8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
Synthesis and Characterization of CuO nanoparticles in
SiO2matrices.
Manas Mondal, Navonil Bose, Palash Patra, Raj Kumar Das, Sampad Mukherjee* (Department of Physics, Bengal Engineering & Science University, Shibpur, Howrah-711 103)
Key words: Sol-gel, Nanoparticle, ultrasonic, glass-ceramic
Introduction:
Since materials are integral part of technological development, the study of material offers great opportunities for advancements. Now-a-days to prepare the new materials having nano dimension is a challenge to the material scientists. Not only the preparation but some typical properties of these materials are also an interesting subject to research workers.
Anything smaller than a nanometer (~ 10-9 meter) in size is just a loose atom or small molecule floating in space as a little dilute speck of vapour. So, nano structures are not just smaller than anything we have made before, they are the smallest solid things which are possible to make. Additionally, the nanoscale is unique because it is the size scale where the familiar day–to-day properties of materials like conductivity, hardness, or melting point meet the more exotic properties of the atomic and molecular world such as wave-particle duality and quantum effects. Particles of any material metal, semiconductor or ceramic having diameter in the range 1-50 nm. constitute nanoscale clusters. The physical properties of the latter neither correspond to those of the free atoms or molecules making up the particle not to those of the bulk solids with identical chemical composition. The clusters are also characterized by a large value of the surface area to volume ratio which signifies that a large fraction of the atoms reside at the grain boundary.
A sol that consists of solid particles suspended in a liquid is a colloidal suspension. A gel is semi rigid colloidal system of at least two components in which both components extend continuously throughout the system. An inorganic gel typically consists of water trapped within a three dimensional network of tiny crystals of an inorganic solid. The crystals are held together by van der Walls force, and the water is both absorbed on the crystals and mechanically enclosed by them (1).If the liquid phase of a gel is removed by heating and pressurizing the gel above critical temperature and pressure of the liquid (supercritical conditions) and allowing the fluid to vent, one obtains an aero gel. The most studied aero gel is silica aero gel, where the solid is SiO2 (silica), which is a covalent network space solid with a three-dimensional array of bonded Si and O atoms.
The original gel can be made by the reaction
Si(OC2H5)4 + 2H2O → SiO2(s) + 4C2H5OH
carried out in the solvent ethanol and yielding a gel with ethanol as the liquid.
With the advent of ultrasonic techniques it is now-a-days a common practice to use this technique to determine the wave propagation behavior such as velocity and attenuation of ultrasonic waves in the material (1– 6) which may be a solid, crystalline or
1
an amorphous or even a liquid and polymer. Ultrasonic measurements are useful to study any property of condensed matter which is sufficiently well coupled to the medium.
To study the kinetics of crystallization and provide a molecular mechanism some assumptions such as (i) nucleation is homogeneous. (ii) at a given temperature the number of nuclei formed per unit time per unit volume is a constant and (iii) there is no impingement of spherulite structure already formed. Considering these assumptions Avrami gives a general equation as
W1/W0 = exp (-k.tm )
Where ‘m’ is known as Avrami constant, k is the constant that includes terms as nucleation frequency, growth vector, shape factors and densities of crystalline and amorphous phases.
In this communication , we prepare silica aero gel and the formation of gel form sol is studied continuously by ultrasonic technique measuring the time dependent behavior of first back wall echo separation and hence to determine the ‘gel time’ of the aero gel. The samples were heat treated at different temperatures for several hours to form the glass and glass ceramic materials and characterized by the XRD, DTA, TDA, FESEM, EDX techniques.
Preparation Technique:
Cu-SiO2 composite was prepared using CuSO4, 5 H2O as starting materials. Copper salt was examined as a precursor for the incorporation of copper into silica.
Composition taken for the preparation of the sample is as follows:
Mixture-1 : TEOS= 0.1352moles. , H2O = 0.6889moles,
EtOH = 0.2157moles, HCl
Mixture-2: CuSO4, 5 H2O =. 0.00562moles,
CH3CN =. 1 mole, HCl
Method:
Suitable amounts (0.1352 moles) of TEOS and ethyel alcohol (0.2157 moles) were dissolved in water (0.6889 moles). Then to prepare a homogeneous aqueous solution of TEOS was stirred by a magnetic stirrer up to 30 min. A few drops of HCL were also added to maintain a pH of near about 4 [7], at which the largest gelation velocities of SiO2 systems are obtained. The temperature was about 270 C.
On the other hand the CuSO4 precursor (0.00562 moles) was dissolved in 1 mole of CH3CN and added to the above mixture. Also a few drops of dilute HCL were added to get more clear and homogeneous solution.
The combined solution was stirred for two hours .The sols evolves then poured in Teflon container and allow it to form of an inorganic network containing a liquid phase
2
(gel). After two hours of stirring, clear green colored sol was obtained, which was then poured in a teflon crucible for gelation. Then we take the reading of ultrasonic data during 10 min. interval.
To examine the effect of temperature and distribution of CuO particles within the silica material, aerogel was heat treated at 4000, 6000, 9000 & 12000C for several hours and then all the samples were grinded by using the mortar and pestle for different characterizations.
