Methods for The Synthesis of Nanocrystalline Silicate Phosphors

The trivalent europium ion (Eu3+) has grabbed the attention of researchers since very long time for many reasons. It is given due attention due to strong luminescence in the red spectral region. Its even number of electrons in the outermost 4f shell has diverted attention of researchers towards it owing to theoretical importance of the structure. Rare-earth doped nanophosphor has been focus of research because in display device or LED applications they overcome the drawback of self-absorption and tunable luminescence. Yttrium orthosilicate (YSO) has been proved as an excellent host material for rare earth elements due to special properties such as water, chemical resistance and visible light transparence. Also, they show superior properties due to thermal stability, wide energy band gap, low cost, non-toxicity, chemical resistance, high temperature strength, low thermal expansion and high conductivity, multi-color phosphorescence, high resistance to acid, alkali and oxygen. Among silicate phosphors, YSO doped with rare earth ions are studied extensively in display applications. YSO has been proved as an excellent cathodoluminescent material and, doped with Eu3+, is a promising candidate for coherent time-domain optical memory and red phosphor for lamps and display applications.

Luminescence properties of Eu3+ doped compounds have been studied extensively in various forms of compounds like in solutions, polymer compounds, liquid crystals, glasses etc. These compounds have been prepared via various conventional methods. Silicate nanophosphors are synthesized by a variety of routes such as solid-state reactions, sol–gel, hydrothermal, co-precipitation and spray pyrolysis. It has also been established that the synthesis method and conditions influence the luminescence properties. In the present study the Eu3+ doped (1-9 mol%) YSO nanophosphor was synthesized sonochemically. The prepared sample was calcined at 11000C and at 13000C. Due to quantum confinement effects incorporation of rare-earth trivalent cations to a host lattice, the properties of the nanophosphor is enhanced substantially. Eu3+ doped YSO phosphor emits in red region with appreciable color purity. The nanophosphor is excited by near-ultraviolet light. Extensive study of YSO: Eu3+ synthesized via sonochemical method has not been reported so far. This study is an effort to probe crystal structure of prepared nanophosphor by studying its photoluminescence.

A reaction is a way of interaction of energy with matter. Controlling a reaction is nothing but controlling matter and energy or both. Preparation of nanoparticles requires efficient control over the chemical reaction leading their synthesis, which is a complex task. Synthesis method or procedure of nanomaterials influences their properties. In order to have nanomaterials prepared according to the need generally several reaction parameters like time, energy input, and pressure are manipulated. But the main constraint in this is to select proper energy source. Recently sonochemical methods involving ultrasonic irradiation have been given importance as they provide rare reaction conditions that are superior to other conventional methods. Acoustic cavitation is the key event that leads to unusual reaction conditions in sonochemical synthesis method. When reaction liquids are exposed to ultrasound, the alternating expansive and compressive acoustic waves (pressure waves) create bubbles called cavities. When an oscillating bubble overgrows (typically tens of mm) accumulating ultrasonic energy, it collapses. The sudden collapse releases the accumulated energy in a very short period of time. This process is nothing but sudden, localized, adiabatic heating and cooling of reaction liquid at a rate more than 1000K in a second. Temperature may reach up to 5000K and pressure can be as high as 1000 bar.

Sonochemical method has been explored widely for the preparation of wide variety of nanomaterials. Various types of nanostructured materials like metal nanoparticles, metal carbonyl compounds, metal oxides, oxides, sulfides, carbides have been synthesized via this technique. This powerful aspect of versatility comes from the fact that ultrasonic irradiation considerably improves the hydrolysis rate, and shock waves can induce unusual morphological changes in nanoparticles. Considering the uniformity in size distribution, higher surface area, reaction time, and phase purity sonochemical method has been proved its superiority over other conventional method. The method has been demonstrated as useful in the direct growth of nanostructured materials on a substrate.

With all these established advantages we tried to synthesis yttrium ortho silicate nanoparticles for the first time using sonochemical method. Nanocrystalline phosphor materials have their applications in UV detectors, coatings in lamps, cathode ray tubes, flat panel displays, electroluminescent, optoelectronic devices, X-ray detectors, photodiodes, bio-detectors, color display, radiation detectors in medical imaging systems, plasma or field emission display with huge industrial applications etc. The 4f electrons are responsible for the unique electronic, optical and chemical characteristics of rare-earth oxides that depend on the particle size and composition. Doping a rare earth trivalent cations to the host system, results in a highly functionalized material with enhanced properties.

Silicate family has superior properties owing to its high thermal stability, wide energy band gap, low cost synthesis, non-toxicity, chemical resistance, high temperature strength, low thermal expansion and high conductivity, multi-color phosphorescence, high resistance to acid, alkali and oxygen. YSO, a member of silicate phosphors family, doped with rare earth ions is widely studied for verity of applications. Silicate nanophosphors are synthesized by a variety of routes such as solid-state reactions, sol–gel, hydrothermal, co-precipitation and spray pyrolysis. Many methods have been developed and widely reported for the synthesis of nanocrystalline silicate phosphors.