Role of the Oxidation State of Cerium on the Ceria Surfaces for Silicate Adsorption
Jihoon Seo,1 Jinok Moon,1,2 Joo Hyun Kim,1 Kangchun Lee,1 Junha Hwang,1,3 Heesung Yoon,1 Dong Kee Yi,4,* and Ungyu Paik1,* 1
WCD Department of Energy Engineering, Hanyang University, Seoul, South Korea
2
Clean/CMP Technology Team, Memory, Samsung Electronics, Hwaseong, South Korea 3
Materials R&D Center, K.C.Tech, Anseong, South Korea
4
Department of Chemistry, Myongji University, Yongin, South Korea
*Corresponding author: Professor Ungyu Paik (
[email protected]) Professor Dong Kee Yi (
[email protected]) Tel. +82-2-2220-0502 Fax +82-2-2281-0502 1
ABSTRACT In this study, we have investigated the role of the Ce oxidation state (Ce3+/Ce4+) on the CeO2 surfaces for silicate adsorption. In aqueous medium, the Ce3+ sites lead to the formation of −OH groups at the CeO2 surface through H2O dissociation. Silicate ions can adsorb onto the CeO2 surface through interaction with the −OH groups (−Ce−OH + −Si−O- ↔ −Ce−O−Si− + OH-). As the Ce3+ concentration increased from 19.3 to 27.6 %, the surface density of −OH group increased from 0.34 to 0.72 OH/nm2. To evaluate the adsorption behaviors of silicate ions onto CeO2 NPs, we carried out an adsorption isothermal analysis, and the adsorption isotherm data followed the Freundlich model. The Freundlich constant for the relative adsorption capacity (KF) and adsorption intensity (1/n) indicated that CeO2 NPs with high Ce3+ concentration show higher adsorption affinity with silicate ions. As a result, we have demonstrated that the Ce oxidation state (Ce3+/Ce4+) on the CeO2 surface can have a significant influence on the silicate adsorption.
Keywords: ceria, particle size, oxidation state, adsorption isotherm, silicate ions. 2
1. INTRODUCTION Ceria (CeO2) nanoparticles (NPs) have been widely used as a promising material in various applications in the fields of catalysis,[1] fuel cells,[2] gas sensors,[3] and chemical mechanical planarization (CMP).[4, 5] In all these applications, the high performance of CeO2 NPs is attributed to their rich O vacancies and low redox potential.[6, 7] It is well known that O vacancies are formed due to the highly mobile nature of the surface oxygens. Two excess electrons, left behind by the formation of O vacancies, are localized on the 4f-state of the nearest Ce ions, leading to a valence change in the Ce ions from Ce4+ to Ce3+.[8, 9] The Ce3+ ions at the surface are active sites for reactions in most applications.[6, 10, 11] In aqueous medium, the Ce3+ ions act as active sites for H2O dissociation, resulting in the formation of −OH groups at the surface.[12] The presence of −OH groups on the CeO2 surface allows for various interactions (e.g., molecular adsorption, desorption) in their many applications. In CMP applications, the OH groups can form a strong Ce−O−Si bonding with silicate ions (−Ce−OH + −Si−O− ↔ −Ce−O−Si− + OH−).[13] The formation of Ce−O−Si bonding directly corresponds with the polishing efficiency of CeO2 NPs.[13] The high affinity and binding energy with silicate ions lead to an increase in the formation of Ce−O−Si bonding, which can increase the removal rate of SiO2 films.[14-17] In our previous study, the surface function effects (e.g., −OH , −NO3 groups) on the interaction between CeO2 and silicate ions were studied through adsorption isotherms and theoretical analyses.[15] Besides the surface functionalities, the surface Ce oxidation state (Ce3+/Ce4+) can have a significant influence on the interactions of CeO2 NPs with silicate ions. However, there is still a lack of understanding on the role of the Ce oxidation state (Ce3+/Ce4+) in the CeO2 NPs for the interactions with silicate ions. Herein, we prepared the CeO2 NPs at different Ce3+ concentrations, and the corresponding 3
adsorption properties of silicate ions onto CeO2 surfaces were studied. The Ce3+ concentration on the surface depends on the size of the CeO2 NPs. The Ce3+ concentration increased with decreasing NP size due to an increase of the surface to volume ratio of the NPs.[18-21] To prepare the CeO2 NPs at different Ce3+ concentration, three different-sized CeO2 NPs (small, mid, and large CeO2, hereafter noted as S-CeO2, M-CeO2, and L-CeO2, respectively) were synthesized in supercritical water (SCW).[22] The adsorption properties of silicate ions onto three different-sized CeO2 NPs were investigated through adsorption isothermal analysis. Adsorption isotherm data for the silicate ions onto CeO2 surfaces were fitted using the Langmuir and Freundlich models.
