loaded activated carbon fiber composites were prepared through a dip-coating method, and used in a pulsed discharge reactor. The crystalline phase information and surface morphology of TiO2/ACF samples were investigated after undergoing calcination at different temperatures. XRD results revealed that the intensity of diffraction peaks from anatase increased with the increase of the calcination temperature. For calcination at 900 °C, the anatase-to-rutile phase transformation was found. Morphology study showed that TiO2 film fractured into irregular flakes on the ACF surface after calcination at high temperatures. Photocatalytic activities of TiO2/ACF samples were tested in a pulsed discharge condition. The sample calcined at 900 °C with ~31 wt.% rutile in the total TiO2 composition showed the highest level of photocatalytic activity compared with samples calcined at lower temperatures. In addition, pore structure and surface properties of the TiO2/ACF sample before and after applying to the pulsed discharge were studied. The amount of micropores increased slightly, while the quantity of mesopores decreased significantly after use, which caused the decrease of the total pore volume and BET surface area of the sample. No new absorption peak was detected in the FTIR spectrum of the TiO2/ACF sample after use compared with the spectrum of the sample before use, indicating that no new compound formed in the process, which was consistent with the XRD analysis of these samples. After applying to the pulsed discharge condition, a small amount of TiO2 fell off from the ACF surface, while the majority of TiO2 retained by welding to grooves on the ACF surface.
Keywords: Composite; Pore structure; Surface property; Crystalline phase; Photocatalytic activity
1. Introduction
Titanium dioxide (TiO2) has been widely studied for its photocatalytic activity and application for decomposition of various environmental pollutants in both gas and liquid phases [1]. Separation of TiO2 fine particles from the mother solution after the wet-chemical synthesis is a challenge in this field. Immobilization of TiO2 photocatalysts on the activated carbon (AC) has been employed to solve this problem [2-4]. Activated carbon fiber (ACF) which has been widely used as a photocatalyst support [5-11] has larger adsorption capacity and higher adsorption rate compared with AC.
On the other hand, ozone is well-known for its destruction of organic compounds in the water treatment processes. The combination of ozonation and photocatalysis has been demonstrated to be very effective in the treatment of organics-contaminated water [12]. In addition, research work indicated that a synergistic effect occurred when the photocatalysis and ozonation treatments were carried out simultaneously [13-15].
In the process of the pulsed discharge, the major reactive species including hydroxyl radicals (•OH), ozone (O3) and hydrogen peroxide (H2O2), and reactive conditions such as ultraviolet (UV) light and shock waves, have been identified by chemical and physical methods for many of these specific processes [16]. Thus the combination of pulsed discharge with TiO2-loaded activated carbon fiber (TiO2/ACF) can not only solve the suspension problem for fine photocatalyst particles, but also utilize ozone and ultraviolet radiation produced in the pulse discharge condition effectively.
In this study, the effect of the calcination temperature on the crystalline phase of TiO2 and the surface morphology of TiO2/ACF samples were investigated. The photocatalytic activity of TiO2/ACF samples were tested using methyl orange (MO) as a contaminant in a pulsed discharge condition. Pore structure and surface properties of TiO2/ACF samples before and after applying to the pulsed discharge were also studied.
2. Experimental
2.1. Preparation of ACF/TiO2 composites
ACF, produced from a rayon precursor, was purchased from Zichuan Co., China. The ACF felt was sheared into pieces (3 mm Ã- 3 mm) to enable fluidization in the discharge reactor. Distilled water with conductivity of less than 5 μS cm-1 was used to prepare all solutions and rinse the ACF. All other chemicals (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd.
Precursor solutions for TiO2/ACF composites were prepared by the following method. Tetrabutyl titanate (17.02 ml) was first dissolved in ethanol (30 ml) by stirring for 30 min at room temperature. Ethanol (28.32 ml), distilled water (7.2 ml) and acetic acid (20 ml) were slowly mixed into the above solution under stirring. The mixture was then stirred vigorously for 3 h to complete the hydrolysis reaction, which produced a TiO2 sol [8, 17]. 2.5 g ACF were added to this TiO2 sol under vigorous stirring. The mixture was then stirred for 1 h. The samples were taken out and separated one by one to avoid conglutination, and dried at 333 K. The dried samples were finally calcined at 1173 K in a flow of high-purity nitrogen for 2 h.
