Effective Immobilization of Nanostructured Materials Aiming Performant Electrochromic Electrodes
Nanotechnology has become the focus of research for many applications. In the field of electrochromism, nanomaterials have been widely studied providing physical and chemical properties to ensure short response times and fast charge compensation during the redox process leading to high performing electrochromic devices. Different syntheses of nanomaterials have been reported both to inorganic and organic materials. The preparation of inorganic nanoparticles such as nickel hydroxide, nickel oxide, Prussian blue, tungsten oxide and conducting polymers and viologens were reported and some examples of the use of these nanoparticles in electrochromic applications can be found.
The development of reliable, low cost methodologies for the immobilization of nanomaterials is required. This work will discuss comparatively with LbLother interesting type of film formation: Electrophoretic Deposition (EPD), that can be achieved by the motion of charged particles towards an opposite charged surface due an external applied electric field, where parameters such as deposition time, distance between electrodes and applied potential can be easily controlled in order to obtain different morphologies and thicknesses. The films formation and performance of them will be compared. The electrostatic layer-by-layer deposition technique is limited due to the lack of electrical connectivity and low ionic diffusion between layers, leading to an irreversible behavior and longer response times, while an electrophoretic deposition technique opens new opportunities of studies in this area overcoming these problems. Thicker films with fast switching times and high durability and reproducibility were obtained.
The comparison between LbL and EPD methodologies was done by studying the electrochromic response of doped nickel hydroxide nanoparticles immobilized onto ITO electrodes. The nanoparticles were synthesized by means of ultrasonic radiation reaching particles of 5 nm in diameter. The LbL electrodes were modified by intercalating PDDA between nanoparticle layers and the EPD was achieved by applying an electric field of 1.15 V cm-1 during different times. It was verified that all electrochromic parameters suffered a huge enhancement, in special the electrode durability was extremely high, the electrode was cycled continuously for over 20 hours keeping its contrast. The remarkable electrochromic properties of the EPD electrodes over LbLelectrodes were attributed to the absence of a non-conductive polymeric layer between the inorganic nanoparticle layers allied to the high roughness obtained by the EPD. Other nanostructures such as WO3 nanoplatelets and Au@Pedot nanoparticles were immobilized by using this promising method. WO3 nanoparticles with 50 nm average size were synthesized in non-aqueous solvent using an ultrasonic irradiation process, deposited by EPD process onto ITO substrates using acetonitrile as solvent. The protic ionic liquid N-methyl-pyrrolidinium tetrafluoroborate was chosen as electrolyte replacing traditional aqueous solvent. Coloration efficiency of films was 52 cm2/C in good agreement with other nanostructured WO3 films. The proton intercalation/desintercalation reaction was achieved satisfactory resulting in fast coloration/bleaching color changing. The results using PIL as electrolyte, instead of H2SO4, show that all electrochromic parameters were improved, including, the cyclic durability that was higher using PIL. Core-shell Au@PEDOT nanoparticles of about 2-8 nm were prepared by one-pot synthesis and immobilized by EPD onto ITO electrodes. The very low hysteresis viewed in the transmittance signal for the EPD modified electrode is related to the effective ionic diffusion through in/out the solid material and this fact can be also corroborated by the very low response time presented, in the order of 60 ms.
The combination of nanoparticles, chomophore organic molecules, transition metal complexes and conducting polymers together with the simplicity of the LbL method for preparing electrodes with defined architectures controlled at nanoscale level, seem to be the novel direction in electrochromics. The LbL deposition is a low cost and easily handled technique where nanoparticles, organic molecules and polymers can be alternately deposited. Depending on their electrochromic behavior, many colorations can be achieved, without counter-electrode modification. The number of publications in this field is increasing sharply indicating that patents and commercial applications in large scale are becoming real in the not so far future. Superficially charged nanostructures can also be employed for electrophoretic deposition (EPD), which can be used alternatively to LBL when the electrical connection of nanomaterials is limited. On the hand, compact thicker films can be prepared easily with the concomitant increase of chromatic contrast.
[i] Vidotti, M., Torresi, S.I.C. (2009) Electrostatic layer-by-layer and electrophoretic depositions as methods for electrochromic nanoparticle immobilization, Electrochimica Acta, 54, (10), 2800-2804.