2. EXPERIMENTAL SECTION Synthesis of CeO2 NPs CeO2 NPs were synthesized as follows. Precursor solutions: To prepare S-CeO2 NPs, ammonium cerium nitrate was dissolved at 20.0 wt% in de-ionized water (DIW). To obtain MCeO2 and L-CeO2 NPs, 37.5 wt% cerium nitrate solution was prepared in DIW. SCW: DIW was preheated to 400, 470 and 350 °C for the S-CeO2, M-CeO2 and L-CeO2 NPs, respectively. Precursor solutions and SCW were simultaneously delivered at a rate of 40 and 160 mL∙min-1, respectively, into the reactor. The reactors were controlled at the same temperature using SCW. The obtained CeO2 NPs were rapidly and continuously collected. They were filtered, washed and dried in an oven at 100 °C for 12 h. Before the characterization, CeO2 NPs were further dried at 100 °C in a vacuum oven for 12 h to remove residual water. Material characterization The CeO2 NPs were characterized using various tools as follows. X-ray diffraction patterns of CeO2 NPs were estimated using an X-ray diffraction analyzer (XRD, Bruker, New D8 4
Advance). The specific surface areas were measured by the Brunauer−Emmett−Teller (BET) method using N2 gas adsorption at 77 K (BET, Quantachrome, Autosorb-1). Electron microscope images were obtained by a transmission electron microscope (TEM, JEOL, JEM2100F). The average sizes (dTEM) of the CeO2 NPs were calculated from the TEM images using the ImageJ analysis software (average 200 particles counted). The Ce3+ concentrations in CeO2 NPs were calculated using an X-ray photoelectron spectrometer (XPS, Thermo Fisher Scientific Co., theta probe base system). The surface characteristics of CeO2 NPs were analyzed using Fourier transform infrared spectroscopy (FT-IR, Nicolet 5700, ThermoElectron) using an attenuated transmission reflectance (ATR, Smart Miracle, Pike Tech.) accessory with a ZnSe crystal. The weight loss of CeO2 NPs was estimated using a thermo gravimetric analyzer with a mass spectrometer (TGA-MS, SDT Q600, TA Instruments) in the temperature range of 40 to 600 °C at a ramping rate of 10 °C/min in N2. It was used to calculate the number of −OH groups per area (#OH/nm2) of CeO2 NPs. Adsorption isotherm The adsorption behaviors of silicate ions on the CeO2 NPs were determined through the solution-depletion method using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 7300 DV, Perkin–Elmer). CeO2 suspensions were prepared as a function of the silicate ions. The pH was adjusted to 7.0 by the addition of a HNO3 and NH4OH solutions. Suspensions were aged for 12 h at room temperature under a mixer. Then, they were centrifuged at 30,000 rpm for 20 min to obtain the unabsorbed silicate ions. Their supernatants were filtered using a 0.02 µm Anotop 25 syringe filter, and then measured using ICP. The adsorption isotherms of silicate ions on the CeO2 NPs were derived from the difference between the added and remaining amount of silicate ions in the supernatants.[20]
5
3. RESULTS AND DISCUSSION We prepared three samples (S-CeO2, M-CeO2, and L-CeO2) at different Ce3+ concentrations to understand the role of the Ce oxidation state (Ce3+/Ce4+) of CeO2 NPs for interactions with silicate ions. The particle sizes (dBET) of the CeO2 NPs were calculated from dBET = 6000/(SSABET∙ρ), where SSABET is the specific surface area (m2/g) and ρ is the density of CeO2 (7.2 g/cm3). The resulting dBET values were 11.8 (S-CeO2), 63.1 (M-CeO2), and 320.5 nm (LCeO2) (Table 1). Figure 1 shows the XRD patterns of the CeO2 NPs. All the CeO2 NPs have a well crystalline cubic fluorite structure (JCPDS 65-5923). The crystallite sizes (dXRD) were calculated using the full-width at half-maximum of the (111) peak from the Scherrer equation, and the lattice parameters were obtained from the (111) diffraction peak position. The dXRD values of the CeO2 NPs are summarized in Table 1. Figure 1 (b) shows a change in the (111) peak in the XRD patterns of the CeO2 NPs. The (111) peak is shifted toward a lower 2 theta value as the size of the CeO2 NPs decreased. As the particle size dXRD of CeO2 decreased from 64.4 to 9.6 nm, the lattice parameter increased from 0.5406 to 0.5425 nm, a 3.5% increase (inset in Fig. 1). The lattice parameter of S-CeO2 (0.5425 nm) was higher than that of bulkCeO2 (0.5403 nm) (JCPDS 65-5923), which is attributed to the increase of O vacancies with an increasing surface to volume ratio.