Apatite-coated titanium dioxide photocatalyst for air purification

Abstract

A multifunctional composite material, titanium dioxide covered with apatite, has been developed for application in air purification and as an antimicrobial, antifungal, and antifouling coating. This composite can absorb and decompose bacteria and various other materials. TiO₂ powder was soaked in a simulated physiological solution containing phosphate ions for periods of about 1 h at 37 °C. The composite material has the following characteristics: (1) the apatite adsorbs contaminants even without exposure to light; (2) material adsorbed by the apatite is decomposed by the titanium dioxide photocatalyst on exposure to light; (3) the apatite is used as an inert spacer, allowing blending of the material with resins, organic coatings, and other organic materials; (4) though the photocatalyst requires some time to fully decompose organic materials, capture of contaminants by the apatite ensures complete decomposition.

Introduction

When exposed to light, a titanium dioxide photocatalyst generates an extremely strong oxidizing power that oxidizes harmful substances, such as microbes, molds, odors, or soils that come into contact, and eliminates them by decomposition into carbon dioxide, water and other small molecules [1], [2], [3], [4]. Titanium dioxide works as a catalyst and does not undergo any change, so that it can theoretically be used indefinitely. However, the uses of titanium dioxide are limited, because, when blended into organic materials such as organic paint, textile, plastics, and paper, it tends to photochemically decompose these materials. There are other problems as well. For example, due to its lack of action to attract substances, it can only decompose substances that happened to have come into contact, and this decomposing action fails to work when there is no impinging light.

In contrast, apatite is chemically inert. It has been used for chromatographic columns by taking advantage of its ability to selectively adsorb protein, for respirators by capitalizing on its ability to adsorb pollen, and for hand creams by commercializing its ability to adsorb bacteria. However, since apatite is only capable of adsorption and is incapable of decomposition, saturation will be reached over time, and the adsorption action weakens as well [5].

We have developed a multifunctional ceramic composite material by covering the surfaces of a titanium dioxide photocatalyst with apatite, where the apatite adsorbs bacteria and organic substances, and the titanium dioxide decomposes them.

A variety of apatite-covering methods, such as sintering and aqueous solution growth, are conceivable. However, since our objective here is to develop an environmental purification (decontamination) material, we want to select a fabrication method which is as environmentally friendly and as low in cost as possible. The wet process using an aqueous solution requires more than a week to produce apatite, leading to increased cost. Since sintering at high temperatures is normally required to obtain crystals of apatite, the process is also energy intensive.

We therefore focused our attention on a type of biomimetic material processing, which came into use in recent years as an artificial bone synthesis method. This method represents an environmentally harmonizing material process that emulates some functions of a living organism. More specifically, this new ceramic synthesis process attempts to synthesize ceramics at room temperature and atmospheric pressure by mimicking in vivo mineralization. In vivo inorganic component synthesis processes are characterized by extremely small loads on the environment and can, therefore, be regarded as ideally environmentally harmonizing materials processes. In addition to being low in energy consumption, the process is free from harmful emissions. Even when its products are discharged into the environment, they tend not to pose any problems.

In artificial bone research, a method to synthesize apatite is being pursued by which apatite is deposited on the surfaces of glass, silica and similar materials from a supersaturated “pseudo-body solution” containing inorganic ions in approximately the same concentrations as their counterparts present in human body fluid. The precipitation of apatite crystals will take 1–2 weeks or longer. For biological materials such as artificial bone, long fabrication time and high cost may not become an impediment because they are generally expensive. However, an environmental material must be low-cost and fast to produce. When emphasis is placed on productivity, it is desired to complete the synthesis in a much shorter time, preferably within several hours.

It is assumed that in the pseudo-body fluid, clusters of calcium phosphate (Ca₉(PO₄)₆) are generated. These clusters are found as basic units in crystal structures of all calcium phosphate compounds that are considered to have a bearing on bone production. We believe that all biological calcium phosphate compounds may be produced through the aggregation of these clusters. As has already been discussed [6], octacalciumphosphate Ca₈H₂(PO₄)₆ (OCP) is under physiological conditions more easily formed than apatite. There is therefore the possibility to accomplish the synthesis in a shorter time by making a detour by way of OCP rather than producing a precipitate of apatite directly. We adjusted the chemical composition in such a manner that OCP would precipitate, and immersed titanium dioxide in a pseudo-body solution held at 37 °C, in order to let apatite be produced on the surface of the titanium dioxide under physiological conditions.