As a classic yellow emission phosphor, YAG:Ce³⁺ is one kind of widely used phosphor due to its high quantum efficiency, good thermal conductivity and wide wave band properties. It has been extensively studied in both the preparation methods and luminescence properties. However, the prepared material always contains Ce⁴⁺ which does not contribute to luminescence theoretically. In order to strengthen the emission intensity, sintering under reduction atmosphere and charge compensation are usually adopted method to increase the Ce³⁺ content in YAG:Ce³⁺ materials.

Generally, cerium compounds exhibit complex features due to hybridization with ligand orbitals and fractional occupancy of the valence 4f orbitals. Although some theory and researches have been elaborated, absolute assignment of the features has not been made. In the study of Edmond Abi-aad et al. [1] , the Ce 3d XPS spectra of the CeO₂ prepared with different alkali solutions (KOH, NH₄OH) have the same shape with six peaks at Bes (Binding Energies) 882.5, 888.7, 898.2, 900.7, 907.6 and 916.5 (±0.2) eV, respectively. In the case of M. Cabala et al. [2] , they observed splitting of cerium 3d core level in two doublets corresponding to configuration 3d⁹4f¹ components at binding energies of 904.5 eV, 886.0 eV and 3d⁹4f² at 900.3 eV, 881.9 eV, respectively. CHAI Chunlin et al. [3] carried out the XPS analysis of sample CeO₂/Si, they believed the peak of 881.34 eV was attributed to Ce3d signal of non stoichiometric CeO₂ and the characteristic peak of Ce³⁺―884.9 eV and 903.4 eV were obtained. In order to determine the oxidation state of Ce in Ce_0.9Fe_0.1O_1.97, Ying Zuo et al. [4] evaluated the XPS spectra of Ce (3d) region and three doublets from CeO₂ and two doublets from Ce₂O₃ could be identified. The assignment of Ce⁴⁺ 3d component (3d_5/2: 882.6 eV, 888.6 eV, 898.3 eV; 3d_3/2: 901.1 eV, 907.3 eV, 916.8 eV) and Ce³⁺ 3d component (3d_5/2: 885.4 eV; 3d_3/2: 903.7 eV) from XPS spectra were collected. A number of related works have been done [5] - [9] .

In this paper, the proportion of Ce³⁺/Ce⁴⁺ was measured using the XPS method. In a word, there will be displacement of the BEs and the analysis should be carried out on the basis of different cerium compounds. In order to confirm the relationship between the luminescence and the ratio of Ce³⁺/Ce⁴⁺ more clearly, a series of YAG:Ce³⁺ phosphors were prepared under different sintering atmospheres. The relationship between the ratio of Ce³⁺/Ce⁴⁺ and the luminescence was studied by XPS as a semi-quantitative method.

YAG:Ce³⁺ phosphors were synthesized via the conventional high temperature solid-state reaction method. The stoichiometric amount of Y₂O₃ (99.999%), Al₂O₃ and Ce₂O₃ (99.999%) were weighted accurately (Y_2.94Al₅O₁₂: 0.06Ce) and then well mixed and grinded using planetary ball mill. The samples were placed in a ceramic heater at 1550˚C for 2 h. The only difference is the sintering atmosphere: with both carbon powder and flowing nitrogen atmosphere, with carbon powder but no nitrogen, with nitrogen atmosphere but no carbon powder, the last with neither of them.

Both emission spectrum and excitation spectrum of the phosphor were measured by a ZOLIX Omni-λ luminescence photoluminescence (PL) spectrophotometer (Xe lamp). We also measured the surface states of cerium electrodes by using a AXIS Ultra DLD X-ray photoelectron spectroscopy (XPS). The Al Kα line was used as an X-ray source. The minimum resolution was 0.48 eV (Ag 3d_5/2). Microscopy of the phosphor was measured on a Hitachi S4800 scanning electron microscope (SEM).

The binding energies were generally using the C 1s peak (284.7 eV) as a standard (Figure 1). According to the standard XPS spectra, the characteristic peak of Ce³⁺ 3d_5/2 and Ce³⁺ 3d_3/2 is 884 eV and 902 eV, respectively. Then for Ce⁴⁺ is eV and respectively. When the chemical environment of atomic changed, there will be displacement of the BEs of inner electrons.

In our case, the instrument automatically identified the Ce3d_5/2 at 899.9eV. Although the displacement of BEs happens, according to related reports [10] , the maximum value would be 17.9 eV. So combined with measured data we inferred the peak of 899.9 eV was consist of Ce³⁺ 3d_5/2 and Ce⁴⁺ 3d_3/2. In consideration of these factors and the obtained data, we identify the 882 eV, 888 eV, 899 eV and 903 eV as the fitting peaks.