[19-21, 23] The formation of O vacancies left two free electrons on the Ce ions at the surface, leading to a reduction of Ce4+ to Ce3+. This change in the oxidation state of the Ce ions leads to a lattice expansion of the CeO2 structure (cubic fluorite) because the ionic radius of Ce3+ (1.143 Å) is bigger than that of Ce4+ (0.970 Å).[19] The TEM images and fast Fourier transformed (FFT) patterns of the CeO2 NPs are shown in Figure 2. The size distributions of the CeO2 NPs were measured from the TEM images (insets in Fig. 2). Both M-CeO2 and L-CeO2 NPs have a faceted shape whereas S-CeO2 NPs have a rounded shape. The corresponding FFT patterns were recorded from the [110] zone axis, 6
showing that the CeO2 NPs are single crystalline. The agglomeration of S-CeO2 was observed because the active sites (e.g., Ce3+ ions) at the surface easily react with other sites on the neighboring CeO2 NPs.[24, 25] On the contrary, less agglomeration was observed for both MCeO2 and L-CeO2. The particle size determination based on TEM images is quite depen dent on the number of measured particle micrographs, not like the colligative measure ment of XRD and BET method, showing the same trend (Table 1). In order to investigate the Ce oxidation state (Ce3+/Ce4+) at the CeO2 surface, XPS analysis was carried out. The Ce 3d5/2 and Ce 3d3/2 spectra are shown in Figure 3. For quantitative calculations of the Ce3+ concentration in CeO2 NPs, All the XPS spectra were deconvoluted into ten separate peaks using the mixed Gaussian–Lorentzian function. The binding energies and area percents of the individual peaks of CeO2 NP are as listed in Table 2. The labels v and u refer to the graphs of Ce 3d5/2 and Ce 3d3/2, respectively. The v0, v2, u0 and u2 peaks are attributed to Ce3+, while the v1, v3, v4, u1, u3, and u4 peaks are related to Ce4+.[26, 27] The concentration of Ce3+ ions in the CeO2 NPs was calculated from the total integrated area of the u and v graphs. As shown in Table 2, the Ce3+ concentration increased from 19.3 to 27.6 % with decreasing the particle size dTEM from 236.0 to 9.9 nm. These results support that smaller CeO2 NPs, having a high surface to volume ratio, are easily reduced and have a higher concentration of Ce3+. CeO2 NPs can be used for various applications. In aqueous media, the Ce3+ ions at the particle surface act as active sites for H2O dissociation, resulting in the formation of −OH groups. To identify the surface density of –OH groups at CeO2 surfaces, we measured the FTIR-ATR spectra of the CeO2 NPs (Figure 4 (a)). The strong peaks in the range of 600-750, 1320, and 1425 cm-1 are associated with the CeO2 stretching frequency,[28] the O−C=O group,[29] and NH4+ species,[30] respectively. A weak band at 1630 cm-1 was observed due to the bending 7
mode of H2O.[31] The –OH stretching bands of CeO2 NPs are observed from 3200 to 3600 cm1
.[32] The intensity of the peak, which is related to the OH groups, significantly increased with
decreasing particle size, which may be attributed to the high concentration of Ce3+ ions on the surface of S-CeO2. To quantitatively analyze –OH groups at CeO2 surfaces, we calculated the number of −OH groups per unit area (#OH/nm2) through the TGA weight loss. Mueller showed that −OH group surface density is given as[33] #OH/nm2 =
2∙(WtT1 −WtT2 )∙NA SSA∙MwH2O
,
(1)
