gangopadhyay R, De A. Conducting polymer nanocomposites: a brief overview. Chem Mater 2000;12(7):2064. [2] Liang L, Liu J, Windisch CF, Exarhos GJ, Lin Y. Direct assembly of large arrays of oriented conducting polymer nanowires. Angew Chem Int Ed 2002;41(19):3665–8. [3] Huang J, Virji S, Weiller BH, Kaner RB. Polyaniline nanofibers: facile synthesis and chemical sensors. J Am Chem Soc 2003;125(2):314–5. [4] Huang SC, Ball IJ, Kaner RB. Polyaniline membranes for pervaporation of carboxylic acids and water. Macromolecules 1998;31(16):5456–64. [5] Soto-Oviedo MA, Araujo OA, Faez R. Antistatic coating and electromagnetic shielding properties of a hybrid material based on polyaniline/organoclay nanocomposite and EPDM rubber. Synth Metals 2006;156(18–20):1249–55. [6] Mottaghitalab V, Xi B, Spinks GM, Wallace GG. Polyaniline fibres containing single walled carbon nanotubes: enhanced performance artificial muscles. Synth Metals 2006;156(11–13):796–803. [7] Zhang ZM, Wan MX. Nanostructures of polyaniline composites containing nano-magnet. Synth Metals 2003;132(2):205–12. [8] Feng XM, Yang G, Xu Q, Zhu JJ. Self-assembly of polyaniline/Au composites: from nanotubes to nanofibers. Macromol Rapid Commun 2006;27(1):31–6. [9] Li XW, Wang GC, Kim KY, Li XX, Lu DM. Surface properties of polyaniline/nano-TiO2 composites. Appl Surf Sci 2004;229(1–4):395–401. [10] Pang SP, Li GC, Zhang ZK. Synthesis of polyanilinevanadium oxide nanocomposite nanosheets. Macromol Rapid Commun 2005;26(15):1262–5. [11] Zhao YB, Zhang ZJ, Liu WM, Dang HX, Xue Q. Controlling synthesis of Biln dendritic nanocrystals by solution dispersion. J Am Chem Soc 2004;126(22):6854–5. [12] Yan HQ, He RR, Pham J, Yang PD. Morphogenesis of onedimensional ZnO nano- and microcrystals. Adv Mater 2003;15(5):402–5. [13] Wang DL, Qian F, Yang C, Zhong ZH. Rational growth of branched and hyperbranched nanowire structures. Nanoletters 2004;4(5):871–4. [14] Stejskal J, Kratochvil P, Armes P. Polyaniline dispersions: 6, stabilization by colloidal silica particles. Macromolecules 1996;29:6814–9. [15] Wang YJ, Wang XH, Li J, Mo ZS, Zhao XJ, Jing XB, Wang FS. Conductive polyaniline/silica hybrids from sol–gel process. Adv Mater 2001;13(20):1582–5. [16] Gomez-Romero P. Hybrid organic–inorganic materials – in search of synergic activity. Adv Mater 2001;13(3):163–74.
European Polymer Journal 43 (2007) 3679–3682
www.elsevier.com/locate/europolj
Short communication
Preparation of dendritic and network PANI/silica composite fibers Xiaocong Wang a
a,*
, Xuefeng Feng b, Yan Zhao a, Ruzhen Zhang a, Donglan Sun
a
College of Science, Tianjin University of Science and Technology, Tianjin 300022, PR China b East China Institute of Technology, JiangXi 344000, PR China Received 24 March 2007; received in revised form 13 May 2007; accepted 21 May 2007 Available online 8 June 2007
Abstract Dendritic and network PANI fibers with controlled diameters from nanosize to sub-micrometer-size were prepared at room temperature. Conducting polyaniline (PANI)/silica composite fibers were synthesized via a sol–gel progress thereafter. Structural characterization was performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). FT-IR and UV–vis were used to verify the incorporation of the silica. 2007 Elsevier Ltd. All rights reserved. Keywords: Conducting polymer; PANI; Composites; Dendritic; Nanofibers
1. Introduction Conducting polymer/inorganic have attracted more and more attention since they can combine the performances of each component usually with synergistic effect, have interesting physical properties and many potential applications [1]. Conducting PANI, as one of the most important conducting polymers, has been intensively studied in recent years. This is mainly due to its high conductivity, ease of preparation, good environmental stability, and a large variety of applications especially in light-emitting and electronic devices [2], chemical sensors [3], separation membranes [4], and antistatic
*
Corresponding author. Fax: +86 22 60600808. E-mail address: [email protected] (X. Wang).
