Deactivation of photocatalytically active ZnO nanoparticle and enhancement of its compatibility with organic compounds by surface-capping with organically modified silica
Research highlights
▶ TEOS and DEDMS were used as the co-precursors of the sol–gel process for preparing ormosil. ZnO nanoparticles were surface-capped by the as-prepared ormosil. The surface properties of the ZnO nanoparticles can be tuned easily and precisely. The ormosil-capped ZnO nanoparticles showed strong hydrophobicity and compatibility with organic phase. And their photocatalytic activity significantly reduced while their UV-shielding ability almost unchanged.
Introduction
Nanosized ZnO as a kind of semiconductor has an absorption edge at a wavelength of about 380 nm and can shield most of ultraviolet-A (UVA) and ultraviolet-B (UVB) rays. With smaller refractive index (2.0) than other metal oxides (for example, titanium dioxide: 2.4–2.7), ZnO nanoparticles have simultaneous high-visible light transparency and high-UV light shielding efficiency, showing great potential as UV-shielding materials in resin, UV-protecting coatings, transparent UV-shielding inorganic-polymer nanocomposites, and sunscreen cosmetics [1], [2], [3], [4]. However, active species are released when photocatalytic active ZnO absorbs light energy equal to or greater than its band gap energy [5], [6], facilitating degradation of organic compounds and hindering the practical application of ZnO as an UV-blocking material. It is therefore imperative to prepare ZnO that has suppressed surface activity and high UV-shielding ability in some practical applications. Fortunately, it is feasible to effectively control the agglomeration and increase the stability of inorganic metallic oxide particulates via surface modification [7], [8], [9], [10]. In order to deactivate the photocatalytic activity of ZnO nanoparticles, inorganic surface modifiers such as alumina and silica are particularly preferred. Silica has special significance for deactivation of ZnO nanoparticles, because it has unique properties such as low chemical activity and high transparency for visible light. Usually, silica coating is prepared from tetraethyl orthosilicate (TEOS) as the starting material for that purpose [11], [12]. Other organic silicon compounds such as polydiethoxysiloxane and silane coupling agent also can be used to generate thin modifying layer on inorganic oxide surface or to functionalize inorganic oxide nanostructures, enabling their dispersion or dissolution in desired solvents [13], [14], [15], [16].
We are especially interested in organically modified silica (ormosil) with combined inorganic–organic composition and structure, because it possesses desired properties such as stability, hydrophobicity, and flexibility, showing promising potential for surface modification of inorganic oxides [17], [18], [19], [20], [21], [22]. Various ormosils can be synthesized from the mixture of TEOS and organosilanes via a hybrid sol–gel route, and their composition can be adjusted by varying the organic groups of organosilanes and the ratio of organosilanes to TEOS. This means that the surface properties of inorganic oxides may be tuned more easily and precisely by making use of surface-modification with ormosils.
Therefore, ZnO nanoparticles have been surface-capped with ormosil to realize photocatalytic deactivation and increase organic compatibility of ZnO nanoparticles as a kind of UV-shielding materials in cosmetics industry. For this purpose, co-precursors TEOS and dimethyldiethoxysilane (DEDMS) were mixed at different molar ratios to produce a series of ormosils. The resultant ormosils were used to surface-cap ZnO nanoparticles. This, hopefully, is to significantly decrease the photocatalytic activity of ZnO nanoparticles and largely increase their compatibility with organic matrices in sunscreen cosmetics.
Section snippets
Materials
TEOS (99%) and DEDMS (98%) used as the co-precursors for the preparation of ormosil, and ammonia solution (25–28%) and anhydrous alcohol used for the surface modification of ZnO nanoparticles were all commercially obtained from Aladdin Reagent Co., Ltd. (Shanghai, China) and used as-received. Deionized water (conductivity less than 10 μS cm−1) prepared at our laboratory was used as solvent in all experiments. Commercial ZnO nanoparticles were provided by Henan Yuguang Gold & Lead Group Company
Characterization of ormosil-capped ZnO nanoparticles by XPS and FTIR
The XPS spectra of Zn 2p, O 1s and Si 2p of uncapped and ormosil-capped ZnO nanoparticles are shown in Fig. 1. With comparing the Zn 2p spectra of uncapped and ormosil-capped ZnO, the result revealed that binding energy of Zn 2p is shifted in varying degrees to high value by ormosil-capping (for example, the binding energy of uncapped Zn 2p3/2 of ZnO is 1021.0 eV, and that of the sample ZT1D2 is 1021.9 eV). Therefore, we can deduce that Si–O–Zn bond forms. The decrease of the electron density
Conclusions
It is feasible to effectively reduce the photocatalytic activity of ZnO nanoparticles and enhance their compatibility with organic phases by introducing ormosil onto the surfaces via sol–gel process in the presence TEOS and DEDMS as co-precursors. The grafting of ormosil onto the surfaces of ZnO nanoparticles is highly dependent on the co-condensation of TEOS and DEDMS during the sol–gel process. Only a small amount of DEDMS is grafted onto the surface of ZnO nanoparticles in the absence of
Acknowledgement
The financial support from the Ministry of Science and Technology of China (grant no. 2007CB607606, in the name of “973” plan) is highly acknowledged.
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