where WtTi is the weight of NPs at the temperature Ti, MwH2O is the molecular weight of H2O, and NA is Avogadro’s constant.[33] Figure 4 (b) shows the TGA graphs of CeO2 NPs in the temperature range 40–600 °C under an N2 environment. The weight loss of CeO2 NPs from 40 °C to 120 °C is attributed to the removal of the physically adsorbed H2O. In the temperature range from 120 °C (T1) to 500 °C (T2), a condensation reaction of −OH groups on the particle surface occurs. Previous work reveals that no free −OH function is available on the surface at 500 °C.[33] The OH surface density of CeO2 NPs was calculated from the TGA weight loss data (Eq. 1) within the temperature range of 120 °C to 500 °C. The characteristics (impurities and OH surface density) of CeO2 NPs, measured by TGA-MS measurement, are shown in Table 1. It was calculated using the weight loss from a reduction of OH groups without impurities. S-CeO2 has the highest value at 0.72 OH/nm2, while M-CeO2 and L-CeO2 have values of 0.68 and 0.34 OH/nm2, respectively. The large amount of −OH groups of S-CeO2 is attributed to the relatively high Ce3+ concentration at the surface (Fig. 3.), where Ce3+ dissociates with H2O to form OH groups.[12] We found that the density of the −OH groups on the CeO2 surface increased with increasing Ce3+ concentration of the CeO2 NPs. OH groups play an important role in the various interactions at the solid/aqueous interface. 8
They can form strong Ce−O−Si bonding with silicate ions.[13, 34] Figure 5 shows the adsorption behaviors of silicate ions onto the CeO2 surface. In the graphs, S-CeO2 has shown the highest slope, and both M-CeO2 and L-CeO2 followed in consecutive order. The Langmuir and Freundlich adsorption isotherms are widely used as models to fit the adsorption equilibrium data. In this study, the Langmuir adsorption assumes that the adsorption of silicate ions occurs on a homogeneous surface via monolayers.[35] It is expressed by the following equation: Ce/Qe = Ce/Qm + 1/(KLQm),
(2)
where Qe is the amount of adsorbed silicate ions per specific surface area of CeO2 at equilibrium (mg/m2), Qm is the amount of maximum adsorbed of silicate ions, Ce is the silicate ions concentration in the solution (mg/L), and KL is related to the adsorption affinity for silicate ions (L/mg). The Freundlich adsorption assumes that the multilayer adsorption of silicate ions occurs on a CeO2 surface via heterogeneous systems.[36] The Freundlich model is calculated using the following equation: Qe = KFCe1/n,
(3)
where Qe is the amount of adsorbed silicate ions per specific surface area of CeO2 at equilibrium (mg/m2) and Ce is the equilibrium concentration of silicate ions in the solution (mg/L). The Freundlich constant KF and 1/n represent the relative adsorption capacity and adsorption intensity, respectively. The values of KF and 1/n can be obtained from the intercept and slope of a linear plot. log Qe = log KF +1/n log Ce
(4)
Table 3 shows the Langmuir and Freundlich isotherm constants. The results show that the correlation coefficient value of the Freundlich model is higher than that of the Langmuir model, 9
which implies that the adsorption behavior of silicate ions onto the CeO2 NPs was better fitted to the Freundlich model than the Langmuir model. This indicates that the multilayer adsorption of silicate ions occurs through interactions between the adsorbed and non-adsorbed silicate ions.[37, 38] To elucidate the adsorption behavior of the silicate ions, an understanding on the adsorption isotherm constants is very important. A higher value of KF represents the relatively higher adsorption capacity of silicate ions. A smaller value of 1/n indicates the higher affinity between CeO2 NP and silicate ions.[39] While increasing the Ce3+ concentration from 19.3 to 27.6 %, the KF values increased from 0.0048 to 0.0570, and the 1/n values decreased from 0.7650 to 0.6771. This result indicates that CeO2 NPs with a high Ce3+ concentration at the surface show higher affinity with silicate ions.
