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SBA-15 by conversion of silver nanoparticles into stable AgCl nanoparticles
Applied Surface Science 265 (2013) 904–911
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Synthesis of photoactive AgCl/SBA-15 by conversion of silver nanoparticles into stable AgCl nanoparticles M. Zienkiewicz-Strzałka ∗ , S. Pikus Department of Crystallography, Faculty of Chemistry, Maria Curie-Skłodowska University, sq. Maria Curie-Skłodowska 3, 20-031 Lublin, Poland
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Article history: Received 22 May 2012 Received in revised form 23 October 2012 Accepted 26 November 2012 Available online 3 December 2012 Keywords: Mesoporous silica SBA-15 Silver chloride Photochemistry Surface plasmon UV–vis
a b s t r a c t In this work the results of synthesis the ordered mesoporous silica (SBA-15) in the presence of stable silver nanoparticles were presented. It has been proven that the proposed method leads to the synthesis of SBA-15 nanocomposite containing silver chloride nanoparticles, formed by the transformation of silver nanoparticles in the acidic conditions. Proposed one-pot procedure is simple and the one requirement is to prepare a stable solution of silver nanoparticles. In this work, silver nanoparticles were obtained during chemical reduction of [Ag(NH3 )2 ]+ ions by formaldehyde. Silver nanoparticles solution can be used as a silver chloride source due to the application of the same polymer as a stabilizer of nanocrystals and structure directing agent of SBA-15. The final AgCl/SBA-15 materials show excellent structural ordering characteristic for this type of materials confirmed by diffraction measurements in range of small angles 2, transmission electron microscopy (TEM) and nitrogen adsorption/desorption measurements. AgCl nanoparticles were identified by diffraction measurements as chlorargyrite phase. The presence of silver nanoparticles in initial solution and their absence after synthesis were confirmed by UV–vis measurements. The photoactivity of obtained AgCl/SBA-15 composite was tested in reaction of organic impurities photodegradation. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Synthesis and applications of materials containing metallic nanoparticles were intensively studied over the last several years. High catalytic activity, some of them, is related to the size of nanoparticles, their shape and surface morphology. In the case of particles with nanometer size on their surface are localized much more atoms (in relation to all atoms in particle), than in clusters with larger dimensions. The important feature of noble metal nanoparticles is also ability to generate a plasmonic oscillations on their surface [1]. Incorporation of metallic nanoparticles in the structure of supports, which are chemically inert in relation to many types of materials and harmless to human, causes that practical applications of these new composites grow significantly. What is more, putting of nanoparticles in the organic and inorganic polymer structure increases their stability and causes that they remain in a state of high dispersion [2,3]. SBA-15 is a material which well fulfills these requirements. In the literature there are many examples of using SBA-15 as a support for metallic nanoparticles [4–10]. Methods of preparation such metallic/silica arrangements include:
∗ Corresponding author. Tel.: +48 0815375641. E-mail address: [email protected] (M. Zienkiewicz-Strzałka). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.145
• Adsorption of the metal ions from solution by solid support and their thermal, chemical or photochemical reduction to the metallic state. • Support wet impregnation of the appropriate ions by filling the pores, evaporation of the solvent and further reduction to metal nanoparticles. • Synthesis of the support with presence of metallic ions. After chemical or thermal reduction, metal ions were transformed to metal nanoparticles. • Simultaneous synthesis of the support and metal precursors during one-pot process. • Deposition of nanoparticles on the support’s surface by spraying solution and condensation of metal vapor in the dispersed solid.
Besides, there is a possibility to use metal nanoparticles as a precursor for new hybrid materials in the inclusion method which constitute synthesis of solid support around the prepared metal nanoparticles (for example synthesis of SBA-15 in the presence of silver nanoparticles). This is a two-stage process where the first step is the synthesis of stable metal nanoparticles solution and in the next step, using them during synthesis of mesoporous support as a solution of metallic precursor. This method allows obtaining materials where support may probably surround metal nanoparticles. It is possible to create metallic-polymer micelles using block copolymers and use this polymer also as a structure directing agent during
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synthesis of the support. The application of the same compounds as a capping and structure directing agent has two general advantages. First of them, the process becomes simply, and secondly there is no necessary to remove stabilizer from nanoparticles solution before synthesis of SBA-15. Other polymers present in synthesis mixture can destroy hexagonal structure of SBA-15, conditioned by Pluronic P123. During synthesis of SBA-15 silver ions were converted to silver chloride particles which constitute photosensitive material widely used in many applications. One of them is the process of removing the organic pollutants from water. The existing methods of organic contaminants removing from water have been effected mostly by adsorption with activated carbons, synthetic resins or other adsorbents [11,12]. This type of solid adsorbents must then be regenerated and adsorbed pollutants should be converts to not harmful products. Solution of this problem can be application of alternative process supported on degradation of organic pollution by illumination during advanced oxidation process (AOPs) [13,14]. During oxidation, active radicals as chemical species with odd number of electrons are produced. These reactive electrophiles with oxidation potential of 2.33 eV react with organic components to form final products of oxidation (CO2 and H2 O). There are many solutions classified as AOPs. They require application of strong oxidants, UV light, ultrasound or electron beam and catalysts like transition metals or photoactive materials (for example TiO2 ) [15]. Another photocatalyst which can be applied in this procedure may constitute silver chloride. Silver chloride is a semiconductor material with indirect band gap of 3.3 eV and does not absorb the light below this value [16]. Also, the presence of clusters at the surface of AgCl is necessary for the absorption of light below the indirect gap value [17]. It is photosensitive material used as a silver chloride electrode which acts as photocatalyst in the presence of a small excess of silver cations in water splitting process [18–21]. Photoactive properties of Ag/AgCl material give rise to utilize it as a plasmonic photocatalyst in decomposition of organic compounds like methylic orange [22,23] or methylene blue [24]. In this work, inclusion method was used to obtain silver chloride nanoparticles incorporated into SBA-15 mesostructure as a result of transformation silver metallic state (silver nanoparticles) under synthesis conditions. We also confirmed the activity of obtained materials in phodtodegradation reaction of organic pollution represented by phenol.
