Enhanced Adsorption Property of TiO2 Based Nanoribbons Produced by Alkaline Hydrothermal Process

TiO2 is a semiconductor material with endless potential for the development of renewable energy as well as in the environmental field application. With various methods, TiO2 nanostructures with various morphology, properties and application can be developed. In this paper, the synthesis of TiO2 based nanoribbons with high adsorption propertyproduced by alkaline hydrothermal methods were investigated. Its morphology, crystal structure and physical properties were characterized using Scanning Electron Microscope (SEM), X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and Brunauer-Emmett-Teller (BET) surface area analysis. The result shows that by controlling the hydrothermal processing time, different morphology and structures of TiO2 nanoribbons with different adsorption properties can be obtained. The nanoribbons produced via alkaline hydrothermal method haswidth 200-300 nm and length up to several microns. It also possesses fair adsorption capacity over dyes (Rhodamine B and Methyl orange) considering its large surface area and high pore volume.


INTRODUCTION
TiO2 based nanostructures has unique physicochemical properties and morphology which can be used for many applications such as catalysis, gas adsorption (Umek et al., 2005), in lithium battery (Kavan et al., 2004;Myung et al., 2011) and solar cells (Pan et al., 2007). Morphology such as nanotubes, nanoribbons, nanobelts, nanorods, and nanowires may form during the process depending on the process parameters. Kasuga et.al were first reported the method namely alkaline hydrothermal in 1998 to produced TiO2 based nanostructures. This method was able to convert amorphous TiO2 into nanotubes and nanoribbons with almost 100% efficiency Kasuga et al., 2005;Bavykin et al., 2006). The application of TiO2 nanoribbon varied such as the photocatalyst nanohybrid with carbon nanotubes for water splitting photoelectrochemical (PEC) enhancement (Ahmed et al., 2020); anode material in combination with SnO2 for rechargeable battery (Li et al., 2016); organic pollutant removal (Ariyanti et al., 2018Ariyanti et al., 2019. Researchers reported low adsorption properties of TiO2 based nanoribbons, which has a typical value of specific surface areain the range of 20-25 m 2 /g and pore volume below 0.1 cm 3 /g (Umek et al., 2005;Bavykin et al., 2006). In this study, the investigation on TiO2 based nanoribbons with high adsorption property using alkaline hydrothermal methods were conducted. The reaction mechanisms that lead to the formation of different types of layered protonated titanate are also discussed. In addition, the ability of layered protonated titanate on dye adsorption is correlated with its morphology and crystal phases.

METHODOLOGY
The starting material for this experiment was TiO2 powder anatase phase with particle size <25nm (purchased from Sigma Aldrich Co.). The synthesize of TiO2 based nanostructure via alkaline hydrothermal method was initiated by the dispersion of 0.2 g anatase powder in the 30mL 10M NaOH solution with continous mixing to achieve the homogenized solution. The well mixed solution then transferred immediately to a hydrothermal autoclave equipped with Teflonlined, sealed properly for safety purposes and placed in an oven at 180 o C for 15-30h. The products after it cooled down then firstly washed with 0.1M HCl, followed by deionized water, and then dried in the oven at 120 o C for 4h.
Samples was first characterized by Scanning Electron Microscope with (SEM-EDS, Philips XL30S FEG) to observed the morphology of the nanostructures. The X-ray diffraction (XRD) was used to analyze its crystal structures and FTIR (Perkin Elmer FT-IR Spectroscopy ATR) was employed to investigate the chemical bonding of the samples. In addition the Brunauer-Emmett-Teller (BET) total surface area was evaluated usingTriStar 3000 surface area analyser along with the average pore diameter using Barrett-Joyner-Halenda (BJH) methodand the cumulative pore volume.
Batch experiment was conducted to evaluated the adsorption capacity of the samples over dyes. Representative dyes such as Rhodamine B (RhB) and Methyl Orange (MO) with purity ≥ 95% and 85% respectively were purchased from Sigma Aldrich Co and used without prior treatment. 50 mg of nanoribbons sample was dispersed and mixedin50 mL RhB or MO solution with concentration of 5ppm for 120 min. Every period of time 2 mL of sample was taken and then filtered with 0.2μm PVDF syringe filter for futher concentration measurement. The concentrations of RhB and MO were determined by the adsorbancemeasurement at the at 553 nm for RhB and 463 nmfor MO in UV-Vis spectrophotometer (Perkin Elmer).

