Preparation of titanium dioxide immobilized on carbon fibers annealed in steam ambient and their photocatalytic properties

Abstract

Composites of titanium dioxide (TiO₂) immobilized on carbon fibers (CFs) were synthesized by a two-step process. Acid-treated CFs were dip-coated in a TiO₂ sol and then annealed under superheated steam ambient to form TiO₂/CF composites. The effects of titanium sol concentration and annealing temperature on the micromorphology and phase structure of the TiO₂/CF were investigated by field emission scanning electron microscopy and X-ray diffraction, respectively. The photocatalytic properties of the as-prepared TiO₂/CFs were evaluated by photocatalytic degradation of acid orange II under ultraviolet light irradiation. The TiO₂/CF composites prepared by superheated steam annealing exhibited a uniform surface morphology and high loading degree. The photocatalytic degradation of acid orange indicated that the TiO₂/CF composites exhibited excellent photocatalytic performance and reached up to 98.7% degradation rate after 2.5 h of irradiation.

Keywords

titanium dioxide carbon fiber sol-gel superheated steam photocatalytic property

Since Fujishima and Honda¹ discovered the splitting decomposition of water in titanium dioxide (TiO₂) electrodes in 1972, photocatalytic degradation of wastewater based on semiconductor photocatalysts has attracted increasing attention due to some advantages of the photocatalysis technology, such as mild reaction conditions, non-toxic end-products (CO₂, H₂O), use of renewable energy, etc.² The benefits of non-toxicity, chemical stability, low cost of the TiO₂ semiconductor photocatalyst have been studied extensively by researchers in the fields of wastewater treatment and air purification.^3–5 However, photocatalyst nanoparticles will bring secondary pollution due to difficult recycling in solutions. To avoid the secondary pollution of TiO₂ nanoparticles to solution, composites, including TiO₂ and flexible fibers, have been prepared by various routes, such as hydrothermal method, sol-gel and electrostatic spinning.^6–8 Among these methods, sol-gel is one of the most popular preparation methods because of its simple procedure, high purity of the products and high adhesive force between nanoparticles and substrates.⁹ Shi et al.¹⁰ prepared TiO₂/carbon fiber (CF) composites by the sol-gel method annealing in air. It was found that as-prepared TiO₂/CF composites have a more outstanding photo-degradation for methyl orange than pure TiO₂, and its photocatalytic performance reduces with increasing of annealing temperature. As is known, the annealing process plays a crucial role in the structures and properties of the resulting products in TiO₂ sol-gel preparation.

Generally, there are three annealing methods that have been used for the sol-gel preparation of TiO₂ photocatalysts, namely annealed in vacuum, air ambient and N₂ atmosphere. In the vacuum annealing process, a few oxygen atoms can easily get away from the TiO₂ crystal, which leads to the formation of oxygen vacancies on the TiO₂ crystal. Due to the nature of electron-donating centers of oxygen vacancies, the concentration of photo-generated holes in the TiO₂ crystal will be decreased, which leads to reduction of the photocatalytic activities of TiO₂.¹¹ For the annealing treatment in air ambient,^12,13 although it can address the limitation of annealing in the vacuum condition, TiO₂ and its supports can be easily oxidized in the existence of oxygen. As for annealed under N₂ atmosphere,^14,15 a large number of surface cracks can been produced on the TiO₂ films and it also lead to the formation of O–Ti–N compounds ascribed to the substitution of O atoms with N atoms in TiO₂ crystal lattices under a high-temperature condition. As a result, the photocatalytic properties of the products will be reduced. However, the annealing method in superheated steam brings a prospect for the sol-gel preparation of TiO₂ film or TiO₂-based composites.