2. Experimental 2.1. Material synthesis The synthesis of AgCl/SBA-15 materials proceeded in two steps. First, stable solution of Ag nanoparticles stabilized by Pluronic P123 was prepared. Ag nanoparticles were synthesized by chemical reduction of [Ag(NH3 )2 ]+ ions by formaldehyde. Diammine silver complex [Ag(NH3 )2 ]+ was prepared as follows: at first 1.25 M sodium hydroxide was added to silver nitrate. Brown precipitate of Ag2 O was received immediately. This precipitate (1 g) was separated from solution and dissolved in concentrated (25%) ammonium hydroxide (2.63 ml). For obtaining silver nanoparticles, 0.9 ml of silver diammine complex was added to the 30 ml of stabilizer (2 g P123/50 ml H2 O) and next 0.25 ml of formaldehyde was dropped to this solution. As a result the green solution of Ag nanoparticles was received (Ag concentration = 0.086 M). The formed Ag nanoparticles were observed as change of color solution from colorless to yellow-green. In the next step, this solution was added to reaction mixture of mesoporous ordered silica. SBA-15 and AgCl/SBA-15 was prepared by a method similar to that reported in literature [25]. In the case of AgCl/SBA-15 composite, 2 g of triblock copolymer Pluronic
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P123 was dissolved with stirring in a 60 ml of HCl solution (2 M), 15 ml deionized H2 O and a certain amounts of silver nanoparticles solution. After 1 h, TEOS (4.56 ml) was dropped to the mixture. The resulting mixture was stirred for 24 h at 35 ◦ C, and then was aged at 95 ◦ C for 48 h under static conditions. The white solid product was recovered by filtration, washed with distilled water and organic components were then removed by calcination at 550 ◦ C in air-atmosphere in muffle furnace for 4 h with 3 ◦ C/min heating rate. 2.2. Evaluation of AgCl/SBA-15 photoactivity To evaluate the catalytic activity of AgCl/SBA-15 materials, process of phenol photodegradation (as a model of organic pollutant) from aqueous solution was performed. The reaction system consists of a mercury vapor lamps (250 W) without fluorescent phosphor coating for emitting of intense ultraviolet radiation. AgCl/SBA-15 material (0.1 g) was added into a 50 ml phenol solution (1 mmol/dm3 ) and protected from light for 24 h to obtain the adsorption equilibrium. The concentration of phenol in initial solution was slightly decreases due to adsorption. The solution was then irradiated by UV–vis-light emitted by mercury vapor lamps for 2 h. During the irradiation, the temperature of the solution was kept at 50 ◦ C without cooling the light source. The concentration of the phenol was determined every 30 min using a UV–vis spectrometry with the use the calibration curve prepared before experiment. The absorbance of phenol was controlled at 270 nm. Also, the experiments without AgCl/SBA-15 and with unmodified SBA-15 materials were performed to confirm that process of phenol photolysis does not occur under the proposed conditions and pure SBA-15 does not exhibit the photocatalytic properties. 2.3. Measurements and calculations The crystalline phases of AgCl/SBA-15 composites were analyzed by X-ray diffraction (XRD) by DRON-3-SEIFERT device with ˚ in the wide range of 2. The low Cu K␣ radiation ( = 1.5418 A) angle XRD patterns of AgCl/SBA-15 were recorded over a 2 range of 0.5–3o . Nitrogen adsorption/desorption measurements were obtained with an ASAP 2405 Micromeritics Inc. USA. The solutions contained silver nanoparticles, solutions after synthesis and phenol solutions were characterized by UV–vis spectrometry (UV–vis Spectrophotometer, SPECORD 200). Transmission electron microscopy images (TEM) were collected on a Tecnai G2 T20 X-TWIN transmission microscope. The size of silver chloride crystallites was determined by X-ray diffraction measurements. The average of crystallites size was calculated through the Scherrer equation by using width of the X-ray lines [26]. SEM/EDX analysis was performed using high resolution Quanta 3D FEG scanning electron microscope (FEI). TEM/EDX measurements were obtained with a high resolution scanning transmission electron microscope Titan G2 60-300 (FEI). 3. Results and discussion The new AgCl/SBA-15 composite can be easily prepared under the proposed procedure. As a result of the synthesis five samples of AgCl/SBA-15 composites with various amount of silver chloride from silver nanoparticles as a precursor were prepared. In the all cases SBA-15 supports exhibit structural ordering according to hexagonal symmetry p6 mm characteristic for this type of materials. Three first and well-resolved reflections registered at small diffraction angles at 2 = 0.9◦ , 1.5◦ and 1.8◦ can be indexed as (1 0 0), (1 1 0) and (2 0 0) reflections of the hexagonal lattice (Fig. 1). The unit cell parameters of SBA-15 calculated from these three peaks are presented in Table 1. Besides, the (1 0 0), (1 1 0) and (2 0 0) peaks
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Table 1 Characteristic data of all synthesized materials and pure SBA-15 as reference. Pore structure parameters of AgCl/SBA-15 derived from the nitrogen adsorption-desorption isotherms and structural parameters determined from the low-angle X-ray diffraction patterns. Sample
SNO1 SNO2 SNO3 SNO4 SNO5 SBA-15 a b c d e f g
Quantities of silver nanoparticles precursor (ml) 1 3 5 8 10 –
SBET (m2 /g)a
VBJH (cm3 /g)b
Smicro (m2 /g)c
Vmicro (cm3 /g)d
DBJH (nm)e
Unit cell parameters of SBA-15 a0 (nm)f
796 791 788 768 759 746
1.00 1.11 1.15 1.18 1.24 0.80
93.3 70.4 47.5 42.6 40.5 174.0
0.04 0.03 0.02 0.02 0.02 0.08
5.9 6.1 6.2 6.3 6.3 4.3
11.4 11.0 10.9 10.2 10.1 11.2
Size of AgCl crystallites (nm)g – – 30.0 29.4 33.0 –
SBET , BET specific surface area. VBJH , BJH total pore volume. Smicro , micropore surface area. Vmicro , micropore volume. DBJH , BJH pore diameter calculated from the desorption branch of isotherms. The unit cell parameter. Average crystallities size estimated from peaks width of AgCl (1 1 1), (2 0 0) and (2 2 0) reflections using Scherrer’s equation.
are shifted slightly to higher angles with the increasing of AgCl content. This indicates that the dimension of unit cell becomes lower with the increasing of silver chloride (Table 1). The wide-angle XRD patterns for AgCl/SBA-15 samples confirmed transformation of silver nanoparticles (silver colloidal solution) to silver chloride particles under acidic conditions required for synthesis of mesoporous SBA-15. In the case of sample SNO3, SNO4 and SNO5 wide range diffraction patterns (Fig. 2) clearly indicate the presence of AgCl nanoparticles in SBA-15 structure. For sample SNO1 amount of silver nanoparticles solution was too small to observe clear AgCl product. For sample SNO2 only very small signal, coming from the most intensity AgCl diffraction peaks (2 0 0) was observed. Silver chloride phase was identified as a chlorargyrite crystallize in cubic crystal system with diffraction peaks indexed as (1 1 1), (2 0 0), (2 2 0) and further (3 1 1), (2 2 0), (4 0 0), (3 1 1), (4 1 0). Additionally, in the case of the same samples the small amount of metallic silver species was observed. X-ray diffraction patterns showed also relation between amount of added precursor and amount of formed AgCl nanoparticles. Fig. 2 shows, that intensity of AgCl peaks grows significantly for samples obtained with larger amounts of silver nanoparticles solution. According to the Scherrer equation, the average crystallite size of
silver chloride was calculated for SNO3–SNO5 samples from diffraction data. Average particle size calculated for three peaks indexed as (1 1 1), (2 0 0) and (2 2 0) were very similar about 30 nm (Table 1). The results confirm that amount of AgCl crystallites was changed in obtained samples rather than their size. Fig. 3 shows the nitrogen adsorption/desorption isotherms and pore size distributions for all synthesized materials. Table 1 summarizes the results of nitrogen adsorption/desorption analyses. All AgCl/SBA-15 composites posses the IV type of isotherms with H1 hysteresis loop in the p/p0 range of 0.65–0.85. The exception is sample SNO1 with the lowest content of silver chloride where the hysteresis loop appears at 0.55 relative pressure. The apparent hysteresis loop between adsorption and desorption confirms the ordered structure of cylindrical mesopores and is due to capillary condensation phenomena in ordered pores. Results confirm the two-dimensional hexagonal structure formed by cylindrical mesopores and our previous XRD observations. The all obtained nanocomposites exhibit good properties described by porous structure parameters. In the all cases they
Fig. 1. The low-angle XRD patterns for AgCl/SBA-15 composites.
Fig. 2. The wide-angle XRD patterns of the AgCl/SBA-15 composites.
