Study on the photocatalysis of TiO
2nanotubes prepared by
methanol-thermal synthesis at low temperature
CHAU THANH NAM1, WEIN-DUO YANG1,* and LE MINH DUC2
1
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung, Taiwan-807, R.O.C
2
Department of Chemical Engineering, Danang University of Technology, Vietnam
Abstract. TiO2 nanotubes were synthesized by a solvothermal process at a low
temperature in a highly alkaline water-methanol mixed solution. Their characteristics were identified by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), specific surface area (BET), Fourier transform infrared spectroscopy (FTIR) and UV-vis absorption spectroscopy. The as-prepared samples was tested by the photodegradation reaction of methylene blue (MB) dye under visible-light irradiation. The ratios of methanol and water, as well as the calcination temperature, affected the morphology, nanostructure, and photocatalytic performance. The methanol solvent plays an important role in improving the crystallization of the anatase phase, which affects the photocatalytic reaction. The titanate nanotubes were synthesized in the methanol-water volume ratios of 10:90, 20:80 and 30:70 were still highly absorbability. The titania nanotubes formed at a calcination temperature of 300 °C using the
methanol-water volume ratio of 30:70 showed the highest photocatalytic performance, much higher than that using the water solvent and TiO2-P25 powder.
Keywords. Solvothermal; nanostructured TiO2; photocatalyst; dye degradation.
1. Introduction
Titanium dioxide (TiO2) is a common metal-oxide semiconductor. This material has many
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applications in key industries such as photocatalysis, solar cells, micro-electronics, and electrochemistry. Many researchers have focused on applied research in the field TiO2
photocatalytic materials, especially in dye wastewater treatment. TiO2 semiconductor
materials are often used in photocatalysis because TiO2 has a low band gap (3.2 eV); thus,
they are extensively tested to combat environmental pollution. In addition, it is important that the various forms of nano-TiO2 be used as photocatalysts because their oxidative power is
very strong. Furthermore, they are biologically and chemically inert and have long-term stability against optical and chemical corrosion.
Titania nanotubes have attracted much interest in scientific research because of their wide range of functional properties. Titania exists in three major crystalline phases: anatase, rutile, and brookite. Each structure has different physical and chemical properties (Aruna et al 2000). The rutile phase can be formed from high-temperature processing of the anatase form.
However, efforts to transform anatase into rutile nanotubes by heat treatment often leads to the collapse of the structure with a loss in structural control (Varghese et al 2003). It is well known that both the TiO2 anatase and rutile phase are composed of TiO6 octahedra, and the
transition phase is achieved by rearranging the octahedra. The arrangement of the TiO6
octahedra through face-sharing initiates the anatase phase, while edge-sharing leads to the rutile phase (Masuda and Kazumi 2009; Yin et al 2001).
There are many methods for synthesizing TiO2 nanotubes, but those with well-shaped anatase
crystals may exhibit the highest photocatalytic activity. Currently, the method of liquid-phase processing at low temperatures is an effective synthesis route for producing nano-crystalline tubes of sodium titanate and TiO2 anatase under hydrothermal conditions (Du et al 2001;
Kasuga et al 1998; Yao et al 2003; Poudel et al 2005; Khan et al 2006; Menzel et al 2006; Huang et al 2009; Bavykin and Walsh 2009). The hydrothermal method has been proved superior for the synthesis of TiO2 nanotubes. The use of solvents other than water under
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solvothermal conditions alters the structure, shape, size, and transformation of titania phases (Yan et al 2010). When using these solvents, alcohol is often utilized and is believed to play an important role in the formation of titania phases (Yan et al 2010).
In recent years, some authors have used the solvothermal method to synthesize TiO2
nanotubes, mainly using ethanol as the solvent. However, researchers have stopped investigating the structure, morphology and states of phase transformation, as well as identifying the characteristics and growth mechanism of the nanostructure of the products (Yan et al 2010; Das et al 2008). They have also predicted that these products will certainly have potential applications in the field of photocatalysis.
