Sensors and Actuators B 24-25 (1995) 347-352
Pore size and pore-size distribution control of porous silica Yao Xi, Zhang Liangying, Wang Sasa Ehxtmnic Material
Research Laboratory, Xi’an Jimtong
Universi!y, Xi’an 710049, China
Abstract Porous silica is a unique matrix material for chemical sensors. In this paper, porous silica is prepared via the sol-gel technique. The porosity, pore size, pore shape and pore-size distribution can be controlled by catalysts and preparation cunditions. A porosity as high as 60-70% can be reached. The pore size can be controlled in the range 2-100 nm. A single narrow peak and double separate peak distributions can be achieved by controlling the pH values. The pore shape can be controlled from narrow-necked and large-abdomened shapes to fine cylindrical pores. Applications of porous silica for chemical sensors are also discussed in this paper. Kqwm& Porous silica
control the pore shape; (6) to discuss the ways of using porous silica for chemical-sensor applications.
The sensing of a chemical environment is achieved mainly in the surface interactions of the sensor material with its chemical surroundings. Therefore, a porous structure is of great importance in developing good chemical sensors. What we need for a high-quality ceramic chemical sensor is a well-sintered ceramic skeleton with high porosity and controllable pore size. Porous silica is a unique matrix material for chemical sensors, combining a lot of excellent physical and chemical properties, such as very high chemical stability, very high thermal stability, very low thermal expansion, good thermal shock resistance, superior aging behaviour, very high chemical inertness, which makes it possible for a wide range of materials to be accommodated, and very high transparency in a wide wavelength range from ultraviolet to infrared. The porous structure of porous silica also has many advantages for chemical-sensor applications, such as the very large surface area, which can enhance the interfacial chemical reaction and thus improve the sensitivity, the various permeabilities of chemical species, which can improve the selectivity, the high thermal shock resistance, which can enhance the reliability, etc. In this paper the way to prepare porous silica via the sol-gel route is presented. The objectives of this work are: (1) to prepare porous silica; (2) to analyse the pore structure; (3) to control the pore size of porous silica; (4) to control the pore-size distribution; (5) to 0925-4005195/$09.50 8 1995 Elsevier SSDI 0925-4005(94)01496-5
2. Preparation of porous silica via the sol-gel route Analytical pure reagent TEOS (tetraethoxysilane) was used to prepare the porous silica gel glass. TEOS was thoroughly mixed with alcohol and water. Various alkaline or acidic catalysts, such as NH,OH, HCl, HNO, and HF, were used to control the hydrolysis and polymerization of the TEOS to form a sol solution. The drying process of the wet gel must be very careful and slow to prevent cracking of the sample due to the very large shrinkage during drying. The dried gel was then heat-treated at 500-1000 “C to remove residual water, alcohol and all organic radicals, to form the final silica gel glass. The typical process is given in Fig. 1.
3. Pore structure control The hydrolysis and condensation reaction in the sol-gel process is very complicated. By controlling the process conditions (such as the pH, the water/TEOS and alcohol,KEOS ratios, gelation temperature, heattreatment temperature) and catalysis, the reaction rates, and thus the porosity, pore size, pore-size distribution and pore shape, can be controlled. The effect of the catalysts on the pore size and its distribution is very strong. Various alkaline and acidic catalysts such as
X. Yao et al. I Sensors and Actuators B 24-25 (1995) 347-352
Stirring 30 min drying for Z-30 days stiff gel fired at 5cKl-8OOC porous silica glass Fig. 1. Flow chart of preparing porous silica glass.
a higher amount of water tends to form very dense spheres with large interstitial pores between them, resulting in the high porosity. 3.1.2. Ammonia (OH-) content Porosity can also be controlled by varying the OHcontent. A higher ammonia concentration results in low porosity. The presence of more OH- groups possibly accelerates the nucleophilic reaction and the number of micropores decreased greatly, thus the porosity decreased. 3.2. Pore-size control The pore size can be controlled in the range 2-100 run by the catalysts, pH values or TEOS/alcohol ratio.