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Fig. 3. (A) Nitrogen adsorption/desorption isotherms of AgCl/SBA-15 samples; (B) pore size distribution function obtained according BJH method.
exhibit large pore volume and BET surface area. The BET surface area decreased slightly for samples with higher content of silver chloride from 796 to 759 m2 /g. At the same time, the total pore volume increase from 1.0 to 1.24 cm3 /g with a slight increase of the average pore size from 5.9 to 6.3 nm. Micropore surface area decrease from 93.3 to 40.5 m2 /g for samples with higher content of AgCl. The presence of micropores, their high surface area and shape of isotherms suggest that most of the cylindrical mesopores of the host silica is open and not blocked by existing silver chloride nanoparticles with widely greater size than pore size in support silica. Decreasing of BET surface area and micropores surface area with the increase of AgCl content may suggest that silver chloride nanoparticles may be surrounded by silica lattice. At first, Ag nanoparticles covered by Pluronic P123 were added to reaction solution and next the addition of TEOS takes place. It suggests that there are possibilities of TEOS condensation occurred around AgCl nanoparticles which allowed preserving good properties described by porous structure parameters. The presence of silver nanoparticles in a precursor solution which was added to the reaction mixture of SBA-15 was confirmed by UV–vis technique. This technique is suitable for determination of silver nanoparticles due to signal generating on UV–vis range of light and corresponds to the localized surface plasmon effect. This effect is related to sensitive changes (by absorption) of electromagnetic waves localized on the surface of metallic particles. Moreover, this effect is not generating by solutions of silver chloride or silver oxide. This makes it possible to confirm the
presence (or absence) of silver nanoparticles in the direct measurement. Position of this characteristic signal is related to the size and shape of silver nanoparticles. Therefore, precursor solution of silver nanoparticles was examined by absorbance measurement. Fig. 4A shows the UV–vis absorption spectra of precursor solution. Yellow-green solution of Ag nanoparticles exhibit absorption peak around 420 nm. It should be emphasized, that reaction solutions after addition of silver precursor were also controlled by UV–vis measurement, and absorption peak primarily noticeable for all samples, gradually disappeared during the progress of reaction. This suggests that silver nanoparticles were transformed to silver chloride in stages. As shown on Fig. 4B, the UV–vis measurements were recorded at specified intervals (10, 20, 40 and 50 min after addition of silver precursor). After 50 min metallic silver nanoparticles were converted almost completely into new product. After synthesis, the solutions were measured again and UV–vis data confirmed completely absence of metallic silver nanoparticles (Fig. 4A). Even in the case of sample where the presence of AgCl is not clearly marked in diffraction patterns (samples SNO1 and SNO2), UV–vis measurements confirmed complete disappearance of signal from Ag colloidal solution. TEM analysis results of AgCl/SBA-15 composite (sample SNO5) are show in Fig. 5. The TEM images confirm that the AgCl/SBA-15 composite have a well-ordered mesoporous channels arranged in 2D hexagonal structure according p6 mm symmetry visible in the direction perpendicular (Fig. 5A and B) and parallel to pore axis (Fig. 5C), characteristic for SBA-15 type materials. The distances
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Fig. 4. (A) UV–vis spectra of precursor solution and solutions after synthesis of SBA-15, (B) UV–vis spectra recorded after addition Ag NP to reaction mixture (10, 20, 40 and 50 min).
between two cylindrical mesopores are ∼8.7 nm, the diameter of mesopores is about 6.2 nm and the thickness of walls about 2.5 nm. These results are in agreement with the XRD patterns and N2 adsorption/desorption isotherms. The AgCl crystallites appear as dark objects on silica surface with diameter in range from 20 to 50 nm (Fig. 5D–G). Moreover, many dispersed AgCl crystallites have
a brighter coating (particularly visible in Fig. 5G) which may suggest that some of them are surrounded by silica material. TEM images confirmed that silver chloride nanoparticles are located on the surface of AgCl/SBA-15. EDX elemental analysis should give an answer for the question of their present in the pores of SBA-15. SEM/EDX spectra generated from the area where the
Fig. 5. TEM images of AgCl/SBA-15 (sample SNO5) focused on: (A–C) well ordered and hexagonal organized mesopores of SBA-15, (D–E) AgCl crystallites located on silica surface, (F–G) AgCl crystallites noticed after changes of contrast during TEM imaging. The AgCl crystallites are more distinct but characteristic ordering of mesopores in SBA-15 then are not visible.
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Fig. 6. EDS spectra of the AgCl/SBA-15 sample, (A) EDS spectrum of the AgCl nanoparticles supported on SBA-15, (B) EDS of the area of AgCl/SBA-15 when AgCl nanoparticles were not visible.
silver nanoparticles were clearly visible (Fig. 6A) revels the signals from silver and chloride atoms. SEM/EDX analysis also confirmed their presence in the case of area where silver chloride was not clearly marked (like Fig. 5C). In this case signals from silver and chloride atoms are significantly smaller but still remains distinct (Fig. 6B and C). The precise definition of location of silver chloride in SBA15 support was possible by TEM/EDX analysis. Fig. 7B shows the map of the distribution and relative proportion (intensity) of silver and chloride elements over the investigated area where the high ordered pores of SBA-15 are also indicated (Fig. 7A). Obtained EDX map suggest that some part of silver chloride atoms are located along the pores of SBA-15 and probably positioned in the pores of silica support. Additionally EDX point spectra (from point 1 on Fig. 7B) was registered and presented in Fig. 7C.
4. Photoactivity test The phenol degradation under UV radiation was used as an exploring process to characterize the photoactive properties of
AgCl/SBA-15. In the all cases the phenol solution was irradiated by UV–vis light upon 2 h. Fig. 8 illustrates the photodegradation effects observed as an amendments of absorbance values in the UV spectra of phenol with our AgCl/SBA-15 material and pure SBA-15 as reference. There are presented also results of behavior the phenol solution without mesoporous ordered silica under illumination by UV–vis radiation. The yield changes of degradation rate are compared in Fig. 9. Presented results show, that new AgCl/SBA-15 material exhibit activity upon UV–vis light and photodegradation is an efficient method for the removal of phenol from water solutions. As shown in Fig. 8C, and Fig. 8, small amount of AgCl modifiers in AgCl/SBA-15 is sufficient to assure almost 60% conversion of pollutants during short time of irradiation, what confirms decreasing of absorbance below 0.7 value. Fig. 8A and B illustrates that absorbance decreased marginally for pure phenol solution and for solution with SBA/15 after irradiation. It means that blank test in the absence of AgCl/SBA-15 showed that phenol solution was decomposed in a very slight extend under the conditions proposed in this work
Fig. 7. TEM/EDX results of AgCl/SBA-15 sample, (A) TEM image of AgCl/SBA-15 with the parallel pores, (B) EDX map of silver and chloride atoms with TEM image, (C) EDX spectra from one point of (B) which confirms the presence of silver chloride in investigated samples.
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Fig. 8. UV–vis spectra recorded during performance of phenol photodegradation process. Spectra show the changes of phenol absorbance after UV–vis light irradiation at intervals of every 30 min for (A) phenol solution without mesoporous ordered silicates, (B) phenol solution with pure SBA-15 and (C) phenol solution with photoactive AgCl/SBA-15 composite.
Fig. 9. Comparison of photodegradation efficiency of phenol solutions in the presence of AgCl/SBA-15 composite, pure SBA-15 and solution without mesoporous ordered silica.
Fig. 10. Comparison of X-ray diffraction patterns of AgCl/SBA-15 sample (SNO5) before and after phenol photodegradation.
