Dispersion of Titanium Oxide Nanoparticles in Aqueous Solution with Anionic Stabilizer via Ultrasonic Wave

This paper aims to reveal the effects of odium hexametaphosphate (SHMP) and polyacrylic acid (PAA) on dispersion of TiO 2 (P25) nanopowder in de-ionic water through ultrasonic horn. We characterized TiO 2 suspension by transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR), zeta potential, and surface contact angle instruments. As shown in the results, particularly, it were noticed that (1) the SHMP has better dispersion than PAA due to electronegativity effect, resulting in that the average particle size of the dispersed titanium dioxide in de-ionic water was roughly 92 nm, and (2) the zeta potential of TiO 2 suspension with SHMP can be achieved by 54mV at pH value of 7.7, causing stronger electrostatic repulsion in the suspension solution, compared with PAA.


Introduction
The optimized utilization of functional particles is very important for the practical applications ranging from catalysts, polishing media, and cooling fluids to cosmetics and sunscreens, which requires robust and cost-effective dispersion and surface functionalization routes.Additionally, nanopowder dispersion for chemicals, food, medicament, paint, and construction has become an important application in various fields.Because the nanoparticle has the volume and surface effects, it has outstanding qualities on light absorption, catalysis, magnetism, and so on.However, the small size of particle is easily agglomerated and difficult to exhibit the advantages of nanoparticles.Therefore, how to improve the dispersion stability of nanoparticles in the liquid phase is a significant issue.TiO 2 nanoparticles can be used for a variety of applications including self-cleaning [1], water treatment [2], antibacterial [3], and air purification [4] due to their effective photocatalytic activity.Furthermore, the TiO 2 -based hybrid nanomaterials [5] are one of the most common nanocomposites, which are widely applied in medicine, lithium batteries, UV-screening, sensors, and hybrid solar cell materials.For these applications, TiO 2 nanoparticles are generally coated on suitable substrates.Among them, the coating process to use TiO 2 suspensions by spraying is the most costeffective method.Producing TiO 2 suspensions necessitates the incorporation of TiO 2 nanoparticles in the liquid phase, the break-up and dispersion of nanoparticle clusters, and subsequently stabilization.However, small particles tend to agglomeration which is generally due to the van der Waals attraction forces between particles which can be counterbalanced by electrostatic and stereo stabilization [6,7], resulting in low or complete absence of chemical activity.
Generally, the combination of both electrostatic and stereo stabilization was termed as electrosteric stabilization [8], which can be achieved using polyelectrolytes.A polyelectrolyte was a polymer chain with numerous dissociable groups [9,10].Many studies have focused on the dispersion stability of TiO 2 particles in water [11][12][13].A lot of applications of PAA were based on the use of thin films and coatings.PAA coatings and thin films were prepared by electrosynthesis [14,15] from acrylic acid monomer solutions.Furthermore, Othman et al. [16] used two common dispersants which were PAA and ammonium poly-methacrylate to find that the 3 wt% of PAA is the optimal dispersion.Vallar et al. [17] found the optimal the dispersion parameters of the oxide slurries by zeta potential measurements.Liufu et al. [18] stated the adsorption of PAA in aqueous suspension onto the Ultrasonic wave has been proven as a useful tool to disperse nanoparticles and to eliminate agglomeration in aqueous suspensions [24].Shock waves caused by collapsing cavitation under ultrasonic irradiation will lead to collisions among particles, whereby the agglomerated particles are eroded and split apart by the collisions [24,25].Ghadimi and Metselaar [26] presented two ultrasonic technologies, namely, ultrasonic probe and ultrasonic bath, respectively, to combine surfactant displayed different stability and characteristic improvements.Chung et al. [27] worked on the stability of nanofluid under various ultrasonic conditions.They revealed that the particulate size of nanofluids is proportional to the volume fraction and decreased with the ultrasonic power.
In the present study, we aim to examine the effects of inorganic and organic stabilizers on dispersion of TiO 2 nanopowder in de-ionic water via probe ultrasonication by using transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR), zeta potential, and surface contact angle instruments to determine the difference for TiO 2 suspension with sodium hexametaphosphate (SHMP) and polyacrylic acid (PAA) stabilizers, respectively.It is expected to provide guidance to develop the TiO 2 /water nanofluid with excellent performance heat exchange process.

Materials. Titanium dioxide (Degussa, P25
) with an average particles size of 21 nm was used as a study material, which consists of more than 70 wt% anatase with a minor amount of rutile and a small amount of amorphous phase and its specifications are shown in Table 1.De-ionic water (18.1 Ω) was used as the continuous phase, which has been mechanically filtered or processed to remove impurities.Sodium hexametaphosphate (99%, SHOWA) and polyacrylic acid (99%, Mw: 2000, ACROS) were, respectively, used as inorganic and organic stabilizers to enhance suspension stability.

