- Email: [email protected]
Optimisation of the surface properties of SBA-15 mesoporous silica for in-situ nanoparticle synthesis
Microporous and Mesoporous Materials 120 (2009) 2–6
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Optimisation of the surface properties of SBA-15 mesoporous silica for in-situ nanoparticle synthesis Roland Benoit a,*, F. Warmont a, V. Meynen b, K. De Witte b, P. Cool b, M. Treguer-Delapierre c, M.-L. Saboungi a a
CRMD UMR6619, Université d’Orléans - CNRS, 1B rue de la Férollerie, 45071 Orléans Cedex 2, France Laboratory of Adsorption and Catalysis, Department of Chemistry, University of Antwerpen Campus Drie Eiken, Universiteitsplein 1, B-2610 Wilrijk, Belgium c ICMCB-CNRS, 87 Av. Dr. A. Schweitzer, 33608 Pessac, France b
a r t i c l e
i n f o
Article history: Received 13 June 2008 Received in revised form 12 December 2008 Accepted 16 December 2008 Available online 25 December 2008 Keywords: Mesoporous silica Bismuth nanoparticles Radiolysis XPS TEM
a b s t r a c t Various methods proposed for preparing nanoparticles within porous matrices involve generally the impregnation of the matrix with a solution of precursors followed by reduction based on chemical, thermal, radiolytic, or photochemical process. In most cases, the amount of solution impregnated is small and not uniformly distributed in the matrix, with a yield less than 30% and concentrated around the matrix surface. We have developed a novel method for preparing composite materials, combining impregnation under partial vacuum with radiolytic reduction. This method has enabled us to obtain SBA-15 mesoporous silicas with 50–70% impregnation and to control the nanoparticle size. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Mesoporous materials [1–3] present an interesting alternative for preparing nanoparticles in-situ. They not only make it possible to control the size and the shape of the particles, but also to optimize the volume fraction of nanoparticles while preserving a good mechanical resistance. These materials are both of fundamental interest, since they enable us to study certain metals under confinement, and of technological interest by facilitating the synthesis of arrays with a high density of nanoparticles. We chose to study bismuth, an elemental semi-metal, well known for its thermoelectric properties on a nanometre scale. For the past few years, special attention has been paid to the properties of bismuth in a confined state, because this element is likely to become one of the most important materials in the thermoelectric industry. Many quantum chemistry calculations [4,5] as well as experimental work carried out on materials containing confined bismuth [6–8] have shown, that in the form of nanoparticles, bismuth metal can reach a figure of merit ZT > 5 [9]. However, the extreme susceptibility of this element to oxidation when its size is lower than 50 nm limits experimental work, in particular for the measurement of the Seebeck coefficient. With current methods of synthesis through water-in-oil emulsion and
* Corresponding author. Tel.: +33 238255377; fax: +33 238255376. E-mail address: [email protected] (R. Benoit). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.12.017
vacuum deposition it is not possible to obtain a dense array of nanoparticles that are not oxidized. We chose a ‘‘bottom up” approach because it is simpler to set up and closer to the industrial conditions. As opposed to physical methods, ‘‘bottom up” approaches are based on the aggregation of molecules or semi-metal atoms. They are based on the reduction of a precursory semi-metal salt in the presence of a stabilizing agent [10]. Foos et al. [11] were the first to show that it was possible to prepare bismuth nanoparticles with sizes lower than 10 nm in aqueous solution starting from BiO+ dissolved in inverse micelles. The reduction is carried out in the presence of sodium borohydride, NaBH4. This approach, however, leads to not very satisfactory results: the distribution of the particles is heterogeneous and their stability is low. Henglein and co-workers [12] showed that the reduction of the bismuth salt could be carried out using ionizing radiation. This method makes it possible to prepare monodisperse nanoparticles with sizes lower than 10 nm. We chose this method and used mesoporous silicas as a chemical reactor in order to synthesize bismuth metal nanoparticles insitu. The growth of the nanoparticles by radiolysis has the advantage of generating homogeneous nucleation centres in this mesoporous medium. In order to impregnate the interior of the mesoporous silicas, we used a method of impregnation under partial vacuum. In order to optimize the impregnation process, we studied the concentration of surface silanol as a function of the temperature at which the surfactant was eliminated and the pH of the impregnation solution.