5. Conclusions (about 5%). The phenol photolysis is here limited and cannot be used as an effective manner for phenol decomposition. It means also that in the presence of SBA-15 in solution no photodegradation progress took place. This suggested that SBA-15 does not exhibit the photoactive properties and phenol conversion depend only on AgCl present in the AgCl/SBA-15 sample. It should be noted that after immersing of AgCl/SBA-15 or pure SBA-15 in phenol solution the phenol concentration was changed by the establishment of the adsorption equilibrium. So that the equilibrium concentration of phenol after adsorption is treated as the as an initial concentration (C0 ), and C is the concentration after UV–vis light irradiation. The XRD pattern of AgCl/SBA-15 sample after irradiations indicates that the ordered structure of support remained unchanged and three first reflections characteristic for SBA-15 are still observed. Additionally, AgCl nanoparticles were reduced to Ag metallic state during interaction with UV light. It is visible in Fig. 10. Most of the AgCl phase after 2 h irradiation was converted to metallic Ag phase (the intensity of peaks for AgCl phase decreased by nearly 70%), causes that further reaction runs less effectively.
In summary, in this study the new composite including AgCl nanoparticles placed on ordered mesoporous silica SBA-15 were synthesized. The procedure proposed in this work is simple and promising for the synthesis of AgCl/SBA-15. The use of SBA-15 as a support has been great interest due to its efficiency no toxicity and inertness for modifiers. The products have been characterized by different methods like XRD, N2 adsorption-desorption isotherm and UV–VIS spectroscopy. It allows confirming that as results of synthesis well defined structure of SBA-15 and small nanoparticles of AgCl were obtained. After photoreaction cycles the structure of support (SBA-15) is still ordered as well as nanocrystals are present. Phenol photodegradation test were carried out to evaluate the activity of the prepared materials upon the UV–vis light. The AgCl/SBA-15 high photoactivity was confirmed in comparing with activity of pure SBA-15 and photolysis of phenol. Used synthesis procedure is very effective because, at first, the same compound (Pluronic P123) as a stabilizer agent for nanoparticles and as structure directing agent was used. It means that Pluronic used as the stabilizer is not waste. The second, great advantages of presented route are possibility to preparation of
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materials with different amounts and various sizes of AgCl crystallites. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. apsusc.2012.11.145. References [1] C. Tabor, R. Murali, M. Mahmoud, M.A. El-Sayed, On the use of plasmonic nanoparticle pairs as a plasmon ruler: the dependence of the near-field dipole plasmon coupling on nanoparticle size and shape, Journal of Physical Chemistry A 113 (2009) 1946–1953. [2] G.Q. Wang, J.F. Gu, S.P. Zhuo, High dispersion Pt nanoparticles using mesoporous carbon support for enhancing conversion efficiency of dye-sensitized solar cells, Chinese Chemical Letters 21 (2010) 1513–1516. [3] I. Sondi, D.V. Goia, E. Matijevic, Preparation of highly concentrated stable dispersions of uniform silver nanoparticles, Journal of Colloid and Interface Science 260 (2003) 75–81. [4] M.H. Huang, A. Choudrey, P. Yang, Ag nanowire formation within mesoporous silica, Chemical Communications 12 (2000) 1063–1064. [5] Z.J. Wang, Y. Xie, C.J. Liu, Synthesis and characterization of noble metal (Pd, Pt Au, Ag) nanostructured materials confined in the channels of mesoporous SBA-15, Journal of Physical Chemistry C 112 (2008) 19818–19824. [6] X.G. Zhao, J.L. Shi, B. Hu, L.X. Zhang, Z.L. Hua, In situ formation of silver nanoparticles inside pore channels of ordered mesoporous silica, Materials Letters 58 (2004) 2152–2156. [7] J. Han, P. Fang, W. Jiang, L. Li, R. Guo, Ag-nanoparticle-loaded mesoporous silica: spontaneous formation of Ag nanoparticles and mesoporous silica SBA15 by a one-pot strategy and their catalytic applications, Langmuir 28 (2012) 4768–4775. [8] I. Yuranov, P. Moeckli, E. Suvorova, P. Buffat, L. Kiwi-Minsker, A. Renken, Pd/SiO2 catalysts: synthesis of Pd nanoparticles with the controlled size in mesoporous silicas, Journal of Molecular Catalysis A: Chemical 192 (2003) 239–251. [9] Y. Zhang, F.L.Y. Lam, X. Hu, Z. Yan, P. Sheng, Fabrication of copper nanowire encapsulated in the pore channels of SBA-15 by metal organic chemical vapor deposition, Journal of Physical Chemistry C 111 (2007) 12536–12541. [10] T. Huang, W. Tu, Modification of functionalized mesoporous silica on the formation and the catalytic performance of platinum nanocatalysts, Applied Surface Science 255 (2009) 7672–7678.
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[11] S.H. Lin, R.S. Juang, Adsorption of phenol and its derivatives from water using synthetic resins and low-cost natural adsorbents: a review, Journal of Environmental Management 90 (2009) 1336–1349. [12] V.C. Srivastava, M.M. Swamy, I.D. Mall, B. Prasad, I.M. Mishra, Adsorptive removal of phenol by bagasse fly ash and activated carbon: equilibrium, kinetics and thermodynamics, Colloids and Surfaces A 272 (2006) 89–104. [13] H. Zhou, D.W. Smith, Advanced technologies in water and wastewater treatment, Journal Environmental Engineering Science 1 (2002) 247–264. [14] S. Esplugas, J. Gimenez, S. Contreras, E. Pascual, M. Rodrıguez, Comparison of different advanced oxidation processes for phenol degradation, Water Research 36 (2002) 1034–1042. [15] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chemical Reviews 95 (1995) 735–758. [16] W. Hayes, I.B. Owen, P.J. Walker, Optically-detected EPR of excitons in AgCl, AgCl:Br and AgBr, Journal of Physics C: Solid State Physics 10 (1977) 1751–1759. [17] C. Hu, T. Peng, X. Hu, Y. Nie, X. Zhou, J. Qu, H. He, Plasmon-induced photodegradation of toxic pollutants with Ag-AgI/Al2 O3 under visible-light irradiation, Journal of the American Chemical Society 132 (2010) 857–862. [18] D. Schurch, A. Currao, S. Sarkar, G. Hodes, G. Calzaferri, The silver chloride photoanode in photoelectrochemical water splitting, Journal of Physical Chemistry B 106 (2002) 12764–12775. [19] M. Lanz, D. Schurch, G. Calzaferri, Photocatalytic oxidation of water to O2 on AgCl-coated electrodes, Journal of Photochemistry and Photobiology A: Chemistry 120 (1999) 105–117. [20] K. Chandrasekaran, J.K. Thomas, The mechanism of the photochemical oxidation of water to oxygen with silver chloride colloids, Chemical Physics Letters 97 (1983) 357–360. [21] C. Lettmann, H. Hinrichs, W.F. Maier, Combinatorial discovery of new photocatalysts for water purification with visible light, Angewandte Chemie International Edition 40 (2001) 3160–3164. [22] P. Wang, B. Huang, X. Qin, X. Zhang, Y. Dai, J. Wei, M.H. Whangbo, Ag@AgCl: a highly efficient and stable photocatalyst active under visible light, Angewandte Chemie International Edition 47 (2008) 7931–7933. [23] P. Wang, B. Huang, Z. Lou, X. Zhang, X. Qin, Y. Dai, Z. Zheng, X. Wang, Synthesis of highly efficient Ag@AgCl plasmonic photocatalysts with various structures, Chemistry – A European Journal 16 (2010) 538–544. [24] C. An, S. Peng, Y. Sun, Facile synthesis of sunlight-driven AgCl:Ag plasmonic nanophotocatalyst, Advanced Materials 22 (2010) 2570–2574. [25] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Frederickson, B.F. Chmelka, G.D. Stucky, Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 Angstrom pores, Science 279 (1998) 548–552. [26] A.L. Patterson, The Scherrer formula for X-ray particle size determination, Physical Review 56 (1939) 978–982.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of photoactive AgCl/SBA-15 by conversion of silver nanoparticles into stable AgCl nanoparticles M. Zienkiewicz-Strzałka ∗ , S. Pikus Department of Crystallography, Faculty of Chemistry, Maria Curie-Skłodowska University, sq. Maria Curie-Skłodowska 3, 20-031 Lublin, Poland
