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Facile synthesis of porous hollow iron oxide nanoparticles supported on carbon nanotubes
Materials Letters 67 (2012) 245–247
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile synthesis of porous hollow iron oxide nanoparticles supported on carbon nanotubes Yingsi Wu, Hao Yu ⁎, Feng Peng, Hongjuan Wang School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 9 August 2011 Accepted 26 September 2011 Available online 1 October 2011 Keywords: Porous hollow iron oxide Nanoparticles Acid etching Kirkendall effect Carbon nanotubes
a b s t r a c t Porous hollow iron oxide nanoparticles (PHNPs) supported on carbon nanotubes (CNTs) were facilely synthesized by etching Fe@FexOy/CNT with dilute nitric acid aqueous solution at ambient temperature without the assistance of any surfactants and ligands. The mean diameter of hollow iron oxide nanoparticles was about 17 nm, with a wall thickness of about 4 nm. The formation mechanism of PHNPs is discussed based on the characterization results from TEM, XRD and H2-TPR. The combination of nanoscale Kirkendall effect and selective acid etching is proposed to be responsible for the formation of CNT supported PHNPs, through a transformation from core/void/shell structures to hollow nanoparticles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Materials with hollow spherical structure in nanoscale are prospective in the fields of medicine, sensing, optics or catalysis [1–4]. Among these materials, hollow iron oxide nanoparticles, including magnetite (Fe3O4) and maghemite (γ-Fe2O3), are of special interest, due to their potential applications in magnetic resonance imaging [5], drug delivery [6], catalysis [7] and separation [8]. Several approaches have been developed to prepare hollow iron oxide micro-/nano-spheres, including hydro/solvothermal method [6], Kirkendall effect [9], acid etching [10], sonochemical method [11], and ultrasonic spray pyrolysis [12]. However, most of the methods require high temperatures, external fields with high energy input, or the assistance with surfactant/template/ligand reagents to obtain hollow structures, which increase the energy consumption or cytotoxicity. In this letter, we report a facile approach to prepare porous hollow iron oxide nanoparticles (PHNPs) supported on CNTs, without using any organic surfactant/template/ligand and at ambient temperature. Bio-compatible CNT supports [13,14] prevent the PHNPs from aggregation. The formation mechanism of PHNPs is discussed based on the characterization results from TEM, XRD and H2-TPR.
containing 0.2 g HNO3-functionalized CNTs (Shenzhen Nanotech Port Co. Ltd.), 1.5 g FeSO4·7H2O and 10 mL distilled water was formed in ultrasonic bath, and then mixed with 50 mL H2O2 aqueous solution (30%) dropwise under strong stirring. The suspension was refluxed at 80 °C for 4 h to obtain Fe(OH)3/CNT precursor. Subsequently, the precursor was reduced in H2 at 450 °C for 2 h. After exposed to air, the resulting sample is denoted as Fe@FexOy/CNT. To form iron oxide PHNPs, 0.25 g Fe@FexOy/CNT was added to 80 mL HNO3. After sonication for 30 min, the solids were filtered, washed and dried at 110 °C. The resulting CNT supported PHNPs are denoted as FexOy/CNT-m, where m represents concentration of acid. The loadings of Fe were measured by atomic adsorption spectroscopy (Hitachi Z-5000). Transmission electron micrographs (TEM) were obtained with a FEI Tecnai G 2 12 operated at 100 kV and a JEOL JEM-2010 operated at 200 kV. X-ray diffraction (XRD) analysis was performed on a D8-advance X-ray diffractometer (German Bruker) with Cu target (40 kV, 40 mA) at a scan rate of 0.02 °/17.7 s. H2-temperature programmed reduction (H2-TPR) tests were conducted in a Micrometrics ASAP 2010 adsorption instrument. For each measurement, 80 mg sample was reduced under flowing H2/Ar (10%) of 50 mL/min from 50 to 800 °C with a ramping rate of 10 °C/min. 3. Results and discussion
2. Experimental CNT supported iron hydroxides were first prepared by a H2O2 homogeneous oxidation precipitation method [15]. In brief, a suspension ⁎ Corresponding author. Tel./fax: + 86 20 8711 4916. E-mail address: [email protected] (H. Yu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.097
A typical TEM image of Fe@FexOy/CNT is shown in Fig. 1a. Nanoparticles with a mean diameter of 17.8 nm are homogenously dispersed on CNTs. Clear core/void/shell structures can be observed. Smaller particles were completely oxidized and formed hollow structures, agreeing with the observations in literature [9,16]. After the HNO3 treatment, the Fe cores disappeared and hollow nanoparticles
246
Y. Wu et al. / Materials Letters 67 (2012) 245–247
Fig. 1. TEM images of (a) Fe@FexOy/CNT, (b) FexOy/CNT—0.1 M, (c) FexOy/CNT—2 M, and (d) FexOy/CNT—9 M.
