Surface Ce Research Articles

Insensitivity of the extent of surface reduction of ceria on termination: comparison of (001), (110), and (111) faces

Abstract

Study on Ceria–Silica Interaction in Various Ce3+ Concentration for Effective Post Chemical Mechanical Planarization (CMP) Cleaning

Shallow trench isolation (STI) is a technology that isolates transistors by constructing trenches and filling them with silicon dioxide. The excess silica should be removed by a subsequent step, chemical mechanical planarization (CMP) process, without any damage to active area. To prevent the damage during polishing, the silicon nitride is used for a stopping layer.1 Therefore, in the CMP process in STI, oxide-to-nitride polishing selectivity is needed. Since ceria nanoparticles (NPs) has high removal rate on silicon oxide but low removal rate on silicon nitride, they are generally utilized for CMP in STI.2 Additionally, Ce3+ concentration on the ceria NPs is considerably related to the oxide-to-nitride selectivity. It was reported that as the Ce3+ concentration on ceria NPs increased, its oxide-to-nitride selectivity was improved.3 However, after CMP process, there exists large amount of contaminants on the substrates which is mainly originated from the abrasives and organic additives in CMP slurry. Especially, on the surface of silicon oxide, the residual ceria NPs caused problems in post-CMP cleaning. Since they have chemical interaction with silicon oxide as reported, they are not easily removed from the surface. As the demands for high-end semiconductor devices with enhanced performance soar, the contaminants should be successfully removed.4-6 Therefore, in-depth research on ceria-silica interaction for effective post-CMP cleaning receives large attention.We directly investigated the ceria-silica interaction using atomic force microscope (AFM) and quartz crystal microbalance (QCM). Using AFM, the adhesion energy between silicon oxide and ceria NPs was quantitatively measured with respect to Ce3+ concentration on the ceria NPs as shown in Figure 1. We changed the surface Ce3+ concentration on the ceria NPs using hydrogen peroxide and ultrasonication, which was classified as Ceria1 to Ceria4.7 Moreover, the adsorption behavior of ceria NPs on the surface of silicon oxide was observed by QCM depending on the surface Ce3+ concentration. From the results of adsorption rate, the activation energy for the ceria-silica adsorption was studied depending on the Ce3+ concentration on the ceria NPs, which is exhibited in Figure 2.Figure CaptionsFigure 1. Adhesion energy between ceria and silica depending on surface Ce3+ concentration.Figure 2. Plot of the natural logarithm of adsorption rate versus the reciprocal of temperature at different surface Ce3+ concentrations of ceria NPs.References M. C. Kang, J. J. Kim and D.-K. Moon, Jpn. J. Appl. Phys., 44, 5949 (2005).R. Srinivasan, P. V. R. Dandu and S. V. Babu, ECS J. Solid State Sci. Technol., 4, P5029 (2015).K. Kim, D. K. Yi and U. Paik, ECS J. Solid State Sci. Technol., 6, P681 (2017).H. S. Philip Wong, Solid-State Electron., 49, 755 (2005).M. Tsujimura, ECS J. Solid State Sci. Technol., 8, P3098 (2019).C. K. Ranaweera, N. K. Baradanahalli, R. Popuri, J. Seo and S. V. Babu, ECS J. Solid State Sci. Technol., 8, P3001 (2018).J. F. Changjian Ma, Jiaxiang Chen, Yaoyao Wen, Paul O Fasan, Hua Zhang, Nuowei Zhang,* Jinbao Zheng, and Bing-Hui Chen, Ind. Eng. Chem. Res., 56, 9090 (2017). Figure 1

Improved reactivity for toluene oxidation on MnOx/CeO2-ZrO2 catalyst by the synthesis of cubic-tetragonal interfaces

