RESUMO

Active control of nanotribological properties is a challenge. Materials responsive to external stimuli may catalyze this paradigm shift. Recently, the nanofriction of a thin film is modulated by light, ushering in phototribology. This frontier is expanded here, by investigating photoactive nanoparticles in lubricants to confer similar functionality to passive surfaces. Quartz-crystal microbalance (QCM) is employed to assess the phototribological behavior of aqueous suspensions of titanium dioxide nanoparticles. A comparison of dark and illuminated conditions provides the first demonstration of tuning the interfacial friction in solid-nanosuspension interfaces by light. Cyclic tests reveal reversible transitions between higher (dark) and lower friction (illuminated) regimes. These transitions are underpinned by transient states with surface charge variations, as confirmed by Zeta potential measurements. The accumulated surface charge increases repulsion within the system and favors sliding. Upon cessation of illumination, the system returns to its prior equilibrium state. These findings impact not only nanotribology but nanofluidics and nanorheology. Furthermore, the results underscore the need to consider light-induced effects in other scenarios, including the calculation of activity coefficients of photoactive suspensions. This multifaceted study introduces a new dimension to in operando frictional tuning, beckoning a myriad of applications and fundamental insights at the nanoscale.

RESUMO

Bacteria are an old concern to human health, as they are responsible for nosocomial infections, and the number of antibiotic-resistant microorganisms keeps growing. Copper is known for its intrinsic biocidal properties, and therefore, it is a promising material to combat infections when added to surfaces. However, its biocidal properties in the presence of light illumination have not been fully explored, especially regarding the use of microsized particles since nanoparticles have taken over all fields of research and subjugated microparticles despite them being abundant and less expensive. Thus, the present work studied the bactericidal properties of metallic copper particles, in microscale (CuMPs) and nanoscale (CuNPs), in the absence of light and under white LED light illumination. The minimum bactericidal concentration (MBC) of CuMPs against Staphylococcus aureus that achieved a 6-log reduction was 5.0 and 2.5 mg mL-1 for assays conducted in the absence of light and under light illumination, respectively. Similar behavior was observed against Escherichia coli. The bactericidal activity under illumination provided a percentage increase in log reduction values of 65.2% for S. aureus and 166.7% for E. coli when compared to the assays under dark. This assay reproduced the testing CuNPs, which showed superior bactericidal activity since the concentration of 2.5 mg mL-1 promoted a 6-log reduction of both bacteria even under dark. Its superior bactericidal activity, which overcame the effect of illumination, was expected once the nanoscale facilitated the interaction of copper within the surface of bacteria. The results from MBC were supported by fluorescence microscopy and atomic absorption spectroscopy. Therefore, CuMPs and CuNPs proved to have size- and dose-dependent biocidal activity. However, we have shown that CuMPs photoactivity is competitive compared to that of CuNPs, allowing their application as a self-cleaning material for disinfection processes assisted by conventional light sources without additives to contain the spread of pathogens.

Assuntos

Cobre , Staphylococcus aureus , Humanos , Cobre/farmacologia , Cobre/química , Escherichia coli , Iluminação , Antibacterianos/farmacologia , Antibacterianos/química , Bactérias

RESUMO

This paper reports the competition of H2O (residual) and H2 by site (defects) on the Si surface, created by Xe+ ion bombardment. X-ray photoelectron spectroscopy (XPS) in an ultrahigh vacuum system attached to the sample preparation chamber provided the data for the analyses. As hydrogen cannot be detected by XPS, an indirect method to evaluate the O and H cover ratio was developed. The hydrogen passivation effect obtained by the formation of the Si-H bond due to H2 chemisorption limits Si-OH and Si-O-Si bonds, which are products of H2O dissociation. In addition, the results have shown that Xe+ ion bombardment diminished the H2 chemisorption energy barrier onto Si.

