Synthesis of a Flaky CeO2 with Nanocrystals Used for Polishing

It is important to adapt the morphology of CeO2 to different applications. A novel flaky CeO2 with nanocrystals was successfully synthesized using the ordinal precipitation method and calcination. The size of the flaky CeO2 was about 10 μm, and the nanocrystals were about 100 nm. Under the action of the precipitant NH4HCO3, Ce3+ nucleated in large quantities. The nanosized crystals gathered into flakes driven by the surface energy. As the calcination temperature increased, the grains grew slowly by mass transfer due to the slow diffusion of reactants. By adding AlOOH to the starting material, the Al3+ doped into the CeO2 increased the content of Ce3+ in the CeO2, which improved the chemical activity of the CeO2. When the starting material’s Al:Ce ratio was 5:1, the Ce3+ increased to 31.11% in the CeO2, which provided good application potential in the polishing field. After polishing by the slurry of flaky CeO2 for 1 h, the SiC surface roughness reduced from 464 nm to 11 nm.


Introduction
The crystal structure of cerium oxide is an open fluorite structure consisting of Ce 4+ arranged in a face-centered cubic arrangement and O 2− occupying tetrahedral positions.Due to the intrinsic characteristics of atomic hybridization to form a cubic fluorite lattice structure, cerium oxide has intrinsic Ce 3+ and its content is easily affected by doping and the formation of oxygen vacancies [1].Such a structure makes it easy to form oxygen vacancies and perform element doping, and it maintains the cubic structure of fluorite with good chemical stability [2].Due to its excellent properties, cerium oxide is currently used in a wide range of fields, such as CO 2 conversion catalysis [3], photocatalysis [4], ultraviolet absorption [5], biomaterials [6], thermal coatings [7] and polishing abrasives [8].
The issue of how to prepare specific sizes and morphologies of cerium oxide is of great significance when it comes to adapting it to different applications.At present, researchers have successfully prepared CeO 2 from different dimensionalities, including nanospherelike and nanowire-shaped, nanoflakes, and nanocubes through different preparation methods.Among them, two-dimensional cerium oxide materials have huge application potential because of their high specific surface area and high anisotropy.However, they are difficult to synthesize due to a lack of intrinsic driving force during growth.Thus, only several reports discussed the synthesis of two-dimensional cerium oxide.C.H. Lu et al. prepared rhombic cerium hydroxycarbonate through the hydrothermal method [9].L. Chen et al. synthesized larger-sized flaky cerium oxide by preparing precursors from the solution with a low concentration and pyrolysis [10].J. Gong et al. controlled the morphology of the precursor CeOHCO 3 through the hydrothermal method and synthesized equilateral triangular nanosheets of CeO 2 with excellent optical properties [11].
In this paper, a novel flaky CeO 2 composed of nanocrystals was prepared and the nanocrystal formation mechanism was discussed.The potential application of this material was explored in the field of polishing.Since aluminum is difficult to replace in cerium oxide crystals during the preparation process, a small amount of the Al element was doped into the lattice of the CeO 2 to enhance its performance in the polishing process.In contrast, the Al 3+ , which did not dope in the CeO 2 , transformed into Al 2 O 3 after precipitation and calcination.The Al 2 O 3 was mixed with CeO 2 to improve the mechanical strength.
Firstly, a certain amount of Al (NO 3 ) 3 was dissolved in deionized water, and then Al (NO 3 ) 3 solution and NH 4 HCO 3 solution as a precipitant were added in parallel flow.Magnetic stirring was used during the reaction.The reaction product was suction-filtrated, dried, and milled to prepare AlOOH.
As AlOOH is an amphoteric oxyhydroxide, it can react with an acid or a base.When the hydrolysis pH value is below 5 or above 10, some AlOOH precipitates will dissolve [12].The AlOOH was dissolved in 1 mol/L Ce (NO 3 ) 3 solution at a pH of 4.3 in the reaction beaker, where the 2 mol/L NH 4 HCO 3 solution as a precipitant was added drop by drop to raise the pH value to 7.5.At this stage, part of the AlOOH dissolved into Al 3+ , while the incompletely dissolved AlOOH helped with Ce 3+ nucleation in the early stage of precipitation.After the reaction, the reaction product was aged for 24 h.The precipitates were filtered and dried at 120 • C for 2 h and calcined at different temperatures with a temperature increase rate of 10 • C/min.To investigate the effects of different calcination temperatures and Al contents, the samples were divided into groups numbered 1-4, consistent with 0:1, 1:5, 1:1, and 5:1 of the atomic ratios of Al:Ce, respectively.Then, the samples with 1:1 of Al:Ce were calcined at 350, 700, 900, and 1200 • C.