In this study, the CeO2-ZrO2 solid solutions with cubic-tetragonal interfaces were prepared by an urea hydrolysis method and a solvothermal method. Then they were used as the supports and Mn/Ce-Zr catalysts were prepared by impregnation method. The catalytic performance of the catalysts for toluene oxidation were evaluated by a steady-state measurement and compared with that of Mn/Ce-Zr-ss without the interfaces, in which the support was prepared by co-precipitation. The result was found that the catalyst interfaces apparently enhanced catalytic activity for toluene oxidation. The coexistence of cubic and tetragonal phases and their interfaces were confirmed by XRD and HRTEM. H2-TPR and XPS results indicated that the cubic-tetragonal interfaces led to stronger interaction between surface Mn and Ce species and higher concentration of lattice oxygen produced from the catalyst surface. DRIFT spectra revealed that the catalyst interface promoted the adsorption of toluene and the formation of key intermediates at low temperatures. These would be important factors that caused a higher activity of the catalyst with cubic-tetragonal interfaces. Moreover, the promoting role of the interfaces might be related to the morphology, surface area and interface components of Ce-Zr supports.

Direct and quantitative study of ceria–SiO2 interaction depending on Ce3+ concentration for chemical mechanical planarization (CMP) cleaning

The importance of chemical mechanical planarization (CMP) and post-CMP cleaning is highly critical because both directly affect the yields and performance of semiconductor devices. Ceria nanoparticles are widely used as abrasives for SiO2 CMP because strong ceria–SiO2 interaction enhances CMP performance. However, the interaction is a major drawback in post-CMP cleaning processes where residual ceria nanoparticles must be completely removed from the SiO2 surface. The interaction depends on the cerium oxidation state of the ceria nanoparticles surface. In this study, the ceria–SiO2 interaction was investigated systemically using an atomic force microscope and a quartz crystal microbalance. The analysis revealed that the adhesion energy between ceria and SiO2 and the adsorption rate of ceria nanoparticles on a SiO2 surface increased with the surface concentration of Ce3+. This is direct evidence showing that the ceria–SiO2 interaction becomes stronger as the surface Ce3+ concentration of ceria nanoparticles increases, suggesting that the surface Ce3+ concentration should be reduced for effective post-CMP cleaning.

Unexpected differences between surface and bulk spectroscopic and implied Kondo properties of heavy fermion CeRh2Si2

Ultra-violet angle-resolved photoemission spectroscopy (UV-ARPES) was used to explore the temperature dependence of the Ce-4f spectral responses for surface and bulk in the antiferromagnetic Kondo lattice CeRh2Si2. Spectra were taken from Ce- and Si-terminated surfaces in a wide temperature range, and reveal characteristic 4f patterns for weakly (surface) and strongly (bulk) hybridized Ce, respectively. The temperature dependence of the Fermi level peak differs strongly for both cases implying that the effective Kondo temperature at the surface and bulk can be rather distinct. The greatly reduced crystal–electric-field (CEF) splitting at the surface gives reason to believe that the surface may exhibit a larger effective Kondo temperature because of a higher local-moment effective degeneracy. Further, the hybridization processes could strongly affect the 4f peak intensity at the Fermi level. We derived the k-resolved dispersion of the Kondo peak which is also found to be distinct due to different sets of itinerant bands to which the 4f states of surface and bulk Ce are coupled. Overall our study brings into reach the ultimate goal of quantitatively testing many-body theories that link spectroscopy and transport properties, for both the bulk and the surface, separately. It also allows for a direct insight into the broader problem of Kondo lattices with two different local-moment sublattices, providing some understanding of why the cross-talking between the two Kondo effects is weak.

Single crystalline CeO2 nanotubes

Here, we report a facile synthetic methodology to prepare uniform single crystalline CeO2 nanotubes through a hydrothermal transformation of CeO2 nanorods in aqueous Ce(NO3)3 solution. A chemically driven etching-dissolution-deposition mechanism is proposed, involving the surface Ce3+ hydrolysis and dissolution at tips of nanorods and subsequent redeposition and crystallization on the outer sides of nanorods. Compared to CeO2 nanorods, CeO2 nanotubes exhibited the richer structural defects, higher reducibility and larger surface area, leading to a higher haloperoxidase-like activity.

