Efficient Decoupled Electrolytic Water Splitting in Acid through Pseudocapacitive TiO2.

Water electrolysis remains a key component in the societal transition to green energy. Membrane electrolyzers are the state-of-the-art technology for water electrolysis, relying on 80 °C operation in highly alkaline electrolytes, which is undesirable for many of the myriad end-use cases for electrolytic water splitting. Herein, an alternative water electrolysis process, decoupled electrolysis, is described which performed in mild acidic conditions with excellent efficiencies. Decoupled electrolysis sequentially performs the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), at the same catalyst. Here, H+ ions generated from the OER are stored through pseudocapacitive (redox) charge storage, and released to drive the HER. Here, decoupled electrolysis is demonstrated using cheap, abundant, TiO2 for the first time. To achieve decoupled acid electrolysis, ultra-small anatase TiO2 particles (4.5 nm diameter) are prepared. These ultra-small TiO2 particles supported on a carbon felt electrode show a highly electrochemical surface area with a capacitance of 375 F g-1. When these electrodes are tested for decoupled water splitting an overall energy efficiency of 52.4% is observed, with excellent stability over 3000 cycles of testing. This technology can provide a viable alternative to membrane electrolyzers-eliminating the need for highly alkaline electrolytes and elevated temperatures.

Alternatives to Fluoropolymers for Motion-Based Energy Harvesting: Perspectives on Piezoelectricity, Triboelectricity, Ferroelectrets, and Flexoelectricity.

Fluoropolymers, including polytetrafluoroethylene (PTFE, Teflon), polyvinylidene difluoride (PVDF), and fluorine kautschuk materials (FKMs, Viton) are critical polymers for applications ranging from non-stick coatings, corrosion resistant seals, semiconductor manufacturing, membranes, and energy harvesting technologies. However, the synthesis of these fluoropolymers requires the use of per- and polyfluorinated alkyl substances (PFAS) known colloquially as "forever chemicals," and as such there is a pressing need to develop alternative technologies that can serve the end-use of fluoropolymers without the environmental cost of using PFAS. Further, fluoropolymers themselves fall under the PFAS umbrella. Here, alternative mechanical-to-electrical energy harvesting polymers are reviewed and benchmarked against the leading fluoropolymer energy harvesters. These alternative technologies include nonfluoropolymer piezoelectric polymers, triboelectric nanogenerators (TENGs), ferroelectric elastomers, and flexoelectric polymers. A vision towards sustainable, non-fluoropolymer-based energy harvesting is provided.

Toward the Assembly of 2D Tunable Crystal Patterns of Spherical Colloids on a Wafer-Scale.

Entering an era of miniaturization prompted scientists to explore strategies to assemble colloidal crystals for numerous applications, including photonics. However, wet methods are intrinsically less versatile than dry methods, whereas the manual rubbing method of dry powders has been demonstrated only on sticky elastomeric layers, hindering particle transfer in printing applications and applicability in analytical screening. To address this clear impetus of broad applicability, we explore here the assembly on nonelastomeric, rigid substrates by utilizing the manual rubbing method to rapidly (≈20 s) attain monolayers comprising hexagonal closely packed (HCP) crystals of monodisperse dry powder spherical particles with a diameter ranging from 500 nm to 10 µm using a PDMS stamp. Our findings elucidate that the tribocharging-induced electrostatic attraction, particularly on relatively stiff substrates, and contact mechanics force between particles and substrates are critical contributors to attain large-scale HCP structures on conductive and insulating substrates. The best performance was obtained with polystyrene and PMMA powder, while silica was assembled only in HCP structures on fluorocarbon-coated substrates under zero-humidity conditions. Finally, we successfully demonstrated the assembly of tunable crystal patterns on a wafer-scale with great control on fluorocarbon-coated wafers, which is promising in microelectronics, bead-based assays, sensing, and anticounterfeiting applications.

Controlled Encapsulation of Gold Nanoparticles into Zr-Metal-Organic Frameworks with Improved Detection Limitation of Volatile Organic Compounds via Surface-Enhanced Raman Scattering.

Volatile organic compounds (VOCs) have harmful effects on human health and the environment but detecting low levels of VOCs is challenging due to a lack of reliable biomarkers. However, incorporating gold nanoparticles (Au NPs) into metal-organic frameworks (MOFs) shows promise for VOC detection. In this study, we developed nanoscale Au@UiO-66 that exhibited surface-enhanced Raman scattering (SERS) activity even at very low levels of toluene vapors (down to 1.0 ppm) due to the thickness of the shell and strong π-π interactions between benzenyl-type linkers and toluene. The UiO-66 shell also increased the thermal stability of the Au NPs, preventing aggregation up to 550 °C. This development may be useful for sensitive detection of VOCs for environmental protection purposes.