Chemical Mechanical Polishing (CMP)
To compare the polishing efficiency and effectiveness of the flaky CeO 2 powder and commercial spherical CeO 2 abrasives, the two abrasives were configured with Na 2 CO 3 solution to form a simple polishing slurry of 3 wt.%.After thinning, SiC wafers were cut into 10 mm × 10 mm plates and polished with a flow rate of 10 mL/min and a rotational speed of 240 ppm for 1 h.

Characterization
Thermogravimetric Analysis (TG) and Differential Thermal Analysis (DTA) on a DTG-60H (Shimadzu, Shanghai, China) analyzed the precursors' phase and mass changes with an increasing temperature.The phase analysis of the precursors and powders after calcination at different temperatures was carried out by X-ray diffraction (XRD) on a XRD6100 diffractometer (Shimadzu, Shanghai, China) with Cu Kα radiation (λ = 1.5418Å); the XRD patterns were collected in the range of 2θ = 10 • -90 • .The changes in the chemicalbonding state of the elemental Ce due to changes in the ratio of Ce to Al in the CeO2 were investigated using X-ray photoelectron spectroscopy (XPS) on a Kratos AXIS Supra, with a monochromatic Al Kα X-ray source (hν = 1486.6eV).All the binding energies were related to the C1s peak (284.8 eV).
The morphology of the samples was determined on a field emission scanning electron microscope (FESEM) Apreo S (Thermo Scientific™, Shanghai, China) and a transmission electron microscope (TEM) Tecnai G2 F20 S-TWIN (FEI, Shanghai, China).
The SiC surfaces before and after the polishing were compared to discuss the change in the surface.The surface profile and roughness were measured using a Contour GT-K optical profilometer from Bruker Nano Inc., Billerica, MA, USA.

Thermal Analysis and Composition
As shown in Figure 1, the TG/DSC curves of the 6.5 mg precursors composed of CeOHCO

Thermal Analysis and Composition
As shown in Figure 1, the TG/DSC curves of the 6.5 mg precursors composed of CeO HCO3 and AlOOH ramping up at room temperature-1200 °C showed the first noticeabl mass loss between 0 and 200 °C.The DSC curve showed a prominent heat absorption pea at 192.92 °C.In addition, the second more pr onounced mass loss occurred at 200-318 °C The DTG curve showed a minimum at 191.27 °C, corresponding to the first mass loss an heat absorption.The minimum at 271.67 °C corresponds to the second weight loss.Th TG/DSC curves showed that the precursor had a phase transformation when the calcina tion temperature was above 200 °C and other components changed during 200-318 °C.The theoretical quality change was calculated according to the inferred compositio change.The weight loss curve of the first section with a large slope is related to the re moval of free water and crystalline water from the sample.After a 10% weight loss, th precursor of 6.5 mg should contain CeOHCO3 weight 4.30 mg and Al(OH)3 weight 1.5 mg, with a molar ratio of 1:1.In the second stage of the weight loss, CeOHCO3 is converted into CeO2 and theoretically loses 0.90 mg (13.85%), while later Al(OH)3 is converted int Al2O3 and theoretically loses 0.19 mg (8.31%).The results of the theoretical calculation ar As the temperature increases to 700 °C and 900 °C, the diffraction peaks of CeO2 become sharper, implying better crystallinity.In terms of Scherrer's formula, the average grain size of the sample can be calculated as 29.3 nm at 350 °C, 42.1 nm at 700 °C, and 49.5 nm at 900 °C based on the diffraction peak of about 28.5°.

