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Photoluminescent mesoporous carbon-doped silica from rice husks
Materials Letters 142 (2015) 280–282
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Photoluminescent mesoporous carbon-doped silica from rice husks Yao Liu a, Zhaofeng Wang b, Huidan Zeng c, Caixing Chen a, Jingjing Liu b, Luyi Sun b,n, Weixing Wang a,nn a Ministry of Education Key Laboratory of Enhanced Heat Transfer & Energy Conservation, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China b Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, United States c Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
art ic l e i nf o
a b s t r a c t
Article history: Received 13 October 2014 Accepted 3 December 2014 Available online 13 December 2014
Photoluminescent mesoporous carbon-doped silica with a surface area of 203 m2/g, pore diameter of 6.4 nm and pore volume of 0.33 cm3/g was prepared through the calcination of HCl treated rice husks. The mechanism of the high intensity photoluminescence is mainly owing to the trapped carbon in the silica framework. & 2014 Elsevier B.V. All rights reserved.
Keywords: Rice husks Optical materials and properties Luminescence Mesoporous Carbon-doped silica
1. Introduction Silica based photoluminescent (PL) materials, a promising candidate for phosphors, have drawn tremendous attention because of their wide applications in photonics, biomedicine, and medical diagnostics and therapy [1–5]. In recent years, many new methods have been developed to fabricate silica based PL materials. For example, Green et al. synthesized highly emissive broadband silica based phosphors, which appeared white to the naked eye, using a tetraalkoxysilane sol–gel precursor and a variety of organic carboxylic acids [6]. Narisawa et al. reported long-lived photoluminescence in amorphous Si–O–C ceramics derived from polysiloxanes by de-carbonization at elevated temperatures [7]. By oxidation of crystalline silicon nanoparticles, amorphous hollow silica nanoparticles with strong visible PL was fabricated by Colder et al. [8]. In general, such new methods allow for a better morphology control of the PL materials, but they typically require multiple steps and are sophisticated [3]. For commercial applications, it is highly desirable to develop a simple, fast, and cost effective method to prepare PL materials at a large scale.
n
Corresponding author. Tel.: þ 1 860 486 6895; fax: þ 1 860 486 4745. Corresponding author. Tel.: þ 86 20 87113735; fax: þ 86 20 22236337. E-mail addresses: [email protected] (L. Sun), [email protected] (W. Wang).
Rice husks (RHs), a by-product in rice production, are widely considered as a bio-waste because of their very limited applications [9]. RHs are mainly composed of cellulose, hemicellulose, lignin, and silica [9,10]. The silica in RHs is in hydrated amorphous form and counts ca. 15–28 wt% of dry husks [11]. The co-existence of the two major components in RHs, SiO2 and organic carbon, plus the inherently intimate contact between the two components [12] make RHs a particularly ideal candidate to synthesize carbondoped silica via appropriate thermal treatment. The high surface area and high reactivity of the silica from RHs allow for facile modification of silica for various applications [13,14]. Recently, it was observed that the silica from RHs can emit PL under UV light irradiation [15]. However, the impurities in RHs inevitably lower the quality of the synthesized silica from RHs (lower purity and lower specific surface area) [11–13]. Herein, we aim to optimize the reaction conditions to achieve the strongest possible photoluminescence and introduce mesopores into the PL material for potential applications [12,16]. Mesoporous PL silica was fabricated from RHs through a facile HCl treatment of RHs to eliminate the majority of the metal ions and followed by a calcination process at 550 1C for 6 h.
2. Experimental
nn
http://dx.doi.org/10.1016/j.matlet.2014.12.034 0167-577X/& 2014 Elsevier B.V. All rights reserved.
