Proceedings - Viện Vật lý

_________________________________________________________________________________Những tiến bộ trong Quang học, Quang tử, Quang phổ và Ứng dụng. 8/2006, Cần Thơ, Việt Nam2.1. Preparation of samples.2. Experiment and discussionZnO and Zn 1-x (TM) x O powders were synthesized by forced hydrolysis method.Used chemicals were Zn(CH 3 COO) 2 .2H 2 O, Ni(CH 3 COO) 2 .4H 2 O,Mn(CH 3 COO) 2 .4H 2 O and Cr(NO 3 ) 3 . All of them were of analytical grad purity.Catalyst and other chemicals were propandiol C 2 H 4 (OH) 2 , ammoniac NH 4 OH 80%,pure water, alcohol 99.5% and silicon. After separately washing process, obtainedsolution was annealed for about 2h at different temperatures: 300, 450, 800,1000 o C.The microstructure and crystallization of the products were further examined byscanning electron microscopy (SEM) and x-ray diffraction (XRD). Opticalproperties of samples were investigated using Raman spectrum (RS) andphotoluminescence measurements (PL).2.2. Structure characters of samples.Figure 1 shows SEM images of ZnO powder annealed at 300 0 C (in the left) andat 500 0 C (in the right). One can see that the grains in sample are not uniform. Thediameters of the grains are mainly distributed in the range of 40-70 nm. The averagevalue of grains diameters does not clearly depend on annealing temperatures.Figure 2 shows the XRD patterns of nanocrystalline ZnO powders annealed at500 0 C (in the left) and at 1000 0 C (in the right). They indicate that these ZnOpowder are highly crystallized in wurtzite structure. An analysis showed that thespacing between adjacent lattice planes is slightly smaller than that of conventionalZnO powder. This decrease of lattice parameters of nanocrystalline ZnO powdermay be due to a compressing effect related to a surface energy increase as the grainsurface increases.From the relationship between crystal grain size and X-ray line broadening,described by Scherrer’s equation, the average grain size of the ZnO was evaluatedto be about 35 nm (using the diffraction line 002, at 34,48 0 ). This value is slightlyincreased with the increasing of annealing temperature.After partial substitution of Zn by Cr, the XRD patterns (see figure 3) showthat the samples annealed at 800 0 C (in the left) contain also a small amount ofZnCr 2 O 4 spinel phase. XRD patterns of samples annealed at 400 0 C show noZnCr 2 O 4 spinel phase, in other word, the material is in pure wurtzite phase.52