In this paper, we synthesized TiO2 anatase phase nanotubes using a solvothermal route at low
temperature in aqueous methanol and NaOH using TiO2-P25 nanoparticles as precursors. We
researched the influence of the different solvent ratios of methanol-water on the morphology of TiO2 nanocrystals under solvothermal conditions, as well as the efficiency of the
photocatalytic reactions associated with the degradation of the dye (methylene blue). The different methanol-water ratios have an important influence on the shape and structure of the product phase. In particular, the methanol solvent affects the crystalline anatase phase of the final product (after calcination), which affects the photocatalytic performance in the
degradation of the MB dye.
In addition, we researched the synthesis of TiO2 nanotubes prepared by the solvothermal
method and calcined at different temperatures. The as-prepared samples were compared simultaneously with other forms of nano-TiO2 such as titanate nanotubes and even TiO2–P25
with respect to their behavior in photocatalytic applications of the methylene blue treatment under visible-light irradiation. The synthesis of TiO2 nanotubes by the solvothermal method
using a methanol-water mixed solvent promises to provide efficiency in energy-saving applications because the nanotubes are synthesized and calcined at low temperatures;
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moreover, the nanotubes still allow for the highly efficient generation of products during photocatalysis, as evidenced by the decoloration of organic dye contaminants under visible-light irradiation.
2. Materials and methods
2.1 Reagents
All of the chemicals used were of analytical grade and were used as received without any further purification. HNO3, NaOH, TiO2 (P25) powder, and methylene blue (MB) were
purchased from Across Organics (New Jersey, USA). A volume of 200 ml of MB solution (300 ppm) was prepared in double-distilled water and diluted as required. The chemical structure of MB is shown in figure 1. Methylene blue is a heterocyclic aromatic chemical compound with the molecular formula C16H18N3SCl. At room temperature, it appears as a
solid, odorless, dark-green powder that yields a blue solution when dissolved in water.
2.2 Instrumentation
All of the glass apparatus used were soaked in concentrated HNO3 for 12 h and then
thoroughly washed in tap water and then double-distilled water. The apparatus were then dried in a hot-air oven for 2 h at 90 °C. To achieve high-accuracy weighing, a highly precise digital electronic pH meter was used to measure the pH of the medium. Powder X-ray diffraction patterns (XRD) were recorded using a PANalytical, X Pert PRO X-ray
diffractometer and CuKα (= 1.5406 Å) radiation. A Micromeritics, ASAP 2020 Analyzer was
used for BET analysis. TEM measurement was performed using a Philips, M-200
transmission electron microscope operated at 200 kV. Before being analyzed by TEM, the samples were sonicated in ethanol for 15 mins and then deposited in 2 - 3 drops on thin carbon film supported on a holey copper grid; the samples were then dried at 60 °C
overnight. The Fourier transform infrared spectroscopy (FTIR) study was conducted with a spectrometer of PerkinElmer instruments. UV-vis diffuse reflectance spectra were obtained
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using a Jasco V-600 UV-vis spectrophotometer between 300-800 nm; the instrument was used to measure the red shift in the samples. In addition, all absorbance measurements were carried out using a UV-Vis spectrophotometer (HITACHI, U-2800) equipped with a 1-cm quartz cell. A Gilson micropipette and micro tips were used to handle samples.
