The effect of surface charge on photocatalytic degradation of methylene blue dye using chargeable titania nanoparticles

Herein, a simple approach based on tailoring the surface charge of nanoparticles, NPs, during the preparation to boost the electrostatic attraction between NPs and the organic pollutant was investigated. In this study, chargeable titania nanoparticles (TiΟ2 NPs) were synthesized via a hydrothermal route under different pH conditions (pH = 1.6, 7.0 and 10). The prepared TiΟ2 NPs were fully characterized via various techniques including; transmission electron microscopy (TEM), X-ray diffraction (XRD), N2 adsorption/desorption, X-ray photoelectron spectroscopy (XPS), Ultraviolet–visible spectroscopy (UV-Vis) and dynamic light scattering (DLS). The influence of the preparation pH on the particle size, surface area and band gap was investigated and showed pH-dependent behavior. The results revealed that upon increasing the pH value, the particle size decreases and lead to larger surface area with less particles agglomeration. Additionally, the effect of pH on the surface charge was monitored by XPS to determine the amount of hydroxyl groups on the TiO2 NPs surface. Furthermore, the photocatalytic activity of the prepared TiΟ2 NPs towards methylene blue (MB) photodegradation was manifested. The variation in the preparation pH affected the point of zero charge (pHPZC) of TiO2 NPs, subsequently, different photocatalytic activities based on electrostatic interactions were observed. The optimum efficiency obtained was 97% at a degradation rate of 0.018 min−1 using TiO2 NPs prepared at pH 10.

band gap which requires an ultraviolet illumination 9 and 2) the high electron-hole pair recombination rate 10 . Thus, many efforts have been devoted to surmount the issues associated with the TiO 2 NPs 11 . Therefore, many studies have been published in attempt to overcome such limitations. An alternative approach is to design novel catalysts that exhibit high activity when illuminated by UV or visible light and have low recombination rates. For example, doping TiO 2 NPs with noble metals 12 , transition metals 13 , metalloids 14 or anions 15 is widely utilized approach to minimize the band gap energy and recombination rate. Another approach which was extensively explored to enhance the photocatalytic activity is coupling different semiconductor particles with TiO 2 NPs such as TiO 2 -CdS 16 , TiO 2 -WO 3 17 and TiO 2 -SnO 2 18 . Coupling reduces the recombination rate and increases the energy range of the photoexcitation. Hybridization with conjugated materials has been also investigated, to improve the transportation of the photocarriers during photocatalysis due to their excellent electronic properties. For instance, Zhang et al. utilized graphite layers and C60 to enhance the photocatalytic activity of TiO 2 NPs 19,20 . An alternative approach to prepare an efficient photocatalyst by fabricating core-shell geometry using two semiconductors was explored to enhance the emissive properties of the photocatalyst hence its photocatalytic activity 21 .
Generally, the pH of the solution is a key factor that affects directly the photoeffficiency of photocatalysts. This is due to the fact that the pH governs the surface characteristics and the size of aggregated nanoparticles, along with the charge of organic molecules, and finally governs the adsorption capacity of molecules onto nanoparticles surface and the concentration of hydroxyl reactive radicals 22 . Many attempts were devoted to better exploitation of surface charge properties to achieve superior photo reactivity such as such as surface doping and sensitization, creation of surface heterojunctions, modification with co-catalysts, increase in the accessible surface areas, and usage of surface F effects and exposure of highly reactive facets 23,24 . However, all of these approaches are considered to be laborious, costly and require meticulous design.
Therefore, we propose an alternative and simple approach based on adjusting the pH value of TiO 2 NPs to boost the electrostatic attraction between the surface of NPs and the dye hence improving the photodegradation efficiency. Initially, TiO 2 NPs were prepared using different ethanol: water ratios and calcination temperatures. Yet, the ratio of ethanol: water did not play a major role in the properties of the formed TiO 2 NPs. Considering the effect of calcination temperatures, as the calcination temperature increased a triple phase mix (rutile, brookite and anatase) was obtained. Then, the optimum TiO 2 NPs were further prepared at different pH values and their point of zero charge (pH PZC ) were measured and correlated with their photocatalytic efficiencies.

