Electro-Optics of TiO2 Nanowire Dispersions in PDMS Matrix

Low-cost electro-optical devices with fast, highly controllable and simple operation mechanisms, as well as long term stability are highly desirable for future generations of smart window technologies. There are different types of smart window technologies, for example thermochromics, photochromics, electrochromics, gaschromics, reflective metal hydrides, polymer-dispersed liquid crystals, gel-glass dispersed liquid crystal and dipole particle suspension (DPS) (also called suspended particle devices). From these solutions, the DPS devices allow great control of the transmittance of solar radiation and are also relatively simple. Here we report electro-optical properties of electrospun anatase TiO2 nanowire dispersions in polydimethylsiloxane (PDMS). It was found that changes in optical properties during TiO2 nanowire orientation in PDMS under the influence of an electric field are strongly influenced by nanowire (NW) diameter. It was found for the first time that either positive or negative change in transmittance can be induced by NW alignment parallel to the electric field depending on the NW diameter. Reported experimental findings could be important for smart window applications for the regulation of visible or even infrared transparency. In addition concentrations used in this study were 200 times lower than previously reported for titanium-based DPS devices, because the change in transmittance is attributed to a decrease of light scattering not absorption cross-section during the nanowire orientation along the direction of the electric field. The measurements of the influence of nanowire concentration and electric field strength on the nanowire orientation kinetics show that the device can be operated using electric fields as low as 0.1 V/μm and the magnitude of the change in transmittance can be adjusted by varying the TiO2 nanowire concentration in the PDMS matrix. A transmittance change of 23% was obtained with a DPS device using a suspension of 0.05 wt% electrospun anatase TiO2 in PDMS

Solvothermal Synthesis of One-Dimensional Transition Metal Doped ZnO Nanocrystals and Their Applications in Smart Window Devices

Oxide semiconductor nanowire (NW) suspension based devices have been attracted growing interest in smart window applications due to their great controllability of light transmittance, simplicity and long term stability. Recently, we demonstrated smart window device using the suspension of electrospun TiO2 or solvothermally synthesized ZnO NWs in viscous polydimethylsiloxane (PDMS) matrix. The operating principle of the oxide semiconductor NW and PDMS device is based on the alterable orientation, alignment or spatial distribution of the NWs in an electric field, by changing light scattering cross-section and thus reversibly increasing or decreasing transmittance [1]. One-dimensional (1D) nanostructures such as NWs exhibit a good response to electric field due to the highly anisotropic shape. Herein we report transition metal doped ZnO NW and PDMS based smart window devices. Doping ZnO NWs with other chosen metal ions, such as transition metals, may lead to the emergence of new targeted material properties. For instance, high quality ZnO nanocrystals doped with transition metal cations lead to enhanced optical absorption of light in visible range. The pristine ZnO NWs used in our previous work did not exhibit visible light absorption and transmittance of ZnO NWs and PDMS based smart window device was regulated by changing scattering cross-section. The visible light absorption could increase efficiency of smart window device, because during electrophoretic alignment of NWs towards direction of electric field will change not only scattering, but also visible light absorption cross-section. One-dimensional ZnO nanostructures have been synthesized by wet chemical techniques including microemulsion hydrothermal synthesis, surfactant-assisted hydrothermal orientation growth and alcohol solution refluxing. However, synthesis of high quality transition metal doped ZnO nanowires with small diameter and high aspect ratio is still a challenge. Most wet chemical methods fail to produce high aspect ratio doped ZnO NWs in large quantities. Here we are demonstrating large-scale, single-step, direct solvothermal method and have successfully prepared high aspect ratio single crystalline transition metal (Co, Cu, Fe, Ni, Mn) doped ZnO NWs. The solvothermal synthesis process presented here can be scaled up to macro scale production and the fact that NWs need no further modification increases the technological potential of oxide semiconductor nanowire suspension based devices electro-optical smart window devices

TiO2 Nanowire Dispersions in Viscous Polymer Matrix: Electrophoretic Alignment and Optical Properties

