Hydrothermal synthesis of titanium dioxide nanotube with methylamine for photodegradation of Congo red

Titanium dioxide (TiO2) nanotube photocatalyst is highly desired for the photodegradation of dye in wastewater treatment. A series of titanium dioxide nanotube photocatalysts were successfully synthesized using methylamine as N-ligand via hydrothermal treatment at different hydrothermal temperatures and durations. The effect of these two parameters on the photocatalytic activity of synthesized materials were investigated. TEM micrographs and XRD analysis depicted methylamine assisted the transformation of anatase TiO2 nanoparticles to nanotube via the exfoliation of TiO2 crystallite into layered sheet and promoted the curling of layered sheet. Hydrothermal temperature up to 180°C was able to fully transform the morphology of anatase TiO2 nanoparticles into nanotube. The reaction duration was further modified. Fluorescence analysis showed that 24 h hydrothermal duration gave the slowest electron-hole recombination rate. DR-UV-Vis analysis indicated that the synthesized samples were active under UV region. The photocatalytic performance of the synthesized materials was tested in the photodegradation of Congo red under UV irradiation. The results suggested that among the materials synthesized, TiO2 nanotube synthesized at 180°C, under 24 h hydrothermal duration appeared to be the most superior photocatalyst which gave the highest photocatalytic activity of 77%. Possible mechanism of the TiO2 nanotube formation with methylamine as N-ligand is presented.


Introduction
Titanium dioxide (TiO 2 ) and TiO 2 -based materials have been intensively investigated due to their excellent catalytic performance [1][2][3][4][5]. However, TiO 2 is suffering from its low surface area, hence leads to limited activity. Since last decade, TiO 2 nanotubes (NTs) have attracted much attraction as compared to the commonly used TiO 2 nanoparticles due to its excellent unique features, including high ionexchangeability, large specific surface area and good photocatalytic activity [6]. The current developed fabrication methods of TiO 2 NT include the template-assisted method, sol-gel method, hydrothermal method, and electrochemical anodic oxidation [7][8]. Among these fabrication methods, hydrothermal fabrication is believed to be the most promising due to its environmentally friendly properties, simple setup and cheap apparatus is used. For the other methods, for instance, electrochemical anodic oxidation method, usage of extremely toxic aqueous hydrochloric acid solutions is needed for the anodization of titanium foil [9].
In this study, hydrothermal treatment method was used to synthesize TiO 2 nanotube (TNT). Although hydrothermal synthesis involves simple setup, every single step in the process from choice of TiO 2 precursor to hydrothermal conditions and lastly the post-treatment process of TiO 2 NT is very important in determining the crystallography and morphology as well as the physical-chemical properties of the TiO 2 NT final products. The hydrothermal conditions include the temperature, hydrothermal time and concentration of reactant [3]. According to the study by Weng [10], increasing the hydrothermal time contributes to the lengthening of TNTs. When the hydrothermal duration exceeds 24 h, there is no further increase in the length of TNTs. From the other studies, however, it has been stated that the morphology of the synthesized products is dependent on the hydrothermal duration [11]. It seems that the hydrothermal duration plays an important role in affecting the morphology of TNTs formed which drives the author to further study on the effect of hydrothermal duration and temperature on the formation of TNTs in this study.
During the hydrothermal synthesis of TNTs, agglomeration of individual morphological form titanate into secondary particles occurs. Secondary particles include nanotubular bundles, nanofibers, nanofibers which are linked hierarchically and others [2]. Even though there were extensive studies on the effect of synthesizing condition on the geometry of single nanotube, there are few studies reported on the principles of controlling the agglomeration of nanotubes and treatment of the shape and geometry of secondary particles [12]. In addition, based on the study by Peng et al. [13], the -NH 2 functional group of DETA captured the Ti 4+ in the solution hence facilitate crystal grow of TiO 2 to be formed uniformly on the carbon nanotube. Methylamine, which is also one of the N-atom bearing ligand, is expected to behave the same as DETA, holding the Ti 4+ ion and could assist the titanate layer to curl up and thus promote the formation of TiO 2 NT in this research. Its effectiveness in helping the intermediate TiO 2 nanosheet to curl up, forming nanotubular structure is yet to be known. The mechanism for the formation of TiO 2 NT with methylamine as a N-bearing ligand was proposed in this research.

