Sub-kilogram-scale synthesis of highly dispersible zirconia nanoparticles for hybrid optical resins

Highlights

• A scalable method for synthesis of highly dispersible zirconia nanoparticles

• ZrO₂ nanodispersion with solid content of 37 wt% was stable for over six months.

• The refractive index of transparent hybrid resins was tunable between 1.55 and 1.71.

Abstract

We report the synthesis of highly dispersible zirconia (ZrO₂) nanoparticles in sub-kilogram-scale by hydrothermal process coupled with alkaline hydrogen peroxide (AHP) treatment, with detailed studies on the effects of experimental parameters. Under optimized experimental condition, which was hydrothermal reaction at 110 °C for 6 h with mole ratio of sodium hydrate to zirconium carbonate basic in reactants as 2.3, pure-phase cubic-ZrO₂ nanoparticles are obtained. Followed by AHP treatment of the ZrO₂ nanoparticles at 50 °C for 5 h in 0.4 M hydrogen peroxide and 4 M sodium hydrate, the dispersibility of ZrO₂ nanoparticles in aqueous suspension was enhanced due to the introduced Zr-OH groups on the surface. ZrO₂ nanodispersion with solid content of 37 wt% in aqueous solution exhibited a uniform hydrodynamic size of 14.15 ± 5.834 nm and was stable for over six months without significant aggregates. Transparent hybrid optical resins with tunable refractive index in the range of 1.55–1.71 were prepared by solution blending method using ZrO₂ nanoparticles as composite fillers in polyvinyl alcohol substrate. Compared with pure polyvinyl alcohol resin, the hybrid resin exhibits significant enhanced mechanical properties. Sub-kilogram-scale preparation of highly aqueous dispersible ZrO₂ nanoparticles offers possibility for practical applications.

Introduction

In recent years, the application of nanomaterials in new optical resins and electronic devices has received increasing attention [[1], [2], [3], [4], [5]]. A wide variety of nanomaterials, including carbon nanomaterial [[6], [7], [8]], semiconductor quantum dots (QDs) [9,10], rare-earth doped upconversion nanoparticles [[11], [12], [13]], metal oxide nanomaterials [[14], [15], [16]], have found great potential applications in the field of optoelectronics and beyond [17,18]. Among them, zirconia is a metal oxide having high melting point, high electrical resistivity, high refractive index (RI) and low coefficient of thermal expansion [19]. Due to the low cost, durability, and inactive chemical properties (not easily soluble in strong acid and alkali), ZrO₂ has been one of the most studied ceramic materials [[20], [21], [22]]. In recent years, along with the rapid development of nanoscience and nanotechnology, ZrO₂ nanoparticles (NPs) have been emerging as attractive materials for high RI nanocomposites [15,23,24]. The commercial available ZrO₂ NPs are usually mixed crystals of multiple phases and composed of aggregates with sizes of hundreds of nanometers to several micrometers. On one hand, producing phase-pure ZrO₂ NPs is important because phase purity defines their properties such as hardness, density and RI. Recently, our group reported the synthesis of aqueous ZrO₂ nanoparticles with a controllable crystalline phase (monoclinic and tetragonal) via the reaction of inorganic zirconium salt and sodium hydroxide in acid aqueous surrounding followed by hydrothermal process [23]. However, it is still challenging to realize the controllable preparation of pure cubic ZrO₂ NPs at low-cost and low energy-consumption.

One the other hand, the cost effective methods for controlling the dispersion of ZrO₂ NPs in polymeric hosts is a stumbling block to the large-scale production and commercialization of ZrO₂ based nanocomposites for advanced applications [15]. The commercial available ZrO₂ NPs are usually composed of particle aggregates with size of over hundreds of nanometers. A homogenous dispersion of the inorganic filler on the nanoscale is important in many applications, especially for optical and optoelectronic devices in which the light transmission plays the critical role [11,15,23]. For instance, transparent hybrid optical resins with tunable refractive index using ZrO₂ NPs as fillers are highly required for advanced glasses and encapsulation [15,23]. Therefore, the issues related to scale-up, cost and compatibility of functional NPs must be considered at an earlier stage of development. The proper surface treatment of nanoparticles is the key to solving the problems of dispersion and compatibility.

Nowadays, the mixture of NaOH and H₂O₂, known as alkaline hydrogen peroxide (AHP), has been used as an effective surface treatment for commercial crystalline TiO₂ [25,26]. The AHP treated TiO₂ NPs had significantly enhanced visible-light photocatalytic degradation of dye contaminants compared to untreated TiO₂ NPs [26]. The AHP treatment as a pulping and bleaching method was originally used in papermaking industry [27], and also was used for biomass pretreatment [[28], [29], [30]]. In addition to the TiO₂ NPs related articles mentioned above, this method is rarely reported for the treatment of other NPs. The AHP treated TiO₂ NPs has a large number of hydroxyl groups on the surface to form a uniform and stable aqueous phase nanodispersion. Therefore, with the previous applications of the AHP method, it is worthwhile to try to prepare ZrO₂ nanodispersion on a large scale by the AHP method. At the same time, it should be pointed out that although the ZrO₂ NPs and TiO₂ NPs are both treated by the AHP treatment, the formation mechanism is different. The main reason is that TiO₂ NPs will dissolve in the AHP solution [25], but ZrO₂ NPs will not dissolve. The AHP treatment only can change the surface properties of ZrO₂ NPs. Therefore, the quality of ZrO₂ NPs before and after AHP treatment is not changed much, even if there is a loss, it is produced during the washing process. Therefore, the AHP method is more suitable for treating ZrO₂ NPs than TiO₂ NPs.

Herein, we propose a method for the synthesis of highly dispersible cubic phase ZrO₂ NPs by hydrothermal process coupled with AHP treatment. The agglomerated ZrO₂ NPs treated by the mixture of NaOH and H₂O₂ are highly dispersible in aqueous solution and can be prepared at sub-kilogram-scale in one batch reaction. The as-prepared cubic-ZrO₂ aqueous nanodispersion can be used to prepare thin composite films with high refractive index, and as filler in polymers to enhance optical and mechanical properties, which are both discussed in detail in our study.