Abstract
In this work, titanium dioxide was coated on a metallic substrate using sol electrophoretic technique. Titanium dioxide particles were synthesised by hydrolysis and condensation of a precursor in a colloidal solution. The particles were suspended in the solution using electrostatic [hydrochloric acid (HCl)], steric [polyvinyl butyral (PVB)] and electrosteric [polyethyleneimine (PEI)] stabilisers and were coated on an electrode using an external direct electrical field. The pH, conduction and stability of the suspensions, deposition rate and the current versus time changes were monitored for suspensions containing different stabilisers. Low surface potential of the steric stabilisation and low conduction of the solution led to no film formation for solutions containing PVB. The highest zeta potential was achieved by addition of PEI as the stabiliser. The optimum applied voltage for pure, uniform and crack free titanium dioxide films was 6 V for suspensions containing PEI and 3 V for those containing HCl.
Introduction
Recently, using semiconductors as photocatalyst has attracted the researchers extensively. Semiconductors can be excited by light exposure and cause the oxidation and reduction reactions because of their special electronic structure.1 Photocatalysis is by far one of the most superior technologies in the environmental purification.2 Titanium dioxide is one of the most commonly used semiconductor oxides because of its special electrical and optical properties, low cost, non-toxicity and chemical and thermal stabilities. In addition, TiO2 can be used as a photocatalyst, superhydrophilic and self-cleaning coating. TiO2 has been used for environmental remediation purposes such as purification of water and air and also for solar water splitting.1, 2 A variety of techniques have been employed to deposit TiO2 thin films on different substrates. Among all used methods, sol–gel is one of the most important techniques due to its low working temperature, high uniformity and purity of the final product and the ability of fabricating ceramics and glasses with new compositions.3, 4 Recently, electrophoretic deposition (EPD) technique is used for creating ceramic thin films on conductive substrates. This method is simple, with a low working temperature and inexpensive accessories, and the film thickness and uniformity are quite in control.5 – 8 Deposition of coatings with acceptable uniformity using this technique requires a suspension with an appropriate stability. According to Derjaguin–Landau–Verway–Overbeek theory, zeta potential is a major factor to determine the stability of ceramic suspensions.3, 5 In order to gain a stable suspension, the long range interparticle van der Waals forces must be overcome, which can be performed by creating interparticle electrostatic, steric and electrosteric repulsion forces.9
The aim of this work is to investigate the surface charge of titania particles synthesised via sol–gel technique using titanium isopropoxide as precursor and the ability of different stabilisers in order to obtain stable TiO2 colloidal solution. Hydrochloric acid (HCl), polyethyleneimine (PEI) and polyvinyl butyral (PVB) have been used to prepare the suspensions, and their effects on the surface charge of the produced particles, stability and conductivity of the suspensions were studied.
Experimental
The colloidal solution for deposition of the TiO2 film was prepared using tetraisopropyl orthotitanate (Ti[OCH(CH3)2]4; 99·5%; Merck) as the titanium precursor, 1-butanol (C4H9OH; 99·5%; Merck) as the solvent and deionised water as the hydrolysis–condensation agent. First, 3 mL of deionised water was added to 20 mL of 1-butanol in a 50 mL beaker, followed by dropwise addition of titanium isopropoxide up to 6 mL. During the preparation procedure, the solution was stirred for 1 h at room temperature with 400 rev min−1. This solution was named as sample 1. In order to investigate the effect of different stabilisers, 40 mL of 1-butanol was mixed with an appropriate amount of the desired stabiliser (Table 1), and 4 mL of sample 1 was added to this mixture and stirred for another 30 min (Fig. 1).
Process flow chart used for suspension preparation
Properties of suspensions containing different stabilisers
In order to determine the particles’ mobility in the solution as well as their stability, the surface potentials of the particles were measured using a 3000HS Zetasizer (Malvern). The particle size distribution was determined by dynamic light scattering (DLS) method (Horiba-LB550). The Fourier transform infrared (FTIR) spectroscopy method (Shimadzu 8300) was used in order to study the molecular structure of the colloidal sols.
Next, in order to electrophoretically deposit the films, a sheet of 316L stainless steel was used as the anode, and a sheet of 304 stainless steel with dimensions of 2×60×20 mm was used as the substrate (the cathode). The substrate was polished and then washed with water and acetone before deposition. The electrode spacing was chosen to be 1 cm, and a rectifier was used as the dc power source. A multimeter was used in order to measure the current during the deposition process. The deposition processes were performed with different applied voltages, i.e. 3, 6, 10 and 30 V, at different durations (Table 2). The samples were air dried for 25 h, and afterwards, the films were calcinated and annealed for 2 h at 500°C with a heating rate of 4° min−1 and a cooling rate of 2° min−1.
Quality of films gained at different conditions
(✗) no film formation, (✓) uniform film formation, (*) non-uniform film formation, (…) not studied.
The crystalline structures of the annealed films were investigated using X-ray diffraction (XRD) method (D8 Advance with Cu Kα ; Bruker). The particle size, shape and morphology of the deposited films were studied using a scanning electron microscope (SEM; S360; Oxford).