Experimental
Preparation of TiO 2 NPs. TiO 2 NPs were prepared by sol-gel method via hydrolysis of titanium tetrachloride (TiCl 4 ). In a typical procedure, 4 mL of TiCl 4 was added dropwise into a mixture of ethanol and distilled water with different ratios (1:1, 4:1 and 3:2 v/v). The mixture was refluxed at 80 °C under continuous stirring until a white suspension of TiO 2 NPs was formed after approximately 4 hours. The NPs were precipitated by centrifugation at 5300 rpm for 30 minutes. The precipitate was filtered and washed several times and was dried overnight at 50 °C.
Hydrothermal modification of TiO 2 NPs. Hydrothermally modified TiO 2 NPs were synthesized according to the same procedure in section 2.1 with some modifications using a mixture of ethanol and distilled water (4:1 v/v). The formed NPs were transferred into a 300 mL Teflon lined stainless steel autoclave at 150 °C for 5 hours. The suspension was then centrifuged at 5300 rpm for 30 minutes. The resultant NPs were dried overnight at 50 °C.
Furthermore, the synthesized TiO 2 NPs were calcined at different temperatures: 100 °C, 200 °C, 300 °C, 400 °C and 500 °C. Finally, the effect of pH on the synthesized NPs was investigated at different pH values: 1.6, 7.0 and 10 by adjusting the pH of the ethanol: water ratio prior to introducing TiCl 4 . TiO 2 NPs Characterization. Surface and bulk characteristics of the TiO 2 NPs were evaluated using X-ray powder diffraction (XRD) patterns, obtained using Bruker, D8 ADVANCE diffractometer with Cu K α (λ = 0.154 nm) radiation under 40 kV, 40 mA and a scanning range of 10-80° 2θ. The morphological properties of the NPs were determined using transmission electron microscopy (TEM) with a JEOL JEM 1230 (Japan) operating at 120 KV. The surface analysis was carried out using X-ray photoelectron spectroscopy (XPS) ESCALAB250 xi XPS spectrometer with an Al K α monochromatic source and a charge neutralizer. C 1 s peak at 284.5 eV are used as reference for binding energies. UV-Vis spectroscopy is used for determining the optical properties of the NPs using Agilent Cary 5000 UV-Vis spectrophotometer. N 2 adsorption-desorption isotherms were measured at −195 °C using a model Gemini VII, ASAP 2020 automatic Micromeritics sorptometers (USA), equipped with an out gassing platform. Zeta potential (ζ) measurements were made using Zeta sizer Nano ZS, Malvern Instruments Ltd, Malvern (UK) for point of zero charge (pH PZC ) determination.

MB Photocatalytic experiments.
The photocatalytic reactor is a Pyrex-glass cell with 1.0 L capacity. A 6W Lamp (BoittonInstruments) as the light source (365 nm) was placed in a quartz lamp holder which immersed in the photoreactor cell. Before illumination, the solution was allowed to stir in dark for 60 minutes to achieve adsorption-desorption equilibrium between the dye and photocatalyst. The cell was filled with 0.6 L of 10 mg/L of MB dye solution and 100 mg/L of the photocatalyst. The reactor was cooled down with an electric fan keeping the temperature at 25 °C. Magnetic stirrer was used to introduce fresh air bubbles into the suspension using a pump. Consequently, based on these observations the resultant NPs had a mixed phase of anatase and brookite at the different ratios examined suggesting that the ratio has no influence on the NPs phase. Figure 1B, demonstrates the effect of calcination temperatures on the prepared NPs at constant ethanol: water ratio (4:1 v/v). Patently, diffraction lines brookite and anatase manifested the spectra of temperatures between 100-400 °C. However, as the calcination temperature increased to 500 °C, an additional phase emerged resulting in a triple phase mixture (rutile, brookite and anatase). Thus, the calcination temperature had a significant effect on the NPs crystallinity compared to different solvent ratios. Therefore, TiO 2 NPs prepared at 100 °C and 4:1 v/v ratio were used for further experiments.