4. CONCLUSION In summary, to investigate the role of the Ce oxidation state (Ce3+/Ce4+) on the CeO2 NPs, we prepared three different CeO2 NPs (S-CeO2, M-CeO2, and L-CeO2) at different Ce3+ concentrations. As the particle size dTEM of CeO2 decreased from 236.0 to 9.9 nm, the Ce3+ concentrations at the surface increased from 19.3 to 27.6 % due to an increase in the surface to volume ratio. In aqueous media, these Ce3+ ions at the particle surface act as active sites for the formation of −OH groups through H2O dissociation. Smaller CeO2 NPs, having high Ce3+ concentrations at the surface, have a high surface density of −OH groups. To analyze the adsorption properties of silicate ions onto the CeO2 NPs, adsorption isotherm data were fitted using the Langmuir and Freundlich models. The Freundlich model was better fitted with the experimental data than the Langmuir model when comparing the correlation coefficient value (R2) of models. The Freundlich constants (KF and 1/n), related to the adsorption affinity of silicate ions, indicated that the adsorption affinity of silicate ions increases with decreasing NP 10
size due to the high Ce3+ concentrations and –OH number density. We believe that our study can provide researchers with a detailed understanding of the physicochemical properties of CeO2 NPs for various interactions (e.g., molecular adsorption, desorption) occurring at the solid/aqueous interface in many fields.
Acknowledgement This work was supported by the Technology Innovation Program (201500000001419) funded by the Ministry of Trade, industry & Energy (MI, Korea)
References [1] A. Trovarelli, Catalytic properties of ceria and CeO2-containing materials, Catal Rev, 38 (1996) 439-520. [2] E.P. Murray, T. Tsai, S.A. Barnett, A direct-methane fuel cell with a ceria-based anode, Nature, 400 (1999) 649-651. [3] P. Jasinski, T. Suzuki, H.U. Anderson, Nanocrystalline undoped ceria oxygen sensor, Sensors and Actuators B: Chemical, 95 (2003) 73-77. [4] X.D. Feng, D.C. Sayle, Z.L. Wang, M.S. Paras, B. Santora, A.C. Sutorik, T.X.T. Sayle, Y. Yang, Y. Ding, X.D. Wang, Y.S. Her, Converting ceria polyhedral nanoparticles into singlecrystal nanospheres, Science, 312 (2006) 1504-1508. [5] J. Seo, J. Moon, J.-Y. Bae, K.S. Yoon, W. Sigmund, U. Paik, Control of Adhesion Force Between Ceria Particles and Polishing Pad in Shallow Trench Isolation Chemical Mechanical Planarization, Journal of nanoscience and nanotechnology, 14 (2014) 4351-4356. [6] C.T. Campbell, C.H. Peden, Oxygen vacancies and catalysis on ceria surfaces, Science, 309 (2005) 713-714. 11
[7] T. Sayle, S. Parker, C. Catlow, The role of oxygen vacancies on ceria surfaces in the oxidation of carbon monoxide, Surf Sci, 316 (1994) 329-336. [8] F. Esch, S. Fabris, L. Zhou, T. Montini, C. Africh, P. Fornasiero, G. Comelli, R. Rosei, Electron localization determines defect formation on ceria substrates, Science, 309 (2005) 752755. [9] P. Dutta, S. Pal, M. Seehra, Y. Shi, E. Eyring, R. Ernst, Concentration of Ce3+ and oxygen vacancies in cerium oxide nanoparticles, Chemistry of materials, 18 (2006) 5144-5146. [10] J. Xu, J. Harmer, G. Li, T. Chapman, P. Collier, S. Longworth, S.C. Tsang, Size dependent oxygen buffering capacity of ceria nanocrystals, Chemical Communications, 46 (2010) 