2.3 Preparation TiO2 nanotube
In this study, the solvothermal method was used to synthesize TiO2 nanotube; the synthesis
process is illustrated in figure 2. Solutions of 10 M NaOH were prepared by introducing 28 g NaOH into 70 ml of a mixed solvent of methanol and distilled water. The volume ratios of methanol (MeOH) and distilled water (DIW) were 10:90, 20:80, 30:70, 40:60, 50:50 and 70:30. The use of TiO2 nanoparticles in the solution was divided into two parts: in the first,
only 1.2 g powder was added to 70 ml of 10 M NaOH aqueous methanol at different ratios. In this first experiment, titanate nanotubes were synthesized to determine the ratio between methanol and water that exhibited a high adsorption efficiency for methylene blue. In the second step, the selected ratios were used for the next experiments, followed by the addition of 3 g of TiO2 powder to a mixed solution of methanol and water at the selected ratio to
synthesize titania nanotubes. The experimental conditions of the two cases were as follows: the mixture solution included water, methanol, NaOH and TiO2 powder stirred continuously
for 30 mins with a magnetic stirrer. Then, the mixture was transferred and sonicated for 30 mins. Next, this homogeneous suspension was hydrothermally treated in a stainless
Teflon-lined autoclave at 130 °C for 24 h. After the reaction time was reached, the reactor cooled naturally in air to room temperature. Then, the final reaction products were thoroughly washed with large volumes of double-distilled water to achieve a neutral pH (~7). The
obtained white cotton-batting-like solid was filtrated and separated for drying at 80 °C for 6 h in the form of the obtained titanate. The other part was then immersed in a solution of 0.5 M HNO3 for 24 h at room temperature to ensure that the protonation of the surface and final
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filtration were achieved. The filtrate was washed several times with double-distilled water to achieve a pH of 7 for the product and then dried at 80 °C for 6 h. The dried product was calcined at different temperatures for 2 h to obtain TiO2 nanotubes with the desired
characteristics. Finally, the various forms of the TiO2 nanotubes were synthesized as
described above to remove the MB by photocatalytic degradation methods under visible-light irradiation.
2.4 Photocatalytic degradation of aqueous methylene blue (MB)
In the experiments, 0.01 g of as-prepared TiO2 nanotubes was dispersed in 100 ml of MB
solution with an initial dye concentration (C) of 12 mg L-1 (dose 0.1 g L-1) by stirring for 1 h in the dark. Suspensions of dyes and catalysts were added to a photocatalytic reaction vessel under visible-light irradiation. A 300-W halogen lamp with a 410 nm cut-off glass filter was used as the source of light. The experiment was conducted at room temperature (~25 ± 2 °C) and atmospheric pressure. The reaction temperature was controlled by continuously adding the cool water outside the vessel during the irradiation process. The total time of visible-light irradiation was 2 h. After every 15 mins, approximately 8 ml of solution was removed and immediately centrifuged at 4000 rpm for 10 mins to separate the solid and liquid phases from the solution experiments. The supernatant fluid was collected after centrifugation, and the dye concentration remaining in the solution was measured by UV-vis spectroscopy at 664 nm. The blue color of the solution gradually faded over time due to the adsorption and
degradation of MB. The concentration of the MB solution was determined as a function of irradiation time from the change in absorbance at 664 nm.
3. Results and discussion
3.1 Synthesis and characterization of TiO2 nanotubes
The crystal structure, morphology, and size of the synthesized products were determined by XRD, SEM, and TEM. The purpose of this study was to determine the methanol-water ratio
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providing the highest photocatalytic performance. Therefore, the XRD analysis results for the sample possessing a methanol-water ratio of 30:70 by volume are mainly presented and compared to the samples synthesized with 100 % water. The explanation for this choice is provided in the photocatalyst section.
Titanate is formed by the dissolution and recrystallization of TiO2 in a highly concentrated
NaOH aqueous solution (Du et al 2001; Huang et al 2009). Figures 3 and 4 show the XRD patterns of titanate products that were solvothermally and hydrothermally prepared at 130 °C and calcined at different temperatures to produce TiO2 nanotubes. As a result, the obtained
product presents the most anatase phase nanoparticles after calcining at different temperatures. However, compared with the case using the methanol solvent (methanol-water ratio of 30:70), the XRD pattern of the first case shows a clearer and sharper anatase phase, even when the sample was calcined at a low temperature of 100 °C. This result is entirely consistent with the photocatalytic efficiency presented in the last section, especially when using a
methanol-water ratio of 30:70, which exhibited the highest photocatalytic efficiency. This suggests that the role of the methanol solvent is very important in promoting the formation of the anatase phase during the liquid-phase synthesis of TiO2 nanocrystals. Because the
methanol solvent has a lower boiling temperature (65 °C) than water, it evaporates more quickly when heated to 130 °C, which affects the accelerated formation of anatase phase compared to the case in which only water is used as the solvent. The ratio of the anatase phase increased with the calcination temperature. At a calcination temperature of 500 °C, in particular, the content of the anatase phase is quite high and close to that of the TiO2-P25
nanoparticles.