composites may be promising in biomedical applications, due to the good dispersibility of PHNPs and the biocompatibility of carbon materials. The removal of Fe cores was supported by XRD measurements. As shown in Fig. 2a, the peak at 2θ = 44.6° is assigned as the (110) reflection of metallic-Fe (JCPDS 65–4899), and the reflections from FeO (JCPDS 46–1312), maghemite γ-Fe2O3 (JCPDS 39–1346) or magnetite
Samples
Fe@FexOy/ CNT
Loading (%) 39.4 SBET 102.9 (m2/g)
FeO
(422) (511)
(440)
(004)
(101) (311) (220)
(b)
(220) (400) (110)
(c)
Table 1 Loadings and specific surface areas of the synthesized samples.
Fe
Graphite γ-Fe2O3/Fe3O4
(002)
Intensity (a.u.)
were formed, with about 17 nm diameter and 4 nm thickness, as shown in Fig. 1b–d. The HRTEM image in the inset of Fig. 1c indicates that the shell is composed of polycrystallines. The lattice fringes with intervals of 0.295 nm correspond to the (220) facets of γ-Fe2O3/Fe3O4. Pits were observed on the surfaces of shells, which led to the porosity of nanoparticles, evidenced by the increase of specific surface area as shown in Table 1. They can offer pathways for exchanging materials between the interior and exterior of the nanoparticles, making them potential for drug delivery. The Fe loading decreases as the concentration of acid increase, indicating that concentrated HNO3 would dissolve the oxide shells. Above results conclusively demonstrate that PHNPs can be synthesized via the procedure proposed here. Comparing with the literature methods [6,9–12], it is quite facile because no any organic reagents are involved and all the evacuation process by HNO3 can be operated at ambient temperature. The resultant
(a)
FexOy/CNT— 0.1 M
FexOy/CNT— 2M
FexOy/CNT— 9M
20.7 144.4
6.44 163.7
3.9 157.1
10
20
30
40
50
60
70
80
2 Theta (degree) Fig. 2. XRD patterns of (a) Fe@FexOy/CNT, (b) FexOy/CNT—0.1 M and (c) FexOy/CNT—2 M.
Y. Wu et al. / Materials Letters 67 (2012) 245–247
H2 assumption (a.u.)
FeO Fe3O4
Fe2O3
Fe
FeO
Fe3O4
(a) (b) (c) 200
400
600
800
Temperature (oC) Fig. 3. H2-TPR profiles of (a) Fe@FexOy/CNT, (b) FexOy/CNT—0.1 M and (c) FexOy/CNT—2 M.
247
A formation mechanism of iron oxide PHNPs is proposed in Fig. 4. The H2 reduction process converts Fe(OH)3 to Fe nanoparticles, which are rapidly oxidized to form Fe@FexOy core/shell structures when exposed to air [9,18]. Due to the Kirkendall effect, the outward diffusion of Fe and counter diffusion of vacancy form the void between core and shell [9,18]. On one hand, the cathodic reaction in the micro cell system containing Fe cores, H + ions in the voids and FexOy shells consumes the metallic Fe cores [19]. On the other hand, the oxide-film acts as a passive layer to lower the corrosion rate of shell in HNO3 [20]. The kinetic difference between oxide passive layers and metal cores allows for the formation of PHNPs. The role of passivation by HNO3 was evidenced by conducting a 0.1 M HCl treatment for 30 min, after which all iron species were washed off, indicating the non-selective etching of HCl. Such a selective etching process is analogous to the separation of passive film of bulk Fe by iodine or HNO3, as reported in the early report by Evans [20]. However, to our best knowledge, it is the first time to report its success in preparing hollow structures on nanoscale. 4. Conclusions In summary, CNT-supported iron oxide PHNPs, with 17 nm diameter and 4 nm shell thickness, were synthesized via a selective HNO 3 etching of Fe@Fe xO y core/shells. The formation mechanism involves the rapid cathodic reaction of Fe cores, and the relatively slow corrosion of iron oxides in nitric acid. The resultant nanocomposites containing iron oxide PHNPs are promising materials in magnetic, biomedical, and catalytic fields. Acknowledgments This work was supported by the National Science Foundation of China (No. 20806027) and the Guangdong Provincial National Science Foundation of China (No. 9251064101000020). References
Fig. 4. Schematic procedure of the formation of iron oxide PHNPs on CNTs.
