Silica gel is a synthetic amorphous porous silica(SiO2 ⋅ nH2O), which is of great interest as a highlyactive sorbent or carrier for immobilizing structuresdemonstrating catalytic or sorption activity [1–3]. Itconsists of polymerized silicate particles (globules)bonded to each other in a random manner [4, 5]. Silicagel surface contains a layer of hydroxyl groups of dif�ferent types, with the continuity and uniformity of thelayer being governed by the prehistory of a sample [3,6]. It has been found that, at elevated temperatures,the material is dehydrated to lose the sorption and cat�alytic properties.
The spatial structure of silica gel has been ratherwell characterized by adsorption methods and X�raydiffraction analysis; however, these methods do notenable one to estimate local structural characteristicsof the porous carrier surface. The investigation ofthe thermal aging of silica gels showed that their struc�ture is composed of spherical disperse particles (glob�ules) with sizes of 3–9 nm, as determined by small�angle X�ray scattering [7–9]. Note that, in subsequentworks, the diameters of silica globules (primary parti�cles [10]), which compose silica gel skeleton, wereestimated to be in a wider range of values from 2–10[11] to 3–30 nm [10], which corresponded to silica gelspecific surface areas Ssp of 100–1000 m2/g.
The thermal treatment of silica gels is accompaniedby processes influencing the structure�related chemi�cal characteristics of the material, i.e., dehydrationand dehydroxylation of the surface, removal of intra�globular water, and modification of silica gel structure.
Several hundred studies have been devoted todehydration and dehydroxylation; however, althoughthe common tendency toward a decrease in the con�tent of OH groups on the surface with a rise in the tem�perature of the thermal treatment was noted in all of
the works, the data on their amount and distributionare substantially different. Nevertheless, the majorityof authors are of the opinion that removal of sorbedwater and almost complete dehydroxylation of silicasurface occur upon thermal treatment at T ≤ 600–650°C. It was shown that the content of OH groups,which, for completely hydroxylated silica surfaces ofdifferent origins, amounts to 4.6–6.0 group/nm2 [5,12], diminishes to 1.2–1.3 [5] and 0.4 group/nm2 [13]at 700 and 1000°C, respectively.
The data on variations in the structural characteris�tics of silica gels upon thermal treatment are alsoambiguous. Some authors, who considered predomi�nantly coarse�pore and mesoporous silica gels, believethat, during thermal treatment at T ≤ 700°C, onlyslight changes in Ssp and the total pore volume Vp ofsilica gel take place due to the removal of intraglobularwater [14–16]. At T > 700°C, silica gel undergoesshrinkage; Ssp and Vp noticeably decrease (at T =1090°C Ssp reaches 3 m2/g [17, 18]). Coarse�pore sil�ica gels are most thermostable (structurally stableupon heating to 900°C) [14]. At the same time, whensilica gel is rapidly heated to a high temperature, thereleased water causes a marked catalytic effect, whichnoticeably reduces Ssp of the material [14].
Other authors, who examined predominantlymicroporous carriers, observed a monotonic variationin the structural characteristics of silica gel beginningfrom T > 200°C [5, 13, 19]. However, the mesoporesizes remained almost unchanged up to 700°C [13].When silica was heated in vacuum, the primary spher�ical globules demonstrated “fluidity” (i.e., volume dif�fusion of SiO2 occurred, which led to material sinter�ing). Microporous structure began to disappear whensilica was heated to 700°C [13]. The morphologicalchanges that take place on the silica gel surface duringthermal treatment remain uninvestigated.
Atomic Force Microscopic Study of Variations in the Surface Morphology of Porous Silica upon Thermal Treatment
A. Yu. Shevkina, E. A. Sosnov, and A. A. MalyginSt. Petersburg State Technological Institute (Technical University), Moskovskii pr. 26, St. Petersburg, 190013 Russia
Received February 15, 2011
Abstract—A combination of atomic force microscopy and adsorption methods is used to investigate varia�tions caused by thermal treatment at 200–1100°C in the structure of ShSKG silica gel and its surface mor�phology. It is shown that silica gel is characterized by two hierarchic levels of structural organization. Thestructural characteristics of silica gel begin to vary at a temperature above 800°C. The analytical potential ofatomic force microscopy is illustrated as applied to studying the initial stages of physicochemical transforma�tions of the surface of the coarse�pore silica carrier.
