CRYSTAL GROWTH & DESIGN
Gold-Induced Crystallization of SiO2 and TiO2 Powders Nina Perkas, Vilas G. Pol, Swati V. Pol, and Aharon Gedanken* Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center for AdVanced Materials and Nanotechnology, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel
2006 VOL. 6, NO. 1 293-296
ReceiVed January 17, 2005; ReVised Manuscript ReceiVed July 12, 2005
ABSTRACT: Gold nanoparticles with an average size of ∼5 nm have been deposited with the aid of power ultrasound on the surface of silica submicrospheres and on the surface of mesoporous titania. The sonochemical reduction of the gold ions is carried out by ultrasonic irradiation under argon-hydrogen atmosphere. A unique crystallization process of the silica and titania particles has been observed and characterized by X-ray powder diffraction, transmission electron microscopy, differential scanning calorimetry, and BET measurements. The crystallization process of the substrates (silica or titania) is assisted by the gold nanoparticles yielding the cristobalite phase of silica at a relatively low temperature and the anatase phase of titania at the temperature of the sonication bath (80 °C). 1. Introduction This paper reports on two sonochemical experiments in which nanoparticles of gold are deposited on or inserted into a ceramic substrate and cause its crystallization at a relative low temperature. This manuscript will describe the two cases and compare these two effects. The introduction section presents other cases in which this phenomenon has been discovered. Metal-induced crystallization (MIC) of amorphous silicon is well studied in the literature.1-5 The addition of small amounts of metals has been shown to significantly reduce the solid-phase crystallization temperature (to 500 °C). This process is very significant in electronics because the formation of thin films of transistors leads to excellent electrochemical properties.6 Metals, such as Ni2-4 and Au,7,8 have been applied for the induced crystallization (IC) and have been found to be the most effective and well developed. The gold IC of amorphous silicon8 takes place by the interaction of free electrons of Au atoms that rearrange the silicon-silicon bonding, turning it into a crystalline structure. After the thermal process, the polycrystalline silicon is composed of needlelike crystallites forming a packed and continuous structure. The electron diffraction patterns suggest the formation and existence of gold-related compounds and their later disappearance in the intermediate stage of the process. There is a growing interest in the gold IC of ceramic materials such as amorphous silica, titania, and zirconia. This process has been previously achieved in thin films of gold nanoparticles in ceramic films. These composites have been synthesized by the sol-gel process.9,10 After heat treatment at 500 °C, the gold particles have a spherical shape of about 18 nm. They have a typical cubic structure with a lattice constant of Au. The distribution of gold has been found to be homogeneous with a slightly higher concentration toward the surface.9 All the publications in the literature on the subject of MIC have dealt with thin film structures and do not touch upon bulk or powder materials. In the current work, we report for the first time on IC by gold of powders such as nanosized SiO2 and TiO2. The gold nanoparticles are synthesized sonochemically and deposited onto the corresponding oxide surfaces in a one-stage process. Sonochemistry is an alternative technique employed for coating nanoparticles on ceramic surfaces. It is also used for the insertion of nanoparticles into mesoporous materials. Power * To whom correspondence should be addressed. E-mail: gedanken@ mail.biu.ac.il. Fax: +972-3-5351250. Tel: +972-3-5318315.