2.2. Photocatalyst content in the TiO2/ACF sample
A TiO2/ACF sample and an original ACF sample, 1 g each, were calcined in a muffle oven at 850 °C for 6 h. Two calcined samples were weighed after cooled down, and their ash weight percentages were calculated. The difference between these two ash weight percentages is equal to the weight percentage of TiO2 in the TiO2/ACF sample. The content of TiO2 coated on the ACF was determined to be 41.9 wt.%.
2.3. Characterization of TiO2/ACF samples
X-ray diffraction (XRD) patterns from all samples were obtained using a Holand X'pert PROMPD diffractometer with Cu Kα radiation. The surface morphology of samples was studied using a field emission scanning electron microscope (FE-SEM, Quanta 200, Holand). BET surface areas (SBET) of TiO2/ACF samples were calculated from N2 adsorption isotherms at 77 K using the BET equation, where the isotherms were obtained using the Quantachrome Autosorb-1 automated gas adsorption system [18]. The micropore volume and micropore structure were determined by the HK method. The mesopore volume was determined by BJH desorption pore-size distribution. Functional groups on TiO2/ACF sample surface were characterized by diffuse reflection infrared Fourier transforms (DRIFT) spectroscopic analysis on a Nicolet Magna-380 spectrometer within the range of 400-4000 cm−1 [19].
2.4. MO degradation experiments
The power supply and the pulsed discharge reactor used in this study are the same as those used in the previous work [20]. All experiments were conducted under a fixed applied voltage of 46 kV and a pulse frequency of 100 Hz. The MO solution concentration used was 80 mg L-1 with a pH value of 6.10 and the conductivity of 20 μS cm-1. The reactor was filled with 200 ml MO solution and 0.25 g TiO2/ACF composite for each experiment. The concentration of MO was monitored during experiments by the UV-vis absorption spectroscopy (UV-762). The MO quantity was characterized by the absorption peak at 465 nm. The degradation rate of MO was measured by the following equation:
Degradation rate (%) = (1)
where C0 and C are the initial concentration of MO and its remaining concentration in the solution after the reaction, respectively. All experiments were conducted for three times, and the average values were calculated. All data reported herein had a standard deviation less than 5%.
3. Results and discussion
3.1. XRD results of TiO2/ACF samples
Anatase and rutile are two different crystalline phases of TiO2. Rutile is the stable phase at high temperature with a band gap of 3.0 eV, whereas anatase is usually present at low temperature with a band gap of 3.2 eV. Anatase transforms into the rutile phase at temperature above 800°C [21]. XRD patterns of TiO2/ACF samples calcined at different temperatures for 2 h are shown in Fig. 1. The TiO2 film coated on the ACF by hydrolysis of the alkoxide formed anatase crystals after calcined at 450 °C (Fig. 1a). With the increase of the calcination temperature (Fig. 1a-1d), the intensity of diffraction peaks from anatase increased and the full width at half maximum (FWHM) of these peaks decreased, which indicated the growth of anatase crystals with improved crystallinity. A few peaks from rutile were found in the diffraction pattern from the sample calcined at 900 °C (Fig. 1d). The amount of anatase in the total TiO2 composition was determined to be 69 wt.%, whereas that of rutile was 31 wt.%, which showed that a non-continuous ACF surface with the presence of micropores and grooves may have an inhibition effect for the anatase-to-rutile transformation. The interfacial adhesion between TiO2 and ACF was relatively large due to the rough surface of the ACF, which induced a non-continuous arrangement of TiO2 crystal grains. Therefore, the grain growth was prevented, which further retarded the anatase-to-rutile transformation [7, 22].
Fig. 1. XRD patterns of TiO2/ACF samples calcined at different temperatures for 2 h (A: anatase; R: rutile):
(a) 450 ℃, (b) 600 ℃, (c) 750 ℃, (d) 900 ℃.
