Electrophoretic deposition of hydroxyapatite nanostructured coatings with controlled porosity

Hydroxyapatite (HA) coatings with controlled porosity were prepared by electrophoretic deposition (EPD) method. Carbon black (CB) particles were used as the sacrificial template (porogen agent). Two component suspensions containing different concentrations of HA and CB particles were prepared in isopropanol. It was found that the finer and positively charged HA nanoparticles are heterocoagulated on the coarser and negatively charged CB particles to form CB–HA composite particles with net positive charge. The deposition rate from the suspensions with WR (C_CB/C_HA ratio) of 0.25 was faster than that of those with WR: 0.5 at initial times of EPD. However the situation was reversed at longer EPD times. It was also found that the amount of porosity in the coatings increases as the CB concentration in the suspension increases (15%, 24%, 31%, 43% for the coatings deposited from the suspensions with 20 g/L HA nanoparticles and 0, 5, 10 and 20 g/L CB particles, respectively).     

Electrophoretic deposition of titania nanostructured coatings for photodegradation of methylene blue

The suspensions of titania nanoparticles were prepared in different alcohols (methanol, ethanol, isopropanol and butanol) using polyethyleneimine (PEI) as dispersant. The results of sedimentation, conductivity, zeta potential, FTIR and thermal analysis showed that PEI macromolecules are protonated in the suspensions and then adsorbed on the particles enhancing their positive surface charge and so colloidal stability. Optimum concentration of PEI (PEI^*) was lower in large molecular size alcohols due to its higher adsorption efficiency. PEI^* was 0.75, 0.5, 0.5 and 0.25?g/l in methanolic, ethanolic, isopropanolic and butanolic suspensions, respectively. Electrophoretic deposition (EPD) was performed at 60?V in different alcoholic suspensions. EPD rate was the fastest in the suspensions with PEI^* due to the highest mobility of particles. In contrast to the coatings deposited for 1?min from isopropanolic and butanolic suspensions, those deposited at same conditions from methanolic and ethanolic ones severely cracked during drying owing to their higher thickness and lower PEI contents (acting as the binder). The coatings deposited from methanolic and ethanolic suspensions with PEI^* had finer and rougher microstructures. The sintered coating deposited at 60?V for 10?s from ethanolic suspension with PEI^* had crack-free microstructure with the thickness of ≈?130?μm. This coating degraded ≈?24% of methylene blue from its aqueous solution (30?ml with the concentration of 5?mg/l) within 2?h under UV illumination. Photodegradation of MB on the surface of titania coating obeyed the first order kinetics law.     

Electrophoretic deposition of fiber hydroxyapatite/titania nanocomposite coatings

Hydroxyapatite/titania nanocomposite coatings were electrophoretically deposited from ethanolic suspensions of titania and fiber shaped hydroxyapatite (FHA) nanoparticles. Triethanolamine (TEA) was used to enhance the colloidal stability of particles in suspensions. Electrophoretic deposition (EPD) was performed using the suspensions with different concentrations (wt%) of titania/FHA particles. EPD rate decreased more rapidly with time for suspensions with higher wt% of FHA due to the higher voltage drop over the deposits shaped from them. Stacking of long FHA particles on the substrate during EPD resulted in the formation of coarse pores in the deposits. It was found that titania nanoparticles can more efficiently infiltrate through and fill the pores in TEA containing suspensions due to the stronger electrostatic repulsion force between pore walls (FHA) and titania nanoparticles in them. The coatings deposited from the suspensions with 50 wt% of FHA or more did not crack during drying due to the significant reinforcement action provided by high wt% of FHA in them. Nanocomposite coatings deposited from TEA containing (2 mL/L) suspensions with 50 and 75 wt% of FHA had the best corrosion resistance in simulated body fluid (SBF) solution due to their crack-free microstructure and efficiently filled pores.     

