Abstract
A novel TiO2-wrapped nanofiber composite catalyst, which possessed a unique porous structure and mixed crystalline phase, was prepared by the combination of superficial sol–gel and post-calcination processes. By means of the superficial sol–gel process, TiO2 layers were deposited on the surface of each nanofiber-like cellulose fiber, and then the TiO2-wrapped nanofiber composite catalysts were calcined at different temperatures under a nitrogen atmosphere. With temperature increasing, the original cotton nanofiber composites were converted into porous carbon nanofiber catalysts wrapped by a TiO2 mixed crystalline phase, which was accompanied by a crystal transformation. The photocatalytic activity of the new catalysts was evaluated by the degradation of methylene blue (MB) under ultraviolet (UV) irradiation. The results demonstrated that the new catalysts had good photocatalytic ability, and the TNC-700 catalyst showed a superior photocatalytic ability compared with the other catalysts; the new catalysts had a unique porous structure, high specific surface area, and mixed crystalline phase. Additionally, the synergistic photocatalytic effect of the TiO2 and activated carbon nanofiber resulted in the efficient degradation of organic pollutants in water or air.
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Preparation and Photocatalytic Activity of TiO2-Wrapped Cotton Nanofiber Composite Catalysts
Liruhua Zhang,a Junmin Wan,a,b,* Zhiwen Hu,b and Wenbin Jiang b
A novel TiO2-wrapped nanofiber composite catalyst, which possessed a unique porous structure and mixed crystalline phase, was prepared by the combination of superficial sol–gel and post-calcination processes. By means of the superficial sol–gel process, TiO2 layers were deposited on the surface of each nanofiber-like cellulose fiber, and then the TiO2-wrapped nanofiber composite catalysts were calcined at different temperatures under a nitrogen atmosphere. With temperature increasing, the original cotton nanofiber composites were converted into porous carbon nanofiber catalysts wrapped by a TiO2 mixed crystalline phase, which was accompanied by a crystal transformation. The photocatalytic activity of the new catalysts was evaluated by the degradation of methylene blue (MB) under ultraviolet (UV) irradiation. The results demonstrated that the new catalysts had good photocatalytic ability, and the TNC