3.2. SurfaHYPERLINK "javascript:showjdsw('jd_t','j_')"ce morphology of TiO2/ACF samples
Fig. 2 shows the morphology of TiO2/ACF samples calcined at different temperatures for 2 h. At low temperatures, the majority of TiO2 was coated on the ACF surface as a thin film (Fig. 2a and Fig. 2b). After calcination at 450 °C (Fig. 2a), cracks initiated in the TiO2 film along grooves on the ACF surface, and small TiO2 flakes formed. Large cracks formed in the TiO2 film along several grooves on the ACF surface after calcination at 600 °C (Fig. 2b). In addition, flakes formed on the edge of the detached TiO2 portion. The thickness of the TiO2 film was not uniform due to the presence of grooves on the ACF surface. Therefore, there was a lateral contraction within the TiO2 film, which caused the fracture and detachment. For the samples calcined at high temperatures (Fig. 2c and Fig. 2d), the TiO2 film completely fractured and formed many small flakes. Besides the lateral contraction, the longitudinal contraction within the film, due to the unmatched thermal expansion coefficient of the TiO2 film and ACF as well as the aggregation of TiO2 crystals at high temperatures, can be another major factor to cause the complete fracture. However, TiO2 flakes formed at 900 °C (Fig. 2d) still welded to the groove surface of the ACF on their edges.
Fig. 2. SEM images of TiO2/ACF samples calcined at different temperatures:
(a) 450 °C; (b) 600 °C; (c) 750 °C; (d) 900 °C.
3.3. Photocatalytic activity of TiO2/ACF samples
3.3.1. The effect of TiO2/ACF amount on the degradation of MO
The effect of TiO2/ACF amount on the degradation of MO in a pulsed charge process was studied. Three TiO2/ACF samples were used with a weight of 0.25g, 0.35g and 0.45g, respectively. The MO degradation curves from these samples are very close (Fig. 3a), which shows the TiO2/ACF amount has little effect for MO degradation in the combination system of the pulsed discharge and TiO2/ACF composite. With the increase of TiO2/ACF amount, the amount of MO molecules absorbed by ACF increased, which facilitated the degradation process. However, UV transmittance decreased with the increase of TiO2/ACF amount, which further lowered the photocatalytic efficiency. Overall, TiO2/ACF amount has little effect on the degradation of MO in this process. The amount of the TiO2/ACF sample used in all the following experiments is 0.25 g.
3.3.2. The effect of MO concentration on the degradation of MO
The concentration of contaminants in industrial wastewater is usually uncertain. Study of the treatment efficiency in this combination system with different initial contaminant concentrations would be important in this point of view. Fig. 3b shows the degradation curves for different initial MO concentrations. The degradation rate decreased with the increase of the initial MO concentration, which was due to the decrease of light transmittance. However, a recent study [20] has shown that the absolute amount of degraded contaminants in the same treatment period increased although the degradation rate decreased with the increase of the contaminant concentration. In the high-concentration solution, the concentration gradient from the ACF surface to the body solution was relatively high, which accelerated the transportation of the contaminant molecules towards the ACF surface, and gave rise to a high reaction rate on the surface.
The degradation curves for 60 mg L-1 MO solution and 80 mg L-1 MO solution are close (Fig. 3b). Without further notification, the initial MO concentrations are all 80 mg L-1 in the following experiments.
3.3.3. Photocatalytic activity of TiO2/ACF samples after calcination at different temperatures
The results of MO degradation from TiO2/ACF samples calcined at four different temperatures are shown in Fig. 3c. The sample calcined at 900 °C has the highest degradation rate, while the sample calcined at 450 °C has the lowest degradation rate. In the combination system, the ACF played a role of aggregation of organic contaminant molecules, while TiO2 loaded on the ACF surface acted as the decomposition center where organic contaminant molecules were degraded to CO2 and H2O through the interaction with •OH radicals. The amount of organic contaminant molecules absorbed by the ACF depended on its exposed surface area. Therefore, a TiO2/ACF sample with a large exposed surface area, such as the sample calcined at 900°C (Fig. 2d), showed a relatively high degradation rate. In addition, a previous study [1] revealed that a mixture of anatase and rutile with a certain ration had a higher level of photocatalytic activity than any single phase. A mixture of 70 wt.% anatase and 30 wt.% rutile had been shown with the highest level of photocatalytic activity in the study. This was due to the formation of a rutile layer on the anatase surface, which accelerated the separation of electron-hole pairs. In our study, the sample calcined at 900 °C with a composition of 69 wt.% anatase and 31 wt.% rutile also had the highest MO degradation rate, which is consistent with this previous study [1]. Without further notification, all the samples in the following experiments were calcined at 900 °C.