ultrasound affects chemical changes due to cavitation phenomena involving the formation, growth, and implosive collapse of bubbles in liquid.11 Sonication of the precursor in the presence of an inorganic support such as silica or titania particles provides an alternative means of trapping the nanometer clusters and produces active, supported heterogeneous catalysts. In our recent publication, we have uniformly deposited silver12 nanoparticles on silica spheres and air-stable iron13 nanoparticles on monodispersed carbon spherules; submicron-size particles of titania have been coated by a nanolayer of europium oxide,14 and a nanolayer of MgO15 has been coated on LiMn2O4. The method of deposition/insertion of nanoparticles on/into surfaces of different supports has been developed and described in our previous work,16,17and the advantages of the sonochemical process in forming a homogeneous distribution of the inserted/ deposited nanoparticles have been demonstrated. An early report on the low-temperature crystallization of silica induced by gold nanoparticles has already been published.18 This paper extends this report, finds a similar behavior for the Au/TiO2 composite, and questions the possibility of generalizing this phenomenon of gold-induced crystallization in other amorphous materials. The resulting gold-deposited/inserted silica and titania samples are characterized by X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and BET measurements. 2. Experimental Section The sonochemical deposition of gold nanoparticles on silica spheres is carried out as previously reported.18 The as-prepared Au/SiO2 is heated at 500 °C and at 950 °C under nitrogen for 3 h. The synthesis of Au/TiO2 is carried out by a sonochemical reduction of HAuCl4 under a gas mixture of 95% Ar/5% H2. For the synthesis of Au/TiO2, a solution of HAuCl4 in HCl is dissolved in water or ethylene glycol (EG). Practically, 0.309 g of an aqueous solution of HAuCl4 (17% of Au content) is added to 1 g of TiO2 powder. The amount of reagents correspond to the molar ratio of Au/TiO2 ) 1:47. The pH is adjusted to 6 with NH4OH, followed by addition of a mesoporous (MSP) TiO2 support. The TiO2(MSP) is prepared by the sonochemical method described elsewhere.19 Before the reaction, the mixture is purged with Ar during 1 h for the removal of air traces. The purged solution is sonochemically irradiated under a gas mixture of 95% Ar/5% H2 for 1 h (Ti horn, 20 kHz, 45 W/cm2 at 60% efficiency). The elemental analysis is carried out by energy dispersed X-ray analysis on a JEOL-JSM 840 scanning microscope. The X-ray powder diffraction patterns are obtained using a Bruker D8 diffractometer with
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Figure 1. X-ray diffraction patterns of (a) Au/SiO2 as-prepared, (b) Au/SiO2 heated at 500 °C, 3 h, and (c) Au/SiO2 heated at 950 °C, 3 h. Cu KR radiation. Transmission electron microscopy studies are carried out with a JEOL-JEM 100 microscope. The DSC measurements are made on a Mettler DSC 25. The surface area is measured using a Micromeritics (Gemini 2375) analyzer. The nitrogen adsorption and desorption isotherms are obtained at 77 K after heating the sample at 120 °C for 1 h. The surface area is calculated from the linear part of the BET plot. Pore size distribution is estimated using the BarretJoyner-Halenda (BJH) model with the Halsey equation,20 and the pore volume is measured at the P/Po ) 0.9947 signal point.
3. Results and Discussion 3.1. X-ray Diffraction and Energy-Dispersive X-ray Analysis. The elemental composition of the products is analyzed by energy-dispersive X-ray analysis. The gold content in Au/SiO2 is about 7% (wt). In the Au/TiO2(MSP) sample, the concentration of gold is determined by EDAX as 5 wt %. These values are close to the molar ratio of HAuCl4/SiO2 and HAuCl4/TiO2 in the starting solutions showing that gold is completely incorporated in the support by sonication. The X-ray diffraction patterns of the gold nanoparticles coated on silica spheres are measured to get information about the nature of the product, its crystal structure (the interatomic distance and angle), purity, etc. In Figure 1a, we present the X-ray diffraction pattern of the as-prepared Au/SiO2. The amorphous nature of the product is demonstrated by the absence of any diffraction peaks. The XRD pattern for the heated sample (500 °C, 3 h) is shown in Figure 1b. The peaks at 2θ ) 38.18°, 44.39°, 64.58°, and 77.54° are assigned as (111), (200), (220), and (311) reflection lines corresponding to the face-centered cubic phase metallic gold. The diffraction lines match the literature values of PDF (Powder Diffraction Files) 004-784. No peaks characteristic of impurities are observed. The XRD diffraction patterns of Au/SiO2 annealed at 950 °C under a flow of nitrogen are assigned to two materials (Figure 1c). They are the fcc Au (PDF 004-784) and tetragonal cristobalite (silicon dioxide, PDF 039-1425). The crystallization temperature of amorphous silica forming the crystalline phase has been reported21 as 1300 °C. To check whether the lowtemperature crystallization is due to a size effect or to induced crystallization due to the gold nanoparticles, a bare silica sphere is annealed at 1000 °C in a furnace under a flow of nitrogen. XRD measurements confirm that the silica heated to 1000 °C is amorphous in nature, contrary to the annealed (950 °C) Au/ SiO2, which shows the fcc phase of Au and the tetragonal cristobalite (SiO2). This means that coated gold nanoparticles induce the crystallization of silica at a lower temperature. The
Perkas et al.