coatings [5]. Many PANI/inorganic nanocomposites such as nanofibers and nanotubes [6–8], nanoparticles [9] and nanosheets [10] have been prepared recently. Following the extensive interest in low dimensional materials, dendritic nanomaterial has attracted another increasing attention [11,12]. Due to their unusual structure, superior performances are arisen creating new opportunities for their potential applications in many fields such as nanoscale devices, photovoltaic cells, and chemical sensors [11–13]. One of the promising properties is their extremely low threshold value on forming network, which is rather important in condensed matter physics and materials science. Herein, in this paper, we describe a very simple method to synthesize dendritic and network PANI fibers with controlled diameters from nanosize to
0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.05.039
3680
X. Wang et al. / European Polymer Journal 43 (2007) 3679–3682
sub-micrometer-size at room temperature. The as-prepared PANI thereafter used as template for preparing dendritic and network PANI/silica composite fibers via a self-assembly sol–gel process. The incorporation of silica could give the PANI new functionality, which could result in enhancing its dispersion and improving its processability [14]. This self-assembly sol–gel method employed here is a simple and inexpensive route to synthesize multifunctional fibers and could be extended to prepare other PANI/inorganic composites. 2. Experimental part 2.1. Materials
2.3. Preparation of PANI/silica fibers In a typical experiment to prepare PANI/silica composites, dried dendritic PANI (20 mg) with diameters 80–150 nm was dispersed in ethanol/ water (20 mL, V/V = 7/3) solution, shaking with hand for several minutes, then added tetraethoxysilane (TEOS, 20 mg) in the solution, stirred very slowly with magnetic for 24 h. Then, stopped stirring, and held for several hours till the composites deposited. The resulting precipitate was filtered and washed with deionized water and ethanol for several times to remove the residual TEOS and left to dry at room temperature. 2.4. Characterization
Aniline (An, Beijing Chem. Co.) was distilled under reduced pressure before using. Ammonium Persulfate ((NH4)2S2O8, APS, A.R., Beijing 3rd Chemical Reagents Factory). Salicylic acid (SA) and other reagents were purchased from Beijing Chem. Co. 2.2. Preparation of PANI fibers The typical procedure to prepare dendritic and network polyaniline fibers is as follows: salicylic acid (SA, 0.0268 g) was dissolved in water (19 mL) and stirred for 30 min. Aniline (An, 0.182 g) was added in the solution and stirred for another 30 min. Then, APS (1 mL, 1 mol/L) was added, shaking for several minutes, the reaction was continued for 24 h. During the polymerization process, the color changed to dark green. The resulting PANI precipitate was washed with water and ethanol for several times.
The morphologies of PANI and PANI/silica fibers were investigated with a Hitachi S-4300 field emission scanning electron microscope (SEM) and a JEOL JEM-100CX transmission electron microscope (TEM). The infrared spectras were recorded by a BRUKER EQUINOX 55 FT-IR using KBr disks. The fibers were compressed into pellets to measure the electrical conductivity. It was measured by a stand four-probe method, using a Keithley 196 System DMM digital multimeter. 3. Results and discussion Fig. 1a shows typical SEM image of dendritic and network PANI fibers synthesized at the condition of 0.01 mol/L SA and 0.1 mol/L An. It was found that the diameter of the branches is 80– 150 nm. The lengths of the branches could be several micrometers. TEM image (Fig. 1b) showed that
Fig. 1. Morphologies of dendritic and network PANI fibers: (a) SEM and (b) TEM.