DOI: 10.1134/S1061933X12030106
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The morphology of materials can be studied withnanometric resolution using scanning electronmicroscopy (SEM) and atomic force microscopy(AFM) [20]. SEM has already been used to investigatethe structure of nanomaterials [21–25] (to tell thetruth, the dielectric nature of the examined materialrequires complex methods for sample preparation orspecial experimental procedures that protect from theeffect of sample surface charging [26]). At the sametime, AFM, which is extensively applied in microelec�tronics [27], is rather promising for investigating trans�formations of the porous structure of sorbents underdifferent physicochemical actions [28].
In this work, AFM was used to study local morpho�logical changes occurring on a silica gel surface as aresult of thermal treatment.
EXPERIMENTAL
Coarse�pore silica gel of the ShSKG brand washedfrom iron impurities according to [4] was investigated.Both granulated silica (granule diameter of 4–5 mm)and a silica gel fraction of 0.2–0.4 mm were applied.Samples were thermotreated in a muffle furnace underisothermal conditions at different temperatures in therange of 200– 1100°C for 5 h.
AFM examination of sample surface morphologywas carried out with a Solver P47 Pro scanning probemicroscope (NT�MDT, Russia) in the tapping mode.The sizes of the samples made it possible to mountthem on the microscope stage using the adhesive fixa�tion method [29]. Taking into account that the resto�ration of the hydroxyl cover and the rehydration of cal�cined silica gel surface are very slow processes [30–33], the samples were scanned in nondried air. Theapplication of NSG�01 silicon cantilevers (ResearchInstitute of Physical Problems, Russia) with probe tipdiameters R < 10 nm resulted in lateral and verticalresolutions of ~10 and ~0.2 nm, respectively.
Variations in the structural characteristics of silicagel were investigated by the method of low�tempera�ture nitrogen adsorption. The measurements wereimplemented with a Sorbi N.4.1 sorbtometer (ZAOMeta, Russia). The error in the determination of Sspwas no larger than 3%.
RESULTS AND DISCUSSION
Adsorption experiments have demonstrated(Fig. 1) that Ssp of the material remains almostunchanged to T = 800°C (a slight increase in Ssp of sil�ica by approximately 3% after heating above 200°Cseems to be due to the removal of intraglobular water).At T ≥ 900°C, Ssp drastically decreases to reach, at1100°C, a value of 0.3 m2/g, which agrees with [4, 5,14, 17, 18].
The pore size distribution (Fig. 2) obtained by pro�cessing nitrogen adsorption isotherms suggests theabsence of substantial changes in the porous structure
of silica gel up to 900°C, while, after thermal treat�ment at 1100°C Vp diminishes by more than threeorders of magnitude.
The morphological AFM study of silica gel dehy�drated at 200°C (conditions that provide the removalof physically sorbed water [4, 30, 34–38]) demon�strated that its surface is composed of strongly bondedroundish silica aggregates with sizes of 0.3–0.6 µm(Fig. 3).Enlargement in the scale of the images (Fig. 4,series of images I) reveals a globular structure, espe�cially when the scanning is carried out in the phasecontrast regime according to [39]. Spherical globuleshave sizes of 10–30 nm.
The sizes of individual globules of ShSKG silica gel(10–30 nm), as determined from the AFM data, are ingood agreement with the results of calculations(≈10 nm) based on the theoretical model of silica gel[40]. The larger globule sizes measured experimentallyare explained by the influence of lateral widening dueto deconvolution of the AFM images of the objects[41, 42], which becomes essential at comparable sizesof a measured object and curvature radius R of a probetip (for NSG�01, R ≈ 10 nm [43]). Moreover, somedifference between the real morphology (sphericalglobules united into large aggregates) and model con�cepts (periodic structure composed of spherical glob�ules) can also contribute to the discrepancy betweenthe calculated and experimental sizes of the structuralelements of silica gel.
Silica gel heating at 600°C distorts the structure oflarge aggregates of silica globules (Fig. 4, series ofimages II), which, as a result of removing coordinatelybound water, acquire an irregular structure. The sizesof the associated globules remain almost unchanged(0.2–0.3 µm); however, aggregates are disrupted intosmaller structural fragments with sizes of 30–100 nm.The sizes of the primary globules in them is nearlyequal to those in silica gel calcined at 200°C (10–30 nm). At the same time, it should be noted that,since, for some large fragments of silica structures, dis�
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Fig. 1. Thermal treatment�induced variations in the spe�cific surface area of ShSKG silica gel.
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tinct boundaries of the zones of contact or coalescencebetween the globules are not observed even when scan�ning in the phase contrast regime (Fig. 4, image IIb),degradation of the silica gel surface may already beginat this temperature.