Tribovoltaic Performance of TiO2 Thin Films: Crystallinity, Contact Metal, and Thermoelectric Effects.

Tribovoltaic devices are attracting increasing attention as motion-based energy harvesters due to the high local current densities that can be generated. However, while these tribovoltaic devices are being developed, debate remains surrounding their fundamental mechanism. Here, we fabricate thin films from one of the world's most common oxides, TiO2, and compare the tribovoltaic performance under contact with metals of varying work functions, contact areas, and applied pressure. The resultant current density shows little correlation with the work function of the contact metal and a strong correlation with the contact area. Considering other effects at the metal-semiconductor interface, the thermoelectric coefficients of different metals were calculated, which showed a clear correlation with the tribovoltaic current density. On the microscale, molybdenum showed the highest current density of 192 mA cm-2. This work shows the need to consider a variety of mechanisms to understand the tribovoltaic effect and design future exemplar tribovoltaic devices.

Electrospinning Triboelectric Laminates: A Pathway for Scaling Energy Harvesters.

Herein, a new paradigm of triboelectric polymers-the triboelectric laminate-a volumetric material with electromechanical response comparable to the benchmark soft piezoelectric material polyvinylidene difluoride is reported. The electromechanical response in the triboelectric laminate arises from aligned dipoles, generated from the orientation of contact electrification in the laminates bulk volume. The dipoles form between sequential bilayers consisting of two different electrospun polymer fibers of different diameter. The loose interface between the fiber bilayers ensures friction and triboelectric charging between two polymers. The electric output from the electrospun triboelectric laminate increases with increasing density of the bilayers. This system design has clear benefits over other flexible devices for mechanical energy harvesting as it does not require any poling procedures, and the electromechanical response is stable over 24 h of continuous operation. Moreover, the electromechanically responsive electrospun laminate can be made from all types of polymers, thus providing ample room for further improvements or functionalities such as stretchability, biodegradability, or biocompatibility. The concept of a triboelectric laminate can be introduced into existing triboelectric nanogenerator form factors, to dramatically increase charge harvesting of a variety of devices.

Straightforward Approach for Preparing Durable Antibacterial ZnO Nanoparticle Coatings on Flexible Substrates.

Flexible antibacterial materials have gained utmost importance in protection from the distribution of bacteria and viruses due to the exceptional variety of applications. Herein, we demonstrate a readily scalable and rapid single-step approach for producing durable ZnO nanoparticle antibacterial coating on flexible polymer substrates at room temperature. Substrates used are polystyrene, poly(ethylene-co-vinyl acetate) copolymer, poly(methyl methacrylate), polypropylene, high density polyethylene and a commercial acrylate type adhesive tape. The deposition was achieved by a spin-coating process using a slurry of ZnO nanoparticles in toluene. A stable modification layer was obtained when toluene was a solvent for the polymer substrates, namely polystyrene and poly(ethylene-co-vinyl acetate). These coatings show high antibacterial efficiency causing >5 log decrease in the viable counts of Gram-negative bacteria Escherichia. coli and Gram-positive bacteria Staphylococcus aureus in 120 min. Even after tapping these coated surfaces 500 times, the antibacterial properties remained unchanged, showing that the coating obtained by the presented method is very robust. In contrast to the above findings, the coatings are unstable when toluene is not a solvent for the substrate.

Photocatalytic Activity of TiO2 Coatings Obtained at Room Temperature on a Polymethyl Methacrylate Substrate.

Titanium dioxide (TiO2) coatings have a wide range of applications. Anatase exhibits hydrophilic, antimicrobial, and photocatalytic properties for the degradation of organic pollutants or water splitting. The main challenge is to obtain durable anatase nanoparticle coatings on plastic substrates by using straightforward approaches. In the present study, we revealed the preparation of a transparent TiO2 coating on polymethylmethacrylate (PMMA), widely used for organic optical fibres as well as other polymer substrates such as polypropylene (PP), polystyrene (PS), and polycarbonate (PC). The films were spin-coated at room temperature without annealing; therefore, our approach can be used for thermo-sensitive substrates. The deposition was successful due to the use of stripped ultra-small (<4 nm) TiO2 particles. Coatings were studied for the photocatalytic degradation of organic pollutants such as MB, methyl orange (MO), and rhodamine B (RB) under UV light. The TiO2 coating on PMMA degraded over 80% of RB in 300 min under a 365 nm, 100 W mercury lamp, showing a degradation rate constant of 6 × 10−3 min−1. The coatings were stable and showed no significant decrease in degradation activity even after five cycles.