R. Benoit et al. / Microporous and Mesoporous Materials 120 (2009) 2–6
2. Experimental 2.1. Synthesis method 2.2.1. Mesoporous silica substrates The SBA-15 material was prepared by dissolving 4 g of P123 (EO20PO70EO20, MW 5800, Acros Organics) in 127 ml H2O and 20 ml of concentrated HCl. The temperature of the solution was raised to 40 °C and 9.14 ml of TEOS was added. The molar ratios obtained were 1 TEOS: 5.87 HCl: 194 H2O: 0.017 P123. The solution was stirred for 8 min at 40 °C. The short stirring time facilitates the formation of SBA-15 materials with short mesoporous channels. In this way a good diffusion and high loading is obtained [15,16]. After stirring for 8 min, the stirring was stopped and the mixture aged for 24 h at 40 °C under static conditions. The mixture was then transferred to an autoclave and treated hydrothermally for 24 h at 100 °C. Afterwards, the autoclave was quenched and the solution was filtered, washed three times with 20 ml of water and dried in ambient atmosphere. Finally, the material was calcined in ambient atmosphere at 300 °C, 370 °C, 420 °C, 500 °C, 570 °C or 650 °C for duration of 6 h with a heating ramp of 1 °C/ min and then cooled down slowly. 2.2.2. Formation of bismuth nanoparticle The mesoporous solid was impregnated with a precursor consisting of a salt of bismuthyl perchlorate, BiOClO4, dissolved in distilled water at a concentration of 10 4 mol/l and placed under a
saturated pressure using a partial vacuum system. Fig. 1 illustrates the principle of the assembly of the impregnation system. The salt contained in cell 3 is introduced into cell 1 containing the silica. Valve 5 is closed and the mixture is heated with a partial opening of valve 6. The vacuum measured in the enclosure is 10 3 Torr. This system is heated in order to generate a backward flow, which takes place in the liquid and mainly at the liquid–solid interfaces in the evaporation and condensation cycles. The coexisting liquid and vapour phases diffuse through the mesoporous solid through an ‘‘advanced adsorption” process [13,14]. Two mechanisms occur in the process of filling pores between 2 and 12 nm wide: condensation on the pore walls, and diffusion of the menisci between vapour and liquid phases. The samples were then exposed to gamma radiation to generate the bismuth nanoparticles. In order to avoid the oxidation of the nanoparticles by the remaining solution, the material containing the nanoparticles was dried in a secondary vacuum. However, we noticed that the concentration of nanoparticles in the final material was strongly influenced by the impregnation phase. This step is very sensitive to the hydrophilic properties of the silica, which in turn are largely governed by the concentration of hydrophilic OH groups. The silicas were synthesized at the Laboratory for Adsorption and Catalysis of the Department of Chemistry of the University of Antwerp, Belgium. We worked with SBA-15 hexagonally ordered silica with a total pore volume of 0.805 ml/g, a specific surface area (BET) of 670 m2/g, a micropore volume of 0.08 ml/g and a pore diameter of 7.3 nm. 2.3. Characterisation
12 11 3
10
2
6 5
8 9
3
7
1
4 Fig. 1. Schematic diagram of the assembly of the impregnation system. 1: Impregnation cell containing silica, 2: liquid nitrogen trap, 3: -cell containing the initial salt, 4: heater, 5-6-9-10-11: valves, 7 and 8 - system of primary and secondary pumping, 12 - pressure gauge.
Nitrogen adsorption–desorption isotherms were measured at 196 °C on a Quantachrome 1-MP apparatus. The surface area was determined by the BET method. The total pore volume was measured at a relative pressure P/P0 = 0.95. The micropore volume was calculated by the t-plot method. The pore diameter derived from the adsorption branch with the BJH method was 7.3 nm. XPS characterization was carried out with a Thermo-Fisher 250 ESCALAB. The Al anode of a dual-anode X-ray source was used for excitation of the samples. The samples were cooled with liquid nitrogen during data acquisition. The detector consisted of a double-pass cylindrical mirror coupled with a multi-channel analyzer and operated at a 20 eV band-pass energy. The pumping system consisted of an ion pump and a large and small turbo pump; the system base pressure during acquisition was 5.10 10 Torr. The carbon contaminant at 284.6 eV was taken as the energy reference. The irradiations were carried out with a 60Co gamma source with an activity of 2 kGy/h. Samples exposed to this radiation for 1 h received a dose that was constant to within ±2%.
Figs. 2–4. XPS spectra of the initial silica and silica treated at 420 °C and 650 °C.