The changes in optical properties during TiO2 nanowire orientation in polydimethylsiloxane (PDMS) matrix under the influence of an electric field are strongly influenced by nanowire (NW) diameter. It was demonstrated for the first time that either positive or negative change in transmittance can be induced by NW alignment parallel to the electric field depending on the NW diameter. These effects can be explained by the interplay between scattering and reflectance. Experimental findings reported could be important for smart window applications for the regulation of visible or even infrared transparency, thus reducing the energy consumption by air conditioning systems in buildings and automobiles in the future

Preparation and Characterization of Photocatalytically Active Antibacterial Surfaces Covered with Acrylic Matrix Embedded Nano-ZnO and Nano-ZnO/Ag

In the context of healthcare-acquired infections, microbial cross-contamination and the spread of antibiotic resistance, additional passive measures to prevent pathogen carryover are urgently needed. Antimicrobial high-touch surfaces that kill microbes on contact or prevent their adhesion could be considered to mitigate the spread. Here, we demonstrate that photocatalytic nano-ZnO- and nano-ZnO/Ag-based antibacterial surfaces with efficacy of at least a 2.7-log reduction in Escherichia coli and Staphylococcus aureus viability in 2 h can be produced by simple measures using a commercial acrylic topcoat for wood surfaces. We characterize the surfaces taking into account cyclic wear and variable environmental conditions. The light-induced antibacterial and photocatalytic activities of the surfaces are enhanced by short-term cyclic wear, indicating their potential for prolonged effectivity in long-term use. As the produced surfaces are generally more effective at higher relative air humidity and silver-containing surfaces lost their contact-killing properties in dry conditions, it is important to critically evaluate the end-use conditions of materials and surfaces to be tested and select application-appropriate methods for their efficacy assessment

Counterintuitive Increase in Optical Scattering Efficiency during Negentropic Orientational Transition in Dilute ZnO Nanowire Suspensions

We demonstrate experimentally that the electrophoretic manipulation of a ZnO nanowire (NW) suspension in polydimethylsiloxane (PDMS) causes a remarkable change in optical scattering. Counterintuitively, as an electric field is applied to the suspension and a negentropic orientational transition from a chaotically oriented state to a partially ordered (aligned) state is induced, the geometrical cross-section of the particles decreases whereas the scattering efficiency increases significantly, indicating an increase in the scattering cross-section. The alignment of the longer axis of oblong ZnO nanoparticles in the direction of incident light unexpectedly resulted in up to a 40% decrease in transmittance in the middle of the visible spectral range in the case of 150 mm thick composite films with below 0.1 vol% NW concentration. A prepared prototype smart window device exhibited spontaneous restoration of transmittance, persistent electro-optical performance (0% change in contrast after more than 10 cycles), and temporal stability against nanoparticle sedimentation and agglomeration

Electro-Optics of Electrospun TiO2 Anatase Submicron Wire Based Dipole Particle Suspension Device

Electro-optical properties of electrospun anatase TiO2 submicron wire dispersion in polydimethylsiloxane (PDMS) were investigated. The optical properties of the dispersion were altered by electrophoretic manipulation of submicron wires (SMWs) in the PDMS matrix. SMWs were aligned in the electric field at a field strength of 0.1 V/lm, resulting in a change in transmittance of up to 23%. Transmittance changes can be attributed to a decrease of light scattering cross-sections during the SMW orientation in the electric field

Switchable Optical Transmittance of TiO2 Submicron-Diameter Wire Suspension-Based “Smart Window” Device

In this paper, for the first time, a TiO2 submicron-diameter wire suspension-based smart window device is demonstrated in which combined planar and finger electrodes are utilised to reversibly change the orientation of the nanowires. Electrospun TiO2 anatase submicron-diameter wire suspensions in a viscous polydimethylsiloxane (PDMS) matrix were prepared directly from electrospun submicron-diameter fibre mats by using high-shear mixing, achieving the complete break-up of all electrospun mats and suspending TiO2 submicron-diameter wires uniformly in the PDMS matrix. Suspension was used as an active layer in an electro-optical device where a reversible change in light scattering is achieved by preparing a device consisting of an active layer and combined planar and finger electrode system. Using the constructed device, it was possible to change the alignment or spatial distribution of TiO2 submicron-diameter wires by applying a DC electric field across the planar or finger, electrodes thus changing the transmittance (DT = 25%) of the suspension and demonstrating the potential to use combined planar and finger electrode devices in smart window applications