Synthesis of porous TiO₂ nanowires and their photocatalytic properties

Abstract

Porous titanium dioxide (TiO₂) nanowires were synthesized via a surfactant-free hydrothermal method followed by acid-washing process and calcination. The structures and morphologies of products were characterized by field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) N₂ adsorption-desorption analyses. The analysis of FESEM suggested the precursor was composed of a vast of uniform nanostructures like wires. The nanowire-like precursor was transformed into the porous nanowire after acid-treatment and calcination at 500°C for 2 h in air. The surface area of as-synthesized TiO₂ nanowires calculated by BET is 86.4 m²/g. Furthermore, the photocatalytic properties of synthesized porous TiO₂ nanowires were evaluated through the degradation of methylene blue (MB) and Rhodamine B (RhB). The results clearly suggested that the as-prepared porous TiO₂ nanowires showed remarkable photocatalytic performance on the degradation of RhB and MB due to their small size of nanocrystallites and the porous naonstructure.

Key words： titanium dioxide (TiO₂); nanowire; porous; photocatalyst; photocatalytic performance

Introduction

These days, water pollution has been a major global issue and become more and more serious as the industry develops rapidly. A great number of efforts have been made to develop several approaches including biodegradation, oxidative decomposition and photochemical degradation, for the purpose of removing the organic pollutants [1]. As for the degradation of organic pollutants, semiconductor photocatalysts have been acting as a vital role. As is known to us, titanium dioxide (TiO₂), which belongs to the typical semiconductor of n-type, has been widely investigated due to its unique properties of high durability, high chemical stability and long-term thermal stability, which possess an extensive range of promising applications in photocatalysis [2–4], environmental pollution control [5], and solar energy conversion [6]. Concerning to the practical application of TiO₂ served as photocatalyst, it is well known that the photocatalytic activity of titanium dioxide largely depends on its crystalline structure, surface area, grain size, and defects, etc. Due to the most reactions usually taking place on the surface of TiO₂, the properties are strongly related to the local microstructures and specific surface area [7,8].

In recent years, remarkable efforts have dedicated to the preparation of various nanostructured TiO₂, including nanowires [9], nanorods [10], nanobelts [11], nanotubes [12], ultra-thin nanosheets [13], porous nanospheres [14], and nanoflowers [15], etc. Among these kinds of nanostructures, one-dimensional (1D) structure has been recognized to be the most suitable catalyst and blocks for the construction of nanoscale photocatalyst due to its unique 1D morphology with high specific surface area and the excellent physical and chemical properties conducive to promote the transport and separation of electron. For example, Lai et al. reported single crystalline TiO₂ nanowires with enhanced photocatalytic degradation of dyes [16]. Yao et al. prepared Ag-doped TiO₂ nanowires with excellent photocatalytic by sunlight induced reduction [17]. Bakar et al. synthesized N-doped TiO₂ nanorods using hydrothermal treatment with high photocatalytic activity [18]. It’s well known to us that the structures with small size nanocrystallites and large specific surface area not only inhibit the recombination rate of holes and photoelectrons, but provide much more active reaction sites, which greatly enhance the efficiency of light-harvesting, showing the excellent photocatalytic activity. Therefore, despite the great efforts contributed by many groups, seeking strategies for constructing the architecture of TiO₂ 1D nanostructures with high surface area and specific morphology has still been an attractive ongoing task.

In present work, porous TiO₂ nanowires composed of nanocrystallites were successfully achieved by a hydrothermal method followed by acid-washing and calcination process. The photocatalytic performance of the as-prepared porous TiO₂ nanowires was also studied. The products exhibit much remarkable performance on photocatalytic decomposition of Rhodamine B (RhB) and methylene blue (MB), which is ascribed to their small size of nanocrystallites and the porous naonstructure.