X. Wang et al. / European Polymer Journal 43 (2007) 3679–3682
3681
Fig. 2. Morphologies of dendritic and network PANI/silica fibers: (a) SEM and (b) TEM.
Intensity
514
1236 1297
456
a
1093 816
b
500
1000
1500
2000
Wavenumbers (cm-1) Fig. 3. FT-IR spectra of: (a) the PANI and (b) the PANI/silica composite fibers.
PANI PANI/silica
Absorbance (a.u.)
most of these PANI fibers are solid and inter-connected to form dendritic and network structure, rather than isolated fibers. The diameters of the dendritic PANI fibers could be controlled easily by varying the concentration of An and SA. As Figure S1 (Support information) shows PANI nanofibers with diameters between 30 nm and 60 nm were synthesized when the reactants’ concentration decreased 10 times (using 0.01 mol/L An and 0.001 mol/L SA). The easiness of controlling the diameter of the PANI fibers provides us facility to synthesize PANI/silica composite fibers with different diameters according to the requisition. Fig. 2a is a typical SEM image of PANI/silica composite fibers, it shows many protuberances formed on the dendritic composite nanofibers, and the fibers formed a continuous 3D interconnected network. TEM image (Fig. 2b) indicated all the dendritic composite fibers are solid rather than hollow. The roughly dark structure corresponds to the overlap of several fibers. The little dark spots inside of the fibers are silica nanoparticles. FT-IR spectra are showed in Fig. 3. As one can see, the FT-IR spectrum of dendritic PANI/silica is similar to that of dendritic PANI, besides the characteristic mode of silica at 1093 cm 1 attributed to Si–O–Si stretching vibration. The weak bands at 456 and 816 cm 1 are correspondingly assigned to the vibration of Si–O. The bands of PANI at 514 cm 1 ascribed to the C–N–C torsion, at 1236 cm 1 related to the protonated C–N group and at 1297 cm 1 corresponded to the C–N stretching mode are a bit of different from that of PANI/ silica, which indicates the doped state is changed. The fact that the doped state changed was confirmed by UV–vis spectra too, since the PANI/silica has no absorption at 320 nm which are attributed
a b 300
400
500
600
700
800
Wavelength (nm) Fig. 4. UV–vis spectra of: (a) the PANI and (b) the PANI/silica composite fibers.
to p–p* transitions (Fig. 4), and the absorption peak of the p-polaron transition is broader and shifts to a higher wavelength in the composite fibers. This
3682
X. Wang et al. / European Polymer Journal 43 (2007) 3679–3682
result indicates that the doping level is higher, and the prepared PANI is more delocalized in the PANI/silica composite [8]. The doping of PANI could involve the introduction of holes (p-doped) into their conjugated chains. The inorganic components preferentially interact with the positively charged ammonium and condensed into a solid, continuous framework. The above proposal suggests that the PANI and silica nanoparticles cowork to fabricate composite through a self-assembly sol– gel process [15]. The sol–gel reaction involves the hydrolysis of Si–OR groups produced Si–OH groups that formed covalent bond with organic components. Those previously resultant containing silicate will act as templates in the formation of subsequent PANI/silica composite. The production of composites combined the microstructure and intrinsic properties of the two components. The inorganic molecules will in general contribute their chemical activity and will need the structure support of organic polymer to become solid composite leading to a material with properties superior to the sum of the properties of its components [16]. The PANI content in dendritic PANI/silica composite was ca. 53-wt%, as determined by means of thermogravimetric analysis (TGA). The room conductivity of the dendritic composites is 4.6 · 10 2 S cm 1, much higher than that of PANI (1.3 · 10 2 S cm 1). The conductivity increasing might be caused by the composite have a higher density than PANI alone and the presence of silica tend to condense the network. When PANI/silica is compressed into a disk at higher pressure, the conductivity will increase. In conclusion, dendritic composite fibers of PANI/silica were prepared through a sol–gel process. The diameters of the composite fibers were adjustable in that the diameters of the PANI fibers could be controlled easily by changing reaction condition. We demonstrate a very easy approach to synthesize dendritic or network PANI composites fibers. This approach is applicable to synthesize composites of PANI and other inorganic which easily self-assembled with PANI. More dendritic and network composite fibers such as combined with TiO2, Fe3O4 and Ag are being fabricated by this method in our lab. Acknowledgements This work was supported by Natural Science Foundation of Tianjin Education Committee (No.
