J. Phys. Chem. C 2007, 111, 5835-5845
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FEATURE ARTICLE Multiform Oxide Optical Materials via the Versatile Pechini-Type Sol-Gel Process: Synthesis and Characteristics Jun Lin,* Min Yu, Cuikun Lin, and Xiaoming Liu Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ReceiVed: January 4, 2007; In Final Form: January 26, 2007
This feature article highlights work from the authors’ laboratories on the various kinds of oxide optical materials, mainly luminescence and pigment materials with different forms (powder, core-shell structures, thin film and patterning) prepared by the Pechini-type sol-gel (PSG) process. The PSG process, which uses the common metal salts (nitrates, acetates, chlorides, etc.) as precursors and citric acid (CA) as chelating ligands of metal ions and polyhydroxy alcohol (such as ethylene glycol or poly ethylene glycol) as a cross-linking agent to form a polymeric resin on molecular level, reduces segregation of particular metal ions and ensures compositional homogeneity. This process can overcome most of the difficulties and disadvantages that frequently occur in the alkoxides based sol-gel process. Using the PSG process, we are able to prepare luminescent powder materials that cannot be well synthesized by the solid-state reaction method, environmentally friendly and highly efficient phosphors that lack metal activator ions, core-shell structured monodisperse and spherical optical materials with tunable physical chemical properties, and thin film phosphors and their patterning combined with soft lithography techniques. The extensive applicability of this process and potential material applications are demonstrated.
1. Background As one of the most well-known soft solution processes, so far the sol-gel technique has found a very extensive application for the design and synthesis of various kinds of advanced functional and engineering materials, including powders, films, fibers, and monoliths of almost any shape, size, and chemical composition, and of course, nanostructures and organicinorganic hybrids.1 There are some 30 000 sol-gel related research papers published in ISI-indexed journals and many special books in the last two decades (1986-2006). The solgel process basically involves the synthesis of an inorganic and/ or organic network by a chemical reaction in solution at low (generally ambient) temperatures followed by the transition from solution to colloidal sol and to a multiphasic gel form. According to the different precursors utilized, the sol-gel techniques can be basically divided into three types: (1) the sol-gel route based upon hydrolysis-condensation of metal-alkoxides; (2) the gelation route based upon concentration of aqueous solutions involving metal-chelates, often called as “chelate gel” route; and (3) the polymerizable-complex (PC) route.2 Apart from the tetraethyl orthosilicate (silicon alkoxide), most of the metal alkoxides suffer from high cost, unavailability, toxicity, and fast hydrolysis rate (thus difficult in controlling the homogeneity of different components during experimental processes).3 As a result, in the preparation of multicomponent oxide materials comprising of more than one type of metal ion, the latter two routes (especially the polymerizable-complex route) are very * Corresponding author. E-mail:
[email protected].
frequently used as the alternatives to the metal-alkoxides based sol-gel process.4 The PC technique, also known as the Pechini method5 (later we call it Pechini-type sol-gel process, and abbreviate as PSG), is well-known and used for the synthesis of homogeneous multicomponent metal oxide materials. This method includes a combined process of metal complex formation and in situ polymerization of organics. Normally an R-hydroxycarboxylic acid such as citric acid (CA) is used to form stable metal complexes, and their polyesterification with a polyhydroxy alcohol such as ethylene glycol (EG) or poly(ethylene glycol) (PEG) forms a polymeric resin. Immobilization of metal complexes in such rigid organic polymer networks reduces segregation of particular metal ions, ensuring compositional homogeneity. The calcination of the polymeric resin at a moderate temperature (500-1000 °C) then generates a pure phase multicomponent metal oxides. The above process is clearly shown in Scheme 1.4a,6 Many metal ions, except monovalent cations such as sodium and potassium, form very stable chelate complexes with CA, since CA is a polybasic compound having three carboxylic acid groups and one alcoholic group in one molecule. The potential ability of CA to solubilize a wide range of metal ions in a mixed solvent of EG and H2O is of prime importance, especially for systems involving cations that can be readily hydrolyzed to form insoluble precipitates in the presence of water.4a An advantage of the PSG process is that the viscosity and the molecular weight of the polymer can be tailored by varying the CA/EG molar ratio and the synthesis
10.1021/jp070062c CCC: $37.00 © 2007 American Chemical Society Published on Web 03/31/2007
5836 J. Phys. Chem. C, Vol. 111, No. 16, 2007
Lin et al. SCHEME 1: Basic Principle of Pechini-Type Sol-Gel Process and Multiform Optical Materials Derived from It