4
R. Benoit et al. / Microporous and Mesoporous Materials 120 (2009) 2–6
Fig. 5. XPS spectra of the silica treated with hydrochloric acid solutions from pH 2 to 5. Fig. 6. TEM micrograph showing an SBA-15 sample prior to impregnation.
3. Results and discussion 3.1. Optimization of the surface silanol groups XPS (X-ray photoelectron spectroscopy) was used as a tool to optimize the mesoporous silica heat treatment before impregnation. We measured the carbon concentration as a marker of surfactant removal and the silicon Si2p level for silanol concentration. This level is resolved into a 3/2 and 1/2 doublet. We observed two steps. Between 200 °C and 400 °C a reduction of the carbon concentration was observed, amounting only 1 at.% at 400 °C. This is in agreement with reports in the literature [1] of a DTA peak at around 145 °C for the removal of P123 from the SBA-15 framework. In parallel, the silanol concentration increased slightly from 11 to 14 (±1)%. Between 400 °C and 650 °C, the silanol groups disappeared gradually and were totally removed at 650 °C. The XPS spectra of the initial silica and the silica heated to 420 °C and 650 °C are shown in Figs. 2–4, respectively. Below 420 °C, the silanol concentration at the surface is masked by the presence of the surfactant. When the temperature increases, the concentrations of surfactant and silanol both decrease but the fraction of silanol on the silica surface increases. 420 °C represents the ideal temperature for a minimal carbon concentration and a maximum silanol one.
XPS cannot distinguish between isolated and H-bonded silanols but only gives the total amount of OH present. However, a previous study [17] of the silica surface by IR showed that, over a thickness of 10 nm, similar to that probed by XPS, the concentration of Hbonded silanol was constant between 300 °C and 400 °C and then diminished up to 580 °C, while over the same temperature range that of the isolated silanol increased by at least 10%. Following these results, we used silica heated to 420 °C to study the effects of a hydrochloric acid solution on this silica surface chemistry. We observe an increase of the silanol group concentration between pH 1 and 5, as shown in Fig. 5. However, a prolonged impregnation of the silica before irradiation in a solution of pH < 2 can cause it to dissolve. On the other hand, the bismuth salt used for these experiments becomes insoluble when the pH tends towards 5.These results show that the ideal conditions for impregnation of this silica are a temperature treatment close to 400 °C and a pH between 3 and 4. At this temperature all the surfactant molecules are removed and the hydrophilic properties of the silica are optimised. In order to prepare these mesoporous materials in periodic arrays with a high density of nanoparticles, we developed a new method of synthesis using radiolysis. We chose to treat the entire volume of porous material. This implies a reaction in two stages: impregnation of the bismuth salt in
Fig. 7. TEM micrographs showing the effect of heat treatment on the rate of filling the mesoporous silica.
5
R. Benoit et al. / Microporous and Mesoporous Materials 120 (2009) 2–6
Counts
3000
θ°
hkl
27.3
012
2000 θ°
hkl
37.9
104 hkl 110
θ°
hkl
48.7
202
I=100
1000
I=40 500
θ° 39.6
22
25
30
35
I=23
I=41
40
45
50
55
60
65
Angle θ° Fig. 8. Bragg–Brentano h–2h X-ray diffraction pattern for a sample of SBA-15silica containing bismuth metal nanoparticles at T = 400 °C and pH = 4.
the porous matrix, followed by the reduction by the ionizing radiation. The impregnation of the porous solid was carried out under partial vacuum and with backward flow. The samples were then exposed to gamma radiation to generate the bismuth nanoparticles. In order to avoid the oxidation of the nanoparticles by the remaining solution, the material containing the nanoparticles was dried under a secondary vacuum. Fig. 6 shows a transmission electron microscope (TEM) micrograph of an SBA-15 sample, showing an organized pore architecture after thermal treatment at 420 °C and before impregnation with the salt. 3.2. Dense arrays of bismuth metal nanoparticles In order to check the effect of the heat treatment on the quality of the impregnation, we characterized the silicas containing the bismuth nanoparticles by TEM. We found that the densest nanoparticle arrays were obtained in the silicas treated at 420 °C. The density was particularly sensitive to the heat treatment. Indeed, a variation of 50 °C to higher or lower temperatures, considerably affected the rate of filling the silica mesoporous. Fig. 7(a) and (b) show the effects of heat treatment on the rate of filling the mesoporous silica. The nanoparticles are perfectly preserved safe from the air in these mesoporous arrays. The analyses by X-ray diffraction and XPS showed that these nanoparticles remained in metal form over periods of more than one month. The bismuth nanoparticles obtained have a size centred on 4 nm and preserve their rhombohedral crystal structure. Fig. 8 shows an example of a wide-angle diffraction pattern for a sample of SBA-15. The diffraction patterns were compared and annotated with JCPDF file 05-0519. TEM observations showed that silica treated at 420 °C contained nanoparticles in the form of chains. The nanoparticles are juxtaposed and the channels are filled continuously with a large amount of nanoparticles. On the other hand, the nanoparticles are very few and dispersed in silica treated at 570 °C. The quantity of nanoparticles present in these two silicas is proportional to the initial quantity of salt in the silica after impregnation. We noticed that the stability in air of the bismuth nanoparticles contained in the silica was greater with higher nanoparticle concentrations.