RESUMO

In dry sliding, the coefficient of friction depends on the material pair and contact conditions. If the material and operating conditions remain unchanged, the coefficient of friction is constant. Obviously, we can tune friction by surface treatments, but it is a nonreversible process. Here, we report active control of friction forces on TiO2 thin films under UV light. It is reversible and stable and can be tuned/controlled with the light wavelength. The analysis of atomic force microscopy signals by wavelet spectrograms reveals different mechanisms acting in the darkness and under UV. Ab initio simulations on UV light-exposed TiO2 show a lower atomic orbital overlapping on the surface, which leads to a friction reduction of up to 60%. We suggest that photocontrol of friction is due to the modification of atomic orbital interactions from both surfaces at the sliding interface.

RESUMO

Friction is a ubiquitous manifestation of nature, and when it is studied at the nanoscale, complex and interesting effects arise from fundamental physical and chemical surface properties. Surprisingly, and probably due to the complexity of nanofriction studies, this aspect has not been completely discussed in prior studies. To fully consider the physicochemical influence in nanoscale friction, amorphous carbon films with different amounts of hydrogen and fluorine were prepared, chemically characterized, and evaluated via lateral force microscopy. Hydrogen and fluorine were selected because although they exhibit different physicochemical properties, both contribute to frictional force reduction. Indeed, to explain the experimental behavior, it is necessary to propose a new damping constant unifying both polarizability (physical) and electronegativity (chemical) properties. The satisfactory agreement between theory and experiments may encourage and enhance deeper discussion and new experiments that take into account the chemical peculiarities of frictional behavior relating to nanoscale elastic regimes.

RESUMO

The role played by oxygen vacancies and rare earth (RE) elements in the anatase-to-rutile (A-R) phase transformation of titanium dioxide (TiO2) is still a matter of controversy. Here, we report the A-R transformation of TiO2 thin solid films as obtained by ion beam sputtering a RE-decorated titanium target in an oxygen-rich atmosphere. The samples correspond to undoped, single-doped (Sm, Tm, and Tb), and codoped (Sm:Tb, Sm:Tm, and Sm:Tb:Tm) TiO2 films. In the as-prepared form, the films are amorphous and contain ∼0.5 at. % of each RE. The structural modifications of the TiO2 films due to the RE elements and the annealing treatments in an oxygen atmosphere are described according to the experimental results provided by Raman scattering, X-ray photoelectron spectroscopy, and optical measurements. The A-R transformation depends on both the annealing temperature and the characteristics of the undoped, single-doped, and codoped TiO2 films. As reported in the literature, the A-R transformation can be inhibited or enhanced by the presence of impurities and is mostly related to energetic contributions. The experimental results were analyzed, considering the essential and stabilizing role of the entropy of mixing in the A-R transformation due to the introduction of more and multiple quantum states originated in vacancies and impurities in the anatase phase.

RESUMO

Hydrogenated amorphous carbon thin films (a-C:H) have attracted much attention because of their surprising properties, including ultralow friction coefficients in specific conditions. Adhesion of a-C:H films on ferrous alloys is poor due to chemical and physical aspects, avoiding a widespread application of such a film. One possibility to overcome this drawback is depositing an interlayer-an intermediate thin film-between the carbon-based coating and the substrate to improve chemical interaction and adhesion. Based on this, interlayers play a key role on a-C:H thin-film adhesion through a better chemical network structure at the outermost layer of the a-SiC x:H interlayer, i.e., the a-C:H/a-SiC x:H interface. However, despite the latest important advances on the subject, the coating adhesion continues being a cumbersome problem since it depends on multifactorial causes. Thus, the purpose of this paper is to report a standard protocol leading to surprising good results based on the control of the interfacial chemical bonding by properly biasing the substrate (between 500 and 800 V) during the a-SiC x:H interlayer deposition at an appropriate low temperature, by using hexamethyldisiloxane as precursor. The interlayers and the outermost interfaces were analyzed by a comprehensive set of techniques, including X-ray photoelectron spectroscopy, glow discharge optical emission spectroscopy, and Fourier transform infrared spectroscopy. Nanoscratch tests, complemented by scanning electron microscopy and energy-dispersive X-ray spectroscopy, were used to evaluate the critical load for delamination to certify and quantify the adhesion improvement. This study was important to identify the chemical local bonding of the elements at the interface and its local environment, including the in-depth chemical composition profile of the coating. An important effect is that the oxygen content decreases on increasing substrate bias voltage, improving the adhesion of the film. This is due to the fact that energetic ion hitting the growing interlayer breaks Si-O and C-O bonds, augmenting the content of Si-C and C-C bonds at the outermost interface of the a-SiC x:H interlayer and enhancing the a-C:H coating adhesion. Moreover, the combination of high bias voltage (800 V) and low temperature (150 °C) during the a-SiC x:H interlayer deposition allows good adhesion of a-C:H thin films due to sputtering of light elements like oxygen. Therefore, an appropriated bias and temperature combination can open new pathways in a-C:H thin-film deposition at low temperatures. These results are particularly interesting for temperature-sensible metal alloys, where well-adhered a-C:H thin films are mandatory for tribological applications.