1-Hydroxydodecylidene-1,1-diphosphonic acid flotation of bastnäsite: Performance and mechanism

To separate and enrich rare-earth elements (REE) minerals from their ores through flotation process has become an attracting topic for REE’s strategic position. In the present investigation, the floatability of bastnäsite [(Ce,La)CO3F] was explored by using 1-hydroxydodecylidene-1,1-diphosphonic acid (HDDPA) as a collector. The micro-flotation results showed that bastnäsite displayed an eminent floatability under pH 7.0–10.0, and HDDPA realized an efficient separation of bastnäsite from calcite at pH around 8.0. Zeta potential and solution chemistry inferred that at pH 7.5–9.3, HDDPA3− anions were glad to replace the CO32- ions of Ce(Ⅲ) carbonates in/on bastnäsite interface, resulting in a high flotation recovery of bastnäsite. With increasing OH- concentration, the competition ability of OH- against HDDPA anion intensified, gradually the surface Ce(III) hydroxide species increased, and the floatability of bastnäsite reduced. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) recommended that the surface Ce(III) of bastnäsite interacted with the O atoms of HDDPA’s OP(O)O group to generate a four membered ring configuration, which hydrophobized bastnäsite particles.

Adsorptive desulfurization of thiophene over Ti0.9Ce0.1O2 mixed oxide: A mechanistic study on the basis of XPS, in-situ FT-IR and TPD characterizations

The adsorption of thiophenic compounds over Ti0.9Ce0.1O2, an oxidatively regenerable sulfur adsorbent, was studied by XPS, DRIFT, and TPD-MS. Effect of the oxidative pretreatment on thiophene adsorption over Ti0.9Ce0.1O2 adsorbent, the adsorption site for thiophenic compound, and the role of surface Ce and Ti cations in the adsorptive desulfurization (ADS) were investigated. The results suggest that the surface oxygen plays an essential role in ADS and is a major active site for chemical adsorption of the thiophenic compounds. The surface Ce and Ti were further oxidized by the surface oxygen in the oxidative pretreatment, and then reduced during the adsorption of thiophenic compounds. This phenomenon is attributed to an electron transfer from the adsorbates to the surface Ce and Ti. It appears that the role of the surface Ce and Ti can accept electrons from adsorbate, and also contributes to producing the active oxygen to oxidize the sulfur atom of thiophenic compounds. An oxidative pretreatment, including oxidative regeneration, activates the surface oxygen to forms active oxygen species and oxidizes the surface Ce and Ti. Based on the results obtained, a possible mechanism was proposed for the chemical adsorption, dissociation and surface reaction of thiophene on Ti0.9Ce0.1O2 mixed metal oxide adsorbent.

Adsorptive desulfurization of jet fuels over TiO2-CeO2 mixed oxides: Role of surface Ti and Ce cations

A Ti0.9Ce0.1O2 oxide-based adsorbent with high surface area mesopores was studied for adsorptive desulfurization of jet fuel (JP-5: 1055 ppm-w of sulfur) using several techniques, including GC-PFPD, N2 adsorption-desorption, XRD, XPS and TPD. Ti0.9Ce0.1O2 oxide-based adsorbent effectively adsorbed sulfur and achieved the sulfur reduction of the jet fuel from 1055 ppm-w to lower than 1 ppm-w in multiple cycles of adsorption in a fixed-bed flow system. The spent adsorbent can be regenerated in-situ at the elevated temperature by using air. Some of the sulfur atom in the organic sulfur compounds could be oxidized to form sulfate species by reaction with the surface active oxygen species, while the surface (Ti4+, Ce4+) cations of adsorbent were reduced simultaneously upon adsorption of the sulfur compounds. The formed sulfate species were further removed from the surface during the oxidative regeneration step under an air flow. The deactivation of the adsorbent was potentially caused by sintering and metal cation migration on the surface.

Relationships between the activities and Ce3+ concentrations of CeO2(111) for CO oxidation: A first-principle investigation

CO oxidation at ceria surfaces has been studied for decades, and many efforts have been devoted to understanding the effect of surface reduction on the catalytic activity. In this work, we theoretically studied the CO oxidation on the clean and reduced CeO2(111) surfaces using different surface cells to determine the relationships between the reduction degrees and calculated reaction energetics. It is found that the calculated barrier for the direct reaction between CO and surface lattice O drastically decreases with the increase of surface reduction degree. From electronic analysis, we found that the surface reduction can lead to the occurrence of localized electrons at the surface Ce, which affects the charge distribution at surface O. As the result, the surface O becomes more negatively charged and therefore more active in reacting with CO. This work then suggests that the localized 4f electron reservoir of Ce can act as the “pseudo-anion” at reduced CeO2 surfaces to activate surface lattice O for catalytic oxidative reactions.

