Photoluminescence properties of CdMoO₄ disk and hollow microsphere-like crystals synthesized by hydrothermal conventional method

Abstract

CdMoO₄ disk and hollow microsphere-like crystals were successfully synthesized by the hydrothermal conventional method at 120 °C without any templates or surfactants. The structure, morphology and photoluminescence (PL) properties of these crystals were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), micro-Raman (MR) spectroscopy, ultraviolet–visible (UV–vis) absorbance and PL measurements. XRD patterns and MR spectra indicate that the CdMoO₄ disk and hollow microsphere-like crystals have a scheelite-type tetragonal structure. FESEM micrographs show that the CdMoO₄ disk-like crystals were composed of circular nanoplates orderly packed layer by layer. The growth process of the CdMoO₄ disk-like crystals was investigated and a possible formation mechanism was proposed. Comparison experiments showed that pH values played an important role in the morphological control of CdMoO₄ crystals. Disk-like superstructures can be obtained with pH = 5, whereas hollow microspheres can be obtained with pH = 7. When the pH value is 11, the main products were Cd(OH)₂ instead of CdMoO₄. PL spectra at room-temperature showed that the as-synthesized CdMoO₄ disk-like superstructures and hollow microspheres exhibited intense green PL emission when excited with 300 nm wavelength.

Introduction

The synthesis of hierarchical micro- and nanoscale inorganic materials with special morphology, size, and properties have attracted considerable attention in the past few decades due to its unique shape-dependent properties and potential applications in various fields [1], [2], [3], [4], [5], [6]. Generally, self-assembled superstructures can be formed through the evolution of zero or one-dimensional (0-D or 1-D) primary crystals via the oriented attachment process, Ostwald ripening process, or both by using surfactants as capping reagents or without [7], [8], [9], [10], [11], [12]. Therefore, the design and controllable synthesis of hierarchical inorganic micro-/nanostructures may improve the physical and chemical properties of the nanoscale materials and provide a crucial step toward realization of functional nanosystems.

Recently, the molybdates with scheelite-type structure have received much attention due to its wide applications in various fields, including photoluminescent devices, solid-state optical maser, optical fibers, scintillation detector, humidity sensor, photocatalysts, and chemical reactors [13], [14], [15], [16], [17], [18], [19]. Among these scheelite-type molybdates, cadmium molybdate (CdMoO₄) is an interesting material due to its unique chemical and structure properties, such as electronic excitation with VUV synchrotron radiation [20], pressure-induced phase transformations [21], and ¹¹¹Cd and ¹¹³Cd spin-lattice relaxation [22]. Moreover, as a wide band gap semiconductor of 3.25 eV, CdMoO₄ is expected to be a photocatalyst under ultraviolet radiation [23], [24], [25]. A few recent efforts have been devoted to the exploration of convenient and efficient approaches for the fabrication of CdMoO₄ micro-/nanostructures. For example, Zhou et al. [24] obtained CdMoO₄ hollow microspheres by a facile precipitation reaction in the absence of any templates or additives. Ren et al. [26] have prepared CdMoO₄ crystallites with different morphologies including sphere, cake, and cubic sugar through a low temperature hydrothermal method. Wang et al. [23], [27] reported the preparation of CdMoO₄ hollow microspheres and homogeneous core-shell CdMoO₄ microspheres via an aqueous solution method. However, compared with research on other metal molybdates, the efforts on the synthesis and characterization of CdMoO₄ have been lagging far behind, and there are only few reports on the synthesis of CdMoO₄ hierarchical structures. Therefore, the synthesis of CdMoO₄ hierarchical structures with well-defined morphologies and sizes via appropriate facile methods is of great importance to the potential studies of its physical and chemical properties.

In the present study, we report the controllable synthesis of CdMoO₄ disk-like self-assembled superstructures by a simple hydrothermal conventional method without any surfactants or templates. The growth conditions such as pH value and reaction time are correlated to affect the morphology of the CdMoO₄ products. The photoluminescence (PL) performance of the as-synthesized CdMoO₄ hierarchical structures was also investigated because of its well-known electronic structure.