Ca2Fe2O5 powder antifungal activity to the Candida utilis culture upon its growth.

This study reports the impact of Ca2Fe2O5 porous powder on the yeast Candida utilis-as a fungal model-at different phases of growth, i.e., early exponential (6 h), mid-log (11 h), and stationary (17 h) phases. Ca2Fe2O5 inhibited the cell growth in a time-dependent manner. After 120 min incubation, the fungicidal activity of porous powder was observed, i.e., log reduction of 2.81 and 2.58 for 11 and 17 h cultures, respectively, reaching the maximum of 4 log reduction after 7 days. Nevertheless, the 6 h culture of C. utilis showed enhanced resistance to Ca2Fe2O5 with a ≤ 0.4 log reduction during the 7 days exposure. Our results not only showed that Ca2Fe2O5 has the potential to effectively eliminate the C. utilis cell growth but also indicated the importance of the yeast culture physiological state for resistance to Ca2Fe2O5. To the best of our knowledge, this is the first study that evaluated the fungicidal activity of Ca2Fe2O5 porous powder on C. utilis and the impact of the C. utilis phase of growth on the cell susceptibility.

Low-density PDMS foams by controlled destabilization of thixotropic emulsions.

In the current study we demonstrate a method of preparation of low-density polydimethylsiloxane (PDMS) foams from emulsions by using water-based thixotropic fluids as porogens. Aqueous dispersions of synthetic hectorite clay and nanocellulose were used as thixotropic fluids, enabling the preparation of fine emulsions in bulk form with the droplet size down to few tens of microns by simple hand mixing. Contrary to conventional emulsion templating where stabilization of emulsion is required, a strategy was developed for obtaining foams by using controlled destabilization of an emulsion, induced during the curing of the PDMS matrix phase by adding a carefully selected surfactant in optimized concentration. This strategy enables the preparation of bulk PDMS foams with interconnected porosity in a range of density values, fast and deformation-free drying and uniform porous structure with a range of mechanical properties. Clay microplatelet with clearly defined shape and with mass in the nanogram range is retained in spherical pores as the porogen is removed by evaporation. Foams with density down to 0.353 g/cm3 and thermal conductivity of 0.0745 W/m * K were prepared. Elastic modulus of the prepared foams ranged from 0.156 to 0.379 MPa, a reduction of 94.3-86.3% as compared to pure nonporous PDMS.

Visible-Light Active Flexible and Durable Photocatalytic Antibacterial Ethylene-co-vinyl Acetate-Ag/AgCl/α-Fe2O3 Composite Coating.

When particles are mixed in polymer, particle surfaces become passivated by polymer matrix, leading to significantly reduced photocatalytic and, thus, also reduced antibacterial activity, as the catalytic particles become isolated from the outer environment and microorganisms reaching the surface. Herein, we demonstrate a facile and rapid approach for coating preparation at room temperature, yielding good adhesion of particles in combination with the particles' interface location. Flexible ethylene-co-vinyl acetate Ag/AgCl/α-Fe2O3 composite coatings were prepared by the spin-coating method. The synthesized photocatalytically active coating surface exhibited a distinct and rapid inhibition of bacterial growth, with at least a 7-log reduction of gram-positive bacteria Staphylococcus aureus viability after 30 min of visible-light illumination. We also analyzed the shedding of the Ag-ions and reactive oxygen species production from the composite coating and showed that reactive oxygen species played the main role in the photocatalytic bacterial inactivation, destroying the bacteria cell as proven by the Confocal Laser Scanning Microscopy.

Photochromic TiO2/PEGDA organogels.