Superheated steam is a colorless and odorless dry steam with a higher temperature than that in the same saturated vapor. Without oxygen in the system, the steam can act as a reactant that decomposes ethanol and other organic matters to carbon monoxide or hydrocarbons. The superheated steam can also provide a stable annealing temperature and high heat transfer effect, which can prevent the cracking of the TiO₂ film on the CF surface and the excessive evaporation of water molecules. Combining TiO₂ and flexible CFs with superior electroconductibility and corrosion resistance, the TiO₂/CFs composites are prepared by annealed in superheated steam. Our earlier research showed that the immobilization of powdered TiO₂ on CFs can avoid the secondary pollution of powder materials and improve the efficiency of the TiO₂ photocatalyst.^16,17

Herein, the TiO₂/CF composites are obtained by annealing in superheated steam instead of a N₂ atmosphere. The effects of TiO₂ sol concentration and annealing temperature on the morphology and structure of the TiO₂/CF composites were investigated and their photocatalytic performance was evaluated by degradation of acid orange II under ultraviolet (UV) light irradiation.

Experimental details

Materials

Polyacrylonitrile-based CFs (PAN-CFs) with a diameter of about 5–6.5 µm were provided by Toray Inc. Concentrated nitric acid (65–68%, analytical reagent) and ethanol (analytical reagent) were provided by Hangzhou Gaojing Fine Chemical Co., Ltd. Tetrabutyl titanate (chemically pure) and glacial acetic acid (analytical reagent) were purchased from the Tianjin Yongda Chemical Reagents Development Center. Acetylacetone (analytical reagent) was provided by Wuxi Zhanwang Chemical Co., Ltd. All commercial chemicals were used as received without further purification.

Preparation of TiO₂ supported on CFs

As reported by Samuneva et al.,¹⁸ a uniform, stable and transparent TiO₂ sol can be achieved when the ratio of the solution is located in the impregnated area of the phase diagram of the Ti(OC₄H₉)₄–C₂H₅OH–H₂O system. The concentration of titanium should be within 0.2–0.8 mol L⁻¹. The TiO₂ sol was prepared with the use of tetrabutyl orthotitanate as the titanium precursor and alcohol as the solvent. In a typical synthesis procedure, Ti(OBu)₄ (17.08 mL) was dissolved in a mixture solution of ethanol (x mL, x = 221.52, 96.52, 54.85, 34), acetic acid (2.9 mL) and acetylacetone (0.5 mL). After stirring for 1 h, the solution was hydrolyzed by the addition of a mixture of water (0.9 mL) and ethanol (10 mL) dropwise, with stirring over 1 h. The solution was maintained at room temperature for 3 days to accomplish thorough hydrolysis and the formation of the TiO₂ sol (0.2, 0.4, 0.6 and 0.8 mol·L⁻¹, respectively). The acid-treated CFs were dipped in the TiO₂ sol for 15 min and then pulled out slowly with a uniform pulling rate. After drying in the oven at 80℃, the dried CF substrates were subsequently calcined under a superheated steam ambient at different temperatures for 2 h. The heating rate was set as 3℃·min⁻¹. Finally, the TiO₂-supported composites (TiO₂/CFs) were obtained. These composites were denoted as M2TiO₂/CF, M4TiO₂/CF, M6 TiO₂/CF and M8TiO₂/CF, depending on the concentration of TiO₂ sol. The powdered TiO₂ was prepared by calcination of different concentrations of TiO₂ sol under the same conditions. The products were denoted as M2TiO_2, M4TiO_2, M6TiO₂ and M8TiO₂.

Material characterization

The morphologies of the TiO₂/CF composites were investigated by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800 and SU70). The crystal structure of TiO₂ was determined with an X-ray diffractometer (Bruker AXS, D8-Discover) using a Cu Kα radiation source at 35 kV, with a scan rate of 0.02° s⁻¹ in the 2θ range of 20–80°. The adhesive strength of TiO₂ and the CFs was evaluated with an ultrasonic cleaner (SY3100DH). Ultraviolet-visible (UV-Vis) diffuse reflectance spectra were recorded with an UV-Vis spectrometer (U-3010, Hitachi), and photoluminescence (PL) emission spectra were measured using a Spex 500 fluorescence spectrophotometer with 325 nm radiation for excitation.

