III. RESULTS AND DISCUSSION

The SEM analysis of pristine and core–shell ceria NPs showed that their particle sizes ranged from 100 to 200 nm [Figs. 1(a) and 1(b)]. Bright field TEM images of NPs were also acquired, showing that the pristine ceria NPs were polyhedral whereas the core–shell ceria NPs were nearly spherical owing to the surface coverage by the ultrasmall (∼5 mm) ceria NPs [Figs. 1(c) and 1(d)]. The deposition of these ultrasmall ceria NPs on the pristine ceria NPs, resulting in the increase of the particle size distribution. The thickness of the shell layers composed of ultrasmall ceria NPs was about 10 nm. The HR-TEM analysis of pristine ceria NPs revealed lattice fringes of 0.31, 0.27, and 0.18 nm that, respectively, corresponded to the (111), (002), and (220) planes of ceria NPs [Fig. 1(e)]. ^{Reference Huang, Chen, Jiang, Jin, Zhu and Sun24,Reference Ou, Mori, Togasaki, Takahashi, Ye and Drennan25} The clear (111), (002), and (220) lattice fringes of the ultrasmall ceria NPs evidenced their successful crystalline growth [Fig. 1(f)].

FIG. 1.

(a and b) FE-SEM and (c–f) TEM images of (a, c, and e) pristine and (b, d, and f) core–shell ceria NPs. Insets: particle size distributions.

The EELS data on ceria NPs can provide information about their oxidation states by means of the analysis of core-loss edges such as the M_4,5 edge. ^{Reference Wang, Crozier, Sharma and Adams26–Reference Wu, Wiesmann, Moodenbaugh, Klie, Zhu, Welch and Suenaga28} Recently, Turner et al. studied the valence change of ceria NPs by using HAADF STEM in combination with resolved EELS. ^{Reference Turner, Lazar, Freitag, Egoavil, Verbeeck, Put, Strauven and Van Tendeloo27} Figure 2 shows the HAADF STEM images and the energy-loss near-edge structure of the Ce M_4,5 edge of pristine and core–shell ceria NPs. The spectra were extracted at the positions indicated by the black and red squares in the micrographs of Figs. 2(a) and 2(b), respectively. In the spectra, two sharp peaks are clearly seen, which show the intensity of M₄ and M₅. The relative intensity of the two peaks, ${{{I_{{{\rm{M}}_4}}}} \mathord{\left/ {\vphantom {{{I_{{{\rm{M}}_4}}}} {{I_{{{\rm{M}}_5}}}}}} \right. \kern-\nulldelimiterspace} {{I_{{{\rm{M}}_5}}}}}$ , can reflect the oxidation states of the Ce ions. ^{Reference Wu, Wiesmann, Moodenbaugh, Klie, Zhu, Welch and Suenaga28} In our experiment, the value of this ratio was 1.16 for pristine ceria NPs and 1.01 for core–shell ceria NPs. These results highlight the fact that the ultrasmall ceria NPs comprising the shell of the core–shell ceria NPs contained Ce predominantly as Ce³⁺ ions.

FIG. 2.

(a and b) HAADF STEM images of (a) pristine ceria and (b) core–shell ceria NPs. (c) Ce M_4,5 edge ELNES signatures of pristine and core–shell ceria NPs, acquired by means of EELS.

The XPS analysis was carried out to validate the differences in the oxidation states of pristine and core–shell ceria NPs arising from the increase in the concentration of Ce³⁺ ions on the surface of the latter. Figure 3(a) shows the Ce 3d XPS spectra for the pristine and core–shell ceria NPs. The peaks for Ce 3d are split into the Ce 3d _5/2 and Ce 3d _3/2 ionizations. In the figure, the peaks labeled v, v′, v″, and v‴ belong to the Ce 3d _5/2 ionization, whereas the peaks u, u′, u″, and u‴ belong to the Ce 3d _3/2 ionization. ^{Reference Deshpande, Patil, Kuchibhatla and Seal20,Reference Shyu, Weber and Gandhi29} Among these peaks, v′ and u′ are characteristic of Ce³⁺ ions, whereas v, v″, v‴, u, u″, and u‴ peaks are attributed to the Ce⁴⁺ ions. ^{Reference Shyu, Weber and Gandhi29} The concentrations of Ce³⁺ ions were calculated from the v and u peak areas ^{Reference Deshpande, Patil, Kuchibhatla and Seal20} as follows:

FIG. 3.