18871889. [11] B. Choudhury, P. Chetri, A. Choudhury, Oxygen defects and formation of Ce3+ affecting the photocatalytic performance of CeO2 nanoparticles, RSC Advances, 4 (2014) 4663-4671. [12] L. Kundakovic, D.R. Mullins, S.H. Overbury, Adsorption and reaction of H2O and CO on oxidized and reduced Rh/CeOx(111) surfaces, Surf Sci, 457 (2000) 51-62. [13] L.M. Cook, Chemical Processes in Glass Polishing, J Non-Cryst Solids, 120 (1990) 152171. [14] T.S. Sreeremya, M. Prabhakaran, S. Ghosh, Tailoring the surface properties of cerium oxide nanoabrasives through morphology control for glass CMP, RSC Advances, 5 (2015) 84056-84065. [15] J. Seo, J.W. Lee, J. Moon, W.M. Sigmund, U. Paik, The Role of Surface Chemistry of Ceria Surfaces on the Silicate Adsorption, Acs Applied Materials & Interfaces, (2014). [16] D. Volkov, P.V. Dandu, H. Goodman, B. Santora, I. Sokolov, Influence of adhesion of silica and ceria abrasive nanoparticles on Chemical–Mechanical Planarization of silica surfaces, Applied Surface Science, 257 (2011) 8518-8524. 12
[17] J. Seo, U. Paik, 11 - Preparation and characterization of slurry for chemical mechanical planarization (CMP) A2 - Babu, Suryadevara, in: Advances in Chemical Mechanical Planarization (CMP), Woodhead Publishing, 2016, pp. 273-298. [18] S. Tsunekawa, R. Sahara, Y. Kawazoe, K. Ishikawa, Lattice relaxation of monosize CeO 2− x
nanocrystalline particles, Applied surface science, 152 (1999) 53-56.
[19] S. Tsunekawa, R. Sivamohan, S. Ito, A. Kasuya, T. Fukuda, Structural study on monosize CeO2-x nano-particles, Nanostruct Mater, 11 (1999) 141-147. [20] L. Wu, H. Wiesmann, A. Moodenbaugh, R. Klie, Y. Zhu, D. Welch, M. Suenaga, Oxidation state and lattice expansion of CeO2− x nanoparticles as a function of particle size, Physical Review B, 69 (2004) 125415. [21] X.-D. Zhou, W. Huebner, Size-induced lattice relaxation in CeO2 nanoparticles, Applied Physics Letters, 79 (2001) 3512. [22] T. Adschiri, K. Kanazawa, K. Arai, Rapid and Continuous Hydrothermal Synthesis of Boehmite Particles in Subcritical and Supercritical Water, J Am Ceram Soc, 75 (1992) 26152618. [23] R. Hailstone, A. DiFrancesco, J. Leong, T. Allston, K. Reed, A study of lattice expansion in CeO2 nanoparticles by transmission electron microscopy, The Journal of Physical Chemistry C, 113 (2009) 15155-15159. [24] S.-K. Kim, Y.-H. Kim, U. Paik, J.-G. Park, Influence of Crystalline Structure of Ceria on the Remaining Particles in the STI CMP, Journal of The Electrochemical Society, 154 (2007) H642-H646. [25] Y.-H. Kim, S.-K. Kim, N. Kim, J.-G. Park, U. Paik, Crystalline structure of ceria particles controlled by the oxygen partial pressure and STI CMP performances, Ultramicroscopy, 108 (2008) 1292-1296. 13
[26] S. Deshpande, S. Patil, S.V. Kuchibhatla, S. Seal, Size dependency variation in lattice parameter and valency states in nanocrystalline cerium oxide, Applied Physics Letters, 87 (2005) 133113-133113. [27] C. Korsvik, S. Patil, S. Seal, W.T. Self, Superoxide dismutase mimetic properties exhibited by vacancy engineered ceria nanoparticles, Chemical Communications, (2007) 1056-1058. [28] J.-S. Lee, S.-C. Choi, Crystallization behavior of nano-ceria powders by hydrothermal synthesis using a mixture of H2O2 and NH4OH, Materials Letters, 58 (2004) 390-393. [29] C. Chen, M. Nasrallah, H. Anderson, Synthesis and characterization of (CeO2) 0.8 (SmO1. 5) 0.2