Figure 2. X-ray diffraction patterns of (a) TiO2(MSP) as-prepared, (b) TiO2(MSP) after sonolysis 1 h in EG, and (c) Au/TiO2 prepared in EG.
presence of metal disrupts the amorphous network, reducing the kinetic barrier to the crystallization. According to our experimental results, thermal treatment at low-temperature promotes the crystallization of amorphous SiO2, creating the cristobalite phase. This happens only when the surface is coated with nanoparticles of gold. A similar effect of the considerable lowering of the amorphous-to-crystalline transition temperature is observed for amorphous SiO2 and silver22 (∼800 °C). Following the results obtained for the gold-coated silica particles, we have addressed the question of whether only the surface of silica covered with the gold is crystallized or whether the crystallization is induced through the whole silica sphere. To answer this question Au/SiO2 was annealed at 950 °C for 3 and 24 h. The sample heated for 3 h at 950 °C showed only weak diffractions peaks assigned to cristobalite silica, and highly intense diffractions of fcc Au. A drastic increase in the intensity (by a factor of 5) of the cristobalite phase of silica (and the intensity ratio of SiO2/Au) is observed when Au/SiO2 is heated for 24 h. Our interpretation is that a 3 h heating of Au/SiO2 causes only a few surface layers to crystallize. On the other hand, after 24 h of heating the increase in the intensity of the diffraction peaks of the cristobalite phase of silica is perhaps due to the complete crystallization of the silica spheres. A further increase of the cristabolite diffraction peaks is not observed after heating for more than 24 h. To eliminate the possibility that the ultrasonic waves play a role in the crystallization of silica, we also sonicated bare silica in water for more than 3 h. Only amorphous silica spheres have been obtained. In parallel to the induced crystallization in Au/SiO2 spheres, we have also examined the crystallization of gold nanoparticles inserted into mesoporous (MSP) titania. We wanted to check whether this is a general phenomenon and whether gold will induce crystallization on other surfaces as well. As a result of its amorphous nature, no XRD patterns are detected for MSP titania in the wide angle range of 2θ ) 1070° (Figure 2a). On the other hand, the XRD diffraction patterns of the 5% Au/TiO2 (MSP) synthesized in EG clearly show the appearance of the anatase (TiO2) phase as well as the fcc phase of Au (Figure 2c). The diffraction peaks at 2θ ) 25.28°, 37.80°, and 48.05° are assigned to anatase (JCPDS 00-021-1272), and the peaks at 2θ ) 38.18°, 44.39°, 64.58°, and 77.54° are assigned to the reflection of fcc gold (JCPDS 00-004-784). We obtained the anatase phase by heating TiO2(MSP) at 350 °C for 3 h. In the current study, we have noticed that the sonication process for the insertion of gold nanoparticles into MSP titania,
Au-Induced Crystallization of SiO2 and TiO2 Powders
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Figure 3. X-ray diffraction patterns of (a) TiO2(MSP) as-prepared, (b) TiO2(MSP) after sonolysis 1h in water, and (c) Au/TiO2 prepared in water.
which is conducted at 80 °C, yields the anatase phase. This raises the question whether the crystallization of TiO2 is due to the insertion of gold or to the ultrasonic power? At first glance, it seems that the ultrasonic waves are not responsible for the crystallization since the prepared MSP titania is amorphous and is prepared sonochemically. To further clarify this issue, a control experiment has been carried out in which mesoporous titania has been irradiated with ultrasonic waves under the same experimental conditions without the addition of gold. This experiment does not cause the crystallization of TiO2, as evidenced from the XRD measurements (Figure 2b). This presents clear evidence for the gold effect on inducing the crystallization of TiO2. Similar experiments on the synthesis of Au/TiO2(MSP) have been conducted in water. The XRD analysis of Au/TiO2(MSP) prepared in water demonstrates the crystallization of titania. The XRD patterns of both (fcc) gold and the anatase phase of titania have been found in this sample (Figure 3c). However, in a control experiment, when sonicating preprepared TiO2(MSP) in water without the addition of gold, we have observed XRD patterns assigned to anatase (Figure 3b). This means that for the process carried out in water we do not get an unambiguous answer to the previous question. In other words, the crystallization of titania in water might be due the high-temperature reached during the collapse of the bubble and the formation of OH radicals. However, judging from the intensity of the anatase diffraction peaks, it