4. Conclusion The impregnation under partial vacuum makes it possible to impregnate the core of SBA-15 silicas in less than one hour. Using this method, we can obtain stable networks of nanoparticles in metallic form. However, this process is extremely sensitive to the chemical composition of the surface over a few nm and in particular to the silanol concentration. We noted that a temperature difference of about 50 °C in the heat treatment of the SBA-15 silicas changed the quality of the impregnation significantly. XPS, in contrast with Uv–vis spectrometry or NMR, makes it possible to study these changes on the first atomic layers of a surface in a simple way. We could thus determine the optimal thermal treatment and pH of the impregnation solution to obtain dense arrays of nanoparticles in SBA silica. XPS proves to be an effective tool to optimize this synthesis in different types of silicas. It constitutes a rapid and effective tool for quantifying the chemical state of the silica surface and consequently its absorbent and hydrophilic properties for in-situ synthesis of nanoparticles by impregnation. Mesoporous silica is found to be an effective medium for obtaining a bismuth–metal nanoparticle composite, capable of supporting electronic conduction with a dense array of nanoparticles with at the same time lower thermal conduction due to its intrinsic properties. These two criteria are essential for improving the figure of merit for thermoelectric applications. Acknowledgments This work was supported by INSIDE-PORES NoE project (FP6EU). V. Meynen acknowledges the Fund for Scientific ResearchFlanders. References [1] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [2] G. Lelong, S. Bhattacharyya, S. Kline, T. Cacciaguerra, M.A. Gonzalez, M.-L. Saboungi, J. Phys. Chem. C 112 (2008) 10674. [3] S.A. Bagshaw, E. Prouzet, T.J. Pinnavaia, Science 269 (1995) 1242. [4] S. Golin, Phys. Rev. 166 (1968) 643. [5] H.T. Chu, Phys. Rev. 51 (1995) 5532. [6] Y.-M. Lin, X. Sun, M.S. Dresselhaus, Phys. Rev. B. 62 (2000) 4610.
6
R. Benoit et al. / Microporous and Mesoporous Materials 120 (2009) 2–6
[7] M.S. Dresselhaus, Y.M. Lin, O. Rabin, A. Jorio, A.G. Souza, M.A. Pimenta, R. Saito, G.G. Samsonidze, G. Dresselhaus, Mat. Sci. Eng. C. 23 (2003) 129. [8] M. Hengsberger, P. Segovia, M. Garnier, D. Purdie, Y. Baer, Eur. Phys. J.B. 17 (2000) 603. [9] M.S. Dresselhaus, G. Dresselhaus, X. Sun, Z. Zhang, S.B. Cronin, T. Koga, Phys. Solid State. 41 (1999) 679. [10] R. Benoit, M.-L. Saboungi, M. Treguer-Delapierre, B.H. Milosavljevic, D.J. Meisel, J. Phys. Chem. A. 111 (2007) 10640. [11] E.E. Foos, R.M. Stroud, A.D. Berry, A.W. Snow, J.P. Armistead, J. Am. Chem. Soc. 122 (2000) 7114.
