Effect of NaOH on the preparation of two-dimensional flake-like zirconia nanostructures

doi:10.1016/j.cplett.2020.137755

Chemical Physics Letters

Volume 754, September 2020, 137755

https://doi.org/10.1016/j.cplett.2020.137755 Get rights and content

Highlights

•
The ZrO₂ flakes dominated by (0 0 1) plane were obtained by a clean molten salt method.
•
The as-prepared ZrO₂ lamellae show regular shape and favorable dispersibility.
•
NaOH has a distinct promoting effect on nucleation and growth of zirconia.
•
The changes in phase and shape originate from martensite transformation of ZrO₂.

Abstract

The monolayer flake-like zirconia was prepared by a clean molten salt method, which does not require any adsorbent and template. The effects of NaOH on the composition, morphology and structure of zirconia were studied by XRD, IR, Raman, FE-SEM and TEM. The nanostructures synthesized under 2 wt% NaOH exhibit definite flake-like geometry with a large percentage of (0 0 1) plane, which measure approximately 1.8 μm long and 80 nm thick. This concomitant phase and morphology change originate from the martensite transformation of metastable tetragonal zirconia. The as-prepared zirconia sheet has potential utility in fields as diverse as chemical adsorption and photocatalysis.

Graphical abstract

Introduction

In recent years, the syntheses of monodispersed nanocrystals with tailored morphologies and geometries have become a central issue in the field of materials chemistry. The ability for size and shape control of nanomaterials plays a crucial role in their application potentials. Among them, nano-zirconia is noteworthy for its advantageous optical, electrical, thermal, catalytic and mechanical properties with corresponding wide-ranging applications such as fuel cells, oxygen sensors, transparent optical devices, electrochemical capacitor electrodes, thermal barrier coatings, and bioceramics [1], [2], [3], [4].

As a typical two-dimensional material, flake-like zirconia exhibits several special structural features including large specific surface area, high surface activity, ultra-thin and large percentage of particular facet, making it a promising candidate for adsorbent, catalyst or catalyst carrier, photocatalyst, and shape memory ceramics. The Cu/ZrO₂ composite catalysts prepared by Ding et al. [5] can selectively synthesize glycol from hydrogenation of diethyl oxalate (DEO). The conversion rate of DEO was up to 98%. The sheet-like crystalline zirconia was not only beneficial for molecular diffusion, but also for adsorption and activation of carbonyl. Shehzad et al. [6] found that the adsorption ability of amorphous zirconia nanosheets for As(III) could reach 74.9 mg/g. Adding 1 mg/L of the adsorbent to real water, the drinking water standard of As(III) will be met in 0.25 h. Shen and co-workers [7] fabricated a polyvinylidene fluoride (PVDF)/ZrO₂ composite film using monoclinic layered zirconia sheets as nanofillers. When 1 wt% zirconia nanosheets were added, the breakdown strength of the films was improved from 416 kV/mm to 519 kV/mm. The authors believed that the increased strength was related to the preferred orientation of nanosheets in the direction perpendicular to external electric field. So far, the preparation methods for flake-like zirconia have been mainly focused on hydro-/solvothermal processing [8], [9], sol–gel method [10] and template synthesis [6], [11]. However, some problems such as but not limited to low yield [10], non-single crystals [11] and inhomogeneous size [6] are notable in the flaky structures synthesized by above protocols. As such, the preparation methods and/or product quality restrict the generalization although flake-like zirconia has shown some interesting practicalities. It still remains an enormous challenge to synthesize thin monolayer zirconia lamellae with favorable dispersibility.

Herein, molten salt method was proposed to prepare flake-like zirconia on the basis of the semicrystalline zirconia precursors obtained through solvothermal treatment. Molten salt is a high-temperature ionic liquid containing anions and cations. The ionic property makes molten salt an ideal solvent for high valence transition metal oxides, which are difficult to dissolve in polar solvents [12], [13]. So far, molten salt method has shown extraordinary potential in the anisotropic growth of crystals. But compared with one-dimensional (1D) nanostructures, including nanowires [14], [15], nanorods [16] and nanobelts [17], few reports of two-dimensional (2D) nanostructures have been documented. Specifically, 1D nanostructure can be attributed to the extremely fast growth along a certain facet, while 2D nanostructure is due to the slow growth of a certain facet. In addition to the crystal structure, the final geometry is also affected by the dynamic (e.g. supersaturation) and thermodynamic (e.g. capping agent) factors. This work further confirmed the possibility of synthesizing 2D nanomaterials by molten salt method.

In our previous study, zirconia micro-sheets were prepared with NaCl/NaF salts [18], but the fluorides were toxic and the sheets were thick. Furthermore, high-quality zirconia nanosheets were synthesized through an improved NaCl/Na₃PO₄ salts and solvothermal treatment [19]. The growth and formation processes of zirconia nanosheets that are dependent on heat treatment schedule are systematically investigated. Based on the preliminary research, this work aims to study the effects of NaOH on the chemical composition and structure of zirconia, examine the as-prepared precursors and zirconia lamellae in detail and discuss the phase and geometry evolution of zirconia. We found that NaOH serves as mineralizer and supersaturation regulator, which allows a deep understanding of the preparation of ZrO₂ nanosheets for technological applications.