Fe3O4 (JCPDS 79–0419) can be clearly observed, indicating the formation of Fe@FexOy core/shell structure. The HNO3 treatment led to the diminution of the diffraction peaks from Fe species. However, the reflection from Fe almost totally disappeared, while obvious (220) and (311) reflections of iron oxides can be detected, indicating the selective etching of Fe cores. Previous studies show that the defective oxide layer covering the innermost Fe is either γ-Fe2O3 or Fe3O4 or their mixture [16]. H2-TPR was used to further identify the composition of PHNPs. As shown in Fig. 3, all the samples exhibited three reduction steps, agreeing with the stepwise reduction of iron oxides as: Fe2O3 →Fe3O4 →FeO→Fe [17]. The Fe2O3 →Fe3O4 peak at about 250 °C demonstrated that γ-Fe2O3 exists in the PHNPs. However, the relatively low intensity of the Fe2O3 →Fe3O4 peak suggested the low fraction of Fe2O3 in the PHNPs. It was therefore unlikely that the TPR profiles were caused by pure γ-Fe2O3. Based on above results, we propose that the PHNPs are a mixture of γ-Fe2O3 and Fe3O4.
[1] Caruso F, Caruso RA, Mohwald H. Science 1998;282:1111–4. [2] Dai ZH, Bai HY, Hong M, Zhu YY, Bao JC, Shen J. Biosens Bioelectron 2008;23: 1869–73. [3] Yu XL, Cao CB, Zhu HS, Li QS, Liu CL, Gong QH. Adv Funct Mater 2007;17: 1397–401. [4] Yang M, Ma J, Ding SJ, Meng ZK, Liu JG, Zhao T, et al. Macromol Chem Phys 2006;207:1633–9. [5] Julian-Lopez B, Boissiere C, Chaneac C, Grosso D, Vasseur S, Miraux S, et al. J Mater Chem 2007;17:1563–9. [6] Cao SW, Zhu YJ. J Phys Chem C 2008;112:12149–56. [7] Mori K, Tanimura I, Yamashita H. Bull Chem Soc Jpn 2010;83:1122–6. [8] Wang P, Lo IMC. Water Res 2009;43:3727–34. [9] Khurshid H, Li WF, Tzitzios V, Hadjipanayis GC. Nanotechnology 2011;22:265605. [10] An K, Kwon SG, Park M, Bin Na H, Baik SI, Yu JH, et al. Nano Lett 2008;8:4252–8. [11] Bang JH, Suslick KS. J Am Chem Soc 2007;129:2242–3. [12] Gurmen S, Ebin B. J Alloys Compd 2010;492:585–9. [13] Ali-Boucetta H, Al-Jamal KT, McCarthy D, Prato M, Bianco A, Kostarelos K. Chem Commun 2008:459–61. [14] Usui Y, Aoki K, Narita N, Murakami N, Nakamura I, Nakamura K, et al. Small 2008;4:240–6. [15] Fu XB, Yu H, Peng F, Wang HJ, Qian Y. Appl Catal A-Gen 2007;321:190–7. [16] Wang CM, Baer DR, Amonette JE, Engelhard MH, Antony J, Qiang Y. J Am Chem Soc 2009;131:8824–32. [17] Jozwiak WK, Kaczmarek E, Maniecki TP, Ignaczak W, Maniukiewicz W. Appl Catal A-Gen 2007;326:17–27. [18] Cabot A, Puntes VF, Shevchenko E, Yin Y, Balcells L, Marcus MA, et al. J Am Chem Soc 2007;129:10358–60. [19] Evans UR. Proc R Soc Lond A 1925;107:228–37. [20] Evans UR. J Chem Soc 1927:1020–40.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile synthesis of porous hollow iron oxide nanoparticles supported on carbon nanotubes Yingsi Wu, Hao Yu ⁎, Feng Peng, Hongjuan Wang School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 9 August 2011 Accepted 26 September 2011 Available online 1 October 2011 Keywords: Porous hollow iron oxide Nanoparticles Acid etching Kirkendall effect Carbon nanotubes
a b s t r a c t Porous hollow iron oxide nanoparticles (PHNPs) supported on carbon nanotubes (CNTs) were facilely synthesized by etching Fe@FexOy/CNT with dilute nitric acid aqueous solution at ambient temperature without the assistance of any surfactants and ligands. The mean diameter of hollow iron oxide nanoparticles was about 17 nm, with a wall thickness of about 4 nm. The formation mechanism of PHNPs is discussed based on the characterization results from TEM, XRD and H2-TPR. The combination of nanoscale Kirkendall effect and selective acid etching is proposed to be responsible