The Debye-Scherer equation was used to calculate the crystallite size of the TiO 2 NPs prepared at 100 °C and 4:1 ethanol: water was found to be 4.5 nm. The effect of pH was explored on the NPs by adjusting the pH of the initial reaction solution (ethanol: water medium) at values of 1.6, 7.0 and 10. The morphological properties of the prepared NPs were studied using TEM, Fig. 2. All the NPs exhibited a semispherical morphology with particles size of 12.4 nm, 10.5 nm and 8.7 nm at pH values of 1.6, 7.0 and 10, respectively. Additionally, agglomeration of the NPs decreased as the pH of the solution increased, which can be ascribed to the Van der Waals attractive or electrostatic repulsion forces. Thus, we can assume that the attractive forces were dominant at pH 1.6 and 7.0. However, the repulsion forces outweighed the attractive forces at pH 10 resulting in less agglomerated particles 26 . Figure 3 depicts the N 2 -isotherms of the prepared TiO 2 NPs at different pH values. All NPs exhibited type-IV isotherm plots with different capillary condensation steps, which suggested the existence of mesopores in the prepared NPs.
The TiO 2 BET surface areas were increased upon increasing the pH of preparation, this can be related to the increasing value of the pore volume. At P/P o = 0.8-1.0, the hysteresis loop obtained of type H4 which can be ascribed to the narrow slit like pores due to the large inter aggregated pores enriched by the generation of hollow interiors 11 . Consequently, TiO 2 NPs present a bimodal pore-size distribution ranging in the mesoporous region centered at 3.4 and >100 nm. Moreover, the capillary condensation arises at relatively high pressures and adsorption-desorption saturation is not significant due to the presence of large mesopores 27 . The measurements of N 2 -sorpometry confirmed the dependence of pore diameter, specific surface area, and the pore volume, for TiO 2 NPs on the pH value as shown in Table 1.
X-ray photoelectron spectroscopy was used to confirm the oxidation state and the surface composition of the synthesized TiO 2 NPs at different pH as depicted in Fig. 4A Figure 5A displays the UV-Vis absorption spectra obtained for TiO 2 NPs prepared under different pH conditions. All the NPs exhibited an absorbance at 350 nm, which is characteristic for TiO 2 NPs. The band gap energies were calculated using the Tauc-equation: where E g (the energy of optical band gap), k (constant) and m = 2 in the case of an indirect energy gap. (αhν) 2 was plotted versus hν and the linear portion of the plot was extrapolated to the ordinate as shown in Fig. 5B. The results were found to be 3.54, 3.47 and 3.40 eV for TiO 2 prepared at pH values of 1.6, 7.0 and 10, respectively, which are accepted for the TiO 2 NPs.
MB dye adsorption on the TiO 2 NPs. The amount of organic pollutant adsorbed on the NPs surface is a crucial factor that influences their photocatalytic performance. Therefore, the adsorption of MB dye on the TiO 2 NPs in the dark was monitored by measuring the absorbance values at different time intervals as shown in Fig. 6. After 120 min without UV irradiation, the calculated rate constants were 0.0046, 0.010 and 0.017 min −1 for TiO 2 NPs prepared at pH values of 1.6, 7 and 10, respectively. Furthermore, the adsorption percentages of MB on the NPs surface were 23.6, 71.0 and 87.5% for TiO 2 NPs prepared at pH values of 1.6, 7 and 10, respectively. The variation in the adsorption percentage can be attributed to the nature of the electrostatic forces between MB   dye and the surface of TiO 2 NPs. In the case of the positively charged TiO 2 (pH = 1.6) electrostatic repulsion was dominant between MB dye and the surface of TiO 2 NPs which minimized MB adsorption. However, in the case of the negatively charged TiO 2 NPs (pH = 7.0 and 10.0), electrostatic attraction promoted the adsorption of MB dye on their surface which was more preponderant at pH 10.