The porosity can be controlled mainly by varying the waterjTEOS ratio and the OH- content.
3.2.1. Catalysts The effect of catalysts on the pore size is very strong. Alkaline and acidic catalysts have different effects on the average pore size due to the different catalytic mechanism. The pore-size control also depends on the acid and pH value. Porous silica with a large pore size can be obtained by ammonia catalysis. A higher ammonia content will result in a larger pore size. Micropores with a diameter less than 10 mn can be obtained by HCl catalysis. The average pore size increases when the amount of HCI increases. Usually, the maximum pore diameter of the HCl-catalysed porous silica is less than 4 nm, and the average pore diameter can shift in the range 2-3 nm by changing the HCl amount, but the pore shape cannot be changed in this way. The fluorine anion catalysts are more effective in forming large pores than the HCl. Porous silica with an average pore diameter in the wide range 4-100 nm can be achieved by using HF or NH,F as catalysts, Both the average pore size and pore volume increase when the amount of fluorine anion increases, as shown in Figs. 3 and 4. The maximum pore diameter is about 100 nm when the HFD’EOS mole ratio is 0.1 (Fig. 3(d)). This indicates that HF is the most effective catalyst in forming large pores compared with NaF and NH,F.
3.1.1. WaterlTEOS ratio The change of porosity via the waterD’EOS ratio is illustrated in Fig. 2. At low water/TEOS ratios, the porosity is as low as about 7%, and it increases sharply when the amount of water increases. A porosity as high as 70% can be reached when the water/TEOS molar ratio is between four and eight. Isotherm plot analysis shows that the pore shape cannot be changed by the water/TEOS ratio. This may be due to the acceleration of the gelation process with a higher amount of water. TEM graphs of the gel structure show that
3.2.2. EthanollTEOS The effect of the ethanoUPEOS mole ratio on pore size is not as strong as that of the fluoride anion, but is greater than that of HCI. As shown in Fig. 5, the average pore size decreases by 30 nm when the amount of ethanol increases. Higher ethanol content retards the gelation process significantly; ethanol is a by-product in both hydrolysis and condensation reactions. Excess alcohol may separate the molecular species formed and hinder the progress of cross-linkage. This is in agreement with the work done by Yoldas [l].
Fig. 2. Effect of watenTEOS
ratio on porosity of silica glass.
N&OH, HCl, HNO,, HF and fluorides such as NH,F, NaF have been used. The catalytic mechanisms of different catalysts are quite different, which results in different pore size, pore shape and pore-size distribution. Using HF as a catalyst, the largest pore size around 100 nm with a very narrow distribution can be achieved. 3.1. Porosity control
X. Yao et al. I Sensors and Actuators B 24-25 (1995) 347-352
peak and double separate peak distributions achieved.
3.3.1. WateriTEOS ratio As shown in Fig. 6, by changing the waterPEOS ratio, the pore-size distribution cn be controlled without affecting the pore size significantly. A higher amount of water accelerates the gelation process, thus increasing the hydrolysis ratio.
Pore Diameter(A) Fig. 3. Effect of HF on pore size.
6.0 tc N$FmEOS n-0.001 b=o.Ol C-O.05
PoreDiameter(A) Fig. 4. Effect of WF
on pore size.