Morphology and Microstructure
The excess Ce 3+ in the reaction beaker was nucleated in large quantities using the ordinal precipitation method.Then, these fine grains were aggregated to form large flakes of material, as shown in Figure 3a.
The morphology of the samples calcined at different temperatures is shown in Figure 3b-d.All the powders are flaky-like, mainly octagonal when the temperature is below 900 As the temperature increases to 350 (2) The theoretical quality change was calculated according to the inferred composition change.The weight loss curve of the first section with a large slope is related to the removal of free water and crystalline water from the sample.After a 10% weight loss, the precursor of 6.5 mg should contain CeOHCO 3 weight 4.30 mg and Al(OH) 3 weight 1.54 mg, with a molar ratio of 1:1.In the second stage of the weight loss, CeOHCO 3 is converted into CeO 2 and theoretically loses 0.90 mg (13.85%), while later Al(OH) 3 is converted into Al 2 O 3 and theoretically loses 0.19 mg (8.31%).The results of the theoretical calculation are consistent with the TG curve obtained in the experiment and confirm the composition change mechanism we inferred.
As the temperature increases to 700 • C and 900 • C, the diffraction peaks of CeO 2 become sharper, implying better crystallinity.In terms of Scherrer's formula, the average grain size of the sample can be calculated as 29.3 nm at 350 • C, 42.1 nm at 700 • C, and 49.5 nm at 900 • C based on the diffraction peak of about 28.5 • .