Photocatalytic activities of as-prepared samples

The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic degradation of acid orange II aqueous solution. A total of 0.06 g of the TiO₂/CF sample was placed in a Pyrex tube containing 25 mL of acid orange II aqueous solution (80 mg/L, pH = 3). The photocatalytic reaction was performed with a 500 W mercury lamp as the UV light source. The real-time concentration of acid orange II was detected with a UV-Vis spectrophotometer (U-3310) at a 484 nm maximum absorption wavelength.

Results and discussion

Morphologies of the TiO₂/CF composites

The surface topographies of the CF and TiO₂/CF samples were observed by FE-SEM, as shown in Figure 1. The surface of the nitric acid-treated CFs exhibits a series of parallel deep grooves (Figure 1(a)), which can be ascribed to the strong etching of nitric acid.¹⁹ These grooves will increase the active functional groups and the specific surface area of the CFs that benefit from the immobilization of the TiO₂ film on the CF surface. The morphologies of the TiO₂/CF samples prepared with different TiO₂ sol concentrations are shown in Figures 1(b)–(e). Compared with TiO₂ film on the surface of M8TiO₂/CF (Figure 1(e)) and M2TiO₂/CF (Figure 1(b)), M4TiO₂/CF (Figure 1(c)) and M6TiO₂/CF (Figure 1(d)) are continuous and smooth. Herein, it shown that the TiO₂ film in M6TiO₂/CF was composed of TiO₂ nanoparticles with a diameter of 10–30 nm (Figure 1(f)). The TiO₂ coating on the CFs (Figure 1(e)) is split into numerous flakes, which may be ascribed to the over-thickness of the TiO₂ coating resulting from the excessively high titanium concentration of TiO₂. So, the TiO₂ sol concentration significantly affects the morphologies of the TiO₂/CF composites.

Figure 1.

Field emission scanning electron microscopy (FE-SEM) photograph of carbon fiber (CF) and TiO₂/CF samples: (a) CF after oxidizing; (b) M2TiO₂/CF; (c) M4TiO₂/CF; (d) M6TiO₂/CF; (e) M8TiO₂/CF; (f) high-resolution SEM image of M6TiO₂/CF (100 k).

Evaluation of load rate

The CFs were treated by acid oxidation, washed with deionized water repeatedly and then dried for 2 days at 80℃. The dry weight of the CFs is denoted as W₁. The weight of the TiO₂-loaded CFs is denoted as W₂. The TiO₂-supported CFs were cleaned in an ultrasonic cleaner for 1 h at 55 kHz and then dried at 80℃ for 2 days. The dry weight is denoted as W₃. The initial load rate of TiO₂ is denoted as K₁, the load rate of TiO₂ after ultrasonic treatment is denoted as K₂ and the loading loss rate (denoted as L) of TiO₂ were calculated with equations (1)–(3)

K_{1} = \frac{W_{2} - W_{1}}{W_{2}} \times 100 %

(1)

K_{2} = \frac{W_{3} - W_{1}}{W_{3}} \times 100 %

(2)

L = \frac{W_{2} - W_{3}}{W_{2}} \times 100 %

(3)