The RHs used in this research were obtained from Guangdong Academy of Agricultural Science. Analytical reagent grade
Y. Liu et al. / Materials Letters 142 (2015) 280–282
hydrochloric acid (37 wt%) was purchased from Guangzhou Chemical Reagent Company and used as received. The raw RHs were boiled in 10 wt% HCl solution for 2 h, rinsed with deionized water, and then dried at 100 1C for 24 h. The dried RHs were then calcined in a muffle furnace, which was preheated to 550 1C, for 6 h to prepare mesoporous carbon-doped silica (RHS-550-6). In comparison, high purity silica (RHS-700-2) was prepared by calcining the HCl treated RHs at 700 1C for 2 h. Elemental analyses were performed on a Vario EL III elemental analysis instrument (Elementar Co.). PL spectra were recorded on a Fluorolog-3-P fluorescence spectrometer (HORIBA Jobin Yvon Inc., Edison, NJ) at room temperature. The excitation wavelength was tuned using a monochromator combined with a 450 W Xenon lamp. X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Kratos AXIS Ultra DLD spectrometer with a monochromated Al Kα radiation source (1486.6 eV) operating at 15 kV and 10 mA. Energy-dispersive spectroscopy (EDS) analysis was performed on an Oxford INCA Energy EDS analysis system. The surface area of the samples was measured on a TriStar II 3020 Surface Area and Porosity System. The samples were degassed at 200 1C for 12 h prior to the measurement. X-ray diffraction (XRD) patterns were recorded using a Bruker D8 diffractometer with Bragg–Brentano θ 2θ geometry (20 kV and 5 mA), using a graphite monochromator with Cu Kα radiation.
3. Results and discussion Our earlier research has shown that calcination of acid treated RHs at 700 1C for 2 h can efficiently remove the vast majority of organic components in RHs to prepare high purity silica (elemental analysis yielded a carbon content of 0.084 wt%). Lowering the calcination temperature will result in incomplete degradation of organic components, leaving carbon residues in silica [17]. In this project, we intentionally chose 550 1C as the calcination temperature for 6 h to prepare carbon-doped silica with the strongest possible PL intensity (elemental analysis yielded a carbon content of 0.477 wt%). Fig. 1 presents the PL and photoluminescent excitation (PLE) spectra of the as-prepared carbon-doped silica from RHs. The PL spectrum (red curve) shows the emission peak at 435 nm under a 365 nm light excitation. The emission of the carbon-doped silica with an emission center wavelength at ca. 435 nm, covers most of
Fig. 1. PL and PLE spectra of carbon-doped silica (RHS-550-6) from RHs. Inset: digital picture of the carbon-doped silica under 365 nm UV lamp. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
281
the visible range. When monitored at 435 nm, the PLE spectrum (black curve) in Fig. 1 exhibited a broadband from 225 to 415 nm, with the optimum excitation wavelength at ca. 365 nm. As the near-ultraviolet (NUV) InGaN-based light emitting diode (LED) chips exhibit emission from 350 to 420 nm [18], the broad excitation band of the as-prepared carbon-doped silica from RHs suggests that it could be a good candidate as white phosphors for NUV white LEDs. The florescence decay curve was recorded under 365 nm excitation, by monitoring the emission of 435 nm, as shown in Fig. S1. The phosphorescence decay is a bi-exponential decay process, with an initially fast decay process followed by a slow decay. The biexponential decay curve can be fitted using the following equation: I(t)¼ A1exp( t/τ1)þA2exp( t/τ2), where I(t) is the time-dependent fluorescence intensity, A1 and A2 are the relative amplitude of fast and slow components, τ1 and τ2 correspond to fast and slow time constants. From the bi-exponential fitting of the fluorescence decay curve, it can be calculated that the fast and slow decay lifetimes are 2.49 and 12.54 ns, respectively. With a facile HCl (10 wt%) pretreatment, metal impurities in RHs can be effectively removed [11–13,17]. The mesoporous structure of silica was well maintained as shown in Fig. 2. The surface area characterization revealed that such carbon-doped silica has a Brunauer–Emmett–Teller (BET) surface area of 203 m2/g. The Barret–Joyner–Halenda (BJH) analysis results, as displayed in the inset of Fig. 2, show that the pore diameter and pore volume of the synthesized carbon-doped silica are 6.4 nm and 0.33 cm3/g, respectively, which support the formation of mesoporous silica. The origin of the luminescence of silica based PL materials has not been fully understood. Three possible mechanisms have been proposed, i.e., carbon impurity mechanism, defect mechanism, and charge transfer mechanism [6,19]. With an aim to identify the exact mechanism of the PL emission, X-ray photoelectron spectroscopy (XPS) characterization was carried out to determine the intrinsic structure (chemical composition and bonding) of the obtained product, as shown in Fig. S2. The survey spectrum shows that silica is the main component of the PL material, which was also confirmed by the EDS spectrum as shown in Fig. S3. The existence of carbon in silica was confirmed by the presence of C1s peak in Fig. S2. To better understand the composition and bonding of carbon in the PL material, the high resolution C1s XPS spectrum (Fig. 3) was deconvoluted and three peaks were obtained: the main peak corresponds to the sp2 hybridized graphitic carbon (CQC groups at 284.8 eV); the other two peaks are attributed to the C–O groups in carbonyl (at 286.0 eV) and O–CQO groups in
Fig. 2. N2 sorption isotherms of mesoporous carbon-doped silica from RHs. The inset shows the pore size distribution from the BJH adsorption.