Section snippets

Experimental section

The anhydrous ethanol (C₂H₆O) used in the experiment was guaranteed reagent, purchased from Sinopharm Chemical Reagent Co., Ltd.; ZrCl₄, NaOH, NaCl and Na₃PO₄ were analytical reagents, purchased from Aladdin Biochemical Technology Co., Ltd.; Deionized water was self-made by a circulating ultrapure water system.

In conical flask, 0.14 M ZrCl₄-ethanol solution was formed by dissolving 1.3 g ZrCl₄ to 40 mL ethanol. Next, 0–4 wt% NaOH was added to the solution with continuous stirring for 1 h.

Effect of NaOH dosage on the composition of precursors

The changes of crystalline phase and bonding of precursors are studied by increasing NaOH dosage, while the other experimental parameters kept constant. As can be seen from XRD patterns in Fig. 1(a), the peaks at around 31.0°, 35.0°, 50.9° and 60.4° are in accordance with the standard PDF card no.50–1089, indicating the formation of pure tetragonal zirconia (t-ZrO₂) in all samples. According to the research of Huang and Li et al. [20], [21], the structures of tetragonal and amorphous zirconia

Conclusion

A facile wet-chemical scheme for the preparation of flake-like zirconia is developed via molten salt method. NaOH not only facilitates the nucleation and growth of zirconia, but also significantly affects the microstructure of the product by adjusting supersaturation. When 2 wt% NaOH is added, flake-like zirconia with a length of 1.8 μm and a thickness of 80 nm can be obtained in fused salts. The as-prepared lamellae show single-crystal structure with monoclinic lattice, which is dominated by

CRediT authorship contribution statement

Xuefeng Liu: Data curation, Writing - original draft. Guo Feng: Conceptualization, Methodology. Jianmin Liu: Writing - review & editing. Feng Jiang: Funding acquisition, Supervision. Ting Chen: Formal analysis, Investigation. Jian Liang: Software, Validation. Lifeng Miao: Resources. Weihui Jiang: Funding acquisition, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the National Natural Science Foundation of China [Grant No. 51662016, 51962014]; the General Projects of Key Research & Development Programs in Jiangxi Province, China [Grant No. 20192BBEL50022]; the Youth Science Foundation of Jiangxi Provincial Department of Education, China [Grant No. GJJ180740]; and the Key Science Foundation of Jiangxi Provincial Department of Education, China [Grant No. GJJ180699].

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Citation Excerpt :
However, the exposure of high-energy crystal surfaces in ZrO2 materials has rarely been explored [18]. The adsorption of F on ZrO2 is closely related to the crystal surface exposure, but the relationship between the adsorption effect and the morphology of ZrO2 has not been established [19,20]. In this work, we studied the effect of the LiF dosage on the morphology of ZrO2.
Zirconia nanorods and polyhedral particles were prepared using the molten-salt method. The effects of the LiF dosage on the ZrO₂ crystal morphology were studied using XRD combined with Rietveld refinement, FE-SEM combined with EDS, FTIR, Raman spectroscopy, and high-temperature microscopy. The results show that the ZrO₂ obtained with LiF is in the monoclinic phase. ZrO₂ nanorods were synthesized at low LiF dosages. Polyhedral ZrO₂ particles were synthesized, and the ZrO₂ crystal planes (100) and (200) were exposed to a high LiF dosage. LiF promoted the dissolution of ZrO₂ and was adsorbed onto the ZrO₂ crystal surface. This work provides a new strategy for controlling morphology and crystal surface exposure.

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Research paperEffect of NaOH on the preparation of two-dimensional flake-like zirconia nanostructures

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental section

Effect of NaOH dosage on the composition of precursors

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgement

Chem. Phys. Lett.

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J. Colloid Interface Sci.

Mater. Lett.

J. Mol. Liq.

Chem. Phys. Lett.

Ceram. Int.

Chem. Eng. J.

Solid State Commun.

Chem. Phys. Lett.

Ceram. Int.

Ceram. Int.

Ceram. Int.

Near interface ionic transport in oxygen vacancy stabilized cubic zirconium oxide thin films

Phys. Chem. Chem. Phys.

Polarization-induced interface and Sr segregation of in situ assembled La0.6Sr0.4Co0.2Fe0.8O3−δ electrodes on Y2O3–ZrO2 electrolyte of solid oxide fuel cells. ACS Appl. Mat

Interfaces.

Effect of zirconia morphology on hydrodeoxygenation of phenol over Pd/ZrO2

ACS Catal.

Enhanced breakdown strength and suppressed leakage current of polyvinylidene fluoride nanocomposites by two-dimensional ZrO2 nanosheets

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Research paper
Effect of NaOH on the preparation of two-dimensional flake-like zirconia nanostructures

Polarization-induced interface and Sr segregation of in situ assembled La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ electrodes on Y₂O₃–ZrO₂ electrolyte of solid oxide fuel cells. ACS Appl. Mat

Effect of zirconia morphology on hydrodeoxygenation of phenol over Pd/ZrO₂

Enhanced breakdown strength and suppressed leakage current of polyvinylidene fluoride nanocomposites by two-dimensional ZrO₂ nanosheets

Synthesis of flake nano-ZrO₂ by hydrothermal method