Synthesis of titanium dioxide nanotube
TiO 2 NT was synthesized using hydrothermal method with the presence of methylamine. Commercial anatase titanium dioxide served as the Ti precursor while methylamine hydrochloride was used as Nligand.
In a typical synthesis, methylamine hydrochloride was added to the 7 M NaOH aqueous solution, followed by the addition of TiO 2 into the reaction mixture based on the molar ratio TiO 2 :CH 3 NH 2 of 1:1. The reaction mixture was allowed to stir vigorously (1000 rpm) for one hour. Later, reaction mixture was transferred to a 100 mL Teflon-lined stainless-steel autoclave and kept in an electric oven at 130°C for 24 h. It should be noted that in order to ensure the desired structure to be formed successfully, any disturbance during the reaction, such as opening of the oven door, had to be prevented. After 24 h of hydrothermal duration, the autoclave was taken out and left to cool to room temperature. The white precipitate was obtained by centrifugation, washed thoroughly with sulphuric acid and double distilled water, then dried at 70°C overnight.
Experiment was repeated with different ratio titanium dioxide to methylamine of 1:1, 1:3, 1:5 and 1:8. The samples were notated as xM-TiO 2 NT-y-z, where x, y, and z represent the ratio of methylamine to TiO 2 and M represent methylamine, hydrothermal temperature and duration, respectively. The optimized ratio was then used to synthesize TiO 2 NT at different temperatures (130°C and 180°C) and different hydrothermal durations (6 h, 12 h and 24 h).
TEM (JEOL, JEM-2100F, 200 kV) was used to investigate the morphologic, crystallographic and compositional information on the TiO 2 samples. Meanwhile, the crystallinity and the crystal structure of the synthesized TiO2 samples were examined via XRD (Bruker Advance D8 with Cu Kα radiation; The 2nd International Conference on Chemistry and Material Science (IC2MS) IOP Conf. Series: Materials Science and Engineering 833 (2020) 012075 IOP Publishing doi:10.1088/1757-899X/833/1/012075 3 λ = 1.5406 Å; 40 kV, 40 mA). UV-Vis diffuse reflectance spectra of the prepared samples were evaluated using a UV-Vis spectrometer (Perkin-Elmer Lambda Lambda 35) equipped with a diffuse reflectance attachment with BaSO 4 coated 76 mm integration sphere as a reference material. The reflection in percentage was measured and presented by Kubalka-Munk function. Meanwhile, the electron-hole recombination rate was calculated using a spectrofluorometer (JASCO, FG-8500).

Photocatalytic testing
The synthesized TiO 2 anatase was tested for its photodegradation activity on Congo red under dark condition where the UV radiation is absent and normal condition where the UV radiation is present.
The photocatalytic activity of each of the synthesized TiO 2 NT was calculated by using the formula as follow: where, C 0 = Concentration of Congo red before reaction (ppm) C t = Concentration of Congo red after reaction (ppm) Figure 1 shows the TEM micrographs of the synthesized samples. As shown, with the addition of methylamine, most of the structure of TiO 2 nanosheet had been fully transformed into nanotubes. Both the as-synthesized TiO 2 NT-130°C-24 h and 3M/TiO 2 NT-130°C-24 h had interplanar spacing of 0.35 nm, represented the (1 0 1) lattice plane of anatase TiO 2 [14]. Meanwhile, 3M/TiO 2 NT-130°C-24 h had one additional d-spacing of 0.64 nm, represented (001) lattice plane of nanotube [15].   Figure 2(a) illustrates the XRD patterns of the methylamine modified titanium dioxide nanotube samples. As illustrated, the characteristic of anatase phase of titania could be observed clearly in all the samples at 2θ value of 25.35°, which attributed to the lattice plane (1 0 1) [16]. The intensity of the peak however, decreased as higher ratio of methylamine ligand was added to the titania. Peaks assigned to the tetragonal titania anatase phase were also observed at 37.  Figure 2(b) shows the XRD patterns of materials synthesized at different hydrothermal temperatures, namely 130°C and 180°C. It can be seen that when the hydrothermal temperature was increased from 130°C to 180°C, the intensity of peaks corresponded to anatase phase of titania reduced and peaks corresponded to Na 2 Ti 3 O 7 type of titanate (JCPDS 31-1329) were observed with increasing intensity. Thus, it could be deduced that increase of hydrothermal temperature up to 180˚C was able to fully transform the spherical morphology of anatase TiO 2 particle to nanotube shape. Meanwhile, the modification of reaction condition by varying the hydrothermal duration had insignificant changes within the samples. Increasing hydrothermal duration would not distort the nature of the synthesized materials. Figure 3(a) illustrates the DR-UV-Vis spectra of the synthesized TiO 2 nanotubes. There were two absorption peaks located at 270 -280 nm, which were contributed by amorphous Ti species in all the synthesized samples [17]. Also, an absorption peak at 320 nm was observed, which was associated to anatase TiO 2 [18]. The intensity of the absorption band at 270 nm decreased with increasing xM/TiO 2 ratio, while it was accompanied by an increased in the intensity of adsorption band at 320 nm. This proves that with an increasing amount of methylamine, amorphous TiO 2 could be reduced, transforming into crystalline anatase phase of TiO 2 . Addition of methylamine increased the band gap energy of assynthesized TiO 2 due to quantum size effect [19]. Figure 3(b) elucidates the DR UV-Vis spectra of the samples synthesized at different hydrothermal temperature, namely 130°C and 180°C. Increasing the hydrothermal temperature would increase the intensity of absorption band at 270 nm which assigned to amorphous Ti species and the absorption band at 320 nm which denoted the anatase TiO 2 was fully disappeared. It could be deduced that hydrothermal temperature of 180°C was able to fully transform the morphology of anatase TiO 2 particle into nanotube shape.