Result & Discussion:
The reading for ΔT (time between buffer echo and sample first back wall echo) has been measured from ultrasonic interferometer (MBS 8000/SR 9000 DSP measurement system made by Mates instruments, INC.). The In(ln(Δν)) (where Δν = 1 / (ΔT)) values be plotted against ln‘t’ (time) and this plot is shown in fig. 1. as time passes; the length of the solution shrinkages and finally it starts to form crystalline polymeric 3D network. This procedure had been also followed by others [8] when the change in volume (i.e. weight) is monitored from the change in height in dilatometer. The phenomena of formation of polymeric crystalline network can be reflected in ln( In Δν) versus lnt curve, where this follows Avrami equation [9]. The time at which this networks start to from referred as the ‘gel time’.
The plot of ln(ln(Δν)) versus ln(‘t’) shows clearly two regions, having different values of slope. The first region is due to normal vaporization phenomenon for the sol-state and the 2nd region follows the shrinkage of length of material due to polymerization of crystallization of silicon of Si-O network and water or alcohol trapped between the network escapes to vapour. The linear region is fitted for the value of n = 1,2,3 and the fit is quite good for n = 1 because the value of ‘goodness of fit for n = 1 is 0.99958. So, we consider the Avrami constant as 1 in our case of study. The linear fit is shown in fig (2) with the values of parameters of equation Y=A+Bx. This procedure had been also followed by others [10] when the change in volume (that is weight) is monitored from the change in height in dialatometer. The time at which this networking starts is denoted as ‘gel time’ of the Si-aerogel. In our sample the gel time obtained as 33.11 minutes. Here one thing must be noticed that the velocity change of sol and gel state of sample does not differ very much (within experimental accuracy) and V=(2L* Δν ) applied here to calculate change in length(L) via the change in Δν, considering velocity as constant. So shrinkage of length of the sample is inversely proportional to the change in ‘Δν’ with time.
So we conclude that our experiments gives a new and versatile root for which the ‘gel-time’ and Avrami constant for nucleation of this system can be estimated more accurately as ultrasonic wave gives some molecular level information of the system.
XRD has been done by using Burker AXS-D8-Advanced4. XRD histogram [Fig.9 & Fig.10] of sample (A) and (B) has no sharp peak and confirm the amorphousness of the samples. But histogram [Fig.11 & Fig.12] of sample (C) and (D) show sharp peaks with a noisy background and confirm the existence of glass-ceramic phase. We can compare those peaks of CuO and SiO2 with standard XRD pattern [7].
Fig .10 Powder X-ray diffraction patterns of CuO-SiO2 glass (heat treated at 600oC for 2 hours.)
10203040506070-505101520253035600 CINTENSITY2 θ(IN DEGREE)
10203040506070-505101520253035400 CINTENSITY2 θ(IN DEGREE)
Fig.9 : Powder X-ray diffraction patterns of CuO-SiO2 glass (heat treated at 400oC for 2 hours.)
As a result of the fact that thermodynamically CuO and SiO2 do not favor to combine to form a glass or any discrete compounds (7) heat treatment of the homogeneous aerogel composites is expected to result in formation of copper oxide nanoparticles that are embedded within the silica matrix. In the XRD histogram of the sample (D) the peaks occur at 2θ values of 33o, 36.5o, 38o are due to CuO and the other peaks at 2θ values of 22.4o, 36.75o are due to SiO2 crystalline phase. However, in samples heat treated at ≥ 900oC, peaks corresponding to crystalline CuO can be clearly seen. The peak at 2θ =36.55o for CuO phase in fig.4 is used to calculate the size of respective crystallites by Scherrer calculation which is of the order of 15nm and ensures the formation of nanocomposite.
The peaks of SEM-EDX (Model: SEM-Hithachi S3400N, Hariba Atachement) is shown in fig.13. From the peaks, the atomic weight percentages of different elements in our sample have been determined. The presence of Si, Cu and O in our sample has been detected in SEM-EDX peaks and micrograph, but the amount of Cu is so less that can not be detected by the resolution of this instrument. Another conclusion can be made from this micrograph that the mixture is homogeneous which the previlage for this route of preparation is.
The micrograph for FESEM study (Model: JEOLJSM 6700F) have shown in fig.14. The micrograph is for the sample (D).The scale of this micrograph indicates the segregation of nanocrystallites on the comparatively large sized grains of SiO2 crystallites
References:-
1. S.I.Yatsyk, Russian Ultrasonic, 7 221(1977)
2. M.Red Wood and J.Lamb, Proc. Phys.Soc. (London) B70, 136 (1957).
3. M.Red Wood, Proc.Phys.Soc (London) B70, 136(1957)
4. H.J.Mc skimin, J.Acoust. Soc. Am, 28, 484 (1956)
5. J.Lamb, M.Red wood and Z.Shteinshleifer, Phys. Rev. Letters, 3, 28(1959).
6. S.Hunklinger, W.Arnold, S.Stein, R.Nava and K.Dransfield, Phys. Letters 42A, 253 (1972).
7. Jaya L. Mohanan & Stephanie L. Brock, Chem. Mater, 2003, 15, 2567-2576
8. N.N.Greenword and A.Earnshaw-“Chemistry of the Elements”.-Max.Mac.International ed.P-1167.
9. Introduction to Polymers (2nd Ed.) by Young.R.J & Lovell.P.L.(1st ed. By Chapman & Hall in 1981) p-278.
10. Introduction to Polymers by R.J.Young & P.A.Lovell(2nd edition )(1991)P-279.
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