The results are shown in Table 1. The initial loading rates of TiO₂ for M2TiO₂, M4TiO₂, M6TiO₂ and M8TiO₂ are 4.56% ± 0.12, 10.7 ± 0.31%, 11.2 ± 0.34% and 18.8 ± 0.65%, respectively. The different loading rates may be ascribed to the different concentrations of TiO₂ sol, which led to the different thicknesses of the TiO₂ film on the surface of the CFs. The loading rates of M2TiO₂/CF, M4TiO₂/CF, M6TiO₂/CF and M8TiO₂/CF were 1.26 ± 0.03%, 7.18 ± 0.21%, 7.57 ± 0.23% and 6.94 ± 0.24% after 1 h ultrasonic oscillation. Clearly, the weight loss is larger with the increasing concentrations of TiO₂ sol. This can be attributed to ultrasonic oscillation, which can remove the TiO₂ particles with a weak combination with the surface of the CFs and the stacked TiO₂ on the surface of the TiO₂ film.⁴ It is worth noting that the weight loss in M8TiO₂/CF is much larger than that in M6TiO₂/CF, which may be because there is a just right coverage of TiO₂ film on the surface of CFs in M6TiO₂/CF and the crack in M8TiO₂/CF leads to more loose TiO₂ on surface of CFs, as observed in Figures 1(d) and (e). The adhesive strength of the TiO₂/CFs composite annealed in a superheated steam environment is much better than that prepared under N₂ atmosphere.¹⁷

Table 1.

Variation in loading rate after 1 h ultrasonic vibration and loading loss rate

Sample	K₁ (%)	K₂ (%)	L (%)
M2TiO₂	4.56 ± 0.12	1.26 ± 0.03	3.34 ± 0.09
M4TiO₂	10.7 ± 0.31	7.18 ± 0.21	3.79 ± 0.11
M6TiO₂	11.2 ± 0.34	7.57 ± 0.23	3.96 ± 0.12
M8TiO₂	18.8 ± 0.65	6.94 ± 0.24	12.7 ± 0.43

Crystal structure of TiO₂

The phase structure of as-prepared TiO₂/CF composites was investigated using an X-ray diffractometer. Figures 2(a)–(d) show the XRD patterns of M2TiO₂, M4TiO₂, M6TiO₂ and M8TiO₂ prepared at the different annealing temperatures for 2 h. All diffraction peaks of M2TiO₂ and M4TiO₂ corresponding to the diffraction peaks of anatase and rutile are not found. The intensity of diffraction peaks for M2TiO₂ and M4TiO₂ increase with increasing the annealing temperature, indicating the good crystalline nature of anatase in the TiO₂/CF composites. For the M6TiO₂ and M8TiO₂ samples, as shown in Figures 2(c) and (d), all diffraction peaks can be assigned to the diffraction peaks of anatase prepared at 450–550℃. The diffraction peaks of rutile appeared in M6TiO₂ and M8TiO₂ when the annealing temperature was 600℃, which indicates that the phase transition temperature of anatase to rutile occurs at 550–600℃. The content of anatase in M6TiO₂ and M8TiO₂ can be calculated with the following equation

X_{A} = 1 / (1 + 1.265 I_{R} / I_{A})

(4)

where X_A is the content of anatase, I_A is the maximum diffraction peak of anatase and I_R is the maximum diffraction peak of rutile. The calculation shows that the content of anatase for M6TiO₂ and M8TiO₂ was 59.54% and 58.55%, respectively. A high concentration of tetrabutyl titanate was reported to result in a high phase transition temperature because more carbon covered the surface of TiO₂ from the carbonization of tetrabutyl titanate.²⁰ However, the results in this work show the opposite. This can be attributed to the superheated H₂O steam in the annealing process, which can lead to the oxidation of carbon monoxide and hydrocarbon decomposed from organics, such as alcohol. This phenomenon prevented carbon from covering the surface of TiO₂. The heat treatment temperature and surface defects are the main factors that affect TiO₂ phase transition.²¹ The large amount of H₂O in M6TiO₂ and M8TiO₂ led to a more intense hydrolysis. Therefore, surface-defective TiO₂ can be easily formed at the same annealing temperature. The phase transition from anatase to rutile can be achieved at a low temperature.

Figure 2.

X-ray diffraction patterns of the TiO₂ nanoparticles subjected to different heat treatments for 2 h: (a) M2TiO₂/CF; (b) M4TiO₂/CF; (c) M6TiO₂/CF; (d) M8TiO₂/CF.