Results and discussion
Figure 2a shows that the synthesised particle sizes have a narrow distribution, which is due to the hydrolysis and condensation reactions of the titanium precursor in the solution with a 760 nm mean size. Figure 2b shows the SEM image of the particles. As can be seen, microclusters have been formed due to agglomeration of nanoparticles. The DLS analysis indicated the existence of large particles (∼6 μm), which might be related to the clustering. This may be avoided using ultrasonic agitation that leads to particle separation.
a dynamic light scattering particle size distribution and b SEM image of initial particles synthesised by hydrolysis and condensation reactions
The measured properties of the suspensions containing different stabilisers are represented in Table 1. The positive sign of the measured zeta potential of the suspensions represents the existence of a positive charge on the surfaces of the suspended particles, and its value shows the stability of the suspension. As was expected, in samples 1 and 2, the TiO2 film was deposited on negative electrode (cathode) surface via sol EPD. The high zeta potential of +45·6 mV for sample 1 originated from the positive charge donation of the alcoholic functional groups, which has already been described by Damodaran and Moudgil.10 In this mechanism, pure or diluted alcohol can be ionised to protonated alcohol (
) or protonated water (H3O+) and alkoxide ion (RO−) as described by equations (1) and (2). The protonated alcohol is decomposed and leaves a proton on the surface of particles, while the alkoxide ions are desorbed into the solution. Therefore, the repulsive force created between particles will cause an increase in the stability of the suspension
Sample 3 was stabilised using PEI, and according to the obtained zeta potential, addition of PEI to the suspension introduces positive charges to the surface of the particles as shown in Table 1. Polyethyleneimine is a cationic polyelectrolyte with inherent binding properties, which is widely used for electrosteric stabilisation of ceramic particles in organic media.12 – 14 The highest value of zeta potential was achieved by addition of 2 g L−1 PEI as the stabiliser, which induces both electrostatic and steric stabilisation due to its polymer chains and also its positive charge donation properties.
Polyvinyl butyral was the third stabiliser added to the suspension. In non-aqueous suspensions, PVB is often used as a binder to prevent cracking and increase the adhesion of the particles to each other. The zeta potential obtained from sample 4 showed that PVB slightly decreased the positive charge on the particles. This might have happened due to the negative surface charge that originated from the hydroxyl group of PVB molecules after attaching to the surface of the particles.14 On the other hand, steric stabilisation that originated from the PVB polymer increases the suspension stability, so despite of low zeta potential value, this suspension showed good stability compared with the other samples. But as it was discussed before, the conductivity of sample 4 was very low as well (Table 1) because of the low ionic strength of the particles suspended in 1-butanol.
Performing the deposition process in different voltages with the suspensions containing different stabilisers showed that in case of the suspension having PVB as the stabiliser, no electrical current was established, and, as a result, no film was formed. The low surface potential of the particles in case of steric stabilising sample, compared with samples 1 and 2, and also the low conductivity of the solution in this case, led to no film formation in applied voltages. But in the other two cases, the electrical current was established and its amount was related to the type and concentration of the stabiliser in the suspension. By increasing PEI and HCl concentration in the solution, the conductivity of the solution increases, and this causes higher electrical current flow, which creates more surface charge on the particles. The surface charge of the suspended particles affects the electrochemical stability, the migration speed of the particles during the EPD process and the green density of the final film.15
Using the EPD technique with different voltages at a fixed deposition time showed that the optimum voltage for formation of pure, uniform and crack free TiO2 films via the suspension containing PEI as the stabiliser was 6 V and that of the suspension containing HCl was 3 V. Owing to low deposition rate in the suspensions containing HCl, the required time in order to gain a film with a desired thickness was more than that of the suspension containing PEI. This happens due to the low particle mobility in the solution containing HCl compared with the solution containing PEI. But using HCl as the stabiliser, thin films (⩽500 nm) can be created in shorter time and the deposited film was more uniform than the film obtained from suspension that contained PEI as the stabiliser. The reasons for the low value of the optimum voltage can be described as follows:
intensification of the extra water electrolysis in high voltages; hydrogen bubbles created by this reaction will prevent the film formation on the cathode electrode or may lead to crack formation and non-uniformity in deposited film6
intensification of the anode electrode corrosion in low pH and high applied voltage and time; by increasing the corrosion of the anode electrode, the ambient will be polluted, so the final film may contain impurities
cracking and detachment of the thick films during drying or calcination; the maximum applicable film thickness in this method depends on the initial applied voltage, will increase by increasing the voltage and will approach a constant value by increasing the deposition time. Formation of the capillary stresses due to solvent evaporation and tensile stresses due to the different thermal expansion coefficients of the film and substrate during the calcination process can be considered as two important factors leading to film detachment and crack formation as shown in Fig. 3.16
Ceramic coating derived at 30 V from suspension containing PEI
Generally, decreasing the applied voltage led to more uniformity, ease of thin film formation in low deposition times and prevention of anode corrosion, so the final product was highly pure. On the other hand, by decreasing the voltage, the particle motion speed in the suspension decreased and no current was established.