[12] M. Gutierrez, A. Henglein, J. Phys. Chem. 100 (1996) 7656. [13] D.H. Everette, J. Colloid Interf. Sci. 52 (1975). [14] J.M. Esparzaa, A. Ojedaa, A. Camperoa, I. Domíngueza, Kornhausera, F. Rojas, Coll. Surf. A: Physicochem. Eng. Aspects. 241 (2004) 35. [15] A. Sayari, B. Han, Y. Yang, J. Am. Chem. Soc. 126 (2004) 14348. [16] J. Fan, J. Liu, L. Wang, Y. Chengzhong, B. Tu, D. Zhao, Chem. Commun. 17 (2003) 2140. [17] S. Kondo, M. Muroya, K. Funi, Bulletin of the Chemical Society of Japan. 47 (1974) 553.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Optimisation of the surface properties of SBA-15 mesoporous silica for in-situ nanoparticle synthesis Roland Benoit a,*, F. Warmont a, V. Meynen b, K. De Witte b, P. Cool b, M. Treguer-Delapierre c, M.-L. Saboungi a a
CRMD UMR6619, Université d’Orléans - CNRS, 1B rue de la Férollerie, 45071 Orléans Cedex 2, France Laboratory of Adsorption and Catalysis, Department of Chemistry, University of Antwerpen Campus Drie Eiken, Universiteitsplein 1, B-2610 Wilrijk, Belgium c ICMCB-CNRS, 87 Av. Dr. A. Schweitzer, 33608 Pessac, France b
a r t i c l e
i n f o
Article history: Received 13 June 2008 Received in revised form 12 December 2008 Accepted 16 December 2008 Available online 25 December 2008 Keywords: Mesoporous silica Bismuth nanoparticles Radiolysis XPS TEM
a b s t r a c t Various methods proposed for preparing nanoparticles within porous matrices involve generally the impregnation of the matrix with a solution of precursors followed by reduction based on chemical, thermal, radiolytic, or photochemical process. In most cases, the amount of solution impregnated is small and not uniformly distributed in the matrix, with a yield less than 30% and concentrated around the matrix surface. We have developed a novel method for preparing composite materials, combining impregnation under partial vacuum with radiolytic reduction. This method has enabled us to obtain SBA-15 mesoporous silicas with 50–70% impregnation and to control the nanoparticle size. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Mesoporous materials [1–3] present an interesting alternative for preparing nanoparticles in-situ. They not only make it possible to control the size and the shape of the particles, but also to optimize the volume fraction of nanoparticles while preserving a good mechanical resistance. These materials are both of fundamental interest, since they enable us to study certain metals under confinement, and of technological interest by facilitating the synthesis of arrays with a high density of nanoparticles. We chose to study bismuth, an elemental semi-metal, well known for its thermoelectric properties on a nanometre scale. For the past few years, special attention has been paid to the properties of bismuth in a confined state, because this element is likely to become one of the most important materials in the thermoelectric industry. Many quantum chemistry calculations [4,5] as well as experimental work carried out on materials containing confined bismuth [6–8] have shown, that in the form of nanoparticles, bismuth metal can reach a figure of merit ZT > 5 [9]. However, the extreme susceptibility of this element to oxidation when its size is lower than 50 nm limits experimental work, in particular for the measurement of the Seebeck coefficient. With current methods of synthesis through water-in-oil emulsion and
* Corresponding author. Tel.: +33 238255377; fax: +33 238255376. E-mail address: [email protected] (R. Benoit). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.12.017
vacuum deposition it is not possible to obtain a dense array of nanoparticles that are not oxidized. We chose a ‘‘bottom up” approach because it is simpler to set up and closer to the industrial conditions. As opposed to physical methods, ‘‘bottom up” approaches are based on the aggregation of molecules or semi-metal atoms. They are based on the reduction of a precursory semi-metal salt in the presence of a stabilizing agent [10]. Foos et al. [11] were the first to show that it was possible to prepare bismuth nanoparticles with sizes lower than 10 nm in aqueous solution starting from BiO+ dissolved in inverse micelles. The reduction is carried out in the presence of sodium borohydride, NaBH4. This approach, however, leads to not very satisfactory results: the distribution of the particles is heterogeneous and their stability is low. Henglein and co-workers [12] showed that the reduction of the bismuth salt could be carried out using ionizing radiation. This method makes it possible to prepare monodisperse nanoparticles with sizes lower than 10 nm. We chose this method and used mesoporous silicas as a chemical reactor in order to synthesize bismuth metal nanoparticles insitu. The growth of the nanoparticles by radiolysis has the advantage of generating homogeneous nucleation centres in this mesoporous medium. In order to impregnate the interior of the mesoporous silicas, we used a method of impregnation under partial vacuum. In order to optimize the impregnation process, we studied the concentration of surface silanol as a function of the temperature at which the surfactant was eliminated and the pH of the impregnation solution.