Experimental

Synthesis

All chemicals in this experiment belonged to analytical grade and need not to be further purified. The detailed synthesis process of the porous TiO₂ nanowires like this: 0.3 g ammonium hexafluorotitanate [(NH₄)₂TiF₆] was dissolved in 40 mL aqueous solution of sodium hydroxide (NaOH, 1.0 M^†

1 M= 1 mol/L

⁾). The mixture solution was put into a Teflon-lined autoclave (50 mL) and then kept at 240°C for 5 h. The obtained white precipitate was washed with deionized water for 5 times. After that, the white precipitate was immersed in 0.1 M HCl solution and placed at 28°C for 24 h. Subsequently, the products were washed with deionized water for 5 times until the resulting pH reached neutral, and dried at 50°C in air for 12 h. Finally, the porous TiO₂ nanowires were obtaind through calcination at 500°C in air atmosphere for 2 h.

Characterizations

X-ray diffraction (XRD) was carried out via Shimadzu XRD-6000 with high-intensity Cu Kα radiation. Field emission scanning electron microscopy (FESEM) were performed via Hitachi S-4800. Transmission electron microscopy (TEM) was carried out via high-resolution transmission electron microscopy (HRTEM, JEOL-2010 TEM with an acceleration voltage of 200 kV). Nitrogen adsorption-desorption measurements were recorded by Nova 2000E. The pore-size distribution was determined from the adsorption isotherm curves using the Barett-Joyner-Halenda (BJH) method.

Photocatalysis test

The process of photocatalytic experiments was performed as follows: 15 mg of porous TiO₂ nanowires or commercial TiO₂ powders were added into 50 mL MB or RhB aqueous solution whose concentration is 20 mg/L. Before irradiation, the above suspension first was ultrasonicated at 28°C for 30 min, followed by stirred in dark for ca. 30 min so as to get adsorption equilibrium of molecules. The mixture solution was exposed to ultraviolet (UV) light (Philips, 300 W, Shenzhen, China) with the intensity of 11280 µW/cm². The UV-light is 20 centimeters above the mixture solution. In the whole process of irradiation, the mixture was stirred without stop. After that, at a constant time interval, the mixture suspension (3.0 mL) was sampled and the catalyst particles were removed by centrifugation. The degradation rate of photocatalytic reaction was determined by UV-vis spectrophotometer (Tokyo, Japan, U-3010) through monitoring characteristic absorption of RhB (552 nm) and MB (665 nm).

Results and discussion

Structures and morphologies

Sodium titanate (Na₂Ti₃O₇) nanowire precursor was obtained by a hydrothermal method. The Na₂Ti₃O₇ nanowire precursor were washed with dilute HCl and then calcined to yield porous TiO₂ nanowires. Figure 1(a) presents the typical XRD pattern of the Na₂Ti₃O₇ nanowire precursor. It’s clear that all peaks of the precursor belong to monoclinic phase Na₂Ti₃O₇ (JCPDS 72-0148) [19,20]. When the precursor was acid-washed and then treated by heating at 500°C in air for 2 h, the XRD diffraction peaks in Fig. 1(b) are totally different. All observed diffraction peaks conformed to the values of tetragonal phase TiO₂ (JCPDS 89-4921), which suggested the Na₂Ti₃O₇ nanowires precursor was transformed into tetragonal phase TiO₂ after acid-washed and being heated at 500°C in air. Scherer formula was adopted to estimate the average grain size of TiO₂ crystallites.

(1)D={\displaystyle \frac{\mathrm{0.9}\lambda }{\beta \cos \theta }},

where \lambda is the wavelength of the X-ray beam, \theta is the diffraction angle and \beta is the full width at half maximum. The average TiO₂ crystallites size was ca. 10.7 nm.

Fig.1 XRD patterns of (a) Na₂Ti₃O₇ precursor and (b) TiO₂ product at 500°C for 2 h