Jun Lin was born in Changchun, China, in 1966. He received B.S. and M.S. degrees in inorganic chemistry from Jilin University, China in 1989 and 1992, respectively, and a Ph.D. degree (inorganic chemistry) from the Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences, in 1995. He was then appointed as a research assistant in City University of Hong Kong (1996) and a DAAD research fellow in Institute of New Materials (INM, Germany, 1997), followed by a postdoctoral researcher at Virginia Commonwealth University (U.S.A., 1998) and University of New Orleans (U.S.A., 1999). He came back to China in 2000, and since then has been working as a professor in CIAC. His research interests include bulk and nanostructrued luminescent materials (powder and thin film), surface modification, crystal growth with different morphologies, luminescence and spectral properties of lanthanide ions in inorganic solids via soft chemistry routes, and organic/inorganic semiconducting layered hybrid materials (especially those with perovskite structure). He is the author or coauthor of more than 200 peer-reviewed journal articles in the areas of luminescent and nanostructured materials Min Yu received her B.S. and M.S. in chemistry from Northeast Normal University (China) in 1988 and 1995, respectively. Since 2000 she has been a graduate student in Jun Lin’s group at Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences, and received a Ph.D. degree in 2003. She won the President Prize of Chinese Academy of Sciences and served as an associate professor in Northeast Normal University after graduation. Her scientific interests include preparation and patterning of phosphor films as well as surface modification of silica particles with luminescent layers via sol-gel and soft lithography techniques. Cuikun Lin is a senior graduate student in Jun Lin’s group at Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences. He graduated from Shandong Normal University (China) with a B.S. degree and came to CIAC to study for a Ph.D. degree in 2002, with a research focus on the synthesis of environmentally friendly luminescent materials without metal activator ions and core shell structured pigment materials via Pechini-type sol-gel process. Xiaoming Liu is a Ph.D. candidate in Jun Lin’s group at Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences. He graduated from Jiujiang College and joined Lin’s group in 2003. His research interests include synthesis of powder luminescent materials for field emission displays via soft chemistry process.
temperature, and the materials can be made into powder and thin film forms and others.2 Now the PSG process has been used for the synthesis of electric and magnetic materials rather extensively, including ferroelectric and capacitor materials, superconducting materials, photocatalytic materials, magneto-optical materials, electrolytic materials for solid oxide fuel cells, and so on.2,4-7 The improved material properties for the PSG process with respect to the other methods (such as solid-state reaction method and amorphous citrate method) have been demonstrated by Kakihana in a review article for the synthesis of superconductors and photocatalysts.4a
In the past 5 years, we have extended the application of the PSG process to the systematic synthesis of various kinds of oxide optical materials, mainly luminescence and pigment materials with different forms (powder, core-shell structures, thin film, and patterning).8 The purpose of our research is to reveal the feasibility, versatility, advantages, and disadvantages of this method for the synthesis of such optical materials, in an effort to gain fine control of the material morphology and find novel optical materials. Here in this feature article, we will demonstrate the multiform of the optical materials derived from the PSG precursor solutions, including powder luminescent materials (combined with the spray drying process), monodisperse and spherical core-shell structured phosphor and pigment particles (via the surface modification process), thin film phosphors (via dip-coating), and their patterning (combined with the soft-lithography process). The basic principle of the multiform of optical materials prepared by the PSG process is shown in Scheme 1. Following this scheme, we will discuss each kind of optical (mainly luminescent) material in the following sections. 2. Powder Luminescent Materials A luminescent material, also called a phosphor, is a solid which converts certain types of energy into electromagnetic radiation over and above thermal radiation. The electromagnetic radiation emitted by a luminescent material is usually in the visible range but can also be in other spectral regions, such as
Feature Article the ultraviolet or infrared.9 Nowadays, luminescent materials have found a wide variety of applications, including displays (such as television tubes, computer monitor tubes, and radar screens) and lighting (such as fluorescent lamps), X-rayintensifying, and scintillation.9,10 For all of these purposes, some tens of thousands of phosphors have been synthesized and characterized via various kinds of methods, but only about 50 materials exhibit properties that are sufficient for practical applications. Although activities aimed at producing novel phosphors for classical applications are still going on, now much attention has been shifted and has begun to be focused on the investigation and optimization of topology of phosphor layers, morphology of phosphor particles, light generation and propagation, and so on.11 The most common and useful form of phosphors is powder, which is mixed with organic binders to spread on a substrate for display and lighting application purposes. Conventionally, powder phosphors are prepared by the solid reaction process, i.e., direct mixing the precursor components (oxides and inorganic salts etc.) and firing them at high temperature for long time with repeat grinding process.12 This process suffers from a waste of energy (>1000 °C, >10 h in general), contamination of impurities (from the repeat grinding process and crucibles at high temperature), and inhomogeneous composition and morphology (>3 µm micron particles with irregular shapes, wide size distribution, and serious aggregation) for the final phosphor products.12 As a result, many soft chemistry processes, such as sol-gel, hydrothermal, spray pyrolysis, coprecipitation, high boiling solvent, combustion, and microwave assisted heating have been developed to prepare luminescent powders.12 All of these processes have their own advantages and disadvantages. In general, they can produce phosphors with better morphology (more regular, spherical in some cases) from the nano- (around 10 nm) to microscale (