(a) Ce 3d and (b) O 1s XPS spectra of pristine and core–shell ceria NPs.

Table I lists detailed information on the XPS analysis. The concentration of Ce³⁺ ions in the core–shell ceria NPs was found to be higher than that in the pristine ceria NPs, consistent with the EELS results. This confirmed that the ultrasmall ceria NPs on the surface of the core–shell ceria NPs increased the total concentration of Ce³⁺ ions in the particles.

TABLE I.

Ce 3d and O 1s XPS peak assignments for pristine and core–shell ceria NPs.

In aqueous systems, the presence of Ce³⁺ ions leads to the dissociation of H₂O, resulting in the formation of hydroxyl (OH) groups on the surface. ^{Reference Senanayake, Stacchiola, Evans, Estrella, Barrio, Pérez, Hrbek and Rodriguez30,Reference Zhang, Yu, Grass, Dejoie, Ding, Gaskell, Jabeen, Hong, Shavorskiy and Bluhm31} In particular, these OH groups on the surface function as reactive sites for the adsorption of other materials. ^{Reference Armistead, Tyler, Hambleton, Mitchell and Hockey32} The concentrations of surface OH groups on the pristine and core–shell ceria NPs were determined by means of O 1s XPS analysis [Fig. 3(b)]; the two peaks observed in the XPS spectra were assigned to lattice oxygen O²⁻ and surface OH⁻. ^{Reference Erdem, Hunsicker, Simmons, Sudol, Dimonie and El-Aasser33,Reference Van den Brand, Sloof, Terryn and De Wit34} The amount of OH groups can be analyzed by deconvolution of the O 1s response into OH⁻ and O²⁻ peaks. ^{Reference Erdem, Hunsicker, Simmons, Sudol, Dimonie and El-Aasser33} Based on the ratios between OH⁻ and O²⁻, OH⁻/O²⁻ (see Table I), more OH groups existed on the surfaces of the core–shell ceria NPs than those of the pristine ceria NPs. This result is attributed to the presence of the ultrasmall ceria NPs on the surface of the core–shell ceria NPs, which included high concentrations of Ce³⁺ ions. To confirm the XPS data, we acquired the FT-IR spectra of the pristine and core–shell ceria NPs [Fig. 4(a)]. Bands observed at 600–750 cm⁻¹ were attributed to stretching of CeO₂. ^{Reference Lee and Choi35} In addition, a sharp peak at 1380 cm⁻¹ corresponded to the surface nitrate groups on the pristine ceria NPs. This peak was quite weak for the core–shell ceria NPs because nitrate groups are replaced by hydroxyl groups under conditions of pH 10.5 and above. ^{Reference Nabavi, Spalla and Cabane36} The OH stretching bands of ceria NPs were observed in the range of 3200–3600 cm⁻¹. ^{Reference Daturi, Finocchio, Binet, Lavalley, Fally and Perrichon37} The intensity of this peak substantially increased for the core–shell ceria NPs. The quantification of the OH group content in pristine and core–shell ceria NPs was performed by means of TGA ^{Reference Ek, Root, Peussa and Niinistö38,Reference Kim, Chang, Kong, Kim, Kim and Kim39} and calculated as follows:

where WL(T ₀) and WL(T ₁) are the weights of the ceria particles (wt%) at temperatures T ₀ and T ₁, respectively ^{Reference Ek, Root, Peussa and Niinistö38} ; N _A is Avogadro’s number, ${M_{{{\rm{H}}_2}{\rm{O}}}}$ is the molecular weight of water, and S _BET is the specific surface area of the ceria NPs. ^{Reference Kim, Chang, Kong, Kim, Kim and Kim39} This equation yields the number of hydroxyl groups per unit area of the ceria NPs. Figure 4(b) shows the TGA graphs of pristine and core–shell ceria NPs within the range of 40–600 °C under N₂ environment. The first weight loss was attributed to the elimination of the physisorbed water between 40 and 120 °C. ^{Reference Mueller, Kammler, Wegner and Pratsinis40} The OH groups on the surface of the ceria particles were then removed between 120 and 500 °C. ^{Reference Mueller, Kammler, Wegner and Pratsinis40,Reference Asgari, Haghighi and Shafiei41} The OH/nm² values of pristine and core–shell ceria NPs were thus calculated based on the weight loss observed during heating from T ₀ = 120 °C to T ₁ = 500 °C. The OH/nm² values calculated for pristine and core–shell ceria NPs were 0.44 and 0.71, respectively. This result is in agreement with the O 1s XPS and FT-IR data. The ultrasmall ceria NPs on the surface of core–shell ceria NPs resulted in increased OH surface density.