is clear that the gold further contributes to the crystallization, similar to the effect obtained in EG. 3.2. Electron Microscopy Studies. The TEM images of Au/ TiO2(MSP) reveal 150-200 nm size spherical particles of TiO2(MSP) coated with gold nanoparticles of about 5 nm. Since the average pore size in MSP titania is 2.5 nm, it is assumed that the major part of the particles will be anchored on the outer walls of the MSP structure. The micrographs do not reveal the character of the gold nanoparticles inserted into the support, but it is clear that the outer layer is uniform and homogeneous (Figure 4). The TEM measurements do not show a difference in the morphology of the products sonochemically irradiated in EG or in water. 3.3. BET Measurements. According to the BET measurements, after the insertion of the Au into the TiO2(MSP), its surface area has reduced by 3.5-4-fold, both in EG and in water (Table 1). The same result is observed for the cumulative pore volume, which has been reduced by a factor of 2.4. The average pore diameter is also decreased. Thus, the pore volume is partly blocked by the inserted nanoparticles. As mentioned above, not
Figure 4. Transmission electron micrographs (a) TiO2(MSP) and (b) Au/TiO2. Table 1. BET Measurements of Au/TiO2(MSP)
sample information TiO2(MSP) as-prepared TiO2(MSP) after sonolysis 1 h in EG TiO2(MSP) after sonolysis 1 h in water 5% Au/TiO2(MSP) prepared in EG 5% Au/TiO2(MSP) prepared in water
surface area [m2/g]
avg pore diameter [nm]
pore volume [mL/g]
850 750
2.5 2.5
0.47 0.45
272
2.0
0.18
237
2.0
0.20
201
2.0
0.19
all the gold nanoparticles are inserted into the mesopores, and they are deposited on the outer surface layer of TiO2. The BET results are in agreement with the TEM observations. The partition of the gold particles between the mesopores and the outer surface can be easily explained as a result of the size of the gold nanoparticles, which are relatively large (5 nm), in comparison to the average pore diameter of 2.5 nm. This result indicates that only part of gold particles, those of the smallest size, is inserted into the mesopores. It should be mentioned that after sonolysis of TiO2(MSP) without gold in EG and water, different results have been obtained. After the sonolysis in EG, the surface area and the pore volume hardly change. On the other hand, the sonolysis in water causes a sharp decrease in the surface area and the pore structure. The reason for this can be the partial crystallization of titania. This means that during the synthesis of Au/ TiO2 in a water solution, two factors can influence the crystallization process of TiO2, the ultrasonic field and the gold. The reason that ultrasound radiation in water can cause crystallization of TiO2 and not of SiO2 is related to the lower crystallization temperature of titania. 3.4. DSC Studies. The DSC of the various samples is measured because it shows an exothermic peak at 270 °C
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the support layers. The intermixed layer with a high concentration of Au atoms is formed through a multiphase diffusion across the gold support interface. In addition, a chemical interaction between the silica and the gold also assists in the crystallization. This interaction reduces the activation energy of the amorphous to the crystalline transition. The specific interactions are dominated by the atomic size. The distance between the unit cell parameters for gold (a ) b ) c ) 4.0786 Å) and cristobalite (a ) b ) 4.9732 Å) narrows upon heating the gold. The explanation for titania is different and perhaps even more complicated. However, it is clear that defects and pores play an important role in assisting crystallization, and they are also important when discussing the crystallization of TiO2. 4. Conclusions Figure 5. DSC of (a) TiO2(MSP), (b) TiO2(MSP) after sonolysis, and (c) Au/TiO2(MSP).
attributed to the crystallization process of titania. It is therefore expected that for a sample that was completely crystallized by gold or ultrasonic radiation, this peak will almost disappear. On the other hand, for a sample that is not crystallized, a strong exothermic peak will be detected. Figure 5 depicts the results of the DSC measurements of the various samples. The peak at 270 °C, associated with the crystallization of TiO2(MSP), reduces considerably only after the insertion of gold into titania. After 3 h of sonication of the TiO2(MSP) sample in water or in an EG solution, the second exothermic peak, usually observed at about 450 °C, shifts toward a higher temperature (500 °C) (Figure 5b). In the case of gold insertion, this peak almost disappears (Figure 5c). It is clear that the influence of gold on the energetic changes during