The grain size of TiO₂ can be calculated according to the Scherrer formula, and the results are shown in Table 2

D = \frac{k λ}{β \cos θ}

(5)

where D is the average grain diameter, k is generally 0.89, λ is the X-ray wavelength (Cu Kα target, λ = 1.54056 Å), β is the width of the half-peak (rad) and θ is the diffraction angle (°).

Table 2.

Average crystallite size of TiO₂ annealed at different temperatures

Sample	450℃/nm	500℃/nm	550℃/nm	600℃/nm
M2TiO₂	79	79	84	89
M4TiO₂	77	90	90	98
M6TiO₂	73	74	88	92
M8TiO₂	79	78	88	100

Table 2 shows that the average grain sizes of M2TiO₂, M4TiO₂, M6TiO₂ and M8TiO₂ are in the range of 73–100 nm and increase with the annealing temperature. This change is ascribed to the agglomeration of TiO₂ grains during the annealing process. Similar to the case of molecular motion, a high temperature results in an intense TiO₂ movement, so an easy collision results in an increase in the average grain size.²²

Photocatalytic activities of TiO₂/CFs

To evaluate the photocatalytic activities of the as-prepared TiO₂/CF composites, the experiment for the decoloration of acid orange II was performed under UV light irradiation. The degradation of organic pollutants in photocatalysis mainly depends on photo-generated holes, hydroxyl radicals or superoxide radicals.^23,24 The decoloration of acid orange II was ascribed to the damage of the chromophores and the breakage of conjugated systems. The experimental results are shown in Figure 3. Figures 3(a)–(d) show the photocatalyst properties of the TiO₂/CF prepared at different temperatures. As observed in Figures 3(a)–(d), the concentrations of acid orange II reduce rapidly in the initial 30 min and slow down gradually after 30 min, which is ascribed to the dual effect of adsorption and degradation for acid orange II. Figure 3 shows that all the TiO₂/CF samples obtained by annealing in the superheated steam atmosphere exhibit excellent photocatalytic activity. Particularly for the products annealed at 600℃, the degradation rates of acid orange II over the M4TiO₂/CF and M6TiO₂/CF composites reach up to 96.6% after a 1.5 h reaction. This is higher than that of TiO₂/CF composites prepared by annealing in N₂ atmosphere.²⁵ It may be attributed to the increase of adsorption capacity of the TiO₂/CF composites for H₂O annealed in the superheated steam atmosphere, and the increased adsorption capacity facilitated the conversion of H₂O to ^•OH on the TiO₂ surface. The annealing temperature considerably affected the photocatalytic properties of the TiO₂/CF composites. The photocatalytic activities of the TiO₂/CF composites increased with the annealing temperature. This result may be attributed to the difference in the surface structure and crystalline phase of TiO₂. Notably, the photocatalytic activities of M2TiO₂/CFs decreased with increasing the annealing temperature, which may be ascribed to the transformation of the TiO₂ crystalline phase. TiO₂ sol with a low concentration can be completely transformed to anatase TiO₂ at 450℃. The average grain size will increase with the increase of annealing temperature, which leads to a reduction of photocatalytic activities.

Figure 3.

TiO₂/CF photocatalytic degradation of the acid orange II solution: (a) TiO₂/CF annealed at 450℃; (b) TiO₂/CF annealed at 500℃; (c) TiO₂/CF annealed at 550℃; (d) TiO₂/CF annealed at 600℃.

The chemical stability of TiO₂/CF composites has been further evaluated by cycling runs of M6TiO₂/CF. As shown in Figure 4, the loading loss rate of M6TiO₂/CF is only decreased by 0.45% after four cycles, indicating the strong adhesive force of TiO₂ on the surface of CFs. Moreover, the photocatalytic activity of M6TiO₂/CF composites has hardly reduced. A 93.27% degradation rate was achieved after four cycles (the degradation time of acid orange II is 150 min for each cycle), demonstrating the excellent chemical stability of M6TiO₂/CF.

