Size controlled nanopigments: dimensional tuning of CoAl
2
O
4
through the use of different templating agents
E. Bonometti, G. Benza Capita, L. Operti, E. Diana
Department of Chemistry, University of Turin
Email:
[email protected]
Introduction:
CoAl2O4 is one of the most important blue pigment with spinel-type structure. It is an idiochromatic (self-coloured) ceramic pigment prepared in laboratory by Thenard in 1807 by reaction of cobalt arsenate and cobalt phosphate with Al2O3 at high temperature. It has been applied in decorating porcelains, ceramics, plastics, paints, rubbers, glass, cement, [1,2]. It has great thermal stability, low temperature sinterability and high mechanical resistance. Besides, it is used in heterogeneous catalysis in CO2 reforming of CH4 [3], as CO and CO2 gas sensing in mesoporous form [4] and it is employed in NOx selective reduction [5]
FTIR and Raman spectra
Diffraction measurements were made on a Phillips PanAnalytical diffractometer with a Co λ. All the spectra correspond to pure CoAl2O4 .
Throught Shaerrer’s formula we can calculate average particles dimensions:
FTIR measurements were registered at t.a. in ATR modality by a Brucker Vertex 70
spectrofotometer while Raman spectra were analyzed by a Renishaw μ-Raman (fig. 1-4).
Vibrational spectra are characteristic of spinel structure. The most significant bands are in
465-485 cm
-1, 550-560 cm
-1and 635-670 cm
-1range.
XRPD spectra
[1] D.V. Shangani, G.R. Abrams, P.J. Smith, Trans. J. Br. Ceram. Soc., 80 (1981)
210-214;
[2] J. Alarcon, P. Escribano, R.M. Marin, Br. Cera,m. Trans., 84 (1985), 170-172; [3] L. Ji S. Thang, H.C. Zheng, J. Lin, K.L. Tan, Appl. Catal.. A, : Gen 207, (2001), 247;
[4] C. R. Michael, Sensor and Actuators B: Chemical; 147 (2010), 635-641; [5] N.K. Nga, D.K. Chi, website: http://www.azom.com/article.aspx?
ArticleID=3012
Several methods were used to synthesize nanopigments but no studies were still made to compare results in term of particle size.
Four SOL-GEL synthetic methods are reproduced from literature, slightly modified.
All methods are based on formation of gels that form homogeneous oxide networks. This reduced drastically the need of atomic diffusion during calcination process allowing the formation of desired phases at much lower temperature and shorter calcination time. The difference in these synthesis lies in use of gelling and/or combustion agents (templating
agents)
1) 2-propanol sol-gel method (alcohol): it offers a good stoichiometric control and
production of ultrafine particles with narrow size distribution. Alcohol acts as a stabilizer that limits particle growth and agglomeration. Thermal treatment was at 800°C per 5’;
2) Citrate method (citric acid): this method permits formation of high dispersed oxides
and there is formation of a tridimensional network because of the nature of citrate template. It was used citric acid + metal nitrates as starting materials;
3) Malonate method (malonic acid): malonic acid can be used as fuel for the preparation
of mixed oxides at low temperature because it oxidizes at below 50°C in acidic medium. The difference with citrate is that here the powder is produced not by thermal decomposition of the gel but from spontaneous ignition (at 210°C) of the solidified precursor. This can produce inhomogeneity but this is overtaken adding a controlled amount of malonic acid in the first phase of reaction;
4) Low T° combustion (urea): solid state reactions normally involve high temperatures and long reaction time. Advantages of this method are the possibility of working at lower T°, low reaction time, homogeneous and finer final products with no need of grinding. In our experiment a blue pigment was obtained, but doped with Zn-minimal concentration of Co2+ -in order to economize the entire process. Urea was used as gelling agent. Advantages are the high solubility of this compound in water and the complete degradation after thermal treatment.
In conclusion, as demonstred also in literature, we can deduce that lower temperature had to be employed if particles with lower nanosized dimensions are to be obtained.
CoAl2O4 bands 198 cm-1 412cm-1 480cm-1 519 cm-1 619 cm-1 690 cm-1 753 cm -1 CoAl2O4 sample bands 195 cm-1 231cm-1 484 cm-1 524cm-1 622 cm-1 693 cm-1 X Fig. 1 Fig. 2 Fig. 4 Comb 1 Fig. 3 CG1 CG3 CG2 Comb 2 Comb 3
Raman bands
2θ (+) Corresponding planes 36.41 [2,2,0] 42.98 [3,1,1,] 52.41 [4,0,0] 65.56 [4,2,2] 70.06 [5,1,1] 77.41 [4,4,0] 88.86 [6,2,0] 92.73 [5,3,3] 114.35 [6,4,2] 115.985 [7,3,1]Conclusions:
Changing reaction starting stoichiometry, it can be noted a relevant change in colour intensity and in particles size. In sol- gel and citrate methods an increase in term of Co2+ concentration leads to a color to decrease.
Synthesis:
Templates dimensions particle in nm
SOL-GEL 26.12
CITRATE 34.14
MALONATE 24.80
LOW COMBUSTION 14.79
All colour measurements were evaluated by a colorimeter using CIE L*a*b* space.