Morphology and Microstructure
The excess Ce 3+ in the reaction beaker was nucleated in large quantities using the ordinal precipitation method.Then, these fine grains were aggregated to form large flakes of material, as shown in Figure 3a.
The morphology of the samples calcined at different temperatures is shown in Figure 3b-d.All the powders are flaky-like, mainly octagonal when the temperature is below 900 • C, and grow to hexagonal at 900 • C. Some particles attached to the precursors and the powders after calcination.
The nanoparticles grow gradually to 400 nm for small grains and 1 µm for big grains when the temperature rises to 1200 • C, as shown in Figure 3e,f.
To determine the morphological characteristics of the aluminum oxide and cerium oxide in the sample, EDS was used to detect the composition of the sample calcined at 900 • C. As shown in Figure 4, the morphology of the cerium oxide and aluminum oxide in the product is quite different.Cerium oxide is a large-sized flake material, while aluminum oxide is an agglomerate of nanoparticles.
To observe the nanocrystals in the flaky CeO 2 , a TEM was used to investigate the morphology of CeO 2 calcined at 900 • C. The sizes of the nanocrystal grains in CeO 2 calcined at 900 • C are about 50-150 nm (Figure 5a).It was accounted for that the precursor obtained using ordinal precipitation was composed of nano-sized grains.The nanograins in the flaky material are finer and have a uniform size distribution.The grains grew slowly as the temperature increased to 900 • C. Based on Figure 5b showing the HRTEM images, the highly crystallized cerium oxide nanograins have distinct lattice lines resolved at 3.123 and 3.118 Å, which belong to the (111) crystal planes of CeO 2 crystals.Among several low-index lattice planes of CeO 2 , the (111) crystallographic plane is the one with higher stability [16], which explains that the principal crystallographic planes observed in the TEM images are (111) crystallographic planes.The nanoparticles grow gradually to 400 nm for small grains and 1 µm for big grains when the temperature rises to 1200 °C, as shown in Figure 3e-f.
To determine the morphological characteristics of the aluminum oxide and cerium oxide in the sample, EDS was used to detect the composition of the sample calcined at 900 °C.As shown in Figure 4, the morphology of the cerium oxide and aluminum oxide in the product is quite different.Cerium oxide is a large-sized flake material, while aluminum oxide is an agglomerate of nanoparticles.The nanoparticles grow gradually to 400 nm for small grains and 1 µm for big grains when the temperature rises to 1200 °C, as shown in Figure 3e-f.
To determine the morphological characteristics of the aluminum oxide and cerium oxide in the sample, EDS was used to detect the composition of the sample calcined at 900 °C.As shown in Figure 4, the morphology of the cerium oxide and aluminum oxide in the product is quite different.Cerium oxide is a large-sized flake material, while aluminum oxide is an agglomerate of nanoparticles.The growth mechanism is inferred from the morphological observation of the flaky nanocrystalline CeO 2 material, as shown in Figure 6.As precipitant was added, Ce 3+ rapidly nucleated into fine grains of CeOHCO 3 .At the same time, AlOOH acted as a template for the nucleation sites.AlOOH gradually dissolved into the cerium nitrate solution to release Al 3+ .Driven by the excellent surface energy, many fine grains were aggregated and formed flaky CeOHCO 3 .The pH value kept rising with the continuous addition of the precipitant, and AlOOH was transformed to Al(OH) 3 .Subsequently, during the calcination process, CeOHCO 3 was converted into CeO 2 with an increasing temperature under an atmosphere of air.Al(OH) 3 transformed into Al 2 O 3 , which enhanced the mechanical strength of the abrasives.Tiny grains kept fusing and growing to become large with the increase in the temperature.Normally, CeO 2 particles grow gradually as the temperature increases.However, the grains grow slowly as the temperature increases from 300 • C to about 900 • C. When the temperature increases to 1200 • C, the grains proliferate to the micrometer level.It can be considered that the slow diffusion of reactants limits the chemical reaction, meaning that the CeO 2 grains grow in a small range by mass transfer [17].
To observe the nanocrystals in the flaky CeO2, a TEM was used to investigate the morphology of CeO2 calcined at 900 °C.The sizes of the nanocrystal grains in CeO2 calcined at 900 °C are about 50-150 nm (Figure 5a).It was accounted for that the precursor obtained using ordinal precipitation was composed of nano-sized grains.The nanograins in the flaky material are finer and have a uniform size distribution.The grains grew slowly as the temperature increased to 900 °C.Based on Figure 5b showing the HRTEM images, the highly crystallized cerium oxide nanograins have distinct lattice lines resolved at 3.123 and 3.118 Å, which belong to the (111) crystal planes of CeO2 crystals.Among several lowindex lattice planes of CeO2, the (111) crystallographic plane is the one with higher stability [16], which explains that the principal crystallographic planes observed in the TEM images are (111) crystallographic planes.The growth mechanism is inferred from the morphological observation of the flaky nanocrystalline CeO2 material, as shown in Figure 6.As precipitant was added, Ce 3+ rapidly nucleated into fine grains of CeOHCO3.At the same time, AlOOH acted as a template for the nucleation sites.AlOOH gradually dissolved into the cerium nitrate solution to release Al 3+ .Driven by the excellent surface energy, many fine grains were aggregated and formed flaky CeOHCO3.The pH value kept rising with the continuous addition of the precipitant, and AlOOH was transformed to Al(OH)3.Subsequently, during the calcination process, CeOHCO3 was converted into CeO2 with an increasing temperature under an atmosphere of air.Al(OH)3 transformed into Al2O3, which enhanced the mechanical strength of the abrasives.Tiny grains kept fusing and growing to become large with the increase in the temperature.Normally, CeO2 particles grow gradually as the temperature increases.However, the grains grow slowly as the temperature increases from 300 °C to about 900 °C.When the temperature increases to 1200 °C, the grains proliferate to the micrometer level.It can be considered that the slow diffusion of reactants limits the chemical reaction, meaning that the CeO2 grains grow in a small range by mass transfer [17].