Fig. 3. Effect of the amount of TiO2/ACF composite (a) and the MO solution concentration (b) on MO degradation; photocatalytic activity of TiO2/ACF samples calcined at different temperatures in a pulse discharge process (c).
3.4. The effect of the pulsed charge on the pore structure and surface properties of TiO2/ACF samples
3.4.1. Pore structures of the TiO2/ACF sample before and after use
N2 adsorption isotherms and pore size distribution of the original TiO2/ACF sample and the sample after use for five times in the pulsed discharge condition are shown in Fig. 4. N2 adsorption isotherm from the sample before use (Fig. 4a) was of IUPAC type-I, which indicated the majority of pores were meso- and micro-pores. The curve showed non-platform characteristic in the partion where the relative pressure was less than 0.1, which revealed there were some mesopores in the sample. With the increase of the relative pressure (>0.1), the absorbed volume increased, which showed there was capillary cohesion inside of the mesopores. Even when the relative pressure was around 0.9, the absorption still increased with the increase of the relative pressure, which was due to the capillary cohesion in the macropores. After applying to the pulsed discharge, N2 adsorption isotherm (Fig. 4a) from the sample also showed IUPAC type-I characteristic, which revealed the majority of pores were micro- and meso-pores. An absorption platform appeared when the relative pressure was larger than 0.1, indicating that there was only a small amount of mesopores in the sample.
Fig. 4b shows the pore size distribution of the sample before and after use. The position of the main peak shifted from 0.73 nm (before use) to 0.68 nm (after use), indicating the amount of micropores increased in the sample after use, which could be caused by oxidation of the amorphous carbon on the ACF surface. In addition, the amount of mesopores in the size range of 2-3.4 nm decreased significantly after use, and mesopores in the size range of 3.5-8.6 nm disappeared, which could be due to the formation of larger pores from these mesopores (2-8.6nm) through oxide etching.
The structure parameters of TiO2/ACF samples before and after use are listed in Table 1. The volume of micropores increased after use. However, the total pore volume and BET surface area decreased after use, due to the significant decrease in the quantity of mesopores.
Fig. 4. N2 adsorption isotherms (a) and the pore size distributions (b) from TiO2/ACF samples before and after use
Table 1 Structure parameters of TiO2/ACF samples before and after use
SBET
（cm2g-1）
Micropore volume （cm3g-1）
Mesopore volume
（cm3g-1）
Total pore volume
（cm3g-1）
Coated TiO2
content
(wt.%)
TiO2/ACF
803.1
0.2669
0.0731
0.3414
41.9
Used TiO2/ACF
790.9
0.2725
0.0267
0.3032
40.5
3.4.2. Surface properties of the TiO2/ACF sample before and after use
FTIR spectra of the TiO2/ACF sample before use and after use for five times are shown in Fig. 5. The peak at 3560 cm-1 was from the O-H stretching vibration [23]. The broad band at 3435 cm-1 could be formed by the O-H stretching vibration from the phenolic hydroxyl [24]. Two shoulder peaks at 2925 cm-1 and 2850 cm-1 should be from the symmetric and asymmetric stretching vibration of -CH, -CH2 and -CH3 in aliphatic groups [25, 26]. Peaks in the range of 1650-1860 cm-1 corresponded to the stretching vibration of C=O in carbonyl and carboxyl groups, whereas the peak at 1850cm-1 attributed to anhydride structure. The peak at 1740 cm-1 and 1460 cm-1 corresponded to the bending vibration of C=O and O-H in a carboxyl group [27]. The band around 1639 cm-1 was formed through the superposition of stretching vibration from C=C in an aromatic functional group and C=O in carboxyl, ester, lactone or carbonyl groups [24, 25]. Complex bands in the range of 1400-1600 cm-1 were formed by C=C and different substitution modes of the aromatic group. The bands in the range of 1300-1400 cm-1 were attributed to the deformation vibrations of aromatic ring absorption and the O-H bending vibration of carboxyl and hydroxyl groups [28, 29]. The bands observed in the range of 1000-1200 cm-1 were in accordance with C-O single-bond stretching vibrations from phenols, epoxide structures, aromatic ethers, and γ and δ lactone groups [24, 30]. Considering the electron affinity of C and Ti, the peak at 1060 cm-1 should be attributed to Ti-O-C [4, 31], suggesting there was a slight conjugation between ACF and Ti-O bonds. The hydroxilation in TiO2/ACF composites also caused the distortion observed in the Ti-O stretching bands at 550-700 cm-1 [32, 33].