thin films from polymeric precursors, Journal of the Electrochemical Society, 140 (1993)
3555-3560. [30] A. Vimont, J.-C. Lavalley, L. Francke, A. Demourgues, A. Tressaud, M. Daturi, Infrared study of the surface properties of HTB-type Al-, Cr-, Fe-hydroxyfluorides, The Journal of Physical Chemistry B, 108 (2004) 3246-3255. [31] S.E. Collins, J.M. Cies, E. del Rio, M. Lopez-Haro, S. Trasobares, J.J. Calvino, J.M. Pintado, S. Bernal, Hydrogen interaction with a ceria-zirconia supported gold catalyst. Influence of CO co-adsorption and pretreatment conditions, The Journal of Physical Chemistry C, 111 (2007) 14371-14379. [32] M. Daturi, E. Finocchio, C. Binet, J.C. Lavalley, F. Fally, V. Perrichon, Study of Bulk and Surface Reduction by Hydrogen of CexZr1-xO2 Mixed Oxides Followed by FTIR Spectroscopy and Magnetic Balance, The Journal of Physical Chemistry B, 103 (1999) 4884-4891. [33] R. Mueller, H.K. Kammler, K. Wegner, S.E. Pratsinis, OH surface density of SiO2 and TiO2 by thermogravimetric analysis, Langmuir, 19 (2003) 160-165. [34] P. Munusamy, S. Sanghavi, T. Varga, T. Suntharampillai, Silica supported ceria nanoparticles: a hybrid nanostructure to increase stability and surface reactivity of nano14
crystalline ceria, RSC Advances, 4 (2014) 8421-8430. [35] I. Langmuir, THE CONSTITUTION AND FUNDAMENTAL PROPERTIES OF SOLIDS AND LIQUIDS. PART I. SOLIDS, J Am Chem Soc, 38 (1916) 2221-2295. [36] H. Freundlich, Over the adsorption in solution, J. Phys. Chem, 57 (1906) 385-470. [37] V. Puddu, C.C. Perry, Peptide adsorption on silica nanoparticles: evidence of hydrophobic interactions, ACS nano, 6 (2012) 6356-6363. [38] J.L. Steinbacher, C.C. Landry, Adsorption and release of siRNA from porous silica, Langmuir, 30 (2013) 4396-4405. [39] A.W. Adamson, A.P. Gast, Physical chemistry of surfaces, (1967).
15
Table and Figure captions. a
Table 1. Physicochemical properties of CeO2 NPs with different sizes. Determined from SBET. b
Calculated from the H2O contents in the TGA weight loss; MS results indicated that H2O
contents in the TGA weight loss are 77.98 (S-CeO2), 77.80 (M-CeO2), and 79.70 % (L-CeO2). Table 2. XPS binding energies (eV) and peak areas (%) of individual peaks of Ce 3d for CeO2 NPs. Table 3. Langmuir and Freundlich constants for the adsorption of silicate ions on the surface of CeO2 NPs. Figure 1. (a) XRD patterns of CeO2 NPs. (b) Magnified view of the (111) peak in XRD. The inset presents the lattice parameter of CeO2 NPs as a function of particle size. The curve is obtained by the equation (α = αbulk + 0.036/D) in ref 22. Figure 2. TEM images of (a) S-CeO2, (b) M-CeO2, and (c) L-CeO2. The insets in the figures are particle size distribution, HR-TEM images and FFT patterns. Figure 3. Ce3d XPS spectra of (a) S-CeO2, (b) M-CeO2, and (c) L-CeO2. -1
Figure 4. (a) FTIR-ATR spectra of CeO2 NPs in the region of 4000-600 cm . (b) TGA graphs o
of CeO2 NPs scanned at a heating rate of 10 C/min in an N2 atmosphere; Step1: The desorption of physically adsorbed H2O. Step2: The dehydroxylation of CeO2 NPs. Figure 5. (a) Langmuir plots and (b) Freundlich plots for adsorption isotherm of the silicate ions on the surface of CeO2 NPs.