RESUMO

Mesoporous films obtained by dip-coating, combining polycondensation of silicate species and organization of amphiphilic mesophases, were decorated with iron-based nanoparticles from iron aqueous solution. High-resolution transmission electron microscopy (HR-TEM) images and energy dispersive X-rays spectroscopy (EDS) analysis show that the nanoparticles have diameters between 5 to 10 nm and they are deposited in an ordered sequence exclusively on top of the mesoporous structure. Afterward, by irradiating the material with a hydrogen ion beam, the iron-based compound is functionalized, i.e., its physical-chemical properties are modified. X-ray photoelectron spectroscopy (XPS) showed that the iron oxides and hydroxides are reduced to metallic iron. The deposition of organized iron-based nanoparticles and further ion beam functionalization might open perspectives on the study and fabrication of complex new materials.

RESUMO

The dissolution of nickel ferrite in oxalic acid and in ferrous oxalate-oxalic acid aqueous solution was studied. Nickel ferrite was synthesized by thermal decomposition of a mixed tartrate; the particles were shown to be coated with a thin ferric oxide layer. Dissolution takes place in two stages, the first one corresponding to the dissolution of the ferric oxide outer layer and the second one being the dissolution of Ni(1.06)Fe(1.96)O(4). The kinetics of dissolution during this first stage is typical of ferric oxides: in oxalic acid, both a ligand-assisted and a redox mechanism operates, whereas in the presence of ferrous ions, redox catalysis leads to a faster dissolution. The rate dependence on both oxalic acid and on ferrous ion is described by the Langmuir-Hinshelwood equation; the best fitting corresponds to K(1)(ads)=25.6 mol(-1) dm(-3) and k(1)(max)=9.17x10(-7) mol m(-2) s(-1) and K(2)(ads)=37.1x10(3) mol(-1) dm(-3) and k(2)(max)=62.3x10(-7) mol m(-2) s(-1), respectively. In the second stage, Langmuir-Hinshelwood kinetics also describes the dissolution of iron and nickel from nickel ferrite, with K(1)(ads)=20.8 mol(-1) dm(3) and K(2)(ads)=1.16x10(5) mol(-1) dm(3). For iron, k(1)(max)=1.02x10(-7) mol of Fe m(-2) s(-1) and k(2)(max)=2.38x10(-7) mol of Fe m(-2) s(-1); for nickel, the rate constants k(1)(max) and k(2)(max) are 2.4 and 1.79 times smaller, respectively. The factor 1.79 agrees nicely with the stoichiometric ratio, whereas the factor 2.4 implies the accumulation of some nickel in the residual particles. The rate of nickel dissolution in oxalic acid is higher than that in bunsenite by a factor of 8, whereas hematite is more reactive by a factor of 9 (in the absence of Fe(II)) and 27 (in the presence of Fe (II)). It may be concluded that oxalic acid operates to dissolve iron, and the ensuing disruption of the solid framework accelerates the release of nickel. Copyright 2000 Academic Press.