20060514) and Natural Science Foundation of Tianjin University of Science and Technology (No. 0218152). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.eurpolymj.2007.05.039. References [1] Gangopadhyay R, De A. Conducting polymer nanocomposites: a brief overview. Chem Mater 2000;12(7):2064. [2] Liang L, Liu J, Windisch CF, Exarhos GJ, Lin Y. Direct assembly of large arrays of oriented conducting polymer nanowires. Angew Chem Int Ed 2002;41(19):3665–8. [3] Huang J, Virji S, Weiller BH, Kaner RB. Polyaniline nanofibers: facile synthesis and chemical sensors. J Am Chem Soc 2003;125(2):314–5. [4] Huang SC, Ball IJ, Kaner RB. Polyaniline membranes for pervaporation of carboxylic acids and water. Macromolecules 1998;31(16):5456–64. [5] Soto-Oviedo MA, Araujo OA, Faez R. Antistatic coating and electromagnetic shielding properties of a hybrid material based on polyaniline/organoclay nanocomposite and EPDM rubber. Synth Metals 2006;156(18–20):1249–55. [6] Mottaghitalab V, Xi B, Spinks GM, Wallace GG. Polyaniline fibres containing single walled carbon nanotubes: enhanced performance artificial muscles. Synth Metals 2006;156(11–13):796–803. [7] Zhang ZM, Wan MX. Nanostructures of polyaniline composites containing nano-magnet. Synth Metals 2003;132(2):205–12. [8] Feng XM, Yang G, Xu Q, Zhu JJ. Self-assembly of polyaniline/Au composites: from nanotubes to nanofibers. Macromol Rapid Commun 2006;27(1):31–6. [9] Li XW, Wang GC, Kim KY, Li XX, Lu DM. Surface properties of polyaniline/nano-TiO2 composites. Appl Surf Sci 2004;229(1–4):395–401. [10] Pang SP, Li GC, Zhang ZK. Synthesis of polyanilinevanadium oxide nanocomposite nanosheets. Macromol Rapid Commun 2005;26(15):1262–5. [11] Zhao YB, Zhang ZJ, Liu WM, Dang HX, Xue Q. Controlling synthesis of Biln dendritic nanocrystals by solution dispersion. J Am Chem Soc 2004;126(22):6854–5. [12] Yan HQ, He RR, Pham J, Yang PD. Morphogenesis of onedimensional ZnO nano- and microcrystals. Adv Mater 2003;15(5):402–5. [13] Wang DL, Qian F, Yang C, Zhong ZH. Rational growth of branched and hyperbranched nanowire structures. Nanoletters 2004;4(5):871–4. [14] Stejskal J, Kratochvil P, Armes P. Polyaniline dispersions: 6, stabilization by colloidal silica particles. Macromolecules 1996;29:6814–9. [15] Wang YJ, Wang XH, Li J, Mo ZS, Zhao XJ, Jing XB, Wang FS. Conductive polyaniline/silica hybrids from sol–gel process. Adv Mater 2001;13(20):1582–5. [16] Gomez-Romero P. Hybrid organic–inorganic materials – in search of synergic activity. Adv Mater 2001;13(3):163–74.