RESULTS AND DISCUSSION
The alkaline hydrothermal reaction is a straightforward process initiated by mixing the as purchased TiO2 with 10 M NaOH to form a well dispersed solution followed by heat treatment through hydrothermal process at 180 o C. The afterwards HCl treatment yields the TiO2 based nanostructures. Figure 1 show the morphology of TiO2 based nanostructure prepared at different times in comparison with the untreated TiO2. Irregular thick nanoribbons were produced after 15 h of hydrothermal reaction ( Figure 1B). Based on this SEM images it observed that the width of this nanoribbons range of 50-400 nm with up to several microns inlength. Meanwhile, by setting up hydrothermal process for 20 h, thin ordered nanoribbons were produced ( Figure 1C) with width observed in the range of 200-300 nm and several microns in length. Further, by prolonging the process time up to 30 h regular nanowires were produced with width of 50-100 nm and length of several microns ( Figure 1D). Figure 2 shows the XRD analysis of the nanoribbons prepared at 15 and 20 h. The structure was proven to be a monoclinic of hydrogen trititanate (H2Ti3O7) with peaks at 11, 24.5, 33, and 48.4º (JCPDS 47-0561). The nanowires produced from hydrothermal process for 30 h appeared to be a mixture of H2Ti3O7 (JCPDS 47-0561), B-TiO2 (JCPDS 46-1238) and H2Ti5O11•H2O (JCPDS 44-0131). According to some researches, by taking the route of hydrothermal process, there are several possibilities of structures may be produced with mostly layered protonated titanates such as H2Ti3O7, H2Ti2O4(OH)2, H2Ti4O9•H2O, TiO2-B or H2Ti5O11•H2O. Thus are depending on the parameters such as temperature and operation time used during alkaline hydrothermal treatment and the acid washing treatment (Bavykin et al., 2006;Gao et al., 2013).
In general, there are two reaction steps in sequence during the hydrothermal process, (i) formation of layered titanate and (ii) dissolution followed by re-crystallisation along with the ion exchange. The chemical reactions are suggested below (Gao et al., 2013;Lai et al., 2015): TiO2 dissolved in 10 M NaOH solution for 15-20 h: 3TiO2 + 2NaOH → Na2Ti3O7 + H2O Washing with HCl for 24 h: Na2Ti3O7 → 2Na + + Ti3O7 2− 2Na + +Ti3O7 2− +H + +Cl -→ H + + Ti3O7 2− + 2Na + + Cl − 2H + + Ti3O7 2− → H2Ti3O7 The formation of B-TiO2 with 30 h hydrothermal treatment is due to H2Ti3O7 reorganization as a result of heat treatment and continuous water loss (Ariyanti et al., 2018;Zhu et al., 2001). Figure 3 shows the FT-IR spectra of TiO2 based nanostructures prepared at variable of times. The peaks situated at 443 cm -1 for all samples, corresponding to the Ti-O-Ti vibrational frequency. Meanwhile, Ti-O stretching vibrations of hydrogen titanates were also detected in vibration mode at 908 cm -1 (Bavykin et al., 2006), which only found in the samples prepared at 20 and 30 h. Furthermore, in all samples, at 3210 cm -1 the board spectrums were observed indicating the presence of -OH groups in the surface, and strong absorption at 1633 cm -1 were also found which attributes to the deformation vibration of H-O-H bonds in the physically absorbed water. Another, IR absorption in the range of 1600-3600 cm -1 detected indicates the manifestation of -OH groups and H-O-H molecules that are weakly bounded in different active sites of TiO2 (Li et al., 2005;Shankar et al., 2009;Choudhury et al., 2012;Ariyanti et al., 2018).
Adsorption characterisation of TiO2 based nanostructures prepared at different times were determined using N2 adsorption at 77.35 K with a relative pressure (P/P0) range of 0.0001 to 1 ( Figure  4). Nanoribbons prepared using hydrothermal process for 20 h shows high adsorption of nitrogen. The BET and pore size distribution are shown in Table 1, confirming that the nanoribbons prepared at 20 h of hydrothermal process has a large surface area (89.805 m 2 /g) and high pore volume (0.384 cm 3 /g).   Adsorption capacity of TiO2 based nanostructures prepared at different time was evaluated over adsorption of Rhodamine B and Methyl orange. As depicted in Figure 5, nanoribbons prepared with hydrothermal process for 20 h possess higher adsorption capacity compared to the other samples. Adsorption capacity is largely decided by pore size and volume of adsorbent (Hsieh, and Teng, 2000;Dąbrowski, 2001). Even though the nanoribbons (20h) have relatively small pore size, it still has higher pore volume compared to samplesof 15h and 30h (3 times higher). The high BET surface area is favourable for the dye adsorption process.
It also observed that TiO2 based nanostructures absorbed more concentration of Rhodamine B compared to Methyl Orange. Rhodamine B and Methyl Orange are two different dyes, and it may have a considerable effect on the adsorption. Methyl orange is an azo group with (-N=N-) and sulfonic group on the other side. These types of bonding are associated with high dissociation energy and less reactive (Horikoshi et al., 2003;Luan et al., 2013) compared to the functional groups existing in Rhodamine B.

CONCLUSION
The formation of TiO2 based nanostructures by alkaline hydrothermal technique has been investigated at different processing times. By controlling the processing time, different morphologies and structures can be obtained. Different processing times could lead to different reaction mechanisms that associated with different crystal structures. Furthermore, variation of TiO2 based nanostructures obtained from the alkaline hydrothermal process also decides the adsorption properties. Nanoribbons with width 200-300 nm and length up to several microns possesses fair adsorption capacity over dyes such as Rhodamine B and Methyl orange with their respective adsorption capacity considering its large surface area and high pore volume. titanates and TiO2 nanostructured materials: Synthesis, properties, and applications. Advanced Materials, 18 (21)