Figure 4.

The photocatalytic activity for acid orange II (a) and loading loss rate (b) of M6TiO₂/CF with four cycling runs.

The band gap energy and PL spectra of M6TiO₂/CF

The optical property of the M6TiO₂/CF composite annealed at 600℃ was characterized by UV-Vis diffuse reflectance spectroscopy. The absorption edge of the M6TiO₂/CF composite was located at 410 nm, as shown in Figure 5(a). Based on the equation ahv = A(hv – Eg) ⁿ , the band gap of the M6TiO₂/CF composite was estimated to be 3.21 eV (the inset of Figure 5(a)). The PL spectrum was used for investigating the recombination of charge carriers, as shown in Figure 5(b). The diminished PL intensity observed in the M6TiO₂/CF composite (red line) indicates the reduced charge recombination and longer-lived excitons compared with those of powered TiO₂ annealed at 600℃. Thus, the lower recombination rate and longer lifetime of charge carriers led to the higher photocatalytic activity.

Figure 5.

Ultraviolet-visible diffuse reflectance spectra (a) and photoluminescence emission spectra (b) of M6TiO₂/CF annealed at 600℃. (Color online only.)

The plausible schematic diagram of photo-degradation of TiO₂/CF composites is shown in Scheme 1. The electron at the edge of the valance band will jump to the conduction band under UV light irradiation, which leads to formation of oxidative holes (h⁺) at the edge of the valence band, and an oxidative superoxide radical (^•O₂^–) at the edge of the conduction band due to the combination of electrons and oxygen.²⁶ The holes and superoxide radical will result in the oxidation of acid orange II to non-toxic end-products. In TiO₂/CF composites, the photo-generated electron transfer between TiO₂ and CF will greatly retard the recombination of photo-induced charge carriers and prolong electron lifetime, which may cause better photoactivity properties of TiO₂/CF samples.^27,28

Scheme 1.

Schematic diagram of photocatalytic degradation of acid orange II over TiO₂/CF composites. UV: ultraviolet.

Conclusions

The TiO₂/CF composite was prepared by annealing TiO₂ sol-coated CFs in superheated steam ambient. A homogeneous and continuous TiO₂ film was formed on the surface of PAN-based CFs with 0.6 mol·L⁻¹ TiO₂ sol. The average size of the TiO₂ crystalline grains in the as-prepared TiO₂/CF composite was about 73–100 nm. The TiO₂/CF composites annealed in superheated steam exhibited a better photocatalytic activity for degradation of acid orange II under UV light irradiation compared to the TiO₂/CF composites annealed in N₂ ambient reported in the previous publication. Further, the photocatalytic performance of the M6TiO₂/CF composite annealed at 600℃ was the most best, with a removal rate of 98.7% for acid orange II after a 2.5 h reaction, attributed to the homogeneous and continuous TiO₂ film on the surface of CF, which indicated that the photocatalytic activity of the TiO₂/CF composite is most affected by the concentration of TiO₂ sol.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Zhejiang Provincial Natural Science Funds Projects (grant number Y406310) and Science Technology Plan Projects of Shaoxing City (grant number 2012B70014).

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Preparation of titanium dioxide immobilized on carbon fibers annealed in steam ambient and their photocatalytic properties

Abstract

Keywords

Experimental details

Materials

Preparation of TiO2 supported on CFs

Material characterization

Photocatalytic activities of as-prepared samples

Results and discussion

Morphologies of the TiO2/CF composites

Evaluation of load rate

Crystal structure of TiO2

Photocatalytic activities of TiO2/CFs

The band gap energy and PL spectra of M6TiO2/CF

Conclusions

Footnotes

Declaration of conflicting interests

Funding

References

Preparation of TiO₂ supported on CFs

Morphologies of the TiO₂/CF composites

Crystal structure of TiO₂

Photocatalytic activities of TiO₂/CFs

The band gap energy and PL spectra of M6TiO₂/CF