the crystallization process is stronger than the influence of ultrasonic waves. The enthalpies of the processes related to the exothermic peaks are calculated from the DSC measurements. Their values are 73.6, 66.4, and 12.8 kJ/mol for the as-prepared TiO2(MSP), TiO2(MSP) after sonolysis, and Au/TiO2(MSP), respectively. Measuring the DSC for Au/TiO2(MSP) samples prepared in EG and water solutions, we have observed the similar results. We have also checked the influence of other metals inserted into TiO2(MSP), such as Ru and Ag. The method of their preparation is sonochemistry, and the process is similar to that reported here for Au/TiO2. The only difference is the solvent used in the sonolysis. For the insertion of Ru, it is EG, while for Ag, it is water. No effect has been observed for the crystallization of titania for Ru/TiO2(MSP) and Ag/TiO2(MSP), as has been demonstrated in the case of Au. For Ru/TiO2 and Ag/TiO2, the products obtained by sonochemical irradiation are completely amorphous. Thus, there is no question that gold influences and reduces the crystallization temperature of TiO2, as well as of SiO2. This conclusion is based on the comparison of silver with gold and ruthenium. While the titania sample remained amorphous when deposited by Ag or Ru, it is converted to anatase when gold is deposited on it. The early experiments show that perhaps ultrasound assists in the crystallization process, but it is clear that gold itself plays a role. An attempt to explain the IC by gold will require the evocation of a combination of factors, as otherwise it would be impossible to interpret this unique phenomenon discovered with gold. One of the important factors is the prediction made for the melting point of nanosized gold. Theoretical calculations predicted that a drastic lowering of the gold’s melting point23 will occur below 5 nm particle size. Following this assumption, the nanosized gold melts at a low temperature and diffuses into
We report in this paper on a unique crystallization process of silica particles to their cristobalite phase assisted by the deposition of gold nanoparticles. The crystallization probably starts due to a reduction in the kinetic energy for crystallization. The effect of gold has been also detected in the case of amorphous titania in EG and in a water solution, but in water it is assisted by and combined with the temperature reached by the sonication. Titania has a lower crystallization temperature than silica, and that is why the crystalline phase of anatase can be obtained immediately after the insertion of gold into TiO2. References (1) Yoon, S. Y.; Park, S. J.; Kim, K. H.; Jang, J. Thin Solid Films 2001, 383, 34. (2) Izmajlowicz, M. A. T.; Flewitt, A. J.; Milne, W. I.; Morrison, N. A. J. Appl. Phys. 2003, 94, 7535. (3) Choi, J. H.; Kim, S. S., Park, S. J.; Jang, J. Thin Solid Films 2003, 440, 1. (4) Hu, G. R.; Huang, T. J.; Wu, Y. S. Jpn. J. Appl. Phys., Part 2 2003, 42, L895. (5) Chandra, A.; Clemens, B. M. J. Appl. Phys. 2004, 96, 6776. (6) Joshi, A. R.; Saraswat, K. C. J. Electrochem. Soc. 2003, 150, G443. (7) Ashtikar, M. S.; Sharma, G. L. Jpn. J. Appl. Phys., Part 1 1995, 34, 5520. (8) Andrade, K.; Jang, J.; Moon, B. Y. J. Korean Phys. Soc. 2001, 39, S376. (9) Innocenzi, P.; Brusatin, G.; Martucci, A.; Urabe, K. Thin Solid Films 1996, 279, 23. (10) Epifani M.; Giannini C.; Tapfer L.; Vasanelli L. J. Am. Ceram. Soc. 2000, 83, 2385. (11) Suslick, K. S. Ultrasound: Its Chemical, Physical and Biological Effects; VCH: Weinheim, Germany, 1988. (12) Pol, V. G.; Srivastava, D. N.; Palchik, O.; Palchik, V.; Slifkin, M. A.; Weiss, A. M.; Gedanken, A. Langmuir 2002, 18, 3352. (13) Pol, V. G.; Motiei, M.; Gedanken, A.; Calderon-Moreno, J.; Mastai, Y. Chem. Mater. 2003, 15, 1378. (14) Pol, V. G.; Reisfield, R.; Gedanken, A. Chem. Mater. 2002, 14, 3920. (15) Gnanaraj, J. S.; Pol, V. G.; Gedanken, A.; Aurbach, D. Electrochem. Commun. 2003, 5, 940. (16) Landau, M. V.; Vradman, L.; Herskowitz, M.; Koltypin, Y.; Gedanken, A. J. Catal. 2001, 201, 22. (17) Perkas, N.; Wang, Y.; Koltypin, Y.; Gedanken, A.; Chandrasekaran, S. Chem. Commun. 2001, 12, 988. (18) Wang, Y.; Tang, X.; Yin, L.; Huang, W.; Rosenfeld, Y.; Gedanken, A. AdV. Mater. 2000, 12, 1183. (19) Gregg, S. J.; Sing, K. S. Adsorption Surface Area and Porosity; Academic Press: London, 1982. (20) Sneh, O.; George, S. J. Phys. Chem. 1995, 99, 4639. (21) Garnica-Romo, M. G.; Gonzalez-Hernandez, J.; Hernandez-Landaverde, M. A.; Vorobiev, Y. V.; Ruiz, F.; Martinez, J. R. J. Mater. Res. 2001, 16, 2007. (22) Ercolessi, F.; Andreoni, W.; Tosatti, E. Phys. ReV. Lett. 1991, 66, 911. (23) Shim, J. H.; Lee, B. J.; Cho, Y. W. Surf. Sci. 2002, 512, 262.
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