The diagrams represented in Fig. 4 show the current versus deposition time for different suspensions containing PEI (Fig. 4a) and HCl (Fig. 4b) as stabilisers at 6 V. It can be pointed out that the conduction of the suspension containing HCl was much more than that of the one containing PEI. The total resistance of the system increased due to formation of an insulator ceramic film on the substrate. Therefore, the current decreased by increasing the deposition time.6 As a result, the deposition rate decreased and became zero at high deposition time.
Current versus time changes for suspensions containing a PEI and b HCl at applied voltage 6 V
Figure 5 shows the results of the FTIR spectroscopy of the dried (Fig. 5a) and sintered (Fig. 5b) coatings obtained from a suspension containing HCl at optimum deposition conditions. Each peak stands for a special bond as described in the figure.
Fourier transform infrared spectroscopy of a dried and b heat treated (at 500°C for 2 h) coatings gained from suspension containing HCl at optimum deposition conditions
In Fig. 5a, the wide peak seen in the range of 400–800 cm−1 is related to Ti–O and Ti–O–Ti groups, which represents the hydrolysis condensation reaction of the precursor. The absorption band at 1103·2 cm−1 was formed due to the stretching vibration of the O–C–C bonds of the isopropyl groups in the titanium isopropoxide as the titanium precursor.17 The peak formed at 1458·1 cm−1 must be related to the asymmetric vibration of the C–H bond of the remaining organic materials in the coating. The absorption gained due to the stretching and flexural vibration modes of Ti–OH bond can be seen at 1627·8 and 3417·6 cm−1 respectively.18 The 1627·8 cm−1 peak can also be related to flexural vibration mode of –OH of the H2O molecule.19 The 2 week peaks near 2939·3 cm−1 can be related to asymmetric stretching vibration mode of the C–H bond of the methyl groups existing in titanium precursor or alcohol in the system. The absorption band near 3417·6 cm−1 might be formed because of the existence of –OH bonds due to the presence of water, titanium hydroxide, alcohol from the hydrolysis condensation process and used solvent (1-butanol).
In order to investigate the processes carried out during heat treatment and calcination of the coatings, FTIR analysis was carried out on a film deposited from a suspension containing HCl with optimum conditions and heat treated at 500°C for 2 h (Fig. 5b). The results showed that titanium hydroxide of the film converted to titanium oxide in the presence of oxygen in the air during the heat treatment process. The increase in the peak depth at 400–800 cm−1 represents the complete formation of the Ti–O and Ti–O–Ti bonds, which has been carried out due to oxidation and crystallisation of the amorphous particles deposited on the film. The decrease in the depth of the peak in the absorption range of the –OH bonds shows the removal of water and alcohol of the system and the decrease in titanium hydroxide content of the coating. Moreover, the existence of the peak at 1458·1 cm−1 after heat treatment indicates that the organic materials cannot be removed completely by heat treatment at 500°C. The existence of the absorption bands at 1627·8 and 3425·3 cm−1 might be related to the absorption of humidity because of the high porosity content and high surface area of particles in the film. During calcination process, the carbon derived from titanium precursors and remaining organic materials reacts with the oxygen at high temperature and leaves the system in the form of gas. This fact leads to the removal of the stretching vibration of the O–C–C bonds (1103·2 cm−1) and the asymmetric stretching vibration mode of the C–H bond (2939·3 cm−1). It is worth mentioning that the removal of the carbon in the form of gas causes porosity formation in the film, which may have desirable or destructive effects in the final properties of the deposited film.
The XRD pattern of the films obtained from a suspension containing HCl at optimum applied voltage and calcined at 500°C is shown in Fig. 6. The crystal size of the phases was calculated using Scherrer's equation and the (101) peak of anatase20
X-ray diffraction pattern of detached film obtained from suspension containing HCl at optimum applied voltage and calcined at 500°C for 2 h
Image (SEM) of film obtained from suspension containing HCl at optimum applied voltage and calcined at 500°C for 2 h
Conclusion
Sol–gel technique was used in order to create initial suspension of TiO2 particles in 1-butanol media. Positive charge donation of the alcoholic functional groups of the solvent caused the high stability of the initial suspension. Addition of HCl led to an increase in zeta potential of the suspension due to adsorption of H+ ions on the surface of the particles. A zeta potential of +57·2 mV achieved by addition of PEI indicates both electrostatic and steric stabilisation due to its polymeric chains and also positive charge donation properties. The lower surface potential of the particles in case of steric stabilisation was also gained due to the donation of negative surface charge that originated from the hydroxyl group of PVB molecules after attaching to the surface of particles.
The obtained results show that PEI can be used as the most effective stabilising agent due to the highest gained zeta potential.
Footnotes
Acknowledgements
The authors appreciate the financial support of Research Committee of Shiraz University in the form of research grant no. 90-GR-ENG-104 given to Professor M. Javidi.