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Y. Liu et al. / Materials Letters 142 (2015) 280–282
U.S. Environmental Protection Agency (P3 Award, SU-83529201). W.W. thanks the National Natural Science Foundation of China (21176093) for funding. L.S. acknowledges the National Science Foundation, (Partnerships for Research and Education in Materials DMR-1205670), the Air Force Office of Scientific Research (No. FA9550-12-1-0159), and the Faculty Large Grant from the University of Connecticut (4623050) for partial support for this research.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.12.034.
References Fig. 3. C1s high resolution spectrum of mesoporous carbon-doped silica from RHs.
carboxyl (at 288.6 eV) [20]. In comparison, the silica from the higher temperature treatment (700 1C for 2 h, RHS-700-2) possesses a very low content of carbon of 0.084 wt% [17], exhibiting a weak PL emission (Fig. S4), although it has a similar amorphous structure, surface area, and pore size distribution (Figs. S5 and S6). As such, it is believed that the trapped carbon is the major cause of the high intensity PL [6]. 4. Conclusions In summary, photoluminescent mesoporous carbon-doped silica prepared through the calcination of HCl treated RHs at 550 1C for 6 h has been demonstrated to exhibit strong PL. The mechanism of the high intensity PL is mainly owing to the trapped carbon. Acknowledgments: This project is sponsored by the National Undergraduate Innovation and Entrepreneurial Training Program (201410561066) and the
[1] Nobre SS, Cattoen X, Ferreira RAS, Man MWC, Carlos LD. Phys Status Solidi – Rapid Res Lett 2010;4:55–7. [2] Bardosova M, Pemble ME, Povey IM, Tredgold RH. Adv Mater 2010;22:3104–24. [3] Zhang C, Lin J. Chem Soc Rev 2012;41:7938–61. [4] Wen P, Ninghui W, Guofeng L, Guiling N. Mater Sci Forum 2011;675–677:1093–6. [5] Rossi LM, Shi LF, Quina FH, Rosenzweig Z. Langmuir 2005;21:4277–80. [6] Green WH, Le KP, Grey J, Au TT, Sailor MJ. Science 1997;276:1826–8. [7] Narisawa M, Kawai T, Watase S, Matsukawa K, Dohmaru T, Okamura K, et al. J Am Ceram Soc 2012;95:3935–40. [8] Colder A, Huisken F, Trave E, Ledoux G, Guillois O, Reynaud C, et al. Nanotechnology 2004;15:L1–4. [9] Sun L, Gong K. Ind Eng Chem Res 2001;40:5861–77. [10] Chen H, Wang W, Martin JC, Oliphant AJ, Doerr PA, Xu JF, et al. ACS Sustain Chem Eng 2013;1:254–9. [11] Wang WX, Martin JC, Fan XT, Han AJ, Luo ZP, Sun LY. ACS Appl Mater Interfaces 2012;4:977–81. [12] Zhao L, Ming T, Chen H, Gong L, Chen J, Wang J. Phys Chem Chem Phys 2011;13:2387–93. [13] Zhu S, Zhang J, Tang S, Qiao C, Wang L, Wang H, et al. Adv Funct Mater 2012;22:4732–40. [14] Lee T, Othman R, Yeoh FY. Biomass Bioenergy 2013;59:380–92. [15] Ishikawa Y, Kawasaki S, Ishi Y, Sato K, Matsumura A. Jpn J Appl Phys 2012;51. [16] Anedda A, Carbonaro CM, Clemente F, Corpino R, Ricci PC. Mater Sci Eng C: Biomim Supramol Syst 2003;23:1073–6. [17] Wang WX, Martin JC, Zhang N, Ma C, Han AJ, Sun LY. J Nanopart Res 2011;13:6981–90. [18] Wang Z, Wang J, Li Z, Gong P, Ren J, Wang H, et al. RSC Adv 2012;2:11681–6. [19] Lin J, Baerner K. Mater Lett 2000;46:86–92. [20] Beamson G, Briggs D. High Resolution XPS of Organic Polymers. 1992.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Photoluminescent mesoporous carbon-doped silica from rice husks Yao Liu a, Zhaofeng Wang b, Huidan Zeng c, Caixing Chen a, Jingjing Liu b, Luyi Sun b,n, Weixing Wang a,nn a Ministry of Education Key Laboratory of Enhanced Heat Transfer & Energy Conservation, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China b Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, United States c Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
art ic l e i nf o
a b s t r a c t
Article history: Received 13 October 2014 Accepted 3 December 2014 Available online 13 December 2014
Photoluminescent mesoporous carbon-doped silica with a surface area of 203 m2/g, pore diameter of 6.4 nm and pore volume of 0.33 cm3/g was prepared through the calcination of HCl treated rice husks. The mechanism of the high intensity photoluminescence is mainly owing to the trapped carbon in the silica framework. & 2014 Elsevier B.V. All rights reserved.
