Adsorption and photocatalytic degradation of methylene blue by titanium dioxide nanotubes at different pH conditions

Commercial TiO₂ powder (Merck, 99.99%) with microscale, MB were obtained from JHD Fine Chemicals (China, 99%), sodium hydroxide (NaOH, Merck, 99%) and hydrochloric acid (HCl, Merck, 99.99%). Deionised water was supplied by Puris-Evo water system and used in all experiments.

TNTs were prepared by the hydrothermal method at the optimised synthesis parameters [18]. Briefly, approximately 1.7 g commercial TiO₂ powders was added to 157 ml NaOH 10 M solution and stirred for an hour at the room temperature. Next, the mixture was placed in an autoclave and heated at 135 °C for 24 h. After the hydrothermal process, the resultant white precipitates were subsequently filtered and repeatedly washed with acid HCl 0.1 M and deionised water until the pH of the filtrate reached approximately 7.0. The obtained TNTs were dried in air at 80 °C for 6 h and stored in airtight brown bottles until required for use in the following experiments.

The phase, composition, and crystal structure of the materials were determined by x-ray diffraction (XRD) pattern, using a Bruker D8-Advance 5005 with Cu-Kα radiation (λ = 0.154064 nm). The morphology of hydrothermally synthesised TNTs was observed by transmission electron microscopy (TEM) images using a transmission electron microscope on a JEM 1400 instrument, JEOL. The chemical state of titanium, and oxygen in materials were analysed by x-ray photoelectron spectroscopy (XPS) using a Leybold spectrometer with an Al Kα monochromatic beam (1486.6 eV, ESCALAB250, Theta Probe XPS system). The specific surface area (S_BET) was calculated from nitrogen adsorption/desorption using the Brunauer–Emmett–Teller (BET) equation, and the pore distribution was determined by the Barrett–Joyner–Halenda equation. Before analysing textural property, the sample was degassed at 120 °C for 2 h. The photocatalytic activity of the material was investigated using an UV–vis spectrometer (U2910, HITACHI, Japan) with the wavelength in the 400–800 nm range.

Firstly, MB solution was adjusted into target pH value by adding 1 M NaOH or 1 M HCl solutions. Secondly, approximately 0.02 g TNTs was introduced and magnetically stirred with 60 ml MB (20 mg l⁻¹) into the closed system with different pH values. The adsorption process was conducted in a dark condition. Thirdly, the solution was centrifuged and the absorption spectrum was calculated to measure the MB concentration. Finally, this solution was poured back into the initial solution and continuously stirred in the dark.

After measuring the adsorption ability of the sample for 60 min in the dark, the MB solution was irradiated under UV light (7 W, λ = 350 nm). The mixture continued to be stirred throughout the irradiation. The absorbance of the sample is determined every 30 min with a maximum absorbance peak (λ_max = 664 nm) for each sample. The photodegradation efficiency (η%) is calculated as follows:

$$\begin{eqnarray}&&\eta \left( \% \right)=100\times \left({C}_{0}-{C}_{t}\right)/{C}_{0},\end{eqnarray} \tag{ 1 }$$

where C₀ and C_t were the concentrations of initial MB after t min of UV light exposure, respectively.

Figure 1(a) presents the morphology of TNTs synthesised by the hydrothermal method. As expected, the synthesised TNTs exhibited a relatively homogeneous size with its length from 200 to 400 nm, average outer diameter 10 ± 2 nm, and average inner diameter 6 ± 2 nm. Figure 1(b) shows HRTEM image of hydrothermally synthesised TNTs, therein, the measurement on lattice spacing (d_spacing) in the micrographs confirmed the crystallisation of hydrothermally synthesised TNTs in the anatase phase with d = 0.352 nm, and 0.238 nm corresponding to (101) and (004) planes, respectively [31]. The result demonstrated the formation of the TiO₂ crystals.

Zoom In Zoom Out Reset image size

XRD result for the commercial TiO₂ and hydrothermally synthesised TNTs is given in figure 2. As expected, all studied materials exhibited characteristic diffraction peaks at 2θ of 25.25°, 37.72°, 47.89°, 54.43°, 55.03°, 62.83°, and 68.82° corresponding to the (101), (004), (200), (105), (211), (204), and (116) crystal lattice planes of the anatase phase of TiO₂, respectively (JCPDS no. 21-1272). Moreover, the presence of diffraction peaks at 2θ = 27.5° and 2θ = 36.14° is attributed to the (110) and (101) crystal lattice planes of the rutile phase of TiO₂, respectively (JCPDS card no. 21-1276). The formation of the anatase phase and the rutile phase indicated the enhancement of the photocatalytic activity for TNTs [32].