R. Benoit et al. / Microporous and Mesoporous Materials 120 (2009) 2–6
2. Experimental 2.1. Synthesis method 2.2.1. Mesoporous silica substrates The SBA-15 material was prepared by dissolving 4 g of P123 (EO20PO70EO20, MW 5800, Acros Organics) in 127 ml H2O and 20 ml of concentrated HCl. The temperature of the solution was raised to 40 °C and 9.14 ml of TEOS was added. The molar ratios obtained were 1 TEOS: 5.87 HCl: 194 H2O: 0.017 P123. The solution was stirred for 8 min at 40 °C. The short stirring time facilitates the formation of SBA-15 materials with short mesoporous channels. In this way a good diffusion and high loading is obtained [15,16]. After stirring for 8 min, the stirring was stopped and the mixture aged for 24 h at 40 °C under static conditions. The mixture was then transferred to an autoclave and treated hydrothermally for 24 h at 100 °C. Afterwards, the autoclave was quenched and the solution was filtered, washed three times with 20 ml of water and dried in ambient atmosphere. Finally, the material was calcined in ambient atmosphere at 300 °C, 370 °C, 420 °C, 500 °C, 570 °C or 650 °C for duration of 6 h with a heating ramp of 1 °C/ min and then cooled down slowly. 2.2.2. Formation of bismuth nanoparticle The mesoporous solid was impregnated with a precursor consisting of a salt of bismuthyl perchlorate, BiOClO4, dissolved in distilled water at a concentration of 10 4 mol/l and placed under a
saturated pressure using a partial vacuum system. Fig. 1 illustrates the principle of the assembly of the impregnation system. The salt contained in cell 3 is introduced into cell 1 containing the silica. Valve 5 is closed and the mixture is heated with a partial opening of valve 6. The vacuum measured in the enclosure is 10 3 Torr. This system is heated in order to generate a backward flow, which takes place in the liquid and mainly at the liquid–solid interfaces in the evaporation and condensation cycles. The coexisting liquid and vapour phases diffuse through the mesoporous solid through an ‘‘advanced adsorption” process [13,14]. Two mechanisms occur in the process of filling pores between 2 and 12 nm wide: condensation on the pore walls, and diffusion of the menisci between vapour and liquid phases. The samples were then exposed to gamma radiation to generate the bismuth nanoparticles. In order to avoid the oxidation of the nanoparticles by the remaining solution, the material containing the nanoparticles was dried in a secondary vacuum. However, we noticed that the concentration of nanoparticles in the final material was strongly influenced by the impregnation phase. This step is very sensitive to the hydrophilic properties of the silica, which in turn are largely governed by the concentration of hydrophilic OH groups. The silicas were synthesized at the Laboratory for Adsorption and Catalysis of the Department of Chemistry of the University of Antwerp, Belgium. We worked with SBA-15 hexagonally ordered silica with a total pore volume of 0.805 ml/g, a specific surface area (BET) of 670 m2/g, a micropore volume of 0.08 ml/g and a pore diameter of 7.3 nm. 2.3. Characterisation
12 11 3
10
2
6 5
8 9
3
7
1
4 Fig. 1. Schematic diagram of the assembly of the impregnation system. 1: Impregnation cell containing silica, 2: liquid nitrogen trap, 3: -cell containing the initial salt, 4: heater, 5-6-9-10-11: valves, 7 and 8 - system of primary and secondary pumping, 12 - pressure gauge.
Nitrogen adsorption–desorption isotherms were measured at 196 °C on a Quantachrome 1-MP apparatus. The surface area was determined by the BET method. The total pore volume was measured at a relative pressure P/P0 = 0.95. The micropore volume was calculated by the t-plot method. The pore diameter derived from the adsorption branch with the BJH method was 7.3 nm. XPS characterization was carried out with a Thermo-Fisher 250 ESCALAB. The Al anode of a dual-anode X-ray source was used for excitation of the samples. The samples were cooled with liquid nitrogen during data acquisition. The detector consisted of a double-pass cylindrical mirror coupled with a multi-channel analyzer and operated at a 20 eV band-pass energy. The pumping system consisted of an ion pump and a large and small turbo pump; the system base pressure during acquisition was 5.10 10 Torr. The carbon contaminant at 284.6 eV was taken as the energy reference. The irradiations were carried out with a 60Co gamma source with an activity of 2 kGy/h. Samples exposed to this radiation for 1 h received a dose that was constant to within ±2%.
Figs. 2–4. XPS spectra of the initial silica and silica treated at 420 °C and 650 °C.
4
R. Benoit et al. / Microporous and Mesoporous Materials 120 (2009) 2–6
Fig. 5. XPS spectra of the silica treated with hydrochloric acid solutions from pH 2 to 5. Fig. 6. TEM micrograph showing an SBA-15 sample prior to impregnation.