Keywords: Rice husks Optical materials and properties Luminescence Mesoporous Carbon-doped silica
1. Introduction Silica based photoluminescent (PL) materials, a promising candidate for phosphors, have drawn tremendous attention because of their wide applications in photonics, biomedicine, and medical diagnostics and therapy [1–5]. In recent years, many new methods have been developed to fabricate silica based PL materials. For example, Green et al. synthesized highly emissive broadband silica based phosphors, which appeared white to the naked eye, using a tetraalkoxysilane sol–gel precursor and a variety of organic carboxylic acids [6]. Narisawa et al. reported long-lived photoluminescence in amorphous Si–O–C ceramics derived from polysiloxanes by de-carbonization at elevated temperatures [7]. By oxidation of crystalline silicon nanoparticles, amorphous hollow silica nanoparticles with strong visible PL was fabricated by Colder et al. [8]. In general, such new methods allow for a better morphology control of the PL materials, but they typically require multiple steps and are sophisticated [3]. For commercial applications, it is highly desirable to develop a simple, fast, and cost effective method to prepare PL materials at a large scale.
n
Corresponding author. Tel.: þ 1 860 486 6895; fax: þ 1 860 486 4745. Corresponding author. Tel.: þ 86 20 87113735; fax: þ 86 20 22236337. E-mail addresses: [email protected] (L. Sun), [email protected] (W. Wang).
Rice husks (RHs), a by-product in rice production, are widely considered as a bio-waste because of their very limited applications [9]. RHs are mainly composed of cellulose, hemicellulose, lignin, and silica [9,10]. The silica in RHs is in hydrated amorphous form and counts ca. 15–28 wt% of dry husks [11]. The co-existence of the two major components in RHs, SiO2 and organic carbon, plus the inherently intimate contact between the two components [12] make RHs a particularly ideal candidate to synthesize carbondoped silica via appropriate thermal treatment. The high surface area and high reactivity of the silica from RHs allow for facile modification of silica for various applications [13,14]. Recently, it was observed that the silica from RHs can emit PL under UV light irradiation [15]. However, the impurities in RHs inevitably lower the quality of the synthesized silica from RHs (lower purity and lower specific surface area) [11–13]. Herein, we aim to optimize the reaction conditions to achieve the strongest possible photoluminescence and introduce mesopores into the PL material for potential applications [12,16]. Mesoporous PL silica was fabricated from RHs through a facile HCl treatment of RHs to eliminate the majority of the metal ions and followed by a calcination process at 550 1C for 6 h.
2. Experimental
nn
http://dx.doi.org/10.1016/j.matlet.2014.12.034 0167-577X/& 2014 Elsevier B.V. All rights reserved.