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Article history: Received 22 May 2012 Received in revised form 23 October 2012 Accepted 26 November 2012 Available online 3 December 2012 Keywords: Mesoporous silica SBA-15 Silver chloride Photochemistry Surface plasmon UV–vis
a b s t r a c t In this work the results of synthesis the ordered mesoporous silica (SBA-15) in the presence of stable silver nanoparticles were presented. It has been proven that the proposed method leads to the synthesis of SBA-15 nanocomposite containing silver chloride nanoparticles, formed by the transformation of silver nanoparticles in the acidic conditions. Proposed one-pot procedure is simple and the one requirement is to prepare a stable solution of silver nanoparticles. In this work, silver nanoparticles were obtained during chemical reduction of [Ag(NH3 )2 ]+ ions by formaldehyde. Silver nanoparticles solution can be used as a silver chloride source due to the application of the same polymer as a stabilizer of nanocrystals and structure directing agent of SBA-15. The final AgCl/SBA-15 materials show excellent structural ordering characteristic for this type of materials confirmed by diffraction measurements in range of small angles 2, transmission electron microscopy (TEM) and nitrogen adsorption/desorption measurements. AgCl nanoparticles were identified by diffraction measurements as chlorargyrite phase. The presence of silver nanoparticles in initial solution and their absence after synthesis were confirmed by UV–vis measurements. The photoactivity of obtained AgCl/SBA-15 composite was tested in reaction of organic impurities photodegradation. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Synthesis and applications of materials containing metallic nanoparticles were intensively studied over the last several years. High catalytic activity, some of them, is related to the size of nanoparticles, their shape and surface morphology. In the case of particles with nanometer size on their surface are localized much more atoms (in relation to all atoms in particle), than in clusters with larger dimensions. The important feature of noble metal nanoparticles is also ability to generate a plasmonic oscillations on their surface [1]. Incorporation of metallic nanoparticles in the structure of supports, which are chemically inert in relation to many types of materials and harmless to human, causes that practical applications of these new composites grow significantly. What is more, putting of nanoparticles in the organic and inorganic polymer structure increases their stability and causes that they remain in a state of high dispersion [2,3]. SBA-15 is a material which well fulfills these requirements. In the literature there are many examples of using SBA-15 as a support for metallic nanoparticles [4–10]. Methods of preparation such metallic/silica arrangements include:
∗ Corresponding author. Tel.: +48 0815375641. E-mail address: [email protected] (M. Zienkiewicz-Strzałka). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.145
• Adsorption of the metal ions from solution by solid support and their thermal, chemical or photochemical reduction to the metallic state. • Support wet impregnation of the appropriate ions by filling the pores, evaporation of the solvent and further reduction to metal nanoparticles. • Synthesis of the support with presence of metallic ions. After chemical or thermal reduction, metal ions were transformed to metal nanoparticles. • Simultaneous synthesis of the support and metal precursors during one-pot process. • Deposition of nanoparticles on the support’s surface by spraying solution and condensation of metal vapor in the dispersed solid.
Besides, there is a possibility to use metal nanoparticles as a precursor for new hybrid materials in the inclusion method which constitute synthesis of solid support around the prepared metal nanoparticles (for example synthesis of SBA-15 in the presence of silver nanoparticles). This is a two-stage process where the first step is the synthesis of stable metal nanoparticles solution and in the next step, using them during synthesis of mesoporous support as a solution of metallic precursor. This method allows obtaining materials where support may probably surround metal nanoparticles. It is possible to create metallic-polymer micelles using block copolymers and use this polymer also as a structure directing agent during
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synthesis of the support. The application of the same compounds as a capping and structure directing agent has two general advantages. First of them, the process becomes simply, and secondly there is no necessary to remove stabilizer from nanoparticles solution before synthesis of SBA-15. Other polymers present in synthesis mixture can destroy hexagonal structure of SBA-15, conditioned by Pluronic P123. During synthesis of SBA-15 silver ions were converted to silver chloride particles which constitute photosensitive material widely used in many applications. One of them is the process of removing the organic pollutants from water. The existing methods of organic contaminants removing from water have been effected mostly by adsorption with activated carbons, synthetic resins or other adsorbents [11,12]. This type of solid adsorbents must then be regenerated and adsorbed pollutants should be converts to not harmful products. Solution of this problem can be application of alternative process supported on degradation of organic pollution by illumination during advanced oxidation process (AOPs) [13,14]. During oxidation, active radicals as chemical species with odd number of electrons are produced. These reactive electrophiles with oxidation potential of 2.33 eV react with organic components to form final products of oxidation (CO2 and H2 O). There are many solutions classified as AOPs. They require application of strong oxidants, UV light, ultrasound or electron beam and catalysts like transition metals or photoactive materials (for example TiO2 ) [15]. Another photocatalyst which can be applied in this procedure may constitute silver chloride. Silver chloride is a semiconductor material with indirect band gap of 3.3 eV and does not absorb the light below this value [16]. Also, the presence of clusters at the surface of AgCl is necessary for the absorption of light below the indirect gap value [17]. It is photosensitive material used as a silver chloride electrode which acts as photocatalyst in the presence of a small excess of silver cations in water splitting process [18–21]. Photoactive properties of Ag/AgCl material give rise to utilize it as a plasmonic photocatalyst in decomposition of organic compounds like methylic orange [22,23] or methylene blue [24]. In this work, inclusion method was used to obtain silver chloride nanoparticles incorporated into SBA-15 mesostructure as a result of transformation silver metallic state (silver nanoparticles) under synthesis conditions. We also confirmed the activity of obtained materials in phodtodegradation reaction of organic pollution represented by phenol.