Photochromic materials can be used for modulation of the visible and infrared light transmittance for providing privacy or energy saving by blocking the heat. Titanium dioxide (TiO2) nanoparticles has been well reported as a promising photochromic material. However, a high photochromic response from TiO2 can be observed only when the nanoparticles are dispersed in a strong photogenerated hole scavenger at a liquid state, but polymer composites are less responsive due to lack of hole scavenging capability. However, it is intricate to apply suspensions in real window devices because of possible leaking. Here, we describe the preparation of TiO2 quantum dot (QD)-based gels from polyethylene glycol diacrylate (PEGDA), N,N-Dimethylformamide (DMF), and ethanol (EtOH). Photochromic gels with TiO2 contents (1-5 volume%) show performance comparable to their colloidal counterparts with capable of photodarkening within 30 min with a transmittance change ranging from 35.8 to 84.5% at 550 nm. These gels were capable of fully recovering the initial transmittance when not exposed to ultraviolet (UV) light within 3-8 h. The photochromic gel systems with ethanol shows reasonable stability by decreasing in transmittance recovery only by less than 10% in 10 cycles. A potential application for the developed photochromic gels can be photochromic windows.

Triboelectric behaviour of selected MOFs in contact with metals.

MOFs have been effectively used to magnify the triboelectric charge of polymers. However, so far the individual triboelectric properties and charge transfer mechanisms of MOFs haven't been reported. Triboelectric property investigation for selected MOFs show that the main mechanism for MOF triboelectrification in contact with metals is electron transfer.

Probing Contact Electrification: A Cohesively Sticky Problem.

Contact electrification and the triboelectric effect are complex processes for mechanical-to-electrical energy conversion, particularly for highly deformable polymers. While generating relatively low power density, contact electrification can occur at the contact-separation interface between nearly any two polymer surfaces. This ubiquitousness of surfaces enables contact electrification to be an important phenomenon to understand energy conversion and harvesting applications. The mechanism of charge generation between polymeric materials remains ambiguous, with electron transfer, material (also known as mass) transfer, and adsorbed chemical species transfer (including induced ionization of water and other molecules) all being proposed as the primary source of the measured charge. Often, all sources of charge, except electron transfer, are dismissed in the case of triboelectric energy harvesters, leading to the generation of the "triboelectric series", governed by the ability of a polymer to lose, or accept, an electron. Here, this sole focus on electron transfer is challenged through rigorous experiments, measuring charge density in polymer-polymer (196 polymer combinations), polymer-glass (14 polymers), and polymer-liquid metal (14 polymers) systems. Through the investigation of these interfaces, clear evidence of material transfer via heterolytic bond cleavage is provided. Based on these results, a generalized model considering the cohesive energy density of polymers as the critical parameter for polymer contact electrification is discussed. This discussion clearly shows that material transfer must be accounted for when discussing the source of charge generated by polymeric mechanical energy harvesters. Thus, a correlated physical property to understand the triboelectric series is provided.

Deposition of low-density thick silica films from burning sol-gel derived alcogels.

In the current study we show that the combustion of sol-gel derived alcogels with specifically tailored composition leads to the release of silica nanoparticles from the burning alcogel in a controlled manner which enables direct deposition of the released nanoparticles into low-density silica thick films. The process has some similarities to flame spray pyrolysis but requires no aerosol generator or other sophisticated instrumental setup. By the proper choice of catalysts and mixture of silicon alkoxides for the synthesis of the alcogel, preferential hydrolysis and polycondensation of one of the alkoxides is achieved. This leads to the formation of an alcogel with volatile silica precursor trapped in the gel pores. Resulting alcogels were burned to deposit uniform porous silica films with density of ~0.1 g/cm3 and primary particle size of ~10 nm. Demonstrated method yields silanol-free silica directly, without additional treatment steps and enables straightforward control over the deposition rate and coarseness of the layer by simple adjustment of the composition of the silica alcogel. The maximum layer thickness is limited only by the deposition time (in the current work up to 134 µm). Such technique of porous oxide film preparation could potentially be extended to the preparation of porous films from other oxides by using respective metal alkoxides as precursors.

Antibacterial Activity of Positively and Negatively Charged Hematite (α-Fe2O3) Nanoparticles to Escherichia coli, Staphylococcus aureus and Vibrio fischeri.