Figure 3 shows the XRD patterns of all of the samples, showing all of the peaks
corresponding to the reflections from the (101), (004), (200), (105), (211), and (204) planes of anatase for samples featuring a methanol-water ratio of 30:70 and calcined at different
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temperatures. These closely match the standard reported values (JCPDS file No. 21-1272). From the (101) peaks of all of the samples, we can see that the width of the peak becomes increasingly narrow with increasing calcination temperature. In addition, the samples
featuring the methanol solvent show clearer peaks than those featuring the water solvent. This may be attributed to the improved crystallinity of the samples with increasing calcination temperature. The titanate phase from the XRD pattern also indicates the presence of H2Ti3O7
in the products washed with water by substitution of Na+ by H+. Similarly, the proportion of H2Ti3O7 in the samples synthesized using the methanol solvent appeared stronger compared
to those synthesized using the water solvent.
Figures 3 and 4 show that all of the diffraction peaks of the samples calcined at 100–500 °C could be indexed as belonging to the anatase phase of TiO2, particularly for samples featuring
the methanol solvent. With an increase in the calcination temperature from 300 °C to 500 °C, the peak intensities of anatase increased significantly, indicating the improved crystallization of the anatase phase. Simultaneously, the width of the (101) peak became narrower,
suggesting the growth of anatase crystallites. When using the methanol solvent, three small peaks appeared at 2 ~25.3o, 37.8o and 48.0o for the samples calcined at 100-300 °C, indicating the existence of small amounts of anatase. As shown, the peaks at 25.3o, 37.8o, 48.0o, 53.8o, 54.9o and 62.5o were attributed to the diffractions of the 101, 004, 200, 105, 211, and 204 planes of anatase, respectively (Huang et al 2007). In addition, the synthesis of the TiO2 nanotube powder at low temperatures tends to result in the formation of finer crystallites
with larger specific surface areas. The gradual narrowing of the XRD lines with increasing calcination temperature reflects a corresponding increase in the average grain size (Klug and Alexander 1974).
Here, we compare our results with those from experiments in which TiO2 nanotubes were
synthesized by the hydrothermal method using water as the solvent. Most previous studies
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have confirmed that the hydrothermal temperature is one factor affecting the morphological characteristics of titania nanotubes (Wang et al 2008). The yield of the tubes (up to 80-90 %) increases with hydrothermal temperatures between 100 and 150 °C (Yuan and Su 2004). The surface area and pore volume of the largest titania nanotubes were observed at a synthesis temperature of 130 °C (Ou and Lo 2007). The hydrothermal temperature plays an important role in promoting the nucleation and crystal growth of titanate nanotubes (Lan et al 2005). The combination of a hydrothermal temperature between 130 and 150 °C applied over 24-72 h generates titanate nanotubes of the highest yield and purity (Lan et al 2005; Ma et al 2005). However, at hydrothermal temperatures up to 160 °C, the pore volume and specific surface area of the titanate nanotubes is reduced. The pore volume and surface area of samples becomes smaller at temperatures above 170 °C (Lee et al 2009). Many researchers have studied the phase transition of titanate nanotubes during the heating process. Moreover, the calcination process is believed to affect the phase structure and microstructure of titanate nanotubes (Wang et al 2008). Indeed, the crystallinity of titanate nanotubes increases with the calcination temperature (Weng et al 2006). Above 600 °C, the titanate nanotubes begin to collapse as titania nanocrystals start to grow. The crystallinity of the anatase phase begins to increase at 300 °C (Sreekantan and Lai 2010). The titanate nanotubes are easily destroyed at high temperatures, causing the tubular structures to transform into nanoparticles after heat treatment (Vuong et al 2009). Thus, for TiO2 nanotubes synthesized using a conventional