2. Experimental 2.1. Material synthesis The synthesis of AgCl/SBA-15 materials proceeded in two steps. First, stable solution of Ag nanoparticles stabilized by Pluronic P123 was prepared. Ag nanoparticles were synthesized by chemical reduction of [Ag(NH3 )2 ]+ ions by formaldehyde. Diammine silver complex [Ag(NH3 )2 ]+ was prepared as follows: at first 1.25 M sodium hydroxide was added to silver nitrate. Brown precipitate of Ag2 O was received immediately. This precipitate (1 g) was separated from solution and dissolved in concentrated (25%) ammonium hydroxide (2.63 ml). For obtaining silver nanoparticles, 0.9 ml of silver diammine complex was added to the 30 ml of stabilizer (2 g P123/50 ml H2 O) and next 0.25 ml of formaldehyde was dropped to this solution. As a result the green solution of Ag nanoparticles was received (Ag concentration = 0.086 M). The formed Ag nanoparticles were observed as change of color solution from colorless to yellow-green. In the next step, this solution was added to reaction mixture of mesoporous ordered silica. SBA-15 and AgCl/SBA-15 was prepared by a method similar to that reported in literature [25]. In the case of AgCl/SBA-15 composite, 2 g of triblock copolymer Pluronic
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P123 was dissolved with stirring in a 60 ml of HCl solution (2 M), 15 ml deionized H2 O and a certain amounts of silver nanoparticles solution. After 1 h, TEOS (4.56 ml) was dropped to the mixture. The resulting mixture was stirred for 24 h at 35 ◦ C, and then was aged at 95 ◦ C for 48 h under static conditions. The white solid product was recovered by filtration, washed with distilled water and organic components were then removed by calcination at 550 ◦ C in air-atmosphere in muffle furnace for 4 h with 3 ◦ C/min heating rate. 2.2. Evaluation of AgCl/SBA-15 photoactivity To evaluate the catalytic activity of AgCl/SBA-15 materials, process of phenol photodegradation (as a model of organic pollutant) from aqueous solution was performed. The reaction system consists of a mercury vapor lamps (250 W) without fluorescent phosphor coating for emitting of intense ultraviolet radiation. AgCl/SBA-15 material (0.1 g) was added into a 50 ml phenol solution (1 mmol/dm3 ) and protected from light for 24 h to obtain the adsorption equilibrium. The concentration of phenol in initial solution was slightly decreases due to adsorption. The solution was then irradiated by UV–vis-light emitted by mercury vapor lamps for 2 h. During the irradiation, the temperature of the solution was kept at 50 ◦ C without cooling the light source. The concentration of the phenol was determined every 30 min using a UV–vis spectrometry with the use the calibration curve prepared before experiment. The absorbance of phenol was controlled at 270 nm. Also, the experiments without AgCl/SBA-15 and with unmodified SBA-15 materials were performed to confirm that process of phenol photolysis does not occur under the proposed conditions and pure SBA-15 does not exhibit the photocatalytic properties. 2.3. Measurements and calculations The crystalline phases of AgCl/SBA-15 composites were analyzed by X-ray diffraction (XRD) by DRON-3-SEIFERT device with ˚ in the wide range of 2. The low Cu K␣ radiation ( = 1.5418 A) angle XRD patterns of AgCl/SBA-15 were recorded over a 2 range of 0.5–3o . Nitrogen adsorption/desorption measurements were obtained with an ASAP 2405 Micromeritics Inc. USA. The solutions contained silver nanoparticles, solutions after synthesis and phenol solutions were characterized by UV–vis spectrometry (UV–vis Spectrophotometer, SPECORD 200). Transmission electron microscopy images (TEM) were collected on a Tecnai G2 T20 X-TWIN transmission microscope. The size of silver chloride crystallites was determined by X-ray diffraction measurements. The average of crystallites size was calculated through the Scherrer equation by using width of the X-ray lines [26]. SEM/EDX analysis was performed using high resolution Quanta 3D FEG scanning electron microscope (FEI). TEM/EDX measurements were obtained with a high resolution scanning transmission electron microscope Titan G2 60-300 (FEI). 3. Results and discussion The new AgCl/SBA-15 composite can be easily prepared under the proposed procedure. As a result of the synthesis five samples of AgCl/SBA-15 composites with various amount of silver chloride from silver nanoparticles as a precursor were prepared. In the all cases SBA-15 supports exhibit structural ordering according to hexagonal symmetry p6 mm characteristic for this type of materials. Three first and well-resolved reflections registered at small diffraction angles at 2 = 0.9◦ , 1.5◦ and 1.8◦ can be indexed as (1 0 0), (1 1 0) and (2 0 0) reflections of the hexagonal lattice (Fig. 1). The unit cell parameters of SBA-15 calculated from these three peaks are presented in Table 1. Besides, the (1 0 0), (1 1 0) and (2 0 0) peaks
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Table 1 Characteristic data of all synthesized materials and pure SBA-15 as reference. Pore structure parameters of AgCl/SBA-15 derived from the nitrogen adsorption-desorption isotherms and structural parameters determined from the low-angle X-ray diffraction patterns. Sample
SNO1 SNO2 SNO3 SNO4 SNO5 SBA-15 a b c d e f g
Quantities of silver nanoparticles precursor (ml) 1 3 5 8 10 –
SBET (m2 /g)a
VBJH (cm3 /g)b
Smicro (m2 /g)c
Vmicro (cm3 /g)d
DBJH (nm)e
Unit cell parameters of SBA-15 a0 (nm)f
796 791 788 768 759 746
1.00 1.11 1.15 1.18 1.24 0.80
93.3 70.4 47.5 42.6 40.5 174.0
0.04 0.03 0.02 0.02 0.02 0.08
5.9 6.1 6.2 6.3 6.3 4.3
11.4 11.0 10.9 10.2 10.1 11.2
Size of AgCl crystallites (nm)g – – 30.0 29.4 33.0 –
SBET , BET specific surface area. VBJH , BJH total pore volume. Smicro , micropore surface area. Vmicro , micropore volume. DBJH , BJH pore diameter calculated from the desorption branch of isotherms. The unit cell parameter. Average crystallities size estimated from peaks width of AgCl (1 1 1), (2 0 0) and (2 2 0) reflections using Scherrer’s equation.
are shifted slightly to higher angles with the increasing of AgCl content. This indicates that the dimension of unit cell becomes lower with the increasing of silver chloride (Table 1). The wide-angle XRD patterns for AgCl/SBA-15 samples confirmed transformation of silver nanoparticles (silver colloidal solution) to silver chloride particles under acidic conditions required for synthesis of mesoporous SBA-15. In the case of sample SNO3, SNO4 and SNO5 wide range diffraction patterns (Fig. 2) clearly indicate the presence of AgCl nanoparticles in SBA-15 structure. For sample SNO1 amount of silver nanoparticles solution was too small to observe clear AgCl product. For sample SNO2 only very small signal, coming from the most intensity AgCl diffraction peaks (2 0 0) was observed. Silver chloride phase was identified as a chlorargyrite crystallize in cubic crystal system with diffraction peaks indexed as (1 1 1), (2 0 0), (2 2 0) and further (3 1 1), (2 2 0), (4 0 0), (3 1 1), (4 1 0). Additionally, in the case of the same samples the small amount of metallic silver species was observed. X-ray diffraction patterns showed also relation between amount of added precursor and amount of formed AgCl nanoparticles. Fig. 2 shows, that intensity of AgCl peaks grows significantly for samples obtained with larger amounts of silver nanoparticles solution. According to the Scherrer equation, the average crystallite size of
silver chloride was calculated for SNO3–SNO5 samples from diffraction data. Average particle size calculated for three peaks indexed as (1 1 1), (2 0 0) and (2 2 0) were very similar about 30 nm (Table 1). The results confirm that amount of AgCl crystallites was changed in obtained samples rather than their size. Fig. 3 shows the nitrogen adsorption/desorption isotherms and pore size distributions for all synthesized materials. Table 1 summarizes the results of nitrogen adsorption/desorption analyses. All AgCl/SBA-15 composites posses the IV type of isotherms with H1 hysteresis loop in the p/p0 range of 0.65–0.85. The exception is sample SNO1 with the lowest content of silver chloride where the hysteresis loop appears at 0.55 relative pressure. The apparent hysteresis loop between adsorption and desorption confirms the ordered structure of cylindrical mesopores and is due to capillary condensation phenomena in ordered pores. Results confirm the two-dimensional hexagonal structure formed by cylindrical mesopores and our previous XRD observations. The all obtained nanocomposites exhibit good properties described by porous structure parameters. In the all cases they
Fig. 1. The low-angle XRD patterns for AgCl/SBA-15 composites.
Fig. 2. The wide-angle XRD patterns of the AgCl/SBA-15 composites.
