Surfactant-free microwave-assisted hydrothermal synthesis of BaMoO4 hierarchical self-assemblies and enhanced photoluminescence properties

doi:10.1016/j.jcis.2012.05.028

Journal of Colloid and Interface Science

Volume 381, Issue 1, 1 September 2012, Pages 24-29

https://doi.org/10.1016/j.jcis.2012.05.028 Get rights and content

Abstract

BaMoO₄ with 3D hierarchical multilayer disk-like and nest-like architectures self-assembled from 2D nanosheets was successfully synthesized via a microwave-assisted hydrothermal route without any surfactant. The as-prepared products were characterized by X-ray powder diffractometer (XRD), scanning electron microscope (SEM), field emission transmission electron microscope (FE-TEM), and photoluminescence (PL) spectrometer. The results show that the reaction parameters, including pH value, reactant concentration, and molar ratio of $[{Ba}^{2 +}] / [{MoO}_{4}^{2 -}]$ , played important roles on the morphologies of the final products. And the formation mechanism of 3D hierarchical architectures is a stepwise oriented aggregation-based self-assembly process. The superstructure characteristic of 3D nest-like BaMoO₄ architecture was observed in HRTEM image and the corresponding fast Fourier transform (FFT) for the first time, and the superlattice reflections with non-integer indices occurred around the subcell reflections at ±1/6(2a^* + 2c^*). Room temperature photoluminescence spectra of 3D BaMoO₄ architectures reveal a strong and broad blue emission, and the 3D nest-like architectures own the enhanced intensity than multilayer disks.

Graphical abstract

Highlights

► BaMoO₄ hierarchical architectures have been synthesized by microwave heating route. ► 3D BaMoO₄ nest-like architecture particles reveal a strong and broad blue emission. ► A stepwise oriented self-assembly mechanism was proposed. ► The superlattice reflection of 3D BaMoO₄ nest-like architecture was observed firstly.

Introduction

The properties of functional materials are relative to their morphology, microstructure, and dimensionality, so much efforts have been devoted to control these characteristics of materials [1], [2], [3], [4]. Recently, three-dimensional (3D) inorganic materials with specific morphology and size are found to play important roles in determining the chemical and physical properties of materials [5], [6], [7], [8]. For example, 3D flower-like MoS₂ nanostructures exhibit good capability in field emission due to the existence of the open edges in their nanopetals, and CdMoO₄ hollow microspheres exhibit a high photocatalytic activity because of the high active surface area. Hence, exploration of complex 3D architectures has been a hot research topic [9], [10], [11].

Over the past few years, metal molybdates have attracted much attention because of their luminescent behavior, structural determined properties, and potential applications [8], [12], [13]. Among the molybdates materials, BaMoO₄ with a sheelite structure is an important material in electrooptics areas [14]. So far, many methods, including solid-state reaction, precipitation method, hydrothermal method, and microemulsion-mediated route, have been used to prepare a variety of BaXO₄ (X = Mo, W) with 3D architectures [15], [16], [17]. All these synthesis processes, however, usually require high temperature, long reaction time, and/or surfactant. And comparing to the aforementioned processes, microwave-assisted hydrothermal method can provide the following advantages [18], [19], [20], [21]: (i) high heating rates, thus increasing the reaction rates and reducing the reaction times, (ii) excellent control of reaction parameters, (iii) selective activation of target precursor to initiate nucleation and subsequent growth, (iv) improve the reproducibility from batch-to-batch, (v) excellent control size and morphology of product, and (vi) enhance product purity or material properties. On the basis of the advantages of microwave-assisted hydrothermal method for chemical synthesis of inorganic materials, Longo et al. [22] synthesized the hierarchical assembly of CaMoO₄ nano-octahedrons, and Marques et al. [23] discussed the effect of different solvent ratios (water/ethylene glycol) on the growth process CaMoO₄ crystals and indicated that the photoluminescence (PL) emission maximum can be linked to crystal shape/intrinsic defects in the surface and mainly to possible distortions on the tetrahedron [MoO₄] clusters in the lattice caused by the microwave irradiation.

In this paper, we report a facile surfactant-free microwave-hydrothermal approach to synthesize uniform BaMoO₄ hierarchical architectures. And the influence of pH value, reactant concentration, and molar ratio of $[{Ba}^{2 +}] / [{MoO}_{4}^{2 -}]$ on the morphological evolution, microstructure, and optical property of 3D BaMoO₄ hierarchical architectures has been systematically investigated.

Section snippets

Preparation of samples

All reagents were of analytical grade and used without further purification. Na₂MoO₄⋅2H₂O was used as the molybdenum source and Ba(NO₃)₂ as the barium source, and distilled water was used throughout. In a typical synthesis route, 0.5 mmol of Ba(NO₃)₂ was dissolved in 5 mL distilled water, and then 0.5 mmol of Na₂MoO₄·2H₂O was dissolved into the Ba(NO₃)₂ solution and dissolved. The pH value of the mixture was adjusted by HNO₃. Then, the mixture was placed in an autoclave container with a volume of

Results and discussion

The XRD patterns of the as-prepared products obtained at pH = 1.5, 2.0, 3.0, and 4.0 were shown in Fig.1. The products were obtained under microwave-hydrothermal conditions in which the initial concentration of Ba²⁺ was 0.01 M, $[{Ba}^{2 +}] / [{MoO}_{4}^{2 -}] = 1$ , reaction temperature was 100 °C, and reaction time was 1.0 h, respectively. All the diffraction peaks can be readily indexed to the tetragonal phase BaMoO₄ (JCPDS Card No. 29-0193), and no impurity peaks can be detected, indicating the formation of a pure

Conclusions

In conclusion, uniform 3D BaMoO₄ hierarchical architectures with high crystallinity were successfully synthesized by a microwave-assisted hydrothermal method without using any surfactants. The results show that the pH value of the initial solution, the reactant concentration, molar ratio of $[{Ba}^{2 +}] / [{MoO}_{4}^{2 -}]$ played important roles in the formation of 3D nest-like architectures, and the superlattice has been observed in nest-like BaMoO₄. The photoluminescence properties of 3D architectures showed

Acknowledgment

This work was funded by the Project “Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).”

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Surfactant-free microwave-assisted hydrothermal synthesis of BaMoO4 hierarchical self-assemblies and enhanced photoluminescence properties

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Preparation of samples

Results and discussion

Conclusions

Acknowledgment

Appl. Surf. Sci.

Powder Technol.

J. Cryst. Growth

J. Cryst. Growth

J. Colloid Interface Sci.

J. Colloid Interface Sci.

J. Soild State Chem.

Appl. Phys.

J. Phys. Chem. Solids

Science

J. Am. Chem. Soc.

Cryst. Growth Des.

CrystEngComm

J. Phys. Chem. C

J. Phys. Chem. C

Appl. Phys. Lett.

Surfactant-free microwave-assisted hydrothermal synthesis of BaMoO₄ hierarchical self-assemblies and enhanced photoluminescence properties