Zoom In Zoom Out Reset image size

Elemental oxidation state was determined by a high-resolution x-ray photoelectron spectroscopy (HRXPS). Clearly, figure 3 shows that two biding energy (BE) peaks in HRXPS of Ti 2p orbital at 464.3 and 458.59 eV were assigned to Ti 2p_1/2 and Ti 2p_3/2 of Ti⁴⁺ in TiO₂, respectively [33, 34]. In addition, HRXPS spectrum of O 1 s confirms the existence of two states with BE at 532.5 eV and 530.2 eV corresponding to O²⁻ states in TiO₂ [34]. The HRTEM and XRD results are consistent with the XPS result. Therefore, it can be concluded that TNTs were formed with high crystallisation and an excellent complex.

Zoom In Zoom Out Reset image size

The textural property of hydrothermally synthesised TNTs and commercial TiO₂ has been published in our previous study [18]. The Brunauer–Emmett–Teller surface area of hydrothermally synthesised TNTs (83.49 m² g⁻¹) was significantly higher than that of commercial TiO₂ (40 m² g⁻¹, a non-porous material). An analogous result was obtained by Wang et al, Pai et al [35, 36] and Son et al [37] who reported that BET surface area of commercial P-25 TiO₂ Degussa is 56 m² g⁻¹ and 50 m² g⁻¹, respectively. In addition, TNTs can be classified as a mesoporous material because of its pore diameter approximately 4.08 nm (in 2–50 nm range). The result suggested that TNTs possibly remove MB cations from water media through the adsorption and photocatalysis processes.

The MB adsorption capacity and photodegradation efficiency of hydrothermally synthesised TNTs are experimented in different pH values. The effects of solution pH (pH_solution) on the process of MB adsorption onto TNTs are provided in figure 4. The point at zero charge (pH_pzc) of hydrothermally synthesised TNTs determined by the drift method was approximately 6.0, which is consistent with the findings of other scholars, such as pH_pzc of hydrothermally synthesised TNTs = 6.25 [38].

Zoom In Zoom Out Reset image size

Essentially, when pH_solution > pH_pzc, the surface of hydrothermally synthesised TNTs will become negatively charged because of the deprotonation of –OH groups (equation (2)), favouring the adsorption of MB cations from the solution and vice versa [28, 39]. Clearly, the adsorption capacity of MB onto TNTs was negligible when solution pH was lower than 2.0 due to the presence of strong repulsion force (equation (3)). However, the adsorption capacity of hydrothermally synthesised TNTs toward MB increased with an increase in solution pH, with its highest adsorption capacity at pH 6.0 and 7.0. Furthermore, figure 4 indicates that the removal process of MB (adsorption onto TNTs) reached fast equilibrium in approximately 40 min, suggesting that TNTs have a high affinity to MB ions in the solution. However, figure 4 shows that the pH value is higher than the pH_pzc value, TNTs can react with base medium leading to the decrease of the TNTs adsorption.

$$\begin{eqnarray}&&{\rm{Ti}}-{\rm{OH}}+{{\rm{OH}}}^{-}\leftrightarrow {{\rm{TiO}}}^{-}+{{\rm{H}}}_{2}{\rm{O}},\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{\rm{Ti}}-{\rm{OH}}+{{\rm{H}}}^{+}\leftrightarrow {{\rm{TiOH}}}_{2}^{+}.\end{eqnarray} \tag{ 3 }$$

Figure 5 illustrates the effects of contact time and solution pH on MB photocatalytic reaction of hydrothermally synthesised TNTs. To describe the kinetic rate of MB photocatalytic degradation, the obtained experimental data in this study were fitted to the Langmuir-Hinshelwood kinetic equation [40, 41]:

$$\begin{eqnarray}&&\displaystyle \frac{d{C}_{0}}{dt}=k{C}_{t},\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&\mathrm{ln}\,\displaystyle \frac{{C}_{0}}{{C}_{t}}=kt,\end{eqnarray} \tag{ 5 }$$

where k (1/min) is the pseudo-first order rate constant, C₀ (mg/l) and C_t(mg/l) are defined in equation (1).