The RHs used in this research were obtained from Guangdong Academy of Agricultural Science. Analytical reagent grade
Y. Liu et al. / Materials Letters 142 (2015) 280–282
hydrochloric acid (37 wt%) was purchased from Guangzhou Chemical Reagent Company and used as received. The raw RHs were boiled in 10 wt% HCl solution for 2 h, rinsed with deionized water, and then dried at 100 1C for 24 h. The dried RHs were then calcined in a muffle furnace, which was preheated to 550 1C, for 6 h to prepare mesoporous carbon-doped silica (RHS-550-6). In comparison, high purity silica (RHS-700-2) was prepared by calcining the HCl treated RHs at 700 1C for 2 h. Elemental analyses were performed on a Vario EL III elemental analysis instrument (Elementar Co.). PL spectra were recorded on a Fluorolog-3-P fluorescence spectrometer (HORIBA Jobin Yvon Inc., Edison, NJ) at room temperature. The excitation wavelength was tuned using a monochromator combined with a 450 W Xenon lamp. X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Kratos AXIS Ultra DLD spectrometer with a monochromated Al Kα radiation source (1486.6 eV) operating at 15 kV and 10 mA. Energy-dispersive spectroscopy (EDS) analysis was performed on an Oxford INCA Energy EDS analysis system. The surface area of the samples was measured on a TriStar II 3020 Surface Area and Porosity System. The samples were degassed at 200 1C for 12 h prior to the measurement. X-ray diffraction (XRD) patterns were recorded using a Bruker D8 diffractometer with Bragg–Brentano θ 2θ geometry (20 kV and 5 mA), using a graphite monochromator with Cu Kα radiation.
3. Results and discussion Our earlier research has shown that calcination of acid treated RHs at 700 1C for 2 h can efficiently remove the vast majority of organic components in RHs to prepare high purity silica (elemental analysis yielded a carbon content of 0.084 wt%). Lowering the calcination temperature will result in incomplete degradation of organic components, leaving carbon residues in silica [17]. In this project, we intentionally chose 550 1C as the calcination temperature for 6 h to prepare carbon-doped silica with the strongest possible PL intensity (elemental analysis yielded a carbon content of 0.477 wt%). Fig. 1 presents the PL and photoluminescent excitation (PLE) spectra of the as-prepared carbon-doped silica from RHs. The PL spectrum (red curve) shows the emission peak at 435 nm under a 365 nm light excitation. The emission of the carbon-doped silica with an emission center wavelength at ca. 435 nm, covers most of
Fig. 1. PL and PLE spectra of carbon-doped silica (RHS-550-6) from RHs. Inset: digital picture of the carbon-doped silica under 365 nm UV lamp. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
281
the visible range. When monitored at 435 nm, the PLE spectrum (black curve) in Fig. 1 exhibited a broadband from 225 to 415 nm, with the optimum excitation wavelength at ca. 365 nm. As the near-ultraviolet (NUV) InGaN-based light emitting diode (LED) chips exhibit emission from 350 to 420 nm [18], the broad excitation band of the as-prepared carbon-doped silica from RHs suggests that it could be a good candidate as white phosphors for NUV white LEDs. The florescence decay curve was recorded under 365 nm excitation, by monitoring the emission of 435 nm, as shown in Fig. S1. The phosphorescence decay is a bi-exponential decay process, with an initially fast decay process followed by a slow decay. The biexponential decay curve can be fitted using the following equation: I(t)¼ A1exp( t/τ1)þA2exp( t/τ2), where I(t) is the time-dependent fluorescence intensity, A1 and A2 are the relative amplitude of fast and slow components, τ1 and τ2 correspond to fast and slow time constants. From the bi-exponential fitting of the fluorescence decay curve, it can be calculated that the fast and slow decay lifetimes are 2.49 and 12.54 ns, respectively. With a facile HCl (10 wt%) pretreatment, metal impurities in RHs can be effectively removed [11–13,17]. The mesoporous structure of silica was well maintained as shown in Fig. 2. The surface area characterization revealed that such carbon-doped silica has a Brunauer–Emmett–Teller (BET) surface area of 203 m2/g. The Barret–Joyner–Halenda (BJH) analysis results, as displayed in the inset of Fig. 2, show that the pore diameter and pore volume of the synthesized carbon-doped silica are 6.4 nm and 0.33 cm3/g, respectively, which support the formation of mesoporous silica. The origin of the luminescence of silica based PL materials has not been fully understood. Three possible mechanisms have been proposed, i.e., carbon impurity mechanism, defect mechanism, and charge transfer mechanism [6,19]. With an aim to identify the exact mechanism of the PL emission, X-ray photoelectron spectroscopy (XPS) characterization was carried out to determine the intrinsic structure (chemical composition and bonding) of the obtained product, as shown in Fig. S2. The survey spectrum shows that silica is the main component of the PL material, which was also confirmed by the EDS spectrum as shown in Fig. S3. The existence of carbon in silica was confirmed by the presence of C1s peak in Fig. S2. To better understand the composition and bonding of carbon in the PL material, the high resolution C1s XPS spectrum (Fig. 3) was deconvoluted and three peaks were obtained: the main peak corresponds to the sp2 hybridized graphitic carbon (CQC groups at 284.8 eV); the other two peaks are attributed to the C–O groups in carbonyl (at 286.0 eV) and O–CQO groups in
Fig. 2. N2 sorption isotherms of mesoporous carbon-doped silica from RHs. The inset shows the pore size distribution from the BJH adsorption.