-_ b C&OHtTEOS
3.3.2. Geiation temperature control Samples were dried at 20 and 60 “C respectively before being fired at the same temperature. A high drying temperature causes a large surface area, large pore volume and narrow pore-size distribution. Large capillary stresses can develop during drying. These stresses will cause the gels to crack unless the drying process is controlled by decreasing the liquid surface energy, eliminating the very small pores, or by obtaining a microdispersed pore size by controlling the rates of ’ hydrolysis and condensation. Thus a narrow pore-size distribution can be obtained at high drying temperatures. 3.3.3. Heat-treatment temperature control The’ effects of heat treatment on the pore-size distribution are shown in Fig. 7. The pore-size distribution has no apparent change when heated under 800 “C, but a single narrow peak can be obtained at 800 “C and correspondingly the average pore diameter enlarges from 18 to 76 A. On heating higher than 800 “C, both the pore volume and the pore size diminish to zero and no pore-size distribution can be observed. This is in agreement with the XRLI results. The gel particles shrink and condense when heated. At 800 “C, crystallites of tridymite develop, the gel particles shrink and the tiny pores coalesce; thus the average pore size increases and its distribution becomes narrow. A further increase in temperature up to 1300 “C may result in the disappearance of amorphous SO* and complete crystallization to form cristobalite and tridymite. The
ml 100 PoreDiameter(A)
Fig. 5. Effect of ethanoL!TEOS ratio on pore size.
3.3. Pore-size distribution control Usually pore size and pore-size distribution control cannot be separated. But by controlling the catalysts, pH value, solvent content, gelation temperature and heat-treatment temperature carefully, single narrow
‘1 PoreDiicter (A) Fig. 6. Effect of waterfI’EOS ratio on pore-size distribution.
X. Yao et al. I Semen
and Actuators B 24-25
Fig. 8. Double-peak pore-size distribution of porous silica by HNO, catalysis.
Rclrtive Pressure (PIPa) Fig. 9. Isothermal plot of porous silica catalysed by HCI.
Radius (A) Fig.
pores which used to be open and connected now become closed, thus the pore volume is so small that no poresize distribution can be observed. 3.3.4. Cata&stcontrol Usually a single narrow peak distribution can be easily achieved by using only one kind of catalyst such as NH,.H,O, HCl, HF, NaF or NH,F. Porous silica catalysed by these catalysts is usually mesoporous with a pore diameter in the range 5-100 nm. But double separate peak distributions can also be achieved under special conditions.. HNO,-catalysed silica gel glass is an example of a double-peak pore-size distribution with micropores less than 5 nm and macropores larger than 0.1 pm (Fig. 8). The second way to get a double separate peak distribution is to use HCI and NH,.H*O simultaneously. Acid and base have different effects on the pore-size distribution due to their different catalytic mechanisms. Addition of HCl decreases the pore size to less than
10 nm; on the other hand, a large amount of ammonia tends to enlarge the pores significantly. Controlling the pH values by the use of HCl and NH3.H,0 simultaneously, double separate peak distributions can also be achieved. 3.4. Pore-shapecontrol Pore shapes are mainly controlled by the type of catalysts. Both pore size and pore shape can be derived from the shape of the isotherm. Also TEM graphs can give direct observation of the internal structure of materials. Fig. 9 is a typical isothermal plot of HCl-catalysed gel glass. The adsorption curve increases slowly with the increase of pressure (P/PO),but the desorption curve falls sharply at moderate pressure. This kind of isotherm shape implies the presence of mesopores of narrownecked and large-abdomened shapes. This is in agreement with the HCl-catalysed gel structure examined by TEM and shown in Fig. 10. The HCl-catalysed gel particles exhibit irregular chains, which may be due to the electrophilic reaction of H’ [2,3].
X. Yao etal.ISemm
Fig. 10. TEh% micrograph of HCkatalysed
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Fig. 12. TEM micrographs of HF-catalyzed gel for HF/TEOS ratios of (a) 0.001, (b) 0.1.
=~=~(P/w) Fig. 11. Effect of HF on pore shape.