Zoom In Zoom Out Reset image size

Table 1 lists the pseudo-first order rate constant determined from the slope of a plot of ln(C₀/C_t) versus t. As provided in figure 5(a), the MB photodegradation efficiency of hydrothermally synthesised TNTs increased when pH_solution increased from 1.0 to 6.0, with the optimal pH_solution being obtained at pH 5.0. Similar to photodegradation efficiency, the photodegradation rate (k) also significantly increased within pH_solution from 1.0 to 6.0, and the highest k (7.08 × 10⁻³ 1 min⁻¹) value was found at pH_solution = 5.0 (table 1). Meanwhile, at pH_solution higher than 6.0, TNTs exhibited the fastest photodegradation rate at pH 11 (k = 5.81 × 10⁻³), but the slowest rate at pH 7 (2.87 × 10⁻³) (figure 5(b) and table 1). The result suggested that the photodegradation process of MB dye molecules by TNTs was strongly dependent on pH_solution.

Figure 6 shows the relationship of the MB adsorption and photodegradation by TNTs in the medium with different pH values. This result confirmed that the pH of the medium strongly affected the adsorption capacity and photodegradation efficiency of hydrothermally synthesised TNTs. Generally, the photocatalytic activity of hydrothermally synthesised TNTs increased significantly in pH value from 1 to 5 and decreased in the pH value from 8 to 10. Although the adsorption ability of hydrothermally synthesised TNTs at pH 6 is greatest but their photocatalytic activity is poor. In addition, in the alkaline medium, the photocatalytic efficiency of hydrothermally synthesised TNTs was almost inferior to that of adsorption capacity but in the medium of the solution with pH 11 and 12, the photocatalytic and adsorption abilities do not comply with this rule. The explanation for this result could be proposed that the TNT surface was completely covered by MB solution leading to the photoexcited receiving ability of hydrothermally synthesised TNTs being disadvantage. Therefore, the MB photodegradation of hydrothermally synthesised TNTs in this pH value range is not good. The MB adsorption and photodegradation efficiencies of hydrothermally synthesised TNTs reach the highest values in the medium with pH 6, while the lowest values of the MB adsorption and photodegradation efficiency occur at pH 1. Results of the adsorption and the photocatalytic activity of TNTs are listed in table 2. Therein, the MB adsorption efficiency of hydrothermally synthesised TNTs in medium with pH 1 and pH 6 is 28% and 91%, respectively. Furthermore, the MB photodegradation efficiency of hydrothermally synthesised TNTs after 300 min of the UV irradiation in medium with pH 1 and pH 6 is 42% and 52%, respectively while the highest photocatalytic efficiency (87%) of TNTs is achieved at pH 5 after 300 min under UV irradiation.

Zoom In Zoom Out Reset image size

The MB intensity change for different pH value conditions could be explained by the existence of the H⁺ or OH⁻ in the medium. These radicals will contribute to the photocatalytic activity of TNTs [5, 42]. When the TNTs are irradiated by UV light, electrons and holes will be generated in the VB of TNTs, and then electrons will transfer to the CB of the TNTs. The existence of the H⁺ or OH⁻ in the medium will contribute to the photocatalytic activity of TNTs as below equations, therein, the contributions of the H + and OH⁻ are expressed clearly in the equations (2), (4), and (6).

$$\begin{eqnarray}&&({\rm{TNTs}})+\,hv\to ({\rm{TNTs}})({e}_{CB}^{-}+{h}_{VB}^{+}),\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{e}^{-}+{{\rm{O}}}_{2}\to {}^{\cdot }{{\rm{O}}}_{2}^{-},\end{eqnarray} \tag{ 7 }$$

$$\begin{eqnarray}&&{}^{\cdot }{{\rm{O}}}_{2}^{-}+{{\rm{H}}}_{{\rm{2}}}{\rm{O}}\to \,{}^{\cdot }{{\rm{HO}}}_{2}+{{\rm{OH}}}^{-},\end{eqnarray} \tag{ 8 }$$

$$\begin{eqnarray}&&{}^{\cdot }{{\rm{HO}}}_{2}+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{H}}}_{2}{{\rm{O}}}_{2}{+}^{\cdot }\mathrm{OH},\end{eqnarray} \tag{ 9 }$$

$$\begin{eqnarray}&&{h}^{+}+{{\rm{OH}}}^{-}\to {}^{\cdot }\mathrm{OH},\end{eqnarray} \tag{ 10 }$$

$$\begin{eqnarray}&&{}^{\cdot }\mathrm{OH}\,+{\rm{MB}}\to {{\rm{CO}}}_{2}+{{\rm{H}}}_{2}{\rm{O}}{\rm{.}}\end{eqnarray} \tag{ 11 }$$