Silver-doped TiO2 prepared by microemulsion method: Surface properties, bio- and photoactivity
Introduction
Silver-doped titanium dioxide nanoparticles became of current interests because of both their effects on the improvement of photocatalytic activity of TiO2 and their effects on antibacterial activity. Noble metals deposited or doped with TiO2 have high Schottky barriers among the metals and thus act as electron traps, facilitating electron–hole separation and promote the interfacial electron transfer process [1], [2]. Silver can trap the excited electrons from titanium dioxide and leave the holes for the degradation reaction of organic species. It also results in the extension of their wavelength response towards the visible region [3]. In addition, silver nanoparticles possess the ability to absorb visible light, due to localized surface plasmon resonance (LSPR) [4]. These properties have led to tremendous range of applications of Ag-TiO2 nanoparticles, for instance, antibacterial textiles, engineering materials, medical devices, food preparation surfaces, air conditioning filters and coated sanitary wares.
The conventional method that has been applied to prepare Ag-TiO2 nanoparticles is sol–gel process with chemical, thermal or photochemical reduction of silver ions. Several approaches to Ag-TiO2 preparation have been proposed: one-step sol–gel route [5], [6], [7], [8], [9], photoreduction of Ag+ in TiO2 suspension [10] and by electrochemical deposition of Ag nanoparticles at the TiO2 surface [11]. Kawahara et al. [11] found that photochemically and electrochemically deposited silver exhibits multicolor photochromism in the nanoporous TiO2 film. The multicolor photochromism is not generally observed for silver nanoparticles, because in the photo-oxidation process the TiO2 matrix plays an important role. The photo-excited electrons on silver are transferred to oxygen molecules via titanium dioxide and non-excited silver [11].
Tran et al. [12] stated that the presence of silver mainly enhances the photocatalytic oxidation of organic compounds that are predominantly oxidized by holes, while it has not insignificant effect on those organic compounds that require hydroxyl radicals for their mineralization. They found that the enhancing effect of silver deposits on TiO2 can initially by predicted by the molecular structure of the substrate to be oxidized. The fewer C–H bonds and/or more CO and C–O bonds a molecule possesses, the more probable the enhancement of mineralization in the presence of silver.
Hamal and Klabunde [13] reported the synthesis and characterization of highly active (in visible light) new nanoparticle photocatalysts based on silver, carbon and sulfur-doped TiO2 prepared by sol–gel route. They found that Ag/(C, S)-TiO2 nanoparticle photocatalysts degrade the gaseous acetaldehyde 10 and 3 times faster than P25 TiO2 under visible and UV light, respectively [13].
Although the precipitation technique is easy, the morphology and the size of the obtained Ag/TiO2 nanoparticles are difficult to control, which results in unreliable properties in real applications. Compared with these methods, the preparation of silver-doped titanium dioxide nanoparticles in w/o microemulsion offers a unique microenvironment in which monodispersed, ultrafine Ag/TiO2 nanoparticles with a narrow size distribution can be obtained. This method has obvious advantages of obtaining nanoparticles with specific diameter and morphology. Water microdroplets surrounded by a monolayer of surfactant in a continuous hydrocarbon phase act as microreactors to synthesize nanoparticles whose growing is controlled inside the water droplet giving rise to a narrow size distribution [14]. Inaba et al. [15] prepared titanium dioxide nanoparticles in a reverse micelle system composed of water, Triton X-100 and isooctane. The TiO2 nanoparticles showed monodispersity, a large surface area and high degrees of crystallinity and thermostability. The particle size of TiO2 was controlled by changing the water content of the reverse micelle solution [15]. A reverse micelle system is also favorable for preparation of nanosilver particles with narrow size distribution. Zhang et al. [14] reported that silver nanoparticles prepared in AOT microemulsion have a smaller average size and a narrower size distribution compared to the particles prepared by using cationic or nonionic surfactant in microemulsion system. In our previous work we also studied the preparation of silver nanoparticles in water/AOT/dodecane microemulsion [16]. We obtained stable monodisperse silver nanoparticles with a narrow size distribution. The diameter size of the silver nanoparticles was in the range 5–10 nm [17]. Based on literature data, microemulsion appears as favorable environment for nanoparticles preparation, but Ag-TiO2 preparation in the microemulsion system has not been investigated.
The aim of this study was to investigate a reverse micelle system for Ag-TiO2 nanocomposites synthesis and to characterize the obtained nanoparticles. Here we report the preparation method of silver-doped titanium dioxide nanoparticles in the water-in-oil microemulsion system of water/AOT/cyclohexane. Sodium bis-(2-ethylhexyl) sulfosuccinate (AOT), as the most common surfactant used to form reverse micelles, were chosen for our investigation. The effect of silver content used for preparation on the photocatalyst structure, surface area, crystallinity, and efficiency of removal of model organic compound and model microorganisms from aqueous phase were systematically investigated.
Section snippets
Materials and instruments
Titanium isopropoxide (pure p.a.) was purchased from Aldrich and used as titanium source for the preparation of TiO2 nanoparticles. A commercial form of TiO2 (P25, ca. 80% anatase, 20% rutile) from Degussa was used for the comparison of the photocatalytic activity. Silver nitrate (pure p.a.) was provided by POCh and used as the starting material for the silver nanoparticles. Hydrazine anhydrous and ascorbic acid (99%) were purchased from Aldrich and used as reducing agents. Cyclohexane was used
Photocatalytic activity of pure TiO2 and Ag-doped TiO2 under UV and visible light
The photocatalytic activity of the as-prepared nanocomposites was studied by examining the reaction of phenol degradation. No phenol was degraded in the absence of illumination indicating that there was no dark reaction at the surface of Ag-TiO2. Also the reference test in the absence of photocatalysts under visible light showed the lack of phenol degradation. Sample labeling, silver amount and reducing agent used in the preparation procedure, as well as photocatalysts characteristics are given
Conclusions
The Ag-TiO2 nanoparticles have been prepared using a water-in-oil microemulsion system of water/AOT/cyclohexane. It was found that microemulsion method allowed obtaining Ag-TiO2 nanoparticles with narrow Ag particle size distribution regardless Ag content in the reaction system. XRD and the BET measurements corroborate that these doped materials are made up of the homogeneous anatase crystalline phase and have high surface areas fluctuating from 92 to 158 m2/g depending on silver amount. It was
Acknowledgments
This research was financially supported by Polish Ministry of Science and Higher Education (grant no. N N523 487634), KAKENHI (Grant-in-Aid for Scientific Research) on Priority Area “Strong Photon-Molecule Coupling Fields” (No. 470) and the Global Center of Excellence (GCOE) Program “Catalysis as the Basis for Innovation in Material Science” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. STEM analyses of samples were supported by Hokkaido Innovation
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