16
SBET (m2/g) dBETa (nm) dTEM (nm) dXRD (nm) S-CeO2 M-CeO2 L-CeO2
70.6 13.2 2.6
11.8 63.1 320.5
9.9 56.1 236.0
9.6 40.5 64.4
Lattice parameter (nm) 0.5425 0.5417 0.5406
#OH/nm2b 0.72 0.68 0.34
a
Table 1. Physicochemical properties of CeO2 NPs with different sizes. Determined from SBET. b
Calculated from the H2O contents in the TGA weight loss; MS results indicated that H2O
contents in the TGA weight loss are 77.98 (S-CeO2), 77.80 (M-CeO2), and 79.70 % (L-CeO2).
17
Ce 3d3/2
Ce 3d5/2 v0 Peak assignment
Ce3+ Binding energy (eV) 880.7 S-CeO2 Peak area (%) 2.3 Binding energy (eV) 880.1 M-CeO2 Peak area (%) 0.7 Binding energy (eV) 880.0 L-CeO2 Peak area (%) 1.0
v1
v2
v3
v4
u0
u1
u2
u3
u4
Ce4+ 882.8 17.1 882.7 20.1 882.9 24.5
Ce3+ 885.7 15.8 885.9 14.2 886.7 10.9
Ce4+ 889.5 11.3 889.6 10.9 889.8 7.2
Ce4+ 898.1 12.8 898.2 11.7 898.3 16.1
Ce3+ 899.4 4.0 899.0 2.8 899.4 3.2
Ce4+ 901.2 15.4 901.0 16.0 901.2 13.5
Ce3+ 904.8 5.5 904.1 6.3 903.6 4.2
Ce4+ 908.1 4.7 907.9 5.8 907.8 4.7
Ce4+ 916.9 11.2 917.1 11.7 917.0 14.8
Ce3+ (%) 27.6 23.9 19.3
Table 2. XPS binding energies and peak areas of individual peaks of Ce 3d for CeO2 NPs.
18
Langmuir constant 2
Qm (mg/m ) KL (L/mg) S-CeO2 M-CeO2 L-CeO2
9.2165 4.5004 1.8372
0.0016 0.0014 0.0009
Freundlich constant 2
R
KF
1/n
R2
0.9388 0.9363 0.9243
0.0570 0.0207 0.0048
0.6771 0.7135 0.7650
0.9646 0.9639 0.9797
Table 3. Langmuir and Freundlich constants for the adsorption of silicate ions on the surface of CeO2 NPs.
19
(a)
(b)
Figure 1. (a) XRD patterns of CeO2 NPs. (b) Magnified view of the (111) peak in XRD. The inset presents the lattice parameter of CeO2 NPs as a function of particle size. The curve is obtained by the equation (α = αbulk + 0.036/D) in ref 22.
20
(a)
Counts
Counts
(c)
Counts
(b) 6
8
10
12
14
16
20
Particle Size (nm)
40
60
80
100 120
Particle Size (nm)
100 150 200 250 300 350 400
Particle Size (nm)
Figure 2. TEM images of (a) S-CeO2, (b) M-CeO2, and (c) L-CeO2. The insets in Figures are particle size distribution, HR-TEM images and FFT patterns.
21
v1
(c)
v2 v3
v4 u0
v0 v1
(b)
(a) v0
u4 u2 u3
v2 v3
v4 u1 u0 u2 u3
u4
v2 v3
v4 u1 u0 u2 u3
u4
v0 v1
u1
Figure 3. Ce3d XPS spectra of (a) S-CeO2, (b) M-CeO2, and (c) L-CeO2.
22
(a)
(b)
-1
Figure 4. (a) FTIR-ATR spectra of CeO2 NPs in the region of 4000-600 cm . (b) TGA graphs o
of CeO2 NPs scanned at heating rate 10 C/min in N2 atmosphere; Step1: The desorption of physically adsorbed H2O. Step2: The dehydroxylation of CeO2 NPs.
23
7
Qe (mg/m2)
(a)
S-CeO2 M-CeO2 L-CeO2
6 5 4 3 2 1 0 0
200
400
600
800
1000
1200
1000
1200
Ce (mg/L) 7
S-CeO2 M-CeO2 L-CeO2
Qe (mg/m2)
(b) 6 5 4 3 2 1 0 0
200
400
600
800
Ce (mg/L) Figure 5. (a) Langmuir plots and (b) Freundlich plots for adsorption isotherm of the silicate ions on the surface of CeO2 NPs.
24