FIG. 4.

(a) FT-IR spectra and (b) TGA curves of pristine and core–shell ceria NPs.

As we have mentioned, surface OH groups function as reactive sites. Especially in CMP applications, Ce–OH groups react with silicate anions on the surface of a SiO₂ film, resulting in the formation of Ce–O–Si bonds. ^{Reference Seo, Moon, Kim, Lee, Hwang, Yoon, Yi and Paik12,Reference Dandu, Peethala and Babu23} Adsorption isotherms of silicate anions on pristine and core–shell ceria NPs were plotted to elucidate the interactions between silicate anions and OH groups on the surfaces of ceria NPs. Figures 5(a) and 5(b) show the adsorption isotherms plotted for the adsorption of silicate anions onto pristine and core–shell ceria NPs at pH 7. The experimental data were analyzed by means of the Langmuir and Freundlich models. ^{Reference Langmuir42,Reference Freundlich43}

FIG. 5.

Adsorption isotherms of pristine and core–shell ceria NPs as a function of silicate concentration at pH 7: (a) Langmuir and (b) Freundlich plots.

The Langmuir model can be expressed by the following equation:

and the Freundlich model can be expressed by the following equation:

where C _e (mg/L) is the equilibrium concentration of silicate ions in solution, Q _e (mg/m²) is the amount of silicate anions adsorbed on the ceria NPs at equilibrium, Q _m is the maximum adsorbed amount (mg/m²), and b is the Langmuir constant (L/mg). ^{Reference Langmuir42} The Freundlich constant K _F is related to the adsorption capacity, and 1/n is correlated to the adsorption intensity. ^{Reference Freundlich43}

Table II lists the constants calculated from the two models. The higher correlation coefficient (R ²) of the Freundlich model indicates that this model fits the data better than the Langmuir model. The Langmuir and Freundlich models, respectively, assume homogeneous and heterogeneous adsorption of silicate anions on the ceria NPs. ^{Reference Langmuir42,Reference Freundlich43} It is therefore reasonable to conclude that silicate anions were heterogeneously adsorbed on the surfaces of the ceria NPs as multilayers. In the case of core–shell ceria NPs, at the silicate concentration of 600 mg/L, the amount of adsorbed silicate anions was 2.82 mg/m²; contrastingly, this value was 1.92 mg/m² for pristine ceria NPs. In addition, the value of K _F determined for the core–shell ceria NPs was higher than that of the pristine ceria NPs, indicating the higher adsorption capacity of the core–shell ceria NPs. These results support the conclusion that the concentrations of Ce³⁺ ions and OH groups on the surface of core–shell ceria NPs were greater owing to the presence of the ultrasmall ceria NPs.

TABLE II.

Langmuir and Freundlich constants for the adsorption of silicate ions on pristine and core–shell ceria NPs.

In CMP applications, ceria NPs are used as abrasives for the removal of silicate glass because they form Ce–O–Si bonds that aid in removing SiO₂. ^{Reference Cook5,Reference Seo, Moon, Kim, Lee, Hwang, Yoon, Yi and Paik12,Reference Dandu, Peethala and Babu23} To identify the effect upon silicate glass removal of the increase in surface OH groups of core–shell ceria NPs, arising from their higher concentration of Ce³⁺ ions, we applied both pristine and core–shell ceria NPs to the CMP process. The SiO₂ removal rate of the core–shell ceria NPs was higher than that of the pristine ceria NPs (Fig. S1) due to the high affinity of the core–shell ceria NPs to silicate anions, in agreement with the adsorption isotherm results. In addition, the surface properties such as nanotopography of SiO₂ film after CMP are shown in Fig. S2. Although the standard deviation (root mean square, Rms) of the nanotopography was slightly decreased, the difference was quite small.