Energy-Guided Shape Control Towards Highly Active CeO2

The shape of nanosized CeO2, obtained via polyvinylpyrrolidone (PVP) micelles, was controlled by microwave (MW)-aided synthesis combined with different combinations of energy input/transfer, including ultrasound (US), ultraviolet (UV) and pressure (P). Whereas ceria nanoflakes resulted from standard solvothermal synthesis, CeO2 nanoparticles were obtained from MW, MW + US and MW + UV. New CeO2 “nanospindles” (with aspect ratio of 2) resulted from MW + US + UV, and nanorods (with aspect ratio of 11) emerged from MW + P. All ceria morphologies, even nanospindles and nanorods, were mesoporous agglomerates of small CeO2 nanocrystals (6–8 nm size), but they still exhibited different specific surface area (SSA) and Ce3+/Ce4+ ratio. Underlying reasons of how the different synthesis routes affect the ceria morphology are discussed. Among the six types, CeO2 nanorods (MW + P) exhibited the highest SSA (196 m2 g−1) and the most surface defects (Ce3+: 26.4%), resulting in excellent catalytic performance in imine synthesis and CO oxidation.

Hydrogen generation by ammonia decomposition over Co/CeO2 catalyst: Influence of support morphologies

The fabrication of nanomaterial is a crucial issue in heterogeneous catalysis for on-site generation of hydrogen in proton exchange membrane fuel cell to achieve excellent performance for ammonia decomposition, and their effects of morphologies are still mysterious in the structure-reactivity relationship. In order to disclose it, three kinds of CeO2 supports with three-dimensionally ordered mesoporous structure (3DOM), nanotubes (NT) and nanocubes (NC) were synthesized by nanocasting of a mesoporous silica KIT-6 template with cubic Ia3d symmetries, hydrothermal method with and without urea, respectively. Various characterization methods (XRD, BET, H2-TPR, CO-TPD, TEM and XPS) were used to characterize the structure-reactivity relationship of catalysts. The Co/CeO2-3DOM catalyst had higher H2 producing rate (4.2 mmol/min･gcat) than Co/CeO2-NC (3.5 mmol/min･gcat) and Co/CeO2-NT (3.2 mmol/min･gcat) under the reaction conditions of 500 °C and GHSV = 6000 mL/gcat･h. The Co/CeO2-3DOM catalysts presented Co nanoparticles with mean size of 5.2 nm, and the highest surface Co concentration (5.12%) and Ce3+/Ce4+ ratio (0.53). Its high activity is attributed to higher surface area and more surface oxygen vacancies. The specific surface area and surface oxygen vacancy are significantly affected by the morphology of CeO2 support. The more mechanism insight of the structure-activity relationship for ammonia decomposition has been revealed.

Nanoisozymes: The Origin behind Pristine CeO2 as Enzyme Mimetics.

It is known that the interplay between molecules and active sites on the topmost surface of a solid catalyst determines its activity in heterogeneous catalysis. The electron density of the active site is believed to affect both adsorption and activation of reactant molecules at the surface. Unfortunately, commercial X-ray photoelectron spectroscopy, which is often adopted for such characterization, is not sensitive enough to analyze the topmost surface of a catalyst. Most researchers fail to acknowledge this point during their catalytic correlation, leading to different interpretations in the literature in recent decades. Recent studies on pristine Cu2 O [Nat. Catal. 2019, 2, 889; Nat. Energy 2019, 4, 957] have clearly suggested that the electron density of surface Cu is facet dependent and plays a key role in CO2 reduction. Herein, it is shown that pristine CeO2 can reach 2506/1133 % increase in phosphatase-/peroxidase-like activity if the exposed surface is wisely selected. By using NMR spectroscopy with a surface probe, the electron density of the surface Ce (i.e., the active site) is found to be facet dependent and the key factor dictating their enzyme-mimicking activities. Most importantly, the surface area of the CeO2 morphologies is demonstrated to become a factor only if surface Ce can activate the adsorbed reactant molecules.