In the current study, the antibacterial activity of positively and negatively charged spherical hematite (α-Fe2O3) nanoparticles (NPs) with primary size of 45 and 70 nm was evaluated against clinically relevant bacteria Escherichia coli (gram-negative) and Staphylococcus aureus (gram-positive) as well as against naturally bioluminescent bacteria Vibrio fischeri (an ecotoxicological model organism). α-Fe2O3 NPs were synthesized using a simple green hydrothermal method and the surface charge was altered via citrate coating. To minimize the interference of testing environment with NP's physic-chemical properties, E. coli and S. aureus were exposed to NPs in deionized water for 30 min and 24 h, covering concentrations from 1 to 1000 mg/L. The growth inhibition was evaluated following the postexposure colony-forming ability of bacteria on toxicant-free agar plates. The positively charged α-Fe2O3 at concentrations from 100 mg/L upwards showed inhibitory activity towards E. coli already after 30 min of contact. Extending the exposure to 24 h caused total inhibition of growth at 100 mg/L. Bactericidal activity of positively charged hematite NPs against S. aureus was not observed up to 1000 mg/L. Differently from positively charged hematite NPs, negatively charged citrate-coated α-Fe2O3 NPs did not exhibit any antibacterial activity against E. coli and S. aureus even at 1000 mg/L. Confocal laser scanning microscopy and flow cytometer analysis showed that bacteria were more tightly associated with positively charged α-Fe2O3 NPs than with negatively charged citrate-coated α-Fe2O3 NPs. Moreover, the observed associations were more evident in the case of E. coli than S. aureus, being coherent with the toxicity results. Vibrio fischeri bioluminescence inhibition assays (exposure medium 2% NaCl) and colony forming ability on agar plates showed no (eco)toxicity of α-Fe2O3 (EC50 and MBC > 1000 mg/L).

Influence of PDA Coating on the Structural, Optical and Surface Properties of ZnO Nanostructures.

Polydopamine (PDA) is a new biocompatible material, which has prospects in biomedical and sensor applications. Due to functional groups, it can host wide range of biomolecules. ZnO nanostructures are well known templates for optical sensors and biosensors. The combination of ZnO and PDA results in a change of optical properties of ZnO-PDA composites as a shift of photoluminescence (PL) peaks and PL quenching. However, to date, the effect of the PDA layer on fundamental properties of ZnO-PDA nanostructures has not been studied. The presented paper reports on optical and surface properties of novel ZnO-PDA nanocomposites. PDA layers were chemically synthesized on ZnO nanostructures from different solution concentrations of 0.3, 0.4, 0.5 and 0.7 mg/mL. Structure, electronic and optical properties were studied by SEM, Raman, FTIR, diffuse reflectance and photoluminescence methods. The Z-potential of the samples was evaluated in neutral pH (pH = 7.2). The response of the samples towards poly-l-lysine adsorption, as a model molecule, was studied by PL spectroscopy to evaluate the correlation between optical and surface properties. The role of the PDA concentration on fundamental properties was discussed.

Strong, Rapid, and Reversible Photochromic Response of Nb Doped TiO2 Nanocrystal Colloids in Hole Scavenging Media.

Understanding photochromicity is essential for developing new means of modulating the optical properties and optical response of materials. Here, we report on the synthesis and exciting new photochromic behavior of Nb5+ doped TiO2 nanoparticle colloids (NCs). We find that, in hole scavenging media, Nb5+ doping significantly improves the photochromic response time of TiO2 nanoparticles. In the infrared regime, Nb-doped TiO2 NCs exhibit 1 order of magnitude faster photoresponse kinetics than the pristine TiO2. Enhanced photochromic response is observed in the visible light regime as well. The transmittance of Nb-doped TiO2 NCs drops to 10% in less than 2 min when irradiated by UV-light in the 500 nm range. The photochromic reaction is fully reversible. The physical origin of the high reaction rate is the high Nb5+ concentration. As a donor dopant, Nb5+ builds up a significant positive charge in the material, which leads to highly efficient electron accumulation during the UV irradiation and results in a rapid photoresponse. EPR experiments identify a new defect type from Nb5+ doping, which alters the physical mechanisms available for transmittance modulation. Our new NCs are economic to synthesize and highly suitable for switchable photochromic applications, e.g., smart windows for modulating visible light and infrared transmittance in built-environments.

Measuring Piezoelectric Output-Fact or Friction?

Piezoelectric polymers are emerging as exceptionally promising materials for energy harvesting. While the theoretical figures of merit for piezoelectric polymers are comparable to ceramics, the measurement techniques need to be retrofitted to account for the different mechanical properties of the softer polymeric materials. Here, how contact electrification, including friction and contact separation, is often mistaken for piezoelectric charge is examined, and a perspective for how to separate these effects is provided. The state of the literature is assessed, and recommendations are made for clear and simple guidelines in reporting, for both sample geometry and testing methods, to enable accurate determination of piezoelectric figures of merit in polymers. Such improvements will allow an understanding of what types of material manipulation are required in order to enhance the piezoelectric output from polymers and enable the next generation of polymer energy harvester design.