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Figure 2 displays the FESEM images of Na₂Ti₃O₇ nanowires and the corresponding TiO₂ product after acid-washed and heat-treatment. From Fig. 2(a), it can be seen that the precursors are a large amount of uniform nanowires. The length of Na₂Ti₃O₇ nanowires is from 2 to 8 µm. The diameter of the Na₂Ti₃O₇ nanowires is from 15 to 25 nm (Fig. 2(b)). Figures 2(c) and 2(d) display the FESEM images of the porous TiO₂ nanowires after acid-washed and heat-treatment of the precursor. It can be clearly observed that after acid-washing and calcinations, no collapse occurred and the obtained product still maintained the wire-like shape similar to the precursor. Close observation further revealed that a vast of irregular nanopores exist randomly in the nanowires, which was resulted from the decomposition of nanowire precursor. The porous structure of TiO₂ materials has been further investigated by high-magnification and high-resolution TEM. The TEM image (Fig. 2(e)) reveals the nanowire-like architecture of the product. The high-magnification TEM image (Fig. 2(e) inset) indicates many nanopores distributed in the nanowires. From Fig. 2(f), it can be observed that the spacing of lattice fringe is approximate 0.24 nm, which is indexed with the (004) plane tetragonal phase TiO₂.

Fig.2 Representative FESEM images of (a, b) the Na₂Ti₃O₇ nanowire precursor and (c, d) porous TiO₂ nanowires. (e) Typical TEM images and (f) HRTEM image of the porous TiO₂ nanowires. The inset is the corresponding TEM image at high magnification

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To study the texture of the porous TiO₂ nanowires, nitrogen adsorption/desorption measurements were conducted. Figure 3 displays the nitrogen adsorption/desorption isotherms and pore size distribution plot (inset) of the sample. The isotherm of sample shows a hysteresis loop at the p/p₀ ranges from 0.82 to 0.98 (Fig. 3), which is related to the filling up and emptying of mesopore by capillary condensation. The feature clearly suggests that the obtained sample possesses a large number of porosity. The BJH method originated from desorption branch of the nitrogen isotherm was applied to determine the pore size distribution of the porous TiO₂ nanowires. The result showed that the pore size of the as-prepared porous TiO₂ nanowires had an average diameter of 7.4 nm. The pore size distribution of the sample presents a broad peak with the range from 1.7 to 34.8 nm. Moreover, the FESEM images also verify the pores with different sizes in porous TiO₂ nanowires, which conform to the results of stochastic calculation. The surface area calculated of obtained sample is 86.4 m²·g⁻¹. Because of the mesoporous structure and high surface area, the as-synthesized porous TiO₂ nanowires are more likely to provide a large amount of active sites on the surface of the material. Therefore, the as-synthesized porous TiO₂ nanowires are considered to have great potential application in photocatalytic degradation with superior performance.

Fig.3 N₂ adsorption-desorption isotherm and BJH pore size distribution plots (inset) of the porous TiO₂ nanowires. STP: standard temperature and pressure

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Photocatalytic activities of the porous TiO₂ nanowires