XNi/Ni0.05Ce0.20Zr0.75O2 Solid Solution over a CO2 Methanation Reaction

A Ni-modified ceria–zirconia support with Ni impregnation at different Ni-loading catalysts for a CO2 methanation reaction was systematically studied. The corresponding structures of each catalyst were characterized by Brunauer–Emmett–Teller surface area analysis, X-ray powder diffraction, X-ray fluorescence, H2 temperature-programmed reduction, CO2 temperature-programmed desorption, Fourier transform infrared, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric/differential thermal analysis. The results showed that Ni-modified Ce–Zr oxide improved the basic properties as well as the oxygen vacancies. A gradual increase in Ni loading from 15 to 45 wt % was found to increase medium-strong basic sites, and surface Ce3+and Ni0 species along with oxygen vacancies favor high activity. The CO2 methanation activity was related to the amount of Ni loadings where the 45Ni/Ni005CZO catalyst was reported to be the most active catalyst. This is due to the high amount of Ni surface suitable for H2 activation and high medium basic sites to accommodate CO2 activation, leading to high catalytic activity.

Unravelling the Role of Structural Geometry and Chemical State of Well-Defined Oxygen Vacancies on Pristine CeO2 for H2O2 Activation.

Although H2O2 has been often employed as a green oxidant for many CeO2-catalyzed reactions, the underlying principle of its activation by surface oxygen vacancy (Vo) is still elusive due to the irreversible removal of postgenerated Vo by water (or H2O2). The metastable Vo (ms-Vo) naturally preserved on pristine CeO2 surfaces was adopted herein for an in-depth study of their interplay with H2O2. Their well-defined local structures and chemical states were found facet-dependent affecting both the adsorption and subsequent activation of H2O2. It is concluded that a strong adsorption of H2O2 on ms-Vo may not guarantee its subsequent activation. The ms-Vo can be only free for the next catalytic cycle when the electron density of surface Ce is high enough to reduce/break the O-O bond of adsorbed H2O2. This explains the 211.8 and 35.8 times enhancement in H2O2 reactivity when the CeO2 surface is changed from (111) and (110) to (100).

The relationship of morphology and catalytic performance of CeO2 catalysts for reducing nitrobenzene to azoxybenzene under the base-free condition

CeO2 morphology was proposed to be a crucial factor for reducing nitrobenzene to azoxybenzene under the base-free condition. Herein, the structure-activity relationship of CeO2 catalysts was explored to improve the azoxybenzene yield. A series of CeO2 catalysts were synthesized with seven morphologies to obtain different Ce3+ proportion and various surface areas. Notably, the catalytic performance of these samples for reducing nitrobenzene to azoxybenzene enhanced with the increasing Ce3+ proportion. With the highest surface Ce3+ proportion, the Rod-CeO2 catalyst exhibited 100% conversion of nitrobenzene and 89.8% azoxybenzene selectivity in 7 h at 150 °C under 1 MPa CO. Moreover, the preliminary mechanistic analysis indicated that the inhabitation of azoxybenzene to by-product azobenzene resulted in the high selectivity of azoxybenzene.

Effect of Zr-doping on Pd/Ce Zr1−O2 catalysts for oxidative carbonylation of phenol

Abstract Ce–Zr solid solution (CexZr1−xO2, CZO) was prepared by the citric acid sol–gel method. The CZO was then used as a support for Pd/CZO catalysts for the oxidative carbonylation of phenol to diphenyl carbonate. The Pd/CZO catalyst showed enhanced activity and diphenyl carbonate selectivity compared with the Pd/CeO2 catalyst. The catalytic performance of Pd/CZO was influenced by the calcination temperature of the CZO support. X-ray diffraction, scanning electron microscopy, N2 adsorption–desorption measurements, X-ray photoelectron spectroscopy and H2 temperature-programmed reduction measurements were used to investigate the effects of Zr doping and calcination temperature. The catalytic performance of Pd/CZO and Pd/CeO2 for the oxidative carbonylation of phenol was affected by several factors, including the specific surface area, Ce3+ and/or oxygen vacancy content, oxygen species type and Pd(II) content of the catalyst. All these properties were influenced by Zr doping and the calcination temperature of the CZO support.