Electrophoretic deposition of hydroxyapatite-chitosan-CNTs nanocomposite coatings

Three component suspensions of hydroxyapatite (HA), chitosan and CNTs were prepared in ethanol base solution (15 vol% water and 0.05 vol% acetic acid). The adsorption of HA nanoparticles on CNTs was investigated by FTIR and SEM analysis. It was found that HA nanoparticles are adsorbed on CNTs via chemical bonding between -NH₂ groups of chitosan (adsorbed on their surface) and -COOH groups of CNTs. Current density as well as kinetics of EPD was studied at 60 V. It was found that current density increases or remains nearly constant during EPD due to the rise in water electrolysis as deposit grows on the substrate. Deposition weight against EPD time showed a linear trend due to the absence of any voltage drop over the deposit during EPD. The incorporation of chitosan and CNTs in the microstructure of coatings was confirmed by TG/DTA and SEM analysis. CNTs exhibited high efficiency in reinforcing the microstructure of coatings and preventing from their cracking. CNTs incorporation in the coatings improved their mechanical properties (adhesion strength, hardness and elastic modulus) and corrosion resistance.     

Electrophoretic deposition of nanostructured TiO2/alginate and TiO2-bioactive glass/alginate composite coatings on stainless steel

Abstract

Two alginate (Alg) based composite coatings on stainless steel AISI 316L substrates, one containing titania nanoparticles (n-TiO₂) and another one a mixture (50/50 wt-%) of n-TiO₂ and bioactive glass (BG), intended for biomedical applications, were developed by electrophoretic deposition (EPD) from ethanol/water suspensions. Different n-TiO₂ (2–10 g L^?1) and BG (1–5 g L^?1) contents were studied for a fixed alginate concentration (2 g L^?1), and the properties of the electrophoretically obtained coatings were characterised. Coatings with high ceramic content (up to 67 vol.-%) were obtained. The presence of BG particles improves the mechanical properties of the coatings by increasing the adhesion to the substrate and also accelerates the formation of hydroxyapatite after immersion of the coatings in simulated body fluid. The electrochemical behaviour of the coated substrates, evaluated by polarisation curves in Dulbecco’s modified eagle medium at 37°C, confirmed the corrosion protection function of the novel EPD coatings. The present polymer–ceramic composite coatings belong to an emerging family of bioactive, compliant coatings that are promising for a range of biomedical applications.     

Electrophoretic deposition of titania–carbon nanotubes nanocomposite coatings in different alcohols

The suspensions of titania nanoparticles in different alcohols (methanol, ethanol and butanol) were prepared using triethanolamine (TEA) as a dispersant. The optimum concentration of TEA was 16.67, 8 and 0.33 mL/L in methanol, ethanol and butanol, respectively. Two component suspensions of titania (20 g/L) and carbon nanotubes (CNTs) (0.1, 0.2, 0.5 and 1 g/L) were prepared in different alcohols without and with optimum concentration of TEA. The finer and positively charged titania nanoparticles were heterocoagulated on the surface of coarser and negatively charged CNTs and generated the titania–CNT composite particles with the net positive charge. In the presence of TEA, titania nanoparticles completely covered CNTs surface due to their higher positive surface charge. At same CNT concentration, the deposition rate was faster for suspensions with TEA additive due to the faster mobility of the composite particles. The photocatalysis efficiency of coatings for methylene blue degradation increased as CNTs were incorporated in their microstructure.     

Fabrication of porous SiC nanostructured coatings on C/C composite by laser chemical vapor deposition for improving the thermal shock resistance

Recently, fabricating one-dimensional (1D) nanomaterials on C/C composite has been recognized effective to improve the thermal shock resistance of the coated composites. However, the remaining metal catalyst in CVD process and the week bond of 1D nanomaterials with substrate limit the strengthening effect. Herein, laser chemical vapor deposition (LCVD) was proposed for fabricating porous SiC nanostructured coating on C/C composite without metal catalyst. The laser heating resulted in a temperature gradient between the top and bottom of the coating, providing an external driving force for the vertical growth of whiskers with side-branches, forming a porous network nanostructure. The porous nanostructure was beneficial to reduce CTE and effectively relieve thermal stress. After 10 times of thermal shock test from RT to 1723 K, the porous SiC nanostructured coating remained intact. This work provides a novel methodology to produce functional coating on C/C composite with outstanding thermal shock resistance.     

Effect of suspension medium on the electrophoretic deposition of hydroxyapatite nanoparticles and properties of obtained coatings

The suspensions of hydroxyapatite (HA) nanoparticles were prepared in different alcohols. The zeta potential of HA nanoparticles was the highest in butanolic suspension (65.65 mV) due to the higher adsorption of RCH₂OH₂⁺ species via hydrogen bonding with surface P3OH group of HA. Electrophoretic deposition was performed at 20 and 60 V/cm for different times. Deposition rate was faster in low molecular weight alcohols due to the higher electrophoretic mobility of HA nanoparticles in them. The coating deposited from butanolic suspension had the highest adhesion strength and corrosion resistance in SBF solution at 37.5 °C. The surface of this coating was covered by apatite after immersion in SBF solution for 1 week.     