3. Results and discussion 3.1. Optimization of the surface silanol groups XPS (X-ray photoelectron spectroscopy) was used as a tool to optimize the mesoporous silica heat treatment before impregnation. We measured the carbon concentration as a marker of surfactant removal and the silicon Si2p level for silanol concentration. This level is resolved into a 3/2 and 1/2 doublet. We observed two steps. Between 200 °C and 400 °C a reduction of the carbon concentration was observed, amounting only 1 at.% at 400 °C. This is in agreement with reports in the literature [1] of a DTA peak at around 145 °C for the removal of P123 from the SBA-15 framework. In parallel, the silanol concentration increased slightly from 11 to 14 (±1)%. Between 400 °C and 650 °C, the silanol groups disappeared gradually and were totally removed at 650 °C. The XPS spectra of the initial silica and the silica heated to 420 °C and 650 °C are shown in Figs. 2–4, respectively. Below 420 °C, the silanol concentration at the surface is masked by the presence of the surfactant. When the temperature increases, the concentrations of surfactant and silanol both decrease but the fraction of silanol on the silica surface increases. 420 °C represents the ideal temperature for a minimal carbon concentration and a maximum silanol one.
XPS cannot distinguish between isolated and H-bonded silanols but only gives the total amount of OH present. However, a previous study [17] of the silica surface by IR showed that, over a thickness of 10 nm, similar to that probed by XPS, the concentration of Hbonded silanol was constant between 300 °C and 400 °C and then diminished up to 580 °C, while over the same temperature range that of the isolated silanol increased by at least 10%. Following these results, we used silica heated to 420 °C to study the effects of a hydrochloric acid solution on this silica surface chemistry. We observe an increase of the silanol group concentration between pH 1 and 5, as shown in Fig. 5. However, a prolonged impregnation of the silica before irradiation in a solution of pH < 2 can cause it to dissolve. On the other hand, the bismuth salt used for these experiments becomes insoluble when the pH tends towards 5.These results show that the ideal conditions for impregnation of this silica are a temperature treatment close to 400 °C and a pH between 3 and 4. At this temperature all the surfactant molecules are removed and the hydrophilic properties of the silica are optimised. In order to prepare these mesoporous materials in periodic arrays with a high density of nanoparticles, we developed a new method of synthesis using radiolysis. We chose to treat the entire volume of porous material. This implies a reaction in two stages: impregnation of the bismuth salt in
Fig. 7. TEM micrographs showing the effect of heat treatment on the rate of filling the mesoporous silica.
5
R. Benoit et al. / Microporous and Mesoporous Materials 120 (2009) 2–6
Counts
3000
θ°
hkl
27.3
012
2000 θ°
hkl
37.9
104 hkl 110
θ°
hkl
48.7
202
I=100
1000
I=40 500
θ° 39.6
22
25
30
35
I=23
I=41
40
45
50
55
60
65
Angle θ° Fig. 8. Bragg–Brentano h–2h X-ray diffraction pattern for a sample of SBA-15silica containing bismuth metal nanoparticles at T = 400 °C and pH = 4.
the porous matrix, followed by the reduction by the ionizing radiation. The impregnation of the porous solid was carried out under partial vacuum and with backward flow. The samples were then exposed to gamma radiation to generate the bismuth nanoparticles. In order to avoid the oxidation of the nanoparticles by the remaining solution, the material containing the nanoparticles was dried under a secondary vacuum. Fig. 6 shows a transmission electron microscope (TEM) micrograph of an SBA-15 sample, showing an organized pore architecture after thermal treatment at 420 °C and before impregnation with the salt. 3.2. Dense arrays of bismuth metal nanoparticles In order to check the effect of the heat treatment on the quality of the impregnation, we characterized the silicas containing the bismuth nanoparticles by TEM. We found that the densest nanoparticle arrays were obtained in the silicas treated at 420 °C. The density was particularly sensitive to the heat treatment. Indeed, a variation of 50 °C to higher or lower temperatures, considerably affected the rate of filling the silica mesoporous. Fig. 7(a) and (b) show the effects of heat treatment on the rate of filling the mesoporous silica. The nanoparticles are perfectly preserved safe from the air in these mesoporous arrays. The analyses by X-ray diffraction and XPS showed that these nanoparticles remained in metal form over periods of more than one month. The bismuth nanoparticles obtained have a size centred on 4 nm and preserve their rhombohedral crystal structure. Fig. 8 shows an example of a wide-angle diffraction pattern for a sample of SBA-15. The diffraction patterns were compared and annotated with JCPDF file 05-0519. TEM observations showed that silica treated at 420 °C contained nanoparticles in the form of chains. The nanoparticles are juxtaposed and the channels are filled continuously with a large amount of nanoparticles. On the other hand, the nanoparticles are very few and dispersed in silica treated at 570 °C. The quantity of nanoparticles present in these two silicas is proportional to the initial quantity of salt in the silica after impregnation. We noticed that the stability in air of the bismuth nanoparticles contained in the silica was greater with higher nanoparticle concentrations.