Simultaneous removal of NO and Hg0 in flue gas over Co-Ce oxide modified rod-like MnO2 catalyst: Promoting effect of Co doping on activity and SO2 resistance

The Co was firstly used as a dopant to improve the NO and Hg0 removal performance of CeO2 loaded rod-like MnO2 catalyst (Ce/MnO2) and its SO2 resistance. The results showed the doping process effectively improved the relative concentrations of chemisorbed oxygen as well as the contents of surface Ce4+ and Mn4+, enhanced the redox ability of catalyst, which was conducive to the simultaneous removal performance of NO and Hg0 in flue gas. The Co-Ce/MnO2 catalyst (the mole ratio of Co/Ce = 1.0) had optimal catalytic performance among all prepared catalysts in 160–280 °C, its NO and Hg0 removal efficiencies achieved 99.9% and 91.9% at 200 °C and 10,000 h−1, respectively. In addition, the Co-Ce/MnO2 performed stronger resistance to SO2 in comparison with Ce/MnO2, its catalytic activities decreased slightly when SO2 with a concentration of 500 ppm was added into the gas stream, and then recovered close to the initial value after eliminating of SO2. We considered the excellent SO2 tolerance performance of Co-Ce/MnO2 catalyst is assigned to the fact that the Co-modification restrained the sulfation of CeO2 and MnO2 to CeSO4 and MnSO4, alleviated the decreasing of chemisorbed oxygen, and hindered the chemisorption of SO2 on the catalyst. This work provided a promising prospect of Co-modification on Ce/MnO2 as SO2 resistant catalysts for simultaneous removal of NO and Hg0 from flue gas.

Adsorption-enhanced nitrogen-doped mesoporous CeO2 as an efficient visible-light-driven catalyst for CO2 photoreduction

Direct converting of CO2 to valuable fuels via solar-powered photoreduction is a promising “green chemistry” route. The key issue for this approach is the exploitation of low-cost and high-efficiency photocatalysts. Here, a novel nanoflakelet-constructed nitrogen-doped CeO2 with relatively ordered mesostructure (NMCe) is prepared through a facile and cheap urea-assisted template method. In comparison with nanocasted ordered mesoporous CeO2 (OMCe) with the similar textural properties, the NMCe shows improved CO2 adsorption ability and has lowered band gap energy, higher surface Ce3+ concentration and more surface oxygen vacancies. Owing to these characteristics, the NMCe exhibits considerably enhanced photocatalytic performance for visible-light-driven reduction of CO2, where the yields of CO and CH4 are 2.5-fold higher than those on OMCe.

Surface configuration modulation for FeOx-CeO2/γ-Al2O3 catalysts and its influence in CO oxidation

The surface configuration of a catalyst, including the present state, coordination environment and related content of the active and additive species, has dominant effect on its physicochemical properties and catalytic performance. Herein, two types of FeCe/Al catalysts with different surface configurations, FeCe/Al-CI (co-impregnation) and FeCe/Al-SI (stepwise impregnation) were synthesized by adjusting the preparation conditions. Both catalysts together with the reference sample Fe/Al and Ce/Al were characterized by means of XRD, BET, TEM, EPR, UV–vis, Mössbauer spectra, H2-TPR and XPS. The results showed that (1) in both impregnation modes, ceria presented as small particles, while the iron species existed in two forms: the isolate state (tetrahedral coordination), cluster state (octahedral coordination); (2) iron species in different existence state can be characterized qualitatively or quantitatively via EPR, UV–vis and Mössbauer spectra, and the contents of FeOx clusters follows: FeCe/Al-SI < FeCe/Al-CI < Fe/Al. (3) With the help of HNO3 leaching operation, FeOx clusters were identified as the main active species for CO oxidation. Although Fe/Al sample contained the highest concentration of active cluster FeOx species, the deficiency of Fe-O-Ce structures led to the lowest number of surface oxygen, Ce3+ and Fe2+ species, which resulted in poor CO oxidation activity for Fe/Al catalyst.