On the other hand, a control experiment was conducted in which MB dye was irradiated with UV light in the absence of the NPs. The findings showed that no major change in the MB dye concentration was observed and only 17% of the dye was degraded. This indicated that the direct photolysis of MB dye was insignificant in the absence of the NPs.  Photocatalytic activity of TiO 2 NPs. Determining the point of zero charge (pH PZC ) is substantial to predict the charge on the NPs surface during the photodegradation process 34 . Since the photocatalysis occurs on the NPs surface, the performance of the photocatalyst is greatly influenced by the solution pH 35,36 , the pollutant type and the surface ability to adsorb the pollutant 37 . Figure 7 showed the Zeta potential of TiO 2 NPs versus the pH of the solution and the pH of the MB was also indicated (green dotted line). The TiO 2 NPs prepared at pH 1.6, 7.0 and 10 had pH PZC of 7.35, 4.27 and 4.15, respectively. At pH values less than pH PZC the NPs carried with a positive charge, whereas, higher pH values promote the formation of negative charge on the NPs.
Photocatalytic degradations of MB (pH = 6.53) in an aqueous suspension of the TiO 2 NPs prepared at different pH values were performed to evaluate their photocatalytic activity, Fig. 8 shows the degradation efficiency of the TiO 2 NPs. The degradation efficiencies recorded were 73, 93 and 97% for TiO 2 NPs prepared at pH values of 1.6, 7.0 and 10, respectively. The high efficiencies of TiO 2 NPs prepared at pH 7.0 and 10 can be attributed to the presence of negative charge on their surfaces. Thus, the cationic dye MB with a positive charge can be adsorbed on the surface of the highly negatively charged TiO 2 NPs through a strong electrostatic attraction and the electrostatic interaction was beneficial for enhancing the adsorptive property thereby enhancing the degradation efficiencies as in the cases of TiO 2 NPs prepared at pH 7.0 and 10. On the other hand, TiO 2 NPs prepared at pH 1.6 exhibited lower efficiency (73%) due to the electrostatic repulsion between the positively charged NPs and the cationic dye. These results are fully matched with the previously published reports 38 .
The photodegradation reaction follows a pseudo-first-order reaction. The photodegradation rate constant for the photodegradation reaction was determined from the equation:  where C o and C are the initial concentration and the concentration at time t, respectively, and k is the apparent first-order rate constant. A plot of ln C o /C versus time represents a straight line as shown in Fig. 9, where the slope of which upon linear regression equals the apparent first-order rate constant k. The degradation rates obtained were 0.006 min −1 , 0.017 min −1 and 0.018 min −1 , for TiO 2 NPs prepared at pH values of 1.6, 7.0 and 10, which were consistent with the results obtained from the photodegradation. Photodegradation mechanism. The photodegradation mechanism is based on the conversion of organic dyes into harmless gaseous CO 2 , nitrate, ammonium, and sulfate ions. The general photocatalysis degradation of the organic pollutant is given by the following scheme 36     In this work, photo-holes are certainly not concerned by the initial step since the reactant is cationic and not electron donor. By contrast, the OH • radicals can attack the R-S + =R functional group in MB, which is in direct coulombic interaction with titania's surface as evidenced by the influence of the pH. Therefore, the initial step of MB degradation can be ascribed to the cleavage of the bonds of the R-S + =R functional group in MB into R-S(=O)-R, R-SO 2 -R, R-SO 3 H-R to finally produce SO 4 2− and phenol 39 .

Conclusion
A facile preparation of chargeable TiO 2 NPs using modified hydrothermal method at different pH was reported. The synthesized NPs had small semispherical morphology with 12.4-8.7 nm in size and surface areas of 12.4-183.6 m 2 g −1 . Furthermore, the impact of surface charge of NPs on the photocatalytic activity was investigated via XPS to estimate the amount of hydroxyl groups and isoelectric measurements for pH PZE determination Moreover, increasing the preparation pH value resulted in decreasing the particle size and increasing the surface area which played a significant role in addition to surface charge that govern the photo efficiency. The MB photodegradation results using different surface charges of TiO 2 NPs influenced the degradation rate and the adsorption efficiency of the dye. The optimum efficiency obtained was 97% at a degradation rate of 0.018 min −1 using TiO 2 NPs prepared at pH 10. This simple and robust approach can be applied to various types of nanophotcatalysts to manipulate their surface charge, hence enhancing their photocatalytic properties.