for the formation of CNT supported PHNPs, through a transformation from core/void/shell structures to hollow nanoparticles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Materials with hollow spherical structure in nanoscale are prospective in the fields of medicine, sensing, optics or catalysis [1–4]. Among these materials, hollow iron oxide nanoparticles, including magnetite (Fe3O4) and maghemite (γ-Fe2O3), are of special interest, due to their potential applications in magnetic resonance imaging [5], drug delivery [6], catalysis [7] and separation [8]. Several approaches have been developed to prepare hollow iron oxide micro-/nano-spheres, including hydro/solvothermal method [6], Kirkendall effect [9], acid etching [10], sonochemical method [11], and ultrasonic spray pyrolysis [12]. However, most of the methods require high temperatures, external fields with high energy input, or the assistance with surfactant/template/ligand reagents to obtain hollow structures, which increase the energy consumption or cytotoxicity. In this letter, we report a facile approach to prepare porous hollow iron oxide nanoparticles (PHNPs) supported on CNTs, without using any organic surfactant/template/ligand and at ambient temperature. Bio-compatible CNT supports [13,14] prevent the PHNPs from aggregation. The formation mechanism of PHNPs is discussed based on the characterization results from TEM, XRD and H2-TPR.
containing 0.2 g HNO3-functionalized CNTs (Shenzhen Nanotech Port Co. Ltd.), 1.5 g FeSO4·7H2O and 10 mL distilled water was formed in ultrasonic bath, and then mixed with 50 mL H2O2 aqueous solution (30%) dropwise under strong stirring. The suspension was refluxed at 80 °C for 4 h to obtain Fe(OH)3/CNT precursor. Subsequently, the precursor was reduced in H2 at 450 °C for 2 h. After exposed to air, the resulting sample is denoted as Fe@FexOy/CNT. To form iron oxide PHNPs, 0.25 g Fe@FexOy/CNT was added to 80 mL HNO3. After sonication for 30 min, the solids were filtered, washed and dried at 110 °C. The resulting CNT supported PHNPs are denoted as FexOy/CNT-m, where m represents concentration of acid. The loadings of Fe were measured by atomic adsorption spectroscopy (Hitachi Z-5000). Transmission electron micrographs (TEM) were obtained with a FEI Tecnai G 2 12 operated at 100 kV and a JEOL JEM-2010 operated at 200 kV. X-ray diffraction (XRD) analysis was performed on a D8-advance X-ray diffractometer (German Bruker) with Cu target (40 kV, 40 mA) at a scan rate of 0.02 °/17.7 s. H2-temperature programmed reduction (H2-TPR) tests were conducted in a Micrometrics ASAP 2010 adsorption instrument. For each measurement, 80 mg sample was reduced under flowing H2/Ar (10%) of 50 mL/min from 50 to 800 °C with a ramping rate of 10 °C/min. 3. Results and discussion
2. Experimental CNT supported iron hydroxides were first prepared by a H2O2 homogeneous oxidation precipitation method [15]. In brief, a suspension ⁎ Corresponding author. Tel./fax: + 86 20 8711 4916. E-mail address: [email protected] (H. Yu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.097
A typical TEM image of Fe@FexOy/CNT is shown in Fig. 1a. Nanoparticles with a mean diameter of 17.8 nm are homogenously dispersed on CNTs. Clear core/void/shell structures can be observed. Smaller particles were completely oxidized and formed hollow structures, agreeing with the observations in literature [9,16]. After the HNO3 treatment, the Fe cores disappeared and hollow nanoparticles
246
Y. Wu et al. / Materials Letters 67 (2012) 245–247
Fig. 1. TEM images of (a) Fe@FexOy/CNT, (b) FexOy/CNT—0.1 M, (c) FexOy/CNT—2 M, and (d) FexOy/CNT—9 M.