Electrophoretic deposition of TiO2 film on titanium foil for a flexible dye-sensitized solar cell

Electrophoretic deposition (EPD) method is employed to obtain mesoporous TiO₂ film on a titanium (Ti) foil; the film is then mechanically compressed and sintered at 350 °C before being subjected to dyeing. A comprehensive study was made on the mechanistic aspects of the EPD process. The dye-sensitized solar cell (DSSC) using the thus formed TiO₂ film rendered a power conversion efficiency (Eff.) of 6.5%. Effects of various compression pressures on the photovoltaic parameters and on other characteristic parameters of the pertinent DSSCs are studied. Electrochemical impedance spectroscopy (EIS) is applied for the first time, using a novel equivalent model, to study the impedance behavior of the DSSC with this type of TiO₂ film. We also obtain characteristic parameters of the TiO₂ photoanode by using EIS. The coordination number of the TiO₂ film, and the ratio of charge transfer resistances of electron recombination and electron transport are also obtained and analyzed. Moreover, we employ a multilayer approach and increase the film thickness to prepare TiO₂ films with the same coordination number and porosity; DSSCs using such TiO₂ films obtained from P90 and P25 rendered efficiencies of 6.5% and 5.24%, respectively. Scanning electron microscopy (SEM) micrographs are obtained to characterize the TiO₂ films formed by the EPD technique and laser-induced transient technique is used to estimate the electron lifetime in the TiO₂ films.     

Electrophoretic deposition of titanium nitride coatings

Electrophoretic deposition of the titanium nitride (TiN) coatings from suspensions prepared by dispersion of TiN particles in triethanolamine (TEA) containing butanol medium was studied. Effects of the TiN particles concentration (C_TiN) on the weight of the deposited coatings, triethanolamine concentration (C_TEA=0.25, 0.5, 0.75, and 1 mL/L) on the Zeta potential of the TiN particles, suspension electrical conductivity and pH, as well as effects of the deposition voltage (V_d=60, 90, and 120 V) and time (t_d =1, 2, and 3 minutes) on the microstructure and thickness of the deposited coatings were investigated. Variations in deposition current density, effective deposition voltage, electrical resistance, and deposited coating weight versus deposition time were recorded. The morphology of the as‐dried coatings was studied using Scanning Electron Microscope (SEM). The results indicated that by increasing the C_TiN the weight of deposits increases linearly up to 40 g/L. For suspensions containing C_TiN=40 g/L, the optimum C_TEA is obtained to be 0.5 mL/L leading to Zeta potential of 43.25 mV. Uniform and crack‐free as‐dried coatings obtained at V_d and t_d of 90 V and 2 minutes, respectively.     

Porous titania microspheres with uniform wall thickness and high photoactivity

Highly porous titania particles were prepared by depositing thin films of titania, using alternating reactions of TiCl₄ and hydrogen peroxide, on poly(styrene-divinylbenzene) (PS-DVB) template particles via atomic layer deposition (ALD) at 77 °C. The composition of the titania films was verified by XPS analysis and the titania films were directly observed by TEM. TGA/DSC was used to study the thermal decomposition of the polymer template. Porous titania particles with uniform wall thicknesses were successfully obtained after the template PS-DVB was removed by oxidation in air at 400 °C for 24 h. Verification of the resulting porous structure of the titania particles was done by cross-sectional SEM and nitrogen adsorption–desorption analysis. Porous titania particles were treated at different temperatures. XRD analysis was used to determine the microstructure and phase transformation of titania at elevated temperatures. The photocatalytic activity of these porous titania particles was studied by methylene blue decomposition under UV light at room temperature and was found to be comparable to that of commercial anatase titania nanoparticles (~20 nm). Depositing Na₂SO₄ on TiO₂ retarded the TiO₂ phase transformation from anatase to rutile during calcination and, thus, greatly increased the photoactivity of the porous titania particles.     