Optical properties
As for the modification of reaction condition based on hydrothermal duration (6 h, 12 h, 24 h), hydrothermal duration of 6 h and 12 h were too short for the complete formation of TiO 2 nanotube. As a result, mixture of unreacted nanoparticles, nanosheet and nanotube were present within the sample. The current findings strongly suggested that hydrothermal duration of 24 h was important for the complete transformation of spherical anatase TiO 2 nanoparticles into nanotube.
As for the fluorescence analysis, the fluorescence spectra of the as-synthesized TiO 2 and the best photocatalyst (3M/TiO 2 NT-130°C-24 h) were shown in Figure 3(c). With the addition of methylamine, the intensity of the peak decreased, denoting a decrease in the electron-hole recombination rate. A possible explanation to this is due to an increase in the separation effectiveness and longer lifetime of the photoinduced charge carriers [20]. It could be suggested that addition of methylamine as N-ligand suppressed the electron-hole recombination rate by increasing their lifetime.
As the hydrothermal temperature increased up to 180°C, the electron-hole recombination rate was greatly suppressed as evidence in Figure 3(d). Possible explanation could be the crystalline nature of the nanotube enable smooth, fast and efficient electron transfer to the surface hence retarded the electron-hole recombination rate [21]. Hydrothermal duration of 24 h was optimum for the complete transformation of nanoparticles to nanotube. The fluorescence intensity of 3M/TiO 2 NT-180°C-24 h was the lowest as compared to those synthesized at 6 h and 12 h, denoting the slowest recombination rate.

Photocatalytic activity
The photocatalytic performance of the synthesized samples was evaluated via photodegradation of Congo red dye under dark condition and with UV irradiation. The obtained absorbance at 498 nm was used to calculate the photodegradation efficiency of the synthesized samples. As illustrated in Table 1, the photocatalytic activities of the xM/TiO 2 (x = 1, 3,5,8) were higher than the as-synthesized TiO 2 . The addition of methylamine increased the photocatalytic activity of TiO 2 by enhancing nanotubes formation. Methylamine might had assisted the scrolling of the intermediate nanosheet, forming nanotube. Sample 3M/TiO 2 had the best photocatalytic activity of 69.4% which could be attributed to the lowest electron-hole recombination rate.  Table 2 that 3M/TiO 2 NT-180°C-24 h had a higher photocatalytic activity as compared to the sample synthesized at 6 h, which could be due to the reduction in electron-hole recombination rate of nanotube. The photocatalytic activity of the samples decreased drastically with hydrothermal duration, around 19%. The increase in photocatalytic activity of TiO 2 NT might be attributed to the higher amount of nanotube formation from mixture of unreacted particle, nanosheet and nanotube. Apparently, hydrothermal duration of 6 h was too short for the TiO 2 NT formation. Larger amount of nanotube could be formed at 24 h hydrothermal duration with the presence of methylamine that facilitated in scrolling up the edge of intermediate nanosheet.   It should be noted that the transformation process was gradual and it started from the surface region to the centre region. Thus, the Ti-O bonds on the surface were broken in the early stage. During the breaking process of Ti-O bonds, the O atoms were twisted or rotated, creating an angle between Ti and O atoms. This led to the formation of the lepidocrocite-type layers, which were suggested to be the construction units in any layered titanate. It was reported that the surface energy of TiO 2 nanosheet and TiO 2 nanotube was 1 J/m 2 and 500×100 -18 J/m 2 , respectively [22]. Due to the stable lower surface energy, it drives TiO 2 to fold the sheet-like structure into tube-like structure. However, it was found out that the hydrothermal temperature of 130˚C in the current study was not enough to transform all the nanosheet curl up into nanotube structure. Presence of nitrogen atom within the methylamine could have held the Oof the Ti-O bond and assisted the scrolling of nanosheet at the edges, which resulted in higher degree of nanotube formation.

Conclusion
The synthesis of titanium dioxide nanotube using methylamine as N-ligand via hydrothermal method was conducted successfully. Effect of different temperature and hydrothermal duration on the properties and photocatalytic activity were evaluated. At higher hydrothermal temperature, up to 180 °C, there was a reduction in the electron-hole recombination rate as all the anatase TiO 2 nanoparticles had transformed into nanotube. On the other hand, hydrothermal duration of 24 h was needed to allow complete transformation of spherical anatase TiO 2 nanoparticles to nanotube shape. Sample 3M/TiO 2 NT-180°C-24 h appeared to be the best photocatalyst due to its low electron-hole recombination rate, giving photocatalytic activity of 77.1% in photodegradation of Congo red organic dye. Evidently, methylamine was vital in assisting the complete TiO 2 nanotube formation and, hydrothermal temperature and hydrothermal duration were optimum at 180 °C and 24 h, respectively.