A rise in the thermal treatment temperature to900°C has no effect on the sizes of the primary glob�ules of ShSKG silica gel; however, it induces morpho�logical changes at the level of their aggregates. TheAFM data (Fig. 4, series of images III) show that bothlarger spherical particles with sizes of 50–60 nm andextended agglomerates with sizes as large as 0.5 µm,which partly lose their internal globular structure,appear on the surface. These changes in the morphol�ogy seem to be caused by crystallization of silica gelsurface layers, which starts at a temperature above800°C and is accompanied by a reduction in Ssp and Vpvalues (see Figs. 1, 2). Note that the performed X�raydiffraction analysis of the sample did not reveal SiO2crystallization (Fig. 5, curve 1).
Thermal treatment of silica gel at 1100°C (Fig. 4,series of images IV) is accompanied by a fundamentalchange in the surface relief. Large overlapping forma�tions (with lateral sizes as large as 700 nm) areobserved on the surface rather than the globular struc�tures and their aggregates of different sizes. X�ray dif�fraction analysis (Fig. 5, curve 2) showed that a mix�ture of α�quartz and high�temperature β�crystobalite(spatial groups P3(1)21 and P4(1)2(1)2, respectively)is formed at the aforementioned temperature. More�over, some globular structures with sizes of 40–50 nm,which seem to had not time to crystallize, are observedon the surface.
It should be noted that, after thermal treatment,silica gel granules retain their spherical shape(decreasing the diameter by two to three times) and donot coalesce in the sites of the contacts. The formationof the relief elements and crystallization of the mate�rial are induced by local melting of separate globulesdue to the excess surface energy of the substance
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Fig. 4. Results of detailed AFM scanning of the surface of ShSKG silica gel thermotreated at (I) 200, (II) 600, (III) 900, and(IV) 1100°C: (a) topography, (b) scanning in the phase contrast regime, and (c) surface 3D images. Arrows denote separate SiO2globules.
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occurring in the nanosized state [44, 45], despite thefact that, during the thermal treatment, conditions forformation of a SiO2 melt are not reached.
Thus, comparison of the changes in the surfacemorphology of silica gel during its thermal treatmentleads us to distinguish between two levels of the struc�tural organization of the dispersed silica, i.e., globulesand aggregates thereof. The formation and stability ofthe latter seem to be governed by the prehistory of thesilica gel, namely, the formation of SiO2 micelles dur�ing sol–gel synthesis of porous silica.
Note that, in contrast to microporous silica gels,heating of which to 700°C results in porous structuredegradation caused by the volume diffusion of SiO2[13], in the case under consideration, the porousstructure of silica remains stable up to 900°C, possiblydue to the formation of strong interglobular bonds inthe aggregates [14].
The retention of the shape of individual globules,which is observed during the thermal treatment of sil�ica gel to 900°C even upon an increase in their sizes,suggests partial contact recrystallization of the glob�ules. Formation of a low�temperature SiO2 melt,which was assumed in [18], is not observed duringthermal treatment of ShSKG silica gel. The sinteringof the material, which begins at a temperature higherthan 800°C and is accompanied by a reduction in thespecific surface area, takes place at the level of theglobules. The aggregation of the globules results in theformation of extended monolithic (without internalglobular structure) formations, which remain X�rayamorphous even at 900°C. ShSKG silica gel crystal�lizes only at 1100°C.
CONCLUSIONS
The study of variations in the surface morphologyof ShSKG silica gel upon its thermal treatment showedthe following.
1. Silica gel exhibits two hierarchic levels of thestructural organization (globules and their aggre�gates), the thermal stabilities of which are substantiallydifferent.
2. According to the data of low�temperature nitro�gen adsorption and AFM, ShSKG silica gel retains itsstructural characteristics (pore volume and specificsurface area) and the structure of the surface layerupon heating to temperatures no higher than 800°C.As a result of further elevation of the treatment tem�perature, the aggregates partly lose their globularstructure. The aggregates per se are stable up to thetemperature of nanosized SiO2 crystallization(≈1100°C).
3. AFM enables one to visualize the surface topog�raphy and morphology of high�porous materials withnanometric resolution. Moreover, it makes it possibleto record changes in the material morphology at initialstages of sintering, when these changes cannot berevealed by other physicochemical methods.
ACKNOWLEDGMENTS
This work was supported by the Russian Founda�tion for Basic Research, projects nos. 10�03�00658and 11�03�00397.
403530252015 2θ, deg
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Fig. 5. X�ray diffraction patterns of ShSKG silica gel thermotreated at (1) 900 and (2) 1100°C.
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