Electrophoretic deposition of yttria-stabilised zirconia powder from aqueous suspensions for fabricating ceramics with channel-like pores

Electrophoretic deposition with simultaneous gas bubble formation by electrolysis can be used for producing ceramic green bodies, typically few millimetres in thickness, with unidirectionally aligned channel-like pores. The method is successfully applied to yttria-stabilised zirconia. Two types of aqueous suspension compositions are investigated. Suspensions with acetic acid additions are particularly suitable for forming green bodies with fine pore channels. Only small amounts of acetic acid, promoting the gas evolution, are needed for this purpose. Dissolution of yttria in the acidic range has to be considered, but the required low acid concentrations do not measurably affect the yttrium content of the deposits. Yttria dissolution can be minimised by a suspension composition containing an anionic polyelectrolyte and ammonia instead of acetic acid. The ammonia concentration influences the size of the tubular pores of the deposits formed under constant-voltage conditions. Using structured deposition electrodes, the regularity of the pore arrangement can be enhanced.     

Electrophoretic deposition of ethanol steam-reforming catalysts on metal plates for the development of catalytic-wall reactors

A procedure based on electrophoretic deposition (EPD) was developed to coat metal plates with powder catalysts. The method was tested on stainless-steel plates with three Ni-based catalysts for the steam reforming of ethanol. The catalysts (Ni/La₂O₃/γ-Al₂O₃) contained 15% Ni and 8% La, and were prepared using three types of γ-alumina with different textural properties. The powder catalysts were suspended in isopropanol, and EPD deposition was performed with a voltage of 100 V and a distance between electrodes of 2 cm. Deposition time was varied between 3 and 7 min, which gave a thickness of the catalyst layer from around 30 to 100 μm. The morphology of the catalyst layer was dependent on the textural characteristics of the γ-Al₂O₃ used to prepare the catalyst. The activity of the catalyst plates was tested at 773 K using a steam to carbon molar ratio of 4. Significant differences in the selectivity towards ethanol dehydrogenation, reforming, and dehydration to ethylene could be observed between the three catalysts. Carbon deposition on the surface of the plates could be easily determined by SEM/ESEM.     

Electrophoretic deposition of biomimetic zinc substituted hydroxyapatite coatings with chitosan and carbon nanotubes on titanium

In this study, a biomimetic method was used to prepare hydroxyapatite (HAP) and zinc substituted HAP (ZnHAP) nanoparticles, in which silk fibroin was used as template. The morphology of HAP is rod-like, while ZnHAP changes to wrinkled sheets. HAP and ZnHAP nanoparticles were used to coat titanium by EPD with additional chitosan and multiwalled carbon nanotubes. Phase composition, morphology and structure were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared and Raman spectroscopy. The results showed that the composite coatings containing HAP and ZnHAP had homogeneous morphology and good apatite formation ability. The ZnHAP composite coating possessed class 5B adhesive strength using tape test. Furthermore, the ZnHAP composite coating had better corrosion resistance compared to the HAP composite coatings.     

Electrophoretic deposition of electroless nickel coated YSZ core-shell nanoparticles on a nickel based superalloy

In this work, yttria-stabilized zirconia (YSZ) nanoparticles were covered by a thin Ni layer with approximately 10 nm thickness by electroless deposition method to reduce sintering temperature of the ceramic coating which was applied on a Ni based superalloy via electrophoretic deposition (EPD). Suspensions containing the processed Ni-YSZ core-shell nanoparticles in acetone and isopropyl alcohol solvents were stabilized by addition of 0.4 wt% iodine and 1.5 wt% polyethylenimine, respectively, to find more effective stabilization method for EPD. It was seen that the presence of the Ni layer on YSZ nanoparticles improved performance and sticking factor of EPD and uniform coatings were obtained in both suspensions. The Ni-YSZ green coating which was produced by EPD at voltage of 35 V and deposition time of 30 min in acetone with thickness of 41 μm was sintered in 1100 °C and finally a uniform NiO-YSZ coating was formed on the metallic surface.     

Liquid metal corrosion resistant LaPO4 coating with metallophobic characteristics fabricated on 316 stainless steel using electrophoretic deposition technique

A liquid metal corrosion (LMC) resistant and metallophobic lanthanum phosphate (LaPO₄) coating was prepared on SUS316 stainless steel using electrophoretic deposition (EPD) technique. A specific hierarchical surface structure was created on coating surface by adjusting EPD process parameters. LMC test was performed using three different metal melts, Al–Zn–Mg alloy, Mg–Al–Mn alloy, and pure Zinc. Results indicated that steel bare surface was severely attacked by all three melts. The mechanism of corrosion process was explained in each case. After coating, the LaPO₄ covered steel showed an excellent resistance against all three liquid metals. Besides, wetting of steel surface by liquid metals was strongly decreased by application of LaPO₄ surface coating. This can be attributed to the intrinsic metallophobic characteristic of LaPO₄ as well as the hierarchical surface structure developed on coating surface.     