water solvent, the anatase phase is usually observed at an annealing temperature between 300 and 400 °C or above; at approximately 600-700 °C, the samples are shown to transform from tubes into particles (Yu et al 2006; Lee et al 2007; Lee et al 2009). Moreover, the increase in the peak intensity and the decrease in the width of the peaks at 400–700 °C indicates that the improvement of crystallization of anatase phase and the growth of anatase crystallites, respectively. At 700 °C, the rutile phase appears (Lee et al 2009). Many researchers have
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observed that the anatase phase is retained after the heat treatment process and the
crystallization, morphology, pore structure and specific surface area of titania nanotubes is strongly dependent on calcination temperature. TiO2 nanotubes synthesized by the
solvothermal method using a mixed solvent composed of methanol and water also follow these rules. However, the difference here is that at a low calcination temperature of 100 °C, the anatase phase appears in relatively low amounts and if calcined at 500 °C the tubular structures can be observed to transform into particles. This demonstrates how the role of methanol solvent is very important in increasing the anatase crystalline phase content. The nitrogen adsorption-desorption isotherms of titania nanotubes at various ratios of methanol and water and at various calcination temperatures are presented in figures 5 and 6. The specific surface areas (SBET), average pore volumes, and pore sizes of the samples are
summarized in tables 1 and 2. The BDDT classification shows that all of the samples are of type IV with type-H3 hysteresis loops, indicating a mesoporous structure (2-50 nm). In general, the sample synthesized using a methanol-water ratio of 30:70 by volume exhibits a specific surface area greater than that of the remaining samples, including the sample synthesized using only water as a solvent. The hysteresis loops shift to higher relative pressures, indicating that the pore size grows gradually because TiO2 crystal growth leads to
the formation of larger pores. The hysteresis loops of the sample calcined at a temperature of 500 °C move more toward higher relative pressures. This sample exhibits the smallest
specific surface area due to the collapse of the nanotube structures to nanoparticles; this result is consistent with TEM observations. Tables 1 and 2 show that the samples synthesized using methanol solvent and those that were calcined at different temperatures exhibited an SBET and
pore volume higher than that of the precursor P25, which greatly benefits the efficiency of the associated absorbability. Similar to the case in which TiO2 nanotubes were synthesized with
the conventional water solvent, the titanate nanotubes calcined at different temperatures
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exhibited an average pore size of 18.1-31.5 nm when the calcination temperature increased to 400-600 °C (Yu et al 2006). Beyond this temperature range, the pore size of the nanotubes increases steadily with a great decrease in surface area (from 219.2 to 64.3 m2/g) and pore volume (from 0.992 to 0.347 cm3/g) (Das et al 2008). The aggregation of the nanotubes is attributed to the increase in average pore size. It is remarkable that the specific surface area of titanate nanotubes after calcination is higher than that of the Degussa P25 titanium dioxide precursor.
Figure 7 shows the TEM images of the TiO2 nanotube products prepared from the
solvothermal method at a temperature of 130 °C for 24 h using a methanol-water ratio of 30:70 by volume and calcined at different temperatures ranging from 100 °C to 500 °C. The insets show the corresponding selected area electron diffraction (SAED) patterns and the dimensions of the TiO2 nanotubes. The SAED patterns show that the crystallites in the
as-prepared samples exist as polycrystalline phases in the anatase phase. In addition, the TEM pictures show that almost all of the samples exhibit predominantly tube-like structures, except for the sample calcined at a temperature of 500 °C. This sample collapsed into
nanoparticles resembling the structure of the initial TiO2-P25 precursor. The samples
prepared at a calcination temperature of 400 °C were found to be gradually broken into shorter tubes. A large number of nanotubes exhibited an outside diameter of approximately 10 nm, and nanotube lengths of several hundreds of nanometers were obtained. Furthermore, the ends were observed to be open, which is extremely beneficial for adsorption and
photocatalysis.