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Y. Liu et al. / Materials Letters 142 (2015) 280–282
U.S. Environmental Protection Agency (P3 Award, SU-83529201). W.W. thanks the National Natural Science Foundation of China (21176093) for funding. L.S. acknowledges the National Science Foundation, (Partnerships for Research and Education in Materials DMR-1205670), the Air Force Office of Scientific Research (No. FA9550-12-1-0159), and the Faculty Large Grant from the University of Connecticut (4623050) for partial support for this research.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.12.034.
References Fig. 3. C1s high resolution spectrum of mesoporous carbon-doped silica from RHs.
carboxyl (at 288.6 eV) [20]. In comparison, the silica from the higher temperature treatment (700 1C for 2 h, RHS-700-2) possesses a very low content of carbon of 0.084 wt% [17], exhibiting a weak PL emission (Fig. S4), although it has a similar amorphous structure, surface area, and pore size distribution (Figs. S5 and S6). As such, it is believed that the trapped carbon is the major cause of the high intensity PL [6]. 4. Conclusions In summary, photoluminescent mesoporous carbon-doped silica prepared through the calcination of HCl treated RHs at 550 1C for 6 h has been demonstrated to exhibit strong PL. The mechanism of the high intensity PL is mainly owing to the trapped carbon. Acknowledgments: This project is sponsored by the National Undergraduate Innovation and Entrepreneurial Training Program (201410561066) and the
[1] Nobre SS, Cattoen X, Ferreira RAS, Man MWC, Carlos LD. Phys Status Solidi – Rapid Res Lett 2010;4:55–7. [2] Bardosova M, Pemble ME, Povey IM, Tredgold RH. Adv Mater 2010;22:3104–24. [3] Zhang C, Lin J. Chem Soc Rev 2012;41:7938–61. [4] Wen P, Ninghui W, Guofeng L, Guiling N. Mater Sci Forum 2011;675–677:1093–6. [5] Rossi LM, Shi LF, Quina FH, Rosenzweig Z. Langmuir 2005;21:4277–80. [6] Green WH, Le KP, Grey J, Au TT, Sailor MJ. Science 1997;276:1826–8. [7] Narisawa M, Kawai T, Watase S, Matsukawa K, Dohmaru T, Okamura K, et al. J Am Ceram Soc 2012;95:3935–40. [8] Colder A, Huisken F, Trave E, Ledoux G, Guillois O, Reynaud C, et al. Nanotechnology 2004;15:L1–4. [9] Sun L, Gong K. Ind Eng Chem Res 2001;40:5861–77. [10] Chen H, Wang W, Martin JC, Oliphant AJ, Doerr PA, Xu JF, et al. ACS Sustain Chem Eng 2013;1:254–9. [11] Wang WX, Martin JC, Fan XT, Han AJ, Luo ZP, Sun LY. ACS Appl Mater Interfaces 2012;4:977–81. [12] Zhao L, Ming T, Chen H, Gong L, Chen J, Wang J. Phys Chem Chem Phys 2011;13:2387–93. [13] Zhu S, Zhang J, Tang S, Qiao C, Wang L, Wang H, et al. Adv Funct Mater 2012;22:4732–40. [14] Lee T, Othman R, Yeoh FY. Biomass Bioenergy 2013;59:380–92. [15] Ishikawa Y, Kawasaki S, Ishi Y, Sato K, Matsumura A. Jpn J Appl Phys 2012;51. [16] Anedda A, Carbonaro CM, Clemente F, Corpino R, Ricci PC. Mater Sci Eng C: Biomim Supramol Syst 2003;23:1073–6. [17] Wang WX, Martin JC, Zhang N, Ma C, Han AJ, Sun LY. J Nanopart Res 2011;13:6981–90. [18] Wang Z, Wang J, Li Z, Gong P, Ren J, Wang H, et al. RSC Adv 2012;2:11681–6. [19] Lin J, Baerner K. Mater Lett 2000;46:86–92. [20] Beamson G, Briggs D. High Resolution XPS of Organic Polymers. 1992.