Chemical State of Ce
The Ce 3+ in CeO2, rather than the Ce 4+ , is believed to improve the performance in polishing [7].To investigate the effect of the content of the element Al on the chemical state of CeO2, samples 1-4 were quantitatively analyzed by X-ray photoelectron spectroscopy

Chemical State of Ce
The Ce 3+ in CeO 2 , rather than the Ce 4+ , is believed to improve the performance in polishing [7].To investigate the effect of the content of the element Al on the chemical state of CeO 2 , samples 1-4 were quantitatively analyzed by X-ray photoelectron spectroscopy (XPS), as shown in Figure 7. Table 1 lists the detailed information of each XPS peak assignment.For the Ce 3d signal in the samples, it can fit the peaks of Ce 3d 5/2 and Ce 3d 3/2 , and after the curve-fitting, the Ce 3d 2/5 peaks were labeled as v 0 , v 1 , v 2 , v 3 , v 4 , and the Ce 3d 3/2 peaks were labeled as u 0 , u 1 , u 2 , u 3 , u 4 .Moreover, v 0 , v 2 , u 0 , u 2 belonged to Ce 3+ , while the v 1 , v 3 , v 4 , u 1 , u 3 , u 4 peaks belonged to Ce 4+ ions [18].The Ce 3+ content in the samples was calculated by measuring the area of each peak of the Ce 3+ to the area of all the Ce 3d peaks.The Ce 3+ content of the control group was 26.00%.When the content of Al was less, it had no obvious effect on the CeO 2 .With the gradual increase in the Al content at the ratio of Al:Ce of 1:1, the Ce 3+ content reaches 28.48%.When the ratio of Al:Ce rises to 5:1, the Ce 3+ content reaches 31.12% and has enhancement compared with that of the control group.Powder consisting of flaky CeO2 with Al2O3 (about 10 µm, calcined at 900 °C and added AlOOH in the ratio Ce:Al = 1:5) and common spherical CeO2 abrasive (2.3 µm) were used to polish SiC wafers to compare the polishing performance.In Figure 8a, the SiC surface before polishing exhibits surface characteristics of alternating high and low elon- 3 and AlOOH ramping up at room temperature-1200 • C showed the first noticeable mass loss between 0 and 200 • C. The DSC curve showed a prominent heat absorption peak at 192.92 • C. In addition, the second more pr onounced mass loss occurred at 200-318 • C. The DTG curve showed a minimum at 191.27 • C, corresponding to the first mass loss and heat absorption.The minimum at 271.67 • C corresponds to the second weight loss.The TG/DSC curves showed that the precursor had a phase transformation when the calcination temperature was above 200 • C and other components changed during 200-318 • C.

Figure 1 .
Figure 1.Thermal analysis curves of the precursors composed of CeOHCO3 and AlOOH.

Figure 1 .
Figure 1.Thermal analysis curves of the precursors composed of CeOHCO 3 and AlOOH.The XRD patterns of the precursor prepared by ordinal precipitation and powder consisting of CeO 2 and Al 2 O 3 after calcination at different temperatures are shown in Figure 2. The precursor shows diffraction peaks of CeOHCO 3 consistent with 41-0013 of the Joint Committee on Powder Diffraction Standards (JCPDS).This is because NH 4 HCO 3 , the precipitant, provides CO 3 2− ions and creates an alkaline precipitation environment.In addition, there are diffraction peaks of Al(OH) 3 consistent with 24-0006, which are from AlOOH combined with H 2 O [13].

Figure 2 .
Figure 2. XRD patterns of (a) precursor and powder calcined at 200 °C, and (b) powder calcined at different temperatures.

Figure 2 .
Figure 2. XRD patterns of (a) precursor and powder calcined at 200 • C, and (b) powder calcined at different temperatures.CeO 2 with a cubic fluorite structure is confirmed at 200 • C, and the diffraction peaks of the precursor CeOHCO 3 are not observed.CeOHCO 3 transforms into CeO 2 according to

Figure 4 .
Figure 4. EDS images of the sample calcined at 900 °C.

Figure 4 .
Figure 4. EDS images of the sample calcined at 900 °C.

Figure 4 .
Figure 4. EDS images of the sample calcined at 900 • C.

Figure 6 .
Figure 6.Illustration of the formation mechanism of flaky nanocrystal CeO 2 .
• C, Al 2 O 3 consistent with 31-0026 is confirmed and the diffraction peaks of Al(OH) 3 cannot be observed, which means that Al(OH) 3 is decomposed into Al 2 O 3 and H 2 O according to Equation (2) [15] 2Al(OH) 3 →Al 2 O 3 + 3H 2 O