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Fig. 3. (A) Nitrogen adsorption/desorption isotherms of AgCl/SBA-15 samples; (B) pore size distribution function obtained according BJH method.
exhibit large pore volume and BET surface area. The BET surface area decreased slightly for samples with higher content of silver chloride from 796 to 759 m2 /g. At the same time, the total pore volume increase from 1.0 to 1.24 cm3 /g with a slight increase of the average pore size from 5.9 to 6.3 nm. Micropore surface area decrease from 93.3 to 40.5 m2 /g for samples with higher content of AgCl. The presence of micropores, their high surface area and shape of isotherms suggest that most of the cylindrical mesopores of the host silica is open and not blocked by existing silver chloride nanoparticles with widely greater size than pore size in support silica. Decreasing of BET surface area and micropores surface area with the increase of AgCl content may suggest that silver chloride nanoparticles may be surrounded by silica lattice. At first, Ag nanoparticles covered by Pluronic P123 were added to reaction solution and next the addition of TEOS takes place. It suggests that there are possibilities of TEOS condensation occurred around AgCl nanoparticles which allowed preserving good properties described by porous structure parameters. The presence of silver nanoparticles in a precursor solution which was added to the reaction mixture of SBA-15 was confirmed by UV–vis technique. This technique is suitable for determination of silver nanoparticles due to signal generating on UV–vis range of light and corresponds to the localized surface plasmon effect. This effect is related to sensitive changes (by absorption) of electromagnetic waves localized on the surface of metallic particles. Moreover, this effect is not generating by solutions of silver chloride or silver oxide. This makes it possible to confirm the
presence (or absence) of silver nanoparticles in the direct measurement. Position of this characteristic signal is related to the size and shape of silver nanoparticles. Therefore, precursor solution of silver nanoparticles was examined by absorbance measurement. Fig. 4A shows the UV–vis absorption spectra of precursor solution. Yellow-green solution of Ag nanoparticles exhibit absorption peak around 420 nm. It should be emphasized, that reaction solutions after addition of silver precursor were also controlled by UV–vis measurement, and absorption peak primarily noticeable for all samples, gradually disappeared during the progress of reaction. This suggests that silver nanoparticles were transformed to silver chloride in stages. As shown on Fig. 4B, the UV–vis measurements were recorded at specified intervals (10, 20, 40 and 50 min after addition of silver precursor). After 50 min metallic silver nanoparticles were converted almost completely into new product. After synthesis, the solutions were measured again and UV–vis data confirmed completely absence of metallic silver nanoparticles (Fig. 4A). Even in the case of sample where the presence of AgCl is not clearly marked in diffraction patterns (samples SNO1 and SNO2), UV–vis measurements confirmed complete disappearance of signal from Ag colloidal solution. TEM analysis results of AgCl/SBA-15 composite (sample SNO5) are show in Fig. 5. The TEM images confirm that the AgCl/SBA-15 composite have a well-ordered mesoporous channels arranged in 2D hexagonal structure according p6 mm symmetry visible in the direction perpendicular (Fig. 5A and B) and parallel to pore axis (Fig. 5C), characteristic for SBA-15 type materials. The distances
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Fig. 4. (A) UV–vis spectra of precursor solution and solutions after synthesis of SBA-15, (B) UV–vis spectra recorded after addition Ag NP to reaction mixture (10, 20, 40 and 50 min).
between two cylindrical mesopores are ∼8.7 nm, the diameter of mesopores is about 6.2 nm and the thickness of walls about 2.5 nm. These results are in agreement with the XRD patterns and N2 adsorption/desorption isotherms. The AgCl crystallites appear as dark objects on silica surface with diameter in range from 20 to 50 nm (Fig. 5D–G). Moreover, many dispersed AgCl crystallites have
a brighter coating (particularly visible in Fig. 5G) which may suggest that some of them are surrounded by silica material. TEM images confirmed that silver chloride nanoparticles are located on the surface of AgCl/SBA-15. EDX elemental analysis should give an answer for the question of their present in the pores of SBA-15. SEM/EDX spectra generated from the area where the
Fig. 5. TEM images of AgCl/SBA-15 (sample SNO5) focused on: (A–C) well ordered and hexagonal organized mesopores of SBA-15, (D–E) AgCl crystallites located on silica surface, (F–G) AgCl crystallites noticed after changes of contrast during TEM imaging. The AgCl crystallites are more distinct but characteristic ordering of mesopores in SBA-15 then are not visible.
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Fig. 6. EDS spectra of the AgCl/SBA-15 sample, (A) EDS spectrum of the AgCl nanoparticles supported on SBA-15, (B) EDS of the area of AgCl/SBA-15 when AgCl nanoparticles were not visible.
silver nanoparticles were clearly visible (Fig. 6A) revels the signals from silver and chloride atoms. SEM/EDX analysis also confirmed their presence in the case of area where silver chloride was not clearly marked (like Fig. 5C). In this case signals from silver and chloride atoms are significantly smaller but still remains distinct (Fig. 6B and C). The precise definition of location of silver chloride in SBA15 support was possible by TEM/EDX analysis. Fig. 7B shows the map of the distribution and relative proportion (intensity) of silver and chloride elements over the investigated area where the high ordered pores of SBA-15 are also indicated (Fig. 7A). Obtained EDX map suggest that some part of silver chloride atoms are located along the pores of SBA-15 and probably positioned in the pores of silica support. Additionally EDX point spectra (from point 1 on Fig. 7B) was registered and presented in Fig. 7C.
4. Photoactivity test The phenol degradation under UV radiation was used as an exploring process to characterize the photoactive properties of
AgCl/SBA-15. In the all cases the phenol solution was irradiated by UV–vis light upon 2 h. Fig. 8 illustrates the photodegradation effects observed as an amendments of absorbance values in the UV spectra of phenol with our AgCl/SBA-15 material and pure SBA-15 as reference. There are presented also results of behavior the phenol solution without mesoporous ordered silica under illumination by UV–vis radiation. The yield changes of degradation rate are compared in Fig. 9. Presented results show, that new AgCl/SBA-15 material exhibit activity upon UV–vis light and photodegradation is an efficient method for the removal of phenol from water solutions. As shown in Fig. 8C, and Fig. 8, small amount of AgCl modifiers in AgCl/SBA-15 is sufficient to assure almost 60% conversion of pollutants during short time of irradiation, what confirms decreasing of absorbance below 0.7 value. Fig. 8A and B illustrates that absorbance decreased marginally for pure phenol solution and for solution with SBA/15 after irradiation. It means that blank test in the absence of AgCl/SBA-15 showed that phenol solution was decomposed in a very slight extend under the conditions proposed in this work
Fig. 7. TEM/EDX results of AgCl/SBA-15 sample, (A) TEM image of AgCl/SBA-15 with the parallel pores, (B) EDX map of silver and chloride atoms with TEM image, (C) EDX spectra from one point of (B) which confirms the presence of silver chloride in investigated samples.
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Fig. 8. UV–vis spectra recorded during performance of phenol photodegradation process. Spectra show the changes of phenol absorbance after UV–vis light irradiation at intervals of every 30 min for (A) phenol solution without mesoporous ordered silicates, (B) phenol solution with pure SBA-15 and (C) phenol solution with photoactive AgCl/SBA-15 composite.
Fig. 9. Comparison of photodegradation efficiency of phenol solutions in the presence of AgCl/SBA-15 composite, pure SBA-15 and solution without mesoporous ordered silica.
Fig. 10. Comparison of X-ray diffraction patterns of AgCl/SBA-15 sample (SNO5) before and after phenol photodegradation.