Pares of narrow-necked and large-abdomened shapes with a fine network structure of linear chains for gels can be achieved by HCl catalysis, while fine cylindrical pores can be obtained by fluorine anion (HF, NH.,F, NaF) on ammonia catalysis. TEM examination shows that silica gels with dense colloidal particles and large interstices between them are formed by ammonia catalysis. The pore shapes can be changed from narrownecked large-abdomened pores to fine cylindrical pores by controlling the fluorine anion content. Fig. 11 shows the isotherm of samples catalysed by HF. Increases in the F- content can shift the plots to the higher-pressure side and increase the sorption capacities in accordance with the increase in pore size. When the F- content is as low as in Fig. 11(a), the adsorption curve coincides with the desorption curve; this indicates the presence of a large number of micropores. On a further increase in F- content, the shape of the isotherm changes from Fig. 11(b) to (d), and the gel structure changes from irregular chains to spherical particles as shown in Fig. 12. All these changes are in agreement with the poreshape change.
The difference of pore size, pore shape and poresize distribution is caused by the different catalytic mechanisms. Hydrolysis in the basic solution is due to the nucleophilic reaction mechanism. The mechanism of the catalytic effect of F- is similar to that of OH-. It is concluded that the F- anion directly attacks Si(OR), groups by nucleophilic substitution. This involves the temporary expansion of the coordination number of silicon from four to five or six, which causes a rapid hydrolysis and condensation reaction, and thus affects the pore size, pore shape and pore-size distribution.
4. Application in chemical sensors
Preparation of porous silica via the sol-gel route has many advantages, such as high purity, high homogeneity and low firing temperature. The most important is the controllable pore size, porosity, pore shape and poresize distribution, which can satisfy the various needs in chemical-sensor applications.
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4.1. Loading of the porous silica with active
components There are several ways to load the porous silica glass with active sensing components. Ultrafine powder of various sensing components can be loaded in the porous silica at the liquid sol stage of the processing. The loaded wet gel is then dried and heat-treated to form the loaded porous silica. The porous silica glasscan also be loaded by immersing the porous glass sample into liquid precursors of active sensing components. The impregnated sample is then treated to remove liquid solvent and to transform the precursors into the final form of the active component. Chemical vapour infiltration (CVI) is another way to load the porous silica . The porous silica sample is placed in a chemical vapour environment at high temperatures. An active component can be condensed from the vapour precursor into the pores through the percolation passage of the porous structure. Compound semiconductors, such as chlorides, sulfides, selenides and tellurides, can be synthesized through chemical vapour reactions. 4.2. In situ crystallization and growthof active components  Active sensing components, which can be transformed into liquid solution or sol, can be mixed with the TEOS sol at the liquid stage. Then the mixture is kept to form wet and dry gel as the normal gelation process. The dried gel is heated to burn out the solvents and all organic substances from the system, and then treated at the crystallization temperature of the active component. Small crystallites, often of nanometre size, will grow out in situ within the confined space of the skeleton
of porous silica. Various active sensing components are to be tested to form such composite sensors by this method. 5. Conclusions The study of porous silica opens up broad prospects for developing better chemical sensors. Porous silica combines many excellent physicochemical properties, which are very important for a carrier of sensor materials. Preparation of porous silica is very flexible and the porosity, pore size, pore shape and pore-size distribution can be easily controlled by catalysts, preparation conditions such as water/IEOS or alcoholITEOS ratio, pH value, gelation temperature and heat-treatment temperature. Porous silica can be loaded with various active sensing components in various ways to form composite materials. Porous silica is a good candidate matrix material for chemical sensors. Acknowledgement The work is supported by the Chinese National Advanced Materials Research Project.
References [l] B.E. Yoldas, Hydrolytic polycondensation of Si(OcZH& and effect of reaction parameters, J. Non-Cryst. Soli& 83 (1986) 375. [Z] R. Aelion, A. Loebel and F. Eirich, Hydrolysis of ethyl silicate, J. Am. Chem. Sot., 72 (1950) 5705.  K.D. Keefer, Better ceramic through chemistry, Mater.Rex Sm. Symp. Pmt., 32 (1984) 15. [4) S.G. Lu, Ph.D. Disxrtation, Xi’an Jiaotong Univ., 1993. [S] Q.F. Zhou, Ph.D. Di.swtation, Xi’an Jiaotong Univ., 1993.