4. Conclusion The impregnation under partial vacuum makes it possible to impregnate the core of SBA-15 silicas in less than one hour. Using this method, we can obtain stable networks of nanoparticles in metallic form. However, this process is extremely sensitive to the chemical composition of the surface over a few nm and in particular to the silanol concentration. We noted that a temperature difference of about 50 °C in the heat treatment of the SBA-15 silicas changed the quality of the impregnation significantly. XPS, in contrast with Uv–vis spectrometry or NMR, makes it possible to study these changes on the first atomic layers of a surface in a simple way. We could thus determine the optimal thermal treatment and pH of the impregnation solution to obtain dense arrays of nanoparticles in SBA silica. XPS proves to be an effective tool to optimize this synthesis in different types of silicas. It constitutes a rapid and effective tool for quantifying the chemical state of the silica surface and consequently its absorbent and hydrophilic properties for in-situ synthesis of nanoparticles by impregnation. Mesoporous silica is found to be an effective medium for obtaining a bismuth–metal nanoparticle composite, capable of supporting electronic conduction with a dense array of nanoparticles with at the same time lower thermal conduction due to its intrinsic properties. These two criteria are essential for improving the figure of merit for thermoelectric applications. Acknowledgments This work was supported by INSIDE-PORES NoE project (FP6EU). V. Meynen acknowledges the Fund for Scientific ResearchFlanders. References [1] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [2] G. Lelong, S. Bhattacharyya, S. Kline, T. Cacciaguerra, M.A. Gonzalez, M.-L. Saboungi, J. Phys. Chem. C 112 (2008) 10674. [3] S.A. Bagshaw, E. Prouzet, T.J. Pinnavaia, Science 269 (1995) 1242. [4] S. Golin, Phys. Rev. 166 (1968) 643. [5] H.T. Chu, Phys. Rev. 51 (1995) 5532. [6] Y.-M. Lin, X. Sun, M.S. Dresselhaus, Phys. Rev. B. 62 (2000) 4610.
6
R. Benoit et al. / Microporous and Mesoporous Materials 120 (2009) 2–6
[7] M.S. Dresselhaus, Y.M. Lin, O. Rabin, A. Jorio, A.G. Souza, M.A. Pimenta, R. Saito, G.G. Samsonidze, G. Dresselhaus, Mat. Sci. Eng. C. 23 (2003) 129. [8] M. Hengsberger, P. Segovia, M. Garnier, D. Purdie, Y. Baer, Eur. Phys. J.B. 17 (2000) 603. [9] M.S. Dresselhaus, G. Dresselhaus, X. Sun, Z. Zhang, S.B. Cronin, T. Koga, Phys. Solid State. 41 (1999) 679. [10] R. Benoit, M.-L. Saboungi, M. Treguer-Delapierre, B.H. Milosavljevic, D.J. Meisel, J. Phys. Chem. A. 111 (2007) 10640. [11] E.E. Foos, R.M. Stroud, A.D. Berry, A.W. Snow, J.P. Armistead, J. Am. Chem. Soc. 122 (2000) 7114.
[12] M. Gutierrez, A. Henglein, J. Phys. Chem. 100 (1996) 7656. [13] D.H. Everette, J. Colloid Interf. Sci. 52 (1975). [14] J.M. Esparzaa, A. Ojedaa, A. Camperoa, I. Domíngueza, Kornhausera, F. Rojas, Coll. Surf. A: Physicochem. Eng. Aspects. 241 (2004) 35. [15] A. Sayari, B. Han, Y. Yang, J. Am. Chem. Soc. 126 (2004) 14348. [16] J. Fan, J. Liu, L. Wang, Y. Chengzhong, B. Tu, D. Zhao, Chem. Commun. 17 (2003) 2140. [17] S. Kondo, M. Muroya, K. Funi, Bulletin of the Chemical Society of Japan. 47 (1974) 553.