Electrophoretic deposition of carbon nanotubes films as counter electrodes of dye-sensitized solar cells

Carbon nanotubes (CNTs) films have been successfully fabricated by electrophoretic deposition (EPD) technique and used as counter electrodes of dye-sensitized solar cells (DSSCs). The CNTs counter electrodes consisting of a large number of bamboo-like structures with defect-rich edge planes exhibit a highly interconnected network structure with high electrical conductivity and good catalytic activity. A high photovoltaic conversion efficiency of 7.03% is achieved for DSSCs based on the CNTs counter electrodes, which is comparable to the cell based on conventional Pt counter electrode at one sun (AM 1.5G, 100 mW cm⁻²). The results suggest that the present synthetic strategy provides a potential feasibility for the fabrication of low-cost flexible counter electrodes of DSSCs using a facile deposition technique from an environmentally “friendly” solution at low temperature.     

Fabrication of ceramic membranes on porous ceramic supports by electrophoretic deposition

Abstract

Nanoporous alumina membrane and continuous zeolite L membrane were fabricated on the inner surface of microporous alumina tubes. In the former case, an electrophoretic deposition (EPD) technique was used for the deposition of bimodal alumina particles for the subsequent low temperature sintering. In the latter case, the EPD was used for the seeding process of zeolite L particles for the subsequent hydrothermal synthesis. A thin layer of polypyrrole was synthesised on the inside wall of the porous tubes by the chemical polymerisation of pyrrole to give the wall electric conduction for the EPD electrode. The thickness of the coating layers was controlled by altering the applied voltage and deposition time. The interfacial connection of the alumina or zeolite coated layer and the substrate was evaluated by SEM observations before and after the thermal treatment. The nanoporous structure of the alumina membrane was also characterised by a pore size analyser.     

Electrophoretic deposition of nanocrystalline TiO2 particles on porous TiO2-x ceramic scaffolds for biomedical applications

Nanocrystalline TiO₂ coated scaffolds offers the possibility to be used in bone tissue regeneration providing not only space for new tissue formation, but also to enhance bioactivity of the implant. In the present study, direct current electrophoretic deposition (EPD) was chosen as simple and low cost technique to coat 3D porous structure of TiO_2-x ceramic. Suspension for EPD was prepared suspending nanocrystalline TiO₂ particles in isopropanol and adding triethanolamine as dispersant. TiO₂ particles were electrophoretically deposited on the surface of TiO_2-x scaffolds through varying EPD time and applied voltage. The scaffold pore structure was maintained after applying the coating by EPD. The deposition of nanocrystalline TiO₂ coating can be a smart strategy to impart bioactive properties to the 3D scaffold, allowing formation of spherical hydroxyapatite particles on the coated scaffolds after immersion in simulated body fluid. In vitro cell studies does not show cytotoxic effect of nanocrystalline TiO₂ coated scaffolds.     

Effects of Al interlayer coating and thermal treatment on electron emission characteristics of carbon nanotubes deposited by electrophoretic method

The effects of aluminum (Al) interlayer coating and thermal post-treatment on the electron emission characteristics of carbon nanotubes (CNTs) were investigated. These CNTs were deposited on conical-shaped tungsten (W) substrates using an electrophoretic method. The Al interlayers were coated on the W substrates via magnetron sputtering prior to the deposition of CNTs. Compared with the as-deposited CNTs, the thermally treated CNTs revealed significantly improved electron emission characteristics, such as the decrease of turn-on electric fields and the increase of emission currents. The observations of Raman spectra confirmed that the improved emission characteristics of the thermally treated CNTs were ascribed to their enhanced crystal qualities. The coating of Al interlayers played a role in enhancing the long-term emission stabilities of the CNTs. The thermally treated CNTs with Al interlayers sustained stable emission currents without any significant degradation even after continuous operation of 20 h. The X-ray photoelectron spectroscopy (XPS) study suggested that the cohesive forces between the CNTs and the underlying substrates were strengthened by the coating of Al interlayers.