5. Conclusions (about 5%). The phenol photolysis is here limited and cannot be used as an effective manner for phenol decomposition. It means also that in the presence of SBA-15 in solution no photodegradation progress took place. This suggested that SBA-15 does not exhibit the photoactive properties and phenol conversion depend only on AgCl present in the AgCl/SBA-15 sample. It should be noted that after immersing of AgCl/SBA-15 or pure SBA-15 in phenol solution the phenol concentration was changed by the establishment of the adsorption equilibrium. So that the equilibrium concentration of phenol after adsorption is treated as the as an initial concentration (C0 ), and C is the concentration after UV–vis light irradiation. The XRD pattern of AgCl/SBA-15 sample after irradiations indicates that the ordered structure of support remained unchanged and three first reflections characteristic for SBA-15 are still observed. Additionally, AgCl nanoparticles were reduced to Ag metallic state during interaction with UV light. It is visible in Fig. 10. Most of the AgCl phase after 2 h irradiation was converted to metallic Ag phase (the intensity of peaks for AgCl phase decreased by nearly 70%), causes that further reaction runs less effectively.
In summary, in this study the new composite including AgCl nanoparticles placed on ordered mesoporous silica SBA-15 were synthesized. The procedure proposed in this work is simple and promising for the synthesis of AgCl/SBA-15. The use of SBA-15 as a support has been great interest due to its efficiency no toxicity and inertness for modifiers. The products have been characterized by different methods like XRD, N2 adsorption-desorption isotherm and UV–VIS spectroscopy. It allows confirming that as results of synthesis well defined structure of SBA-15 and small nanoparticles of AgCl were obtained. After photoreaction cycles the structure of support (SBA-15) is still ordered as well as nanocrystals are present. Phenol photodegradation test were carried out to evaluate the activity of the prepared materials upon the UV–vis light. The AgCl/SBA-15 high photoactivity was confirmed in comparing with activity of pure SBA-15 and photolysis of phenol. Used synthesis procedure is very effective because, at first, the same compound (Pluronic P123) as a stabilizer agent for nanoparticles and as structure directing agent was used. It means that Pluronic used as the stabilizer is not waste. The second, great advantages of presented route are possibility to preparation of
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materials with different amounts and various sizes of AgCl crystallites. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. apsusc.2012.11.145. References [1] C. Tabor, R. Murali, M. Mahmoud, M.A. El-Sayed, On the use of plasmonic nanoparticle pairs as a plasmon ruler: the dependence of the near-field dipole plasmon coupling on nanoparticle size and shape, Journal of Physical Chemistry A 113 (2009) 1946–1953. [2] G.Q. Wang, J.F. Gu, S.P. Zhuo, High dispersion Pt nanoparticles using mesoporous carbon support for enhancing conversion efficiency of dye-sensitized solar cells, Chinese Chemical Letters 21 (2010) 1513–1516. [3] I. Sondi, D.V. Goia, E. Matijevic, Preparation of highly concentrated stable dispersions of uniform silver nanoparticles, Journal of Colloid and Interface Science 260 (2003) 75–81. [4] M.H. Huang, A. Choudrey, P. Yang, Ag nanowire formation within mesoporous silica, Chemical Communications 12 (2000) 1063–1064. [5] Z.J. Wang, Y. Xie, C.J. Liu, Synthesis and characterization of noble metal (Pd, Pt Au, Ag) nanostructured materials confined in the channels of mesoporous SBA-15, Journal of Physical Chemistry C 112 (2008) 19818–19824. [6] X.G. Zhao, J.L. Shi, B. Hu, L.X. Zhang, Z.L. Hua, In situ formation of silver nanoparticles inside pore channels of ordered mesoporous silica, Materials Letters 58 (2004) 2152–2156. [7] J. Han, P. Fang, W. Jiang, L. Li, R. Guo, Ag-nanoparticle-loaded mesoporous silica: spontaneous formation of Ag nanoparticles and mesoporous silica SBA15 by a one-pot strategy and their catalytic applications, Langmuir 28 (2012) 4768–4775. [8] I. Yuranov, P. Moeckli, E. Suvorova, P. Buffat, L. Kiwi-Minsker, A. Renken, Pd/SiO2 catalysts: synthesis of Pd nanoparticles with the controlled size in mesoporous silicas, Journal of Molecular Catalysis A: Chemical 192 (2003) 239–251. [9] Y. Zhang, F.L.Y. Lam, X. Hu, Z. Yan, P. Sheng, Fabrication of copper nanowire encapsulated in the pore channels of SBA-15 by metal organic chemical vapor deposition, Journal of Physical Chemistry C 111 (2007) 12536–12541. [10] T. Huang, W. Tu, Modification of functionalized mesoporous silica on the formation and the catalytic performance of platinum nanocatalysts, Applied Surface Science 255 (2009) 7672–7678.
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[11] S.H. Lin, R.S. Juang, Adsorption of phenol and its derivatives from water using synthetic resins and low-cost natural adsorbents: a review, Journal of Environmental Management 90 (2009) 1336–1349. [12] V.C. Srivastava, M.M. Swamy, I.D. Mall, B. Prasad, I.M. Mishra, Adsorptive removal of phenol by bagasse fly ash and activated carbon: equilibrium, kinetics and thermodynamics, Colloids and Surfaces A 272 (2006) 89–104. [13] H. Zhou, D.W. Smith, Advanced technologies in water and wastewater treatment, Journal Environmental Engineering Science 1 (2002) 247–264. [14] S. Esplugas, J. Gimenez, S. Contreras, E. Pascual, M. Rodrıguez, Comparison of different advanced oxidation processes for phenol degradation, Water Research 36 (2002) 1034–1042. [15] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chemical Reviews 95 (1995) 735–758. [16] W. Hayes, I.B. Owen, P.J. Walker, Optically-detected EPR of excitons in AgCl, AgCl:Br and AgBr, Journal of Physics C: Solid State Physics 10 (1977) 1751–1759. [17] C. Hu, T. Peng, X. Hu, Y. Nie, X. Zhou, J. Qu, H. He, Plasmon-induced photodegradation of toxic pollutants with Ag-AgI/Al2 O3 under visible-light irradiation, Journal of the American Chemical Society 132 (2010) 857–862. [18] D. Schurch, A. Currao, S. Sarkar, G. Hodes, G. Calzaferri, The silver chloride photoanode in photoelectrochemical water splitting, Journal of Physical Chemistry B 106 (2002) 12764–12775. [19] M. Lanz, D. Schurch, G. Calzaferri, Photocatalytic oxidation of water to O2 on AgCl-coated electrodes, Journal of Photochemistry and Photobiology A: Chemistry 120 (1999) 105–117. [20] K. Chandrasekaran, J.K. Thomas, The mechanism of the photochemical oxidation of water to oxygen with silver chloride colloids, Chemical Physics Letters 97 (1983) 357–360. [21] C. Lettmann, H. Hinrichs, W.F. Maier, Combinatorial discovery of new photocatalysts for water purification with visible light, Angewandte Chemie International Edition 40 (2001) 3160–3164. [22] P. Wang, B. Huang, X. Qin, X. Zhang, Y. Dai, J. Wei, M.H. Whangbo, Ag@AgCl: a highly efficient and stable photocatalyst active under visible light, Angewandte Chemie International Edition 47 (2008) 7931–7933. [23] P. Wang, B. Huang, Z. Lou, X. Zhang, X. Qin, Y. Dai, Z. Zheng, X. Wang, Synthesis of highly efficient Ag@AgCl plasmonic photocatalysts with various structures, Chemistry – A European Journal 16 (2010) 538–544. [24] C. An, S. Peng, Y. Sun, Facile synthesis of sunlight-driven AgCl:Ag plasmonic nanophotocatalyst, Advanced Materials 22 (2010) 2570–2574. [25] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Frederickson, B.F. Chmelka, G.D. Stucky, Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 Angstrom pores, Science 279 (1998) 548–552. [26] A.L. Patterson, The Scherrer formula for X-ray particle size determination, Physical Review 56 (1939) 978–982.