composites may be promising in biomedical applications, due to the good dispersibility of PHNPs and the biocompatibility of carbon materials. The removal of Fe cores was supported by XRD measurements. As shown in Fig. 2a, the peak at 2θ = 44.6° is assigned as the (110) reflection of metallic-Fe (JCPDS 65–4899), and the reflections from FeO (JCPDS 46–1312), maghemite γ-Fe2O3 (JCPDS 39–1346) or magnetite
Samples
Fe@FexOy/ CNT
Loading (%) 39.4 SBET 102.9 (m2/g)
FeO
(422) (511)
(440)
(004)
(101) (311) (220)
(b)
(220) (400) (110)
(c)
Table 1 Loadings and specific surface areas of the synthesized samples.
Fe
Graphite γ-Fe2O3/Fe3O4
(002)
Intensity (a.u.)
were formed, with about 17 nm diameter and 4 nm thickness, as shown in Fig. 1b–d. The HRTEM image in the inset of Fig. 1c indicates that the shell is composed of polycrystallines. The lattice fringes with intervals of 0.295 nm correspond to the (220) facets of γ-Fe2O3/Fe3O4. Pits were observed on the surfaces of shells, which led to the porosity of nanoparticles, evidenced by the increase of specific surface area as shown in Table 1. They can offer pathways for exchanging materials between the interior and exterior of the nanoparticles, making them potential for drug delivery. The Fe loading decreases as the concentration of acid increase, indicating that concentrated HNO3 would dissolve the oxide shells. Above results conclusively demonstrate that PHNPs can be synthesized via the procedure proposed here. Comparing with the literature methods [6,9–12], it is quite facile because no any organic reagents are involved and all the evacuation process by HNO3 can be operated at ambient temperature. The resultant
(a)
FexOy/CNT— 0.1 M
FexOy/CNT— 2M
FexOy/CNT— 9M
20.7 144.4
6.44 163.7
3.9 157.1
10
20
30
40
50
60
70
80
2 Theta (degree) Fig. 2. XRD patterns of (a) Fe@FexOy/CNT, (b) FexOy/CNT—0.1 M and (c) FexOy/CNT—2 M.
Y. Wu et al. / Materials Letters 67 (2012) 245–247
H2 assumption (a.u.)
FeO Fe3O4
Fe2O3
Fe
FeO
Fe3O4
(a) (b) (c) 200
400
600
800
Temperature (oC) Fig. 3. H2-TPR profiles of (a) Fe@FexOy/CNT, (b) FexOy/CNT—0.1 M and (c) FexOy/CNT—2 M.
247
A formation mechanism of iron oxide PHNPs is proposed in Fig. 4. The H2 reduction process converts Fe(OH)3 to Fe nanoparticles, which are rapidly oxidized to form Fe@FexOy core/shell structures when exposed to air [9,18]. Due to the Kirkendall effect, the outward diffusion of Fe and counter diffusion of vacancy form the void between core and shell [9,18]. On one hand, the cathodic reaction in the micro cell system containing Fe cores, H + ions in the voids and FexOy shells consumes the metallic Fe cores [19]. On the other hand, the oxide-film acts as a passive layer to lower the corrosion rate of shell in HNO3 [20]. The kinetic difference between oxide passive layers and metal cores allows for the formation of PHNPs. The role of passivation by HNO3 was evidenced by conducting a 0.1 M HCl treatment for 30 min, after which all iron species were washed off, indicating the non-selective etching of HCl. Such a selective etching process is analogous to the separation of passive film of bulk Fe by iodine or HNO3, as reported in the early report by Evans [20]. However, to our best knowledge, it is the first time to report its success in preparing hollow structures on nanoscale. 4. Conclusions In summary, CNT-supported iron oxide PHNPs, with 17 nm diameter and 4 nm shell thickness, were synthesized via a selective HNO 3 etching of Fe@Fe xO y core/shells. The formation mechanism involves the rapid cathodic reaction of Fe cores, and the relatively slow corrosion of iron oxides in nitric acid. The resultant nanocomposites containing iron oxide PHNPs are promising materials in magnetic, biomedical, and catalytic fields. Acknowledgments This work was supported by the National Science Foundation of China (No. 20806027) and the Guangdong Provincial National Science Foundation of China (No. 9251064101000020). References
Fig. 4. Schematic procedure of the formation of iron oxide PHNPs on CNTs.