Figures 2-5 show the XPS spectra of Ce 3d of four YAG:Ce³⁺ samples, by a binding energy scan from 880 to 910 eV at a rate of 100 meV per step and a 40 eV pass energy. The dwell time was 120 ms. All the spectra were fitted and the areas of each fitting peak were measured. It is well known that the fraction of Ce³⁺ ions can be obtained by calculating the ratio of peak areas: f (Ce³⁺) = area (Ce³⁺)/area (Ce³⁺ + Ce⁴⁺). The linear background was subtracted before curve fitting.

Figure 2 showed the XPS patterns of Ce3d sintered in air. Four peaks of binding energy were observed to be located at 888.131 eV for Ce⁴⁺ 3d_5/2, 890.301 eV for Ce³⁺ 3d_5/2, 900.611 eV for Ce⁴⁺ 3d_3/2 and 904.47 eV for Ce³⁺ 3d_3/2, respectively. Table 1 showed the calculated result. The ratio of Ce³⁺ is about 68.14% while Ce⁴⁺ is about 31.86%. As we know, O₂ will oxidize some Ce³⁺ to Ce⁴⁺ at high temperature in air. So the ratio of Ce³⁺ is low sintered in air.

The XPS patterns of Ce3d sintered in nitrogen atmosphere was showed in Figure 3. Peaks at 887.5111 eV was for Ce⁴⁺ 3d_5/2, 890.151eV for Ce³⁺ 3d_5/2, 900.131 eV for Ce⁴⁺ 3d_3/2 and 904.061 eV for Ce³⁺ 3d_3/2, respectively. Table 2 showed the calculated result. The ratio of Ce³⁺ is about 75.33% while Ce⁴⁺ is about 24.67%. The concentration of O₂ was low in nitrogen atmosphere. This reason will prevent some Ce³⁺ oxidized to Ce⁴⁺. So the ratio of Ce³⁺ sintered in nitrogen is bigger than sintered in air.

Figure 4 indicated the XPS patterns of Ce3d sintered in carbon monoxide atmosphere. Peaks at 889.181eV was for Ce⁴⁺ 3d_5/2, 891.811 eV for Ce³⁺ 3d_5/2, 902.131 eV for Ce⁴⁺ 3d_3/2 and 905.601 eV for Ce³⁺ 3d_3/2, respectively. Table 3 showed the calculated result. The ratio of Ce³⁺ is about 77.55% while Ce⁴⁺ is about 22.45%. Some Ce⁴⁺ was reduced to Ce³⁺ by carbon monoxide in carbon monoxide atmosphere under high temperature. So the ratio of Ce³⁺ sintered in carbon monoxide atmosphere is bigger than sintered in the two kinds of atmosphere referred above.

Figure 5 showed the XPS patterns of Ce3d sintered in carbon monoxide and nitrogen mixed atmosphere. The peaks at 885.262 eV and 899.923 eV were for Ce⁴⁺ 3d_5/2, and 888.669 eV, 903.507 eV were for Ce³⁺ 3d_5/2, respectively. The calculated result was showed in Table 4. The ratio of Ce³⁺ is about 88.46% while Ce⁴⁺ is about 11.54%. In the mixed atmosphere, oxidation of some Ce³⁺ to Ce⁴⁺ were prevented due to the lack of O₂, while some Ce⁴⁺ were reduced to Ce³⁺ by carbon monoxide. So the ratio of Ce³⁺ was the largest sintered in mixed atmosphere.

Figure 6 showed the emission spectra of samples prepared under different sintered atmosphere. The YAG: Ce³⁺ phosphor sintered under carbon monoxide and nitrogen atmosphere showed the strongest luminescence. The sample has the biggest ratio of Ce³⁺/(Ce³⁺+Ce⁴⁺) (88.46%) in the four conditions. The lowest luminescent was sintered in air, which possess the ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) 68.14%. It can be preliminary confirmed that with the increasing of the ratio of Ce³⁺, the emission intensity increases gradually.

Figure 7 gives the SEM images of YAG:Ce³⁺ phosphors prepared with: a) normal atmosphere and b) carbon monoxide and nitrogen, respectively. We can see that the different sintering atmosphere has no influence on the SEM pictures. Both of them have some conglomeration phenomenon which is common in high temperature solid state reaction method and the crystallites were irregular spheres.

In conclusion, XPS was used as a semi-quantitative method to determined the mole ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) in phosphor. The percentage of Ce³⁺ was 88.46%, 77.55%, 75.33% and 68.14% under four different sintering atmosphere. It confirmed that under the same preparation conditions and within the quenching concentration, the high ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) could enhance the emission intensity in phosphor.

This work was financially supported in part by Qianqian Hu (Shanghai Jiao Tong University) for assistance in XPS analysis. The authors also received supports from Professor Guorong Chen and Liang Li (East China University of Science and Technology) for general discussions.