Preparation and characterization of new materials based on TiO2 and silver.

Abstract

Evidence is presented for the RF-plasma pretreatment of polyester enhancing the TiO2 induced generation of oxidative species/radicals under a low intensity actinic/visible light irradiation. After 30 min RF-plasma pretreatment, the fastest bacterial inactivation was detected by (a) the largest ratio of oxidized-functionalities / reduced C-functionalities on the bacteria contacted polyester as determined by XPS, b) the sample optical absorption as seen by DRS and (c) the highest concentration surface OH-radicals monitored by the fluorescence of the hydroxy-terephthalic acid. Evidence for the TiO2-polyester self-cleaning was found by XPS by the lack of bacteria destruction on the polyester-TiO2. A further proof of self-cleaning was the sample ability to inactivate bacterial in a repetitive way. Evidence is presented by Ti4+/Ti3+ redox reactions occurring in the photocatalyst during bacterial inactivation. DC-magnetron sputtering (DCMS) and DCMS-pulsed coating of Ag-films on polyester was carried out to induce E. coli inactivation and compared with highly intensity pulse plasma power magnetron sputtering (HIPIMS). The amounts of Ag needed to inactivate E. coli by HIPIMS sputtering were an order of magnitude lower than with DCMS and DCMSP indicating a significant saving of noble metal. Concomitantly a faster E. coli inactivation was observed compared to samples sputtered with DCMS and DCMSP. By DCMS and DCMSP the thicker layers needed to inactivate E. coli comprised larger Ag-aggregates compared to the thinner Ag-layers sputtered by HIPIMS. Longer sputtering times by DCMS, DCMSP and HIPIMS lead to optically darker Ag-deposits up to the absorption edge of silver of ~1000 nm. Mass spectroscopic analyses and X-ray photoelectron spectroscopy indicated that HIPIMS produced a much higher amount of Ag1+ and Ag2+ compared to DCMS and DCMSP due to the higher peak discharge current employed in the former case, the higher density generated of e-/m3 and the much higher generation of Ag-ions compared to DCMS sputtering. Zr-Ag-N films were deposited on polyester by direct current pulsed magnetron sputtering (DCMSP) in an Ar + N2 atmosphere. ZrN on the polyester surface interacts with Ag leading to Zr-Ag-N films. The Ag-atoms are shown by TEM to be immiscible with the ZrN-layer. These composite films were more active in E. coli inactivation compared to the Ag-films. The E. coli inactivation kinetics on Zr-Ag-N polyester surfaces was about 4 times compared to samples sputtering only Ag under vacuum on the polyester. Sputtering Zr in N2 atmosphere presented no antibacterial activity by itself when applied for short times below one min. The Zr-Ag-N polyester sample sputtered for 20 s at 0.3 Amp led to the fastest antibacterial E. coli inactivation kinetics within 90 min. but the E. coli inactivation kinetics on Zr-Ag-N was accelerated two times under low intensity visible/actinic light (400–700 nm, 4 mW/cm2). The sample consisted of Ag-particles with sizes of 15–40 nm, within a layer thickness of 30–45 nm covering ~60–70% of the polyester fiber in the direction of the AgO/Ag-ion-flux from the Ag-target. Ag sputtering for 20 s leads to the optimal ratio of Ag-loading/Ag cluster size with the highest amount of Ag-sites held in exposed positions on the polyester surface. The Ag-nanoparticles sputtered for times >20 s agglomerated to bigger units leading to longer bacterial inactivation times. The increase in thickness of the Zr-Ag-N at longer sputtering times lead to a concomitant increase in the rugosity and hydrophobic character of the Zr-Ag-N sputtered layers. The Zr-Ag-N films showed a uniform metal distribution and a semi-transparent gray-brown color. The photo-induced charge transfer from Ag2O and ZrO2 is discussed considering the relative positions of the electronic bands of the two oxides. Taking into consideration the interfacial charge transfer mechanism (IFCT) to explain the photo-induced electron injection. Surface techniques analysis such as X-ray fluorescence (XRF), diffuse reflectance spectroscopy (DRS), electron microscopy (EM), X-ray diffraction (XRD), contact angle (CA) measurements and X-ray photoelectron spectroscopy (XPS) were applied to describe the microstructure of the surface of TiO2, Ag, ZrN and Zr-Ag-N and provide evidence for the destruction of E. coli on these surfaces.