Preparation of Suspension.
The experiments were carried using 0.05 wt%, 0.01 wt%, and 0.005 wt% of titanium dioxide mixing with 20 mL of de-ionic water at the temperature of 25 ∘ C. The samples were then dispersed by an ultrasonic horn (Hoyu Ultrasonic Clash, 250 W, 23 kHz, Taiwan) in a vessel of 25 mL for 480 sec, which is based on obtaining smaller secondary average size of dispersed TiO 2 particle, followed by adding the different amounts (1 wt%, 3 wt%, 5 wt%, and 7 wt%) of stabilizer and stirring for 24 hrs.To keep the dispersions from overheating during sonication, the dispersions were sonicated in a cooling-water bath maintained at a constant temperature of 25 ∘ C.

Characterization.
After dispersing TiO 2 capped with anionic surfactant through the ultrasonic dispersion process, the dynamic light scattering analyzer (Malvern Zetasizer Nano Series) was applied for measuring the particle size and distribution and zeta potential, where values were averaged by three times of measurement after preparing each suspension and standing for 30 min; Fourier transform infrared spectroscopy (FTIR) (Perkin Elmer Spectrum One) was employed to examine the adsorption of stabilizer on titanium oxide particles, where samples were prepared when the dispersion solution was centrifuged for 30 min with angular speed of 5000 cycle min −1 and the resulted powders were eluted with distilled water and afterward dried in an oven at 100 ∘ C for 12 h, followed by mixing with KBr by grinding to fabricate pellets; a pH meter (InLab 439/120) is an electronic device for measuring the pH value of dispersion solution; and the surface contact angle of dispersion liquid on the glass was analyzed by the instrument of Kruss DSA10.

Concentration of TiO 2 Nanoparticles in Aqueous Solution.
We prepared three concentrations (i.e., 0.005, 0.01, and 0.05 wt%) of TiO 2 suspensions without stabilizer to reveal the effect of amount of TiO 2 on the particle size in the dispersion solution.As seen from Figure 1, after dispersing by ultrasonic horn for 480 sec, the particle size of suspension was gradually becoming small with decreasing the concentration of TiO 2 in de-ionic water, where pH values are 5.98 ± 0.14, 5.63 ± 0.01, and 5.11 ± 0.25, respectively, for 0.005, 0.01, and 0.05 wt% of TiO 2 suspensions in de-ionic water in the absence of stabilizer.Figure 2 is TEM images to be used for monitoring the amount of titanium oxide dependent on the aggregation of particle.As shown in the figure, it could be found that the secondary particle size is increased with increasing concentration of particle in suspension, where average particle sizes of dispersed TiO 2 are ranged from 175 nm to 250 nm in de-ionic water in the absence of stabilizer.This suggests that the high power of ultrasonic horn is necessary to overcome the interparticle force for higher amount of TiO 2 in de-ionic water.In this work, the suitable concentration of TiO 2 particle for nanofluid was determined by 0.005 wt% after dispersing through ultrasonic probe for 480 sec to obtain the smaller size of dispersed particles size in aqueous solution with stabilizer.

Surfactant Amount for Dispersing TiO 2 Nanoparticles.
As shown in Figure 3, it was observed for the effect of stabilizer amount on the size of dispersed TiO 2 particles through the ultrasonic dispersion process.With changing the amount of SHMP and PAA stabilizers, respectively, we found that 5 wt% of stabilizer was an appropriate amount to disperse TiO 2 in de-ionic water because the smaller average particle size could be obtained.In particular, it was shown that the average particle size of dispersed TiO 2 in de-ionic water in the presence of SHMP stabilizer was about 92 nm, which was validated by TEM image, as presented in Figure 4(a).
Figure 5 is FTIR analysis for the TiO 2 particles capped without and with 5 wt% of PAA and SHMP stabilizers, respectively, after dispersing in DI-water via sonication.As viewed from the spectrum, SHMP and PAA stabilizers displayed a strong absorption band at 1650 cm −1 and 1250 cm −1 , respectively, indicating -C=O and -P=O stretching vibration for carboxylic acids and phosphoric acid [28]; meanwhile, the FTIR spectrum of TiO 2 particles showed intense broadband in the vicinity of 400 to 800 cm −1 [29].These findings proved that stabilizers were adsorbed onto the surface of TiO 2 particles.