The left inset in figure 7 is an enlarged picture of a tube wall. For the sample calcined at a temperature of 500 °C, the TEM image (figure 7(f)) shows nanoparticles measuring 15 nm in average diameter. The above TEM images revealed that all samples synthesized using the mixed methanol-water solvent and calcined at different temperatures showed a nearly 1D
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nanostructure with the same diameter as the nanotubes, except for the sample calcined at a temperature of 500 °C, which was formed of nanoparticles. The above examples are very beneficial for interpreting the effectiveness of the photocatalytic reactions that are presented in detail in a later section. Moreover, the introduction of methanol into an aqueous NaOH solution still promotes the formation of nanotubes. This reveals the positive effect of alcohols in the hydrothermal method used to synthesize titania nanotubes (Yan et al 2010).
Figure 8 shows the FTIR spectra of the different samples. As seen from this figure, the range of 3000-3650 cm-1 the bands due to adsorbed water and hydroxyl groups can be observed in all of the samples, which can be attributed to the stretching vibration of –OH from water. In particular, the content of water adsorbed in the titania nanotubes was significantly higher than in TiO2-P25 due to the increase of the specific surface areas and pore volumes. The band
observed of all samples at 1630 cm-1 can be assigned to molecular water bending mode (Maira et al 2001). A distinct broad bands in the 400-800 cm-1 region were assigned to Ti-O and Ti-O-Ti skeletal frequency region (Guo et al 2007). A difference here compared to the TiO2 precursor sample that is the samples of titania nanotubes began to appear a new peak (figures 8b-g) at 1384 cm−1, even with the sample using 100 % solvent of water also appeared this peak. It is probable that during protonation of the surface TiO2 under acidic solution to form Ti-OH2+ groups. Compared with the previous studies, the titania nanotube products were synthesized by hydrothermal method without the protonation stage not occurred this absorption peak. For the samples use the mixed solvent of methanol and water in volume ratio at 30:70, the intensity of absorption bands at 1384 cm-1 decreased gradually after increase the calcination temperature. When the calcination temperature is higher than 400 0C (figures 8h-i), this peak also disappeared due to at high temperatures to remove a water molecules.
The UV-vis diffuse reflection spectra of TiO2-P25 powder and the products obtained at
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different calcination temperatures ranging from 100 °C to 500 °C are shown in figure 9. The products using the mixed solvent of methanol and water in various volume ratios are shown in figure 10, which shows that compared with pure TiO2-P25, all of the samples also
generally exhibit high absorption in the visible region. The band gaps of the samples can be calculated from the intercepts of the UV-vis spectra using the following equation: Eg =1240/.
The optical absorption edge of the above samples show an appreciable red shift towards longer wavelengths compared with that of the raw TiO2, indicating a decrease in the energy
band gap. These results are fully consistent with the high photocatalytic efficiency exhibited by the samples in comparison with the TiO2-P25 sample, which will be discussed later. For
the samples synthesized at 130 °C for 24 h using the mixed solvent of methanol and water in various volume ratios, the optical absorption edges show a red shift toward the visible light region. These samples exhibited the greater red shift than that observed for the pure TiO2-P25
sample. This is entirely consistent with the photocatalysis results presented in a later section, in which exhibited the higher photocatalytic efficiency.