Fe3O4 (JCPDS 79–0419) can be clearly observed, indicating the formation of Fe@FexOy core/shell structure. The HNO3 treatment led to the diminution of the diffraction peaks from Fe species. However, the reflection from Fe almost totally disappeared, while obvious (220) and (311) reflections of iron oxides can be detected, indicating the selective etching of Fe cores. Previous studies show that the defective oxide layer covering the innermost Fe is either γ-Fe2O3 or Fe3O4 or their mixture [16]. H2-TPR was used to further identify the composition of PHNPs. As shown in Fig. 3, all the samples exhibited three reduction steps, agreeing with the stepwise reduction of iron oxides as: Fe2O3 →Fe3O4 →FeO→Fe [17]. The Fe2O3 →Fe3O4 peak at about 250 °C demonstrated that γ-Fe2O3 exists in the PHNPs. However, the relatively low intensity of the Fe2O3 →Fe3O4 peak suggested the low fraction of Fe2O3 in the PHNPs. It was therefore unlikely that the TPR profiles were caused by pure γ-Fe2O3. Based on above results, we propose that the PHNPs are a mixture of γ-Fe2O3 and Fe3O4.
[1] Caruso F, Caruso RA, Mohwald H. Science 1998;282:1111–4. [2] Dai ZH, Bai HY, Hong M, Zhu YY, Bao JC, Shen J. Biosens Bioelectron 2008;23: 1869–73. [3] Yu XL, Cao CB, Zhu HS, Li QS, Liu CL, Gong QH. Adv Funct Mater 2007;17: 1397–401. [4] Yang M, Ma J, Ding SJ, Meng ZK, Liu JG, Zhao T, et al. Macromol Chem Phys 2006;207:1633–9. [5] Julian-Lopez B, Boissiere C, Chaneac C, Grosso D, Vasseur S, Miraux S, et al. J Mater Chem 2007;17:1563–9. [6] Cao SW, Zhu YJ. J Phys Chem C 2008;112:12149–56. [7] Mori K, Tanimura I, Yamashita H. Bull Chem Soc Jpn 2010;83:1122–6. [8] Wang P, Lo IMC. Water Res 2009;43:3727–34. [9] Khurshid H, Li WF, Tzitzios V, Hadjipanayis GC. Nanotechnology 2011;22:265605. [10] An K, Kwon SG, Park M, Bin Na H, Baik SI, Yu JH, et al. Nano Lett 2008;8:4252–8. [11] Bang JH, Suslick KS. J Am Chem Soc 2007;129:2242–3. [12] Gurmen S, Ebin B. J Alloys Compd 2010;492:585–9. [13] Ali-Boucetta H, Al-Jamal KT, McCarthy D, Prato M, Bianco A, Kostarelos K. Chem Commun 2008:459–61. [14] Usui Y, Aoki K, Narita N, Murakami N, Nakamura I, Nakamura K, et al. Small 2008;4:240–6. [15] Fu XB, Yu H, Peng F, Wang HJ, Qian Y. Appl Catal A-Gen 2007;321:190–7. [16] Wang CM, Baer DR, Amonette JE, Engelhard MH, Antony J, Qiang Y. J Am Chem Soc 2009;131:8824–32. [17] Jozwiak WK, Kaczmarek E, Maniecki TP, Ignaczak W, Maniukiewicz W. Appl Catal A-Gen 2007;326:17–27. [18] Cabot A, Puntes VF, Shevchenko E, Yin Y, Balcells L, Marcus MA, et al. J Am Chem Soc 2007;129:10358–60. [19] Evans UR. Proc R Soc Lond A 1925;107:228–37. [20] Evans UR. J Chem Soc 1927:1020–40.