To estimate the porous TiO₂ nanowires activity as the photocatalyst, the mixture was treated by UV light irradiation for a period of time. The intensity of absorption peak of the solution was measured to calculate photodagradation rate. When porous TiO₂ nanowires act as catalyst, during the process of irradiation, the absorbance spectra of RhB solution was shown in Fig. 4(a). It can be clearly observed that at 665 nm, the maximum absorbance decreases rapidly with increasing the irradiation time. After the mixture was irradiated by UV light for 7 min, the photodagradation rate reaches approximate 25.6%. After irradiation for 49 min, no obvious absorption peak was observed. The degradation ratio of RhB can be calculated using 1−C_t/C₀, where C_t is the concentration of dyes and C₀ is the initial concentration. The plots of irradiation time vs degradation ratio shows the degradation ratio of RhB can reaches up to 98.76%. Since no obvious absorption peak can be observed, the RhB in mixture can be considered to be almost totally decomposed. Figure 4(a) (inset) also exhibits photodagradation of RhB solution without any catalyst when the solution was exposed at different duration time under the same experiment condition. It’s clearly found that after 49 min irradiation, there was almost no degradation in RhB solution, indicating that the as-synthesized porous TiO₂ nanowires are promising to act as excellent photocatalyst to remove RhB in water. Meanwhile, the plots of irradiation time vs degradation ratio were also achieved without catalyst, see Fig. 4(a) (inset). The result suggests that after the same duration time, no obvious degradation of RhB solution happened. Figure 4(b) shows characteristic absorption peak of MB solution with the catalyst. It clearly shows that the porous TiO₂ nanowires can serve as excellent catalyst for degradation of MB. Figure 4(b) (inset) demonstrates that the degradation ratio of MB solution reaches up to 97.98% after irradiation for 56 min. The photocatalytic performances of TiO₂ materials with different morphologies are listed in Table 1. The degradation ratio of the porous TiO₂ nanowires is higher than the previous reported values obtained under UV light irradiation with commercial TiO₂ powders (P25) [14], porous TiO₂ nanospheres [14], anatase nano-TiO₂ [21], and hierarchical TiO₂ nanoflowers [22]. Therefore, the as-prepared porous TiO₂ nanowires have great potential application in wastewater treatment. It is widely acknowledged that the catalytic process is primarily associated with the adsorption and desorption of molecules that usually occur on the surface of the catalyst. The possible reasons of the high performance TiO₂ nanowires may be ascribed to three aspects. First, the small size of nanocrystallites in the porous TiO₂ nanowires can effectively promote the transportation of carrier and greatly decrease the recombination rate between photoelectrons and holes, thus enhancing photocatalytic activities. Secondly, the porous structure of the TiO₂ nanowires facilitates the transportation of the pollutant molecules onto the surface of TiO₂ nanocrystallites and stores more molecules. Finally, the porous TiO₂ nanowires have the highest specific surface area than other catalysts, reaching up to 86.4 m²·g⁻¹ (as shown in Table 1), which means it can produce more unsaturated surface coordination sites exposed to the solution, provide much more active reaction sites on the surface of material, and effectively promote the separation of electron–hole pairs, resulting in the high photocatalytic activities.

Fig.4 Change of absorbance spectra of (a) RhB, (b) MB in solution with the concentration of 20 mg/L with porous TiO₂ nanowires under UV light irradiation. The insets are plots of photodegradation rate vs different irradiation time of RhB and MB under UV light irradiation with or without catalysts, where C_t is the concentration of dyes and C₀ is the initial concentration

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Fig.5 Recycling results of the catalyst in the photodegradation of (a) MB, (b) RhB, in which C_t is the concentration of MB or RhB and C₀ is the initial concentration

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Tab.1 Comparisons of the photocatalytic performances of TiO₂ materials with different morphologies

materials	specific surface area/（m2·g-1）	pollutants	rate of degradation	Ref.
commercial TiO2 powders	7.81	MB RhB	48.2% (70 min) 49.1% (70 min)	[14]
corous TiO2 nanospheres	26.1	MB RhB	90.2% (70 min) 98.1% (70 min)	[14]
anatase nano-TiO2	75.0	MB RhB	90.3% (60 min) 86.0% (60 min)	[21]
hierarchical TiO2 nanoflowers	35.8	RhB	97.8% (120 min)	[22]
porous TiO2 nanowires	86.4	MB RhB	97.98% (56 min) 98.76% (49 min)	this work

The reusability and stability of the sample are critical problems for developing practical application. The porous TiO₂ nanowires can be easily reused by centrifugation after each reaction. The reusability and stability of as-synthesized porous TiO₂ nanowires was tested through many times reusability of the catalyst. As can be seen in Fig. 5, after four cycling repeatability, the photocatalytic activity of catalyst do not show obvious decline and there are almost the same photodagradation rate in each cycle. Obviously, the porous TiO₂ nanowire prepared in this experiment is a promising photocatalyst due to its excellent photocatalytic activity and good recycle performance.

Conclusions

In conclusion, the synthesis of porous TiO₂ nanowires was successfully achieved through a hydrothermal method without any surfactant combined with acid-treatment and annealing process. Very high photodegradation efficiency (near 100%) for the degradation of MB and RhB was achieved by using porous TiO₂ nanowires as photocatalytic catalyst. The excellent photocatalytic activities can be ascribed to the small size of nanocrystallites, the porous structure and the larger specific surface area. Furthermore, the porous TiO₂ nanowires also show good stability in the process of degrading MB and RhB. Therefore, the porous TiO₂ nanowire prepared in this work is a promising photocatalyst in the degradation of organic pollutants.

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