In general, the above discussions suggest that we can determine the role of methanol in the system. The solvothermal method normally exerts better control than the hydrothermal method over the size and shape distributions and the crystallinity of TiO2 nanoparticles. The
polarity and coordinating ability of the solvent can influence the morphology and the crystallization behavior of the final products (Chen and Mao 2007). The fact that methanol solvent has a lower critical temperature and pressure compared to water is a boon to the processing of materials at much lower temperature and pressure conditions. The addition of methanol solvent to water provides an excellent reaction medium for the hydrothermal processing of nanoparticles because it allows for the modulation of the reaction rate and equilibrium by adjusting the dielectric constant and density of the solvent with respect to pressure and temperature, thus generating higher reaction rates and smaller particles (Byrappa
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and Adschiri 2007). In the hydrothermal treatment of titania powders in basic solution, parameters such as the autoclave pressure influence the morphology and crystal phase of the final products (Poudel et al 2005; Menzel et al 2006). Therefore, at the same temperature used during heat treatment (130 °C), methanol has a low boiling point and might influence the crystal phase. It is likely that the presence of methanol solvent in a system exhibits a synergistic effect to facilitate the face-sharing of adjoining octahedra to form the anatase phase during the recrystallization of TiO6 octahedra. Moreover, according to the growth
process, sheets rolled into nanotubes are speculated to also be composed of the anatase phase. Additionally, because methanol has a small molecular size and solubility in water, drying and calcination at low temperatures (100 °C) will facilitate dehydration to generate the anatase phase (shown at the XRD pattern).
Water molecules also act as bridges between the surface OH groups of different octahedra and connect the octahedra closely during recrystallization (Yanagisawa and Ovenstone 1999). Under hydrothermal conditions, Na+ is gradually released with intercalated H2O molecules
into the interlayer spaces of TiO6 sheets. The inclusion of methanol up to 30 % in the mixed
solvent may not affecting the transformation efficiency of sheets into nanotubes due to the replacement of H2O molecule into the interlayer spaces of TiO6 sheets. The BET analysis
showed that these samples group remain the specific surface area is relatively high. The XRD diagram also showed the samples using of methanol up to 30% appear the anatase phase in the calcined products. Finally, these products are experimentally shown to affect
photocatalytic efficiency with the positive effects (presented in later section).
3.2 Photocatalytic degradation of MB
Samples of titanate nanotubes were synthesized using the solvothermal method and
methanol-water volume ratios of 10:90, 20:80, 30:70, 40: 60, 50:50 and 70:30. The purpose of the first experiment was to choose the titanate nanotube samples that were highly effective
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at adsorbing the methylene blue dye. After 60 mins stirring in the dark, two groups of samples were observed to be relatively distinct: groups of samples with high adsorption efficiency featuring methanol-water volume ratios of 30:70, 20:80 and 10:90 (the adsorption yield of MB solution is from 63.1 to 65.9 %) and sample groups with lower adsorption efficiency featuring methanol-water volume ratios of 40:60, 50:50 and 70:30. The
experimental results regarding titanate nanotube samples are not shown here. Previous studies using a conventional water solvent to synthesize nanotube samples have confirmed that such samples exhibit very high adsorption capacities due to their high specific surface areas (Yu et
al 2006; Lee et al 2009; Xiong et al 2010). The inclusion of 10-30 % methanol in the mixed
solvent may not be sufficiently high to affect the reaction; however, these titanate samples retain high specific surface areas, much like those featuring the conventional water solvent, although the relevant data are not presented here.