By comparison of the two FTIR spectra, it was found that the intensity of some reflection peaks increased slightly after use. No new peaks formed in the sample after applying to the pulsed discharge. The increase of the intensity for the peak at 3435 cm-1 indicated the number increase of phenolic hydroxyl and carboxyl groups, whereas the increase in intensity of the peak at 1022 cm-1 showed the increasing number of C-O groups on the ACF surface. The increase in intensity of bands at 786 cm-1, 663 cm-1 and 636 cm-1 could be caused by the formation of ozonides on the ACF surface. XRD results from the sample before and after use (not shown) also indicated there was no change for the crystalline phases of TiO2 in the process.
Fig. 5. FTIR spectra of TiO2/ACF sample before and after use
3.5. The morphology change of the TiO2/ACF sample after repeat use
SEM images of the TiO2/ACF sample before and after use and their magnifications are shown in Fig. 6. By comparison of Fig. 6a and Fig. 6b, it was found a small amount of TiO2 was washed away from the ACF surface by water and airstream, which was also indicated by the decrease in the amount of TiO2 in the sample after use as shown in Table 1. In addition, there was a slight amount of TiO2 particles in the reaction system after the first cycle found by the turbidity test, while no TiO2 particles were detected after the second cycle. Therefore, loss of TiO2 particles mainly occurred in the first cycle. TiO2 particles lost in the process were small TiO2 particles which had weak interaction with the ACF (Fig. 6c). However, the majority of TiO2 flakes still adhered to the ACF after use (Fig. 6d). The interaction between TiO2 and ACF was relative strong due to the interlock network of the ACF. In addition, the TiO2 flakes welded to the ACF grooves due to the aggregation of TiO2 particles in the calcination process, which not only increased the interaction between TiO2 and ACF, but also improved the mechanical strength of the composite. On the other hand, due to a wide distribution of TiO2 particle size and groove structures on ACF surfaces, lights can easily transmitted through the TiO2/ACF composite, which induced photocatalytic reactions on the surfaces of most TiO2 particles.
Fig. 6. SEM images of the TiO2/ACF sample before (a) and after use (b) in the pulsed discharge condition, and their magnification images (c is a magnification of a spot in image a; d is a magnification of a spot in image b).
4. Conclusion
Photocatalytic TiO2/ACF composites were prepared in a sol through a dip-coating method, and subsequently used in a pulse discharge reactor. After calcination at 900 °C, TiO2 film fractured into flakes which still welded to the grooves on the ACF surface. There were 31 wt.% of rutile and 69 wt.% of anatase in the TiO2 composition in this sample. The photocatalytic activity of this sample was higher than samples calcined at lower temperatures for the degradation of methyl orange. N2 adsorption tests showed the majority of pores in the sample were micropores with a small amount of mesopores. FTIR results revealed there were a large amount of oxygen-containing functional groups on the TiO2/ACF surfaces. Ti-O-C was found in the FTIR spectrum suggesting there was a conjugation between ACF and Ti-O bonds. The surface morphology and properties of the sample had little change even after used in the pulse discharge reactor for several times. The amount of micropores increased slightly, while the quantity of mesopores decreased significantly after applying to the pulsed discharge. However, the overall photocatalytic activity of the TiO2/ACF sample was not affected by the pulse discharge.
