Nanomechanical and structural properties of native cellulose under compressive stress

Cellulose is an important biopolymer with applications ranging from its use as an additive in pharmaceutical products to the development of novel smart materials. This wide applicability arises in part from its interesting mechanical properties. Here we report on the use of high pressure X-ray diffraction and Raman spectroscopy in a diamond anvil cell to determine the bulk and local elastic moduli of native cellulose. The modulus values obtained are 20 GPa for the bulk modulus and 200-355 and 15 GPa for the crystalline parts and the overall elastic (Young's) modulus, respectively. These values are consistent with those calculated from tensile measurements. Above 8 GPa, the packing of the cellulose chains within the fibers undergoes significant structural distortion, whereas the chains themselves remain largely unaffected by compression

Patterning of metal oxide thin films using a H₂/He atmospheric pressure plasma jet

A hydrogen-doped helium atmospheric pressure plasma jet (APPJ) is shown to be effective for the chemical reduction of metal oxides. Copper and tin oxide films (CuO and SnO2) show rapid (<2 seconds) and complete reduction to zero valence metal after exposure to the plasma jet, as revealed by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and Raman spectroscopy. After a total residence time of the plasma jet of 100 seconds, titanium oxide (TiO2) produced a surface decorated with Ti2+, Ti3+ and Ti4+ with proportions of 16, 38 and 46 atom%, respectively, as determined by XPS peak integration. Similarly, with tungsten oxide (WO3), after exposure for a few seconds, W5+ was produced, yielding a deep blue electrically conductive coating. The treatment of these oxide films by this dielectric radio frequency (RF) barrier discharge plasma jet provides a level of redox conversion not seen in any other technique, particularly for TiO2, especially with a comparable power input. The precise nature of the reduction is unclear; however, the involvement of free electrons may have an important role in the reduction process

A novel route to Pt-Bi2O3 composite thin films and their application in photo-reduction of water

A novel homoleptic bismuth(III) β-diketonate (dibenzoylmethane – dbm) complex [Bi(dbm)3]2 has been used as a precursor to thin films of crystalline β-Bi2O3, and hexachloroplatinic acid (H2PtCl6·6H2O) has been demonstrated as a suitable precursor for deposition of platinum nanoparticles, both deposited via aerosol-assisted chemical vapour deposition (AACVD). Thin films of Pt–Bi2O3 were co-deposited from a mixture of [Bi(dbm)3]2 and H2PtCl6·6H2O; the introduction of Pt particles into β-Bi2O3 causes hydrogen to be evolved during photolysis of water over the composite material, a property not found for Pt particles or β-Bi2O3 alone

Visible-Light-Active Iodide-Doped BiOBr Coatings for Sustainable Infrastructure

The search for efficient materials for sustainable infrastructure is an urgent challenge toward potential negative emission technologies and the global environmental crisis. Pleasant, efficient sunlight-activated coatings for applications in self-cleaning windows are sought in the glass industry, particularly those produced from scalable technologies. The current work presents visible-light-active iodide-doped BiOBr thin films fabricated using aerosol-assisted chemical vapor deposition. The impact of dopant concentration on the structural, morphological, and optical properties was studied systematically. The photocatalytic properties of the parent materials and as-deposited doped films were evaluated using the smart ink test. An optimized material was identified as containing 2.7 atom % iodide dopant. Insight into the photocatalytic behavior of these coatings was gathered from photoluminescence and photoelectrochemical studies. The optimum photocatalytic performance could be explained from a balance between photon absorption, charge generation, carrier separation, and charge transport properties under 450 nm irradiation. This optimized iodide-doped BiOBr coating is an excellent candidate for the photodegradation of volatile organic pollutants, with potential applications in self-cleaning windows and other surfaces

Generating, probing and utilising photo-induced surface oxygen vacancies for trace molecular detection

Metal oxide semiconductors (MOS) are extensively used for a wide range of industrial applications [1] – [3] , where defects states in MOS can strongly affect their overall performance, even at very low concentrations [4] , [5] . The functionality of MOS has been reported to be significantly altered through the addition of defects, whereby the materials can become more/less chemically active or the electronic properties are altered. Surface defects, in particular, are often one of the most reactive sites on the surface, greatly influencing MOS photocatalytic activity. Under UV irradiation conditions, interactions with photo- induced charge carriers can generate temporary oxygen vacancies, VO, defect states on the surface of MOS [6] , affecting the material properties during the defects’ lifetime

Surface mapping of photocatalytic activity in heterogeneous TiO2 films

Recent developments in advanced spectroscopy and imaging techniques are shedding light into key charge transfer processes and kinetics in photocatalysis. Complementary photocatalytic reactions with local markers and surface structural mapping are essential to establish property-function correlations in photocatalytic materials. The current work describes a simple approach to probe heterogeneous photocatalytic surfaces using digital colorimetry and the well-established smart ink test, based on the photo-induced reduction of resazurin dye. Surface crystallographic phase mapping is carried out using valence band photoemission spectra (depth resolution of ca. 5 nm). The method has prospective implementation to pixel-by-pixel analysis using high resolution cameras at micron scale