Figure 11 shows the photodegradation ratio of MB under visible-light irradiation at various volume ratios of methanol and water for samples of titania nanotubes calcined at temperature of 300 °C. The results show that the samples using methanol-water volume ratio of 30:70 were the most efficient photocatalysts, even when adsorbing the MB dye in the dark and being irradiated by visible light. The adsorption yield of MB solution is from 48.5 to 53.2 % for the titania nanotube products using methanol-water volume ratios of 10:90, 20:80 and 30:70. Consider that Co is the initial concentration of the dye solution (MB) after stirring in
the dark. In this case, the MB removal percentage was equal to 34 % after an irradiation time of 120 mins. It was found that the photocatalytic performance using the a methanol-water volume ratio of 30:70 (a) is much higher than that using 100 % water solvent (d) and TiO2-P25 powder (e). For the samples shown in figures 11d and 11e, the MB removal
percentages were equal to 24.7 % and 23.5 %, respectively, following 120 mins of irradiation. This is consistent with the results shown in the XRD and UV-vis spectra presented in the
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previous section, which showed that the sample featuring a methanol-water volume ratio of 30:70 exhibited the better intense diffraction peaks for the crystalline anatase phase and that these peaks were red shifted toward the visible-light region to a greater extent than the peaks of the other samples using 100 % water and TiO2-P25 powder. Moreover, the results shown
in table 1 for the SBET analysis also show that the sample featuring a methanol-water ratio of
30:70 by volume exhibited the highest SBET.
The results clearly demonstrate the important role played by the mixed solvent composed of methanol and water in the formation of crystalline anatase. The boiling point of methanol at 65 °C is lower than water, which helps to increase the vapor pressure in the autoclave below 130 °C and thereby accelerates the formation of the crystalline anatase phase. It can be
clearly observed in the XRD patterns that the titania samples using a mixed solvent composed of methanol and water exhibit clearer and sharper anatase peaks. This helps to effectively enhance the photocatalysis of such samples to a greater extent than the samples using the water solvent and TiO2-P25 powder.
To evaluate the photocatalytic activity of the titania nanotubes after calcination at various temperatures, the samples were treated at different temperatures when using various
methanol-water ratios. The samples using a methanol-water volume ratio of 30:70 were then selected and reviewed (figure 12). The photocatalytic activity of the calcined nanotube samples was observed to increase with increasing calcination temperature. When the calcination temperature was 300 °C, the photocatalytic activity reached a maximum (34 %) higher than that of the samples composed of TiO2-P25 powder (23.5 %) after an irradiation
time of 120 mins. However, groups of samples calcined at temperatures ranging from 100 °C to 400 °C generally retained a high adsorption efficiency (the adsorption yield of MB solution is from 36.4 to 47.2 %); this can be ascribed to their higher SBET and pore volumes (table 2).
Moreover, as indicated by the XRD analysis above has shown that at a low calcination
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temperature, the anatase phase also appeared in relatively low amounts; this is beneficial to photocatalytic reactions. Compared with the other samples, the sample calcined at 500 °C showed better crystallization of the anatase phase (see the XRD pattern, figures 3 and 4). However, the TEM images clearly show that the sample’s structure collapsed into
nanoparticles. This could be attributed to the sintering and growth of TiO2 crystallites and the
decrease in the specific surface area and pore volume. Therefore, the photocatalytic activity of the calcined samples decreased significantly.
The results indicate that the role of the methanol solvent is very important in increasing the crystalline anatase content in the resulting particles. The results also show that using methanol solvent allows for highly effective energy savings due to a reduction in the synthesis and calcination temperature and an increase in photocatalytic efficiency. 4. Conclusions
Titanate and titania nanotubes were synthesized by a simple solvothermal method at low temperature using a mixed solvent composed of methanol and water in various ratios at different calcinations temperatures. The nanotubes were produced in a methanol-water mixed solvent, and their morphology was observed via SEM and TEM. The XRD observations revealed that the methanol solvent played an important role in improving the crystallization of the titania anatase phase. The various ratios of methanol to water affected the morphology and structure of the final product, as well as the photocatalytic performance in the
degradation of MB dye.
The samples featuring methanol-water volume ratios of 10:90, 20:80, and 30:70 were observed to remain highly effective in adsorbing MB dye when the initial concentration was 12 mg L-1. It was observed that the adsorption yield of MB solution is from 48.5 to 53.2 % after 60 mins stirring in the dark. When titania nanotubes formed by calcination at different temperatures were used, it was found that the crystalline phase, morphology, structure, and
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