Stability of TiO 2 Nanosuspensions with Stabilizer.
The stability of TiO 2 suspension in aqueous solution is closely related to its electrokinetic properties.The well-dispersed suspension can be obtained with high surface charge density to generate strong repulsive forces [30][31][32].In addition, the salts are containing the multiply charged ions such as polyphosphate, hexametaphosphate, pyrophosphate, and polysilicate anions can be alternatively applied as dispersing salt.The multiple charged ions might be adsorbed by the particle in an aqueous environment and leads to an increase in particle surface charge and zeta potential [6]. Figure 6 shows that zeta potential of TiO 2 particle with stabilizer is greater than that without stabilizer, meaning that the surface charge on particle has been changed, where pH values of suspension were changed from 8.2 to 7.7 and 6.2, respectively, for SHMP and PAA stabilizers.In addition, it was found that the zeta potential of TiO 2 suspension with SHMP is greater than that with PAA.As sketched in Figure 7, this is because SHMP have stronger polarity due to larger electron negativity [33,34], compared to polyacrylates, producing high charge density on the surface of TiO 2 particle For further validation, surface contact angle analysis was performed by dropping the TiO 2 suspension without and with stabilizers of SHMP and PAA, respectively, on glass substrate.As displayed in Figure 8, the surface contact angle of TiO 2 suspension with SHMP was remarkably larger than that with PAA.It implies that the surface contact angle of TiO 2 suspension with SHMP is close to that without stabilizer, meaning that the wettability of TiO 2 suspension with SHMP on glass surface is similar to that without stabilizer due to higher polarity of dispersion solution resulting from the higher electronegativity of SHMP, compared to TiO 2 suspension with PAA.In other words, the glass surface has affinity with the dispersion solution in the presence of PAA.
Figure 9 illustrates zeta potential of dispersed TiO 2 in de-ionic water varying with the amounts of SHMP and PAA stabilizers at pH values of 7.7 and 6.2, respectively.As analyzed from the figure, the maximum zeta potential value of TiO 2 suspension contains 5 wt% of SHMP and PAA stabilizers, respectively, corresponding to average particle size of dispersed TiO 2 in de-ionic water as functions of amount of stabilizer, resulting from the limitation of surface area of TiO 2 particle.The zeta potential value of TiO 2 suspensions with and without stabilizers as functions of pH is shown in Figure 10.As displayed in the figure, the absolute value of zeta potential in the presence of stabilizer is higher than that in the absence of stabilizer.As pH increases, the zeta potential of the particle surface increases, resulting in the fact that the electrostatic repulsion force between particles is sufficient to prevent attraction and collision between particles caused by Brownian motion [35].As indicated by the dash line in the figure, the isolated electrical points (IEP) are roughly located at pH values of 5.7, 3.1, and 2.9, respectively, for the TiO 2    suspension without stabilizer as well as with SHMP and PAA.Additionally, both samples of SHMP and PAA achieved similar zeta potential values at pH values ranging from 7.7 to 10, which may be induced by the extreme thickness of electrical double layer due to the limiting amount of stabilizer and that surface negative charge of dispersed TiO 2 particle is increased by the pH value of suspension.In addition, without stabilizer, the zeta potential of TiO 2 aqueous solution reaches −40 mV, implying that the suspension is stabilized very well, which is in agreement with the previous reports [36,37], resulting from high polarity of surface in the high alkaline solution.Nevertheless, in the practical application, the pH value of suspension would be limited in the suitable range near to 7 for the operating safety.

Conclusion
This study revealed that TiO 2 nanoparticles were dispersed by ultrasonic wave combining with inorganic and organic stabilizers in aqueous solution.As shown in the results, we summarized the following remarks: (1) The dispersion ability of SHMP was better than that of PAA due to the electronegativity of SHMP more than PAA in the TiO 2 nanosuspension.
(2) The suitable amount of PAA and SHMP stabilizers could be determined by the variations of average particle size and zeta potential with the amount of stabilizer to obtain maximum zeta potential of TiO 2 nanosuspension.
(3) With 5 wt% of SHMP, the average particle size of dispersion solution containing 0.005 wt% of TiO 2 particles was about 92 nm in de-ionic water, where zeta potential reaches 54 mV at pH value of 7.7.
Hopefully, these results could be expected to provide guidance to design nanofluid and functional coating with excellent performance.

Figure 1 :
Figure 1: Average particle size of dispersed TiO 2 without stabilizer as function of ultrasonic time for the different amount of dispersing phase.

Figure 2 :
Figure 2: Particle size distribution and TEM images with scale bars of 1 m and 100 nm of suspension containing (a) 0.005 wt%, (b) 0.01 wt%, and (c) 0.05 wt% of TiO 2 in the absence of stabilizer after dispersing by ultrasonic probe for 480 sec.

Figure 3 :Figure 4 :Figure 5 :
Figure 3: Average particle size of dispersed TiO 2 in aqueous solution varying with the amount of (a) SHMP and (b) PAA stabilizers, respectively.

Figure 6 :
Figure 6: Zeta potential of TiO 2 suspension without stabilizer at pH value of 8.2, as well as with SHMP and PAA at pH values of 7.6 and 6.2 respectively.

Figure 7 :
Figure7: Sketch of polarity models and chemical structures of PAA and SHMP, respectively, where  + and  − express the positive and negative charges[33,34], and the colors indicate the different atomics.

Figure 8 :Figure 9 :
Figure 8: Surface contact angles of TiO 2 suspension (a) without and with stabilizers of (b) SHMP and (c) PAA, respectively.

Figure 10 :
Figure 10: Zeta potential of TiO 2 suspension with 5 wt% of (a) SHMP and PAA stabilizers, respectively, as functions of pH value.

Table 1 :
Specification of titanium oxide used in this study.NaPO 3 ) 6 , Na 2 SiO 3 , and C 2 H 5 OH to disperse for TiO 2 particles in aqueous solution.The experimental result show that (NaPO 3 ) 6 was efficient dispersants for TiO 2 particles in aqueous solution.