The Effect of Photoinduced Surface Oxygen Vacancies on the Charge Carrier Dynamics in TiO₂ Films

Metal-oxide semiconductors (MOS) are widely utilized for catalytic and photocatalytic applications in which the dynamics of charged carriers (e.g., electrons, holes) play important roles. Under operation conditions, photoinduced surface oxygen vacancies (PI-SOV) can greatly impact the dynamics of charge carriers. However, current knowledge regarding the effect of PI-SOV on the dynamics of hole migration in MOS films, such as titanium dioxide, is solely based upon volume-averaged measurements and/or vacuum conditions. This limits the basic understanding of hole-vacancy interactions, as they are not capable of revealing time-resolved variations during operation. Here, we measured the effect of PI-SOV on the dynamics of hole migration using time-resolved atomic force microscopy. Our findings demonstrate that the time constant associated with hole migration is strongly affected by PI-SOV, in a reversible manner. These results will nucleate an insightful understanding of the physics of hole dynamics and thus enable emerging technologies, facilitated by engineering hole-vacancy interactions

Intelligent Multifunctional VO2/SiO2/TiO2 Coatings for Self-Cleaning, Energy-Saving Window Panels

Monoclinic vanadium(IV) oxide (VO2) has received much attention for applications as intelligent solar control coatings, with the potential to reduce the need for both heating and air conditioning loads within building infrastructure. Chemical vapor deposition, a high-throughput industrially scalable method, is an ideal technology for the deposition of VO2 thin films on window panels. However, these films suffer from poor adhesion and are chemically susceptible to attack. In addition, the VO2 films with optimum solar modulation are unfortunately translucent, restraining their commercial use in energy-efficient fenestration. In this work, multifunctional, robust, layered VO2/SiO2/TiO2 films were quickly deposited on glass substrates using atmospheric-pressure chemical vapor deposition and fully characterized using structural, vibrational spectroscopy, and electron microscopy techniques. The VO2/SiO2/TiO2 thin films were designed to exhibit excellent solar modulation properties as well as high transparency and resistance to abrasion, compared to single VO2 films of the same thickness. The films also showed self-cleaning properties comparable to those of commercial Pilkington Activ glass, as demonstrated here during the photodegradation of a model organic pollutant (stearic acid). The SiO2 acted as a barrier layer, preventing the diffusion of Ti4+ ions into the VO2 layer but it also promoted the optical properties and allowed for superior thermochromic behavior when compared to single VO2 films. The system was modeled to determine the effect of the individual components on the properties of the overall material. It was found that the deposition of the SiO2/TiO2 overlayer resulted in a dramatic improvement of visible-light transmission (∼30% increase when compared to single-layer analogues) while also doubling the solar modulation of the material

Optimized Atmospheric-Pressure Chemical Vapor Deposition Thermochromic VO2 Thin Films for Intelligent Window Applications

Monoclinic vanadium(IV) oxide (VO2) has been widely studied for energy-efficient glazing applications because of its thermochromic properties, displaying a large change in transmission of near-IR wavelengths between the hot and cold states. The optimization of the reaction between VCl4 and ethyl acetate via atmospheric-pressure chemical vapor deposition (APCVD) was shown to produce thin films of monoclinic VO2 with excellent thermochromic properties (ΔTsol = 12%). The tailoring of the thermochromic and visible light transmission was shown to be possible by altering the density and morphology of the deposited films. The films were characterized by X-ray diffraction, atomic-force microscopy, scanning electron microscopy, ellipsometry, and UV–vis spectrometry. This article provides useful design rules for the synthesis of high-quality VO2 thin films by APCVD

On the apparent visible-light and enhanced UV-light photocatalytic activity of nitrogen-doped TiO₂ thin films

Nitrogen-doped titania (N—TiO₂) thin films were synthesized using atmospheric-pressure chemical vapor deposition (APCVD) using ammonia, tert-butylamine or benzylamine as the nitrogen source. The influence of these precursors on the structural, morphological and optical absorption properties of the films was studied using X-ray diffraction (XRD), Raman spectroscopy, Scanning electron microscopy (SEM) and UV/Vis spectroscopy. The chemical state and location of the nitrogen species in the films was investigated using X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of films with similar structural properties was evaluated during degradation of stearic acid under UVA and visible light illumination. A previous study established a potential photosensitization mechanism involving surface N groups with binding energy of ∼400 eV, which would result in extrinsic enhanced UV activity of the N—TiO₂ films. Here, an empirical approach was adopted in order to establish correlation between structural features, nitrogen content and photocatalytic properties of these films. Within the thickness range considered, the photocatalytic activities of the undoped TiO₂ ilms were consistent with their diffraction features (peak intensities and sharpness). Nevertheless, the activities of the N—TiO₂ films did not follow the same trend but it was consistent with their nitrogen content. Further evidence is provided on the participation of nitrogen species on the enhanced UV activity of N—TiO₂ films and the impact of surface N—O groups such as N—O—Ti—O (or O—N—Ti—O) and bulk substitutional nitrogen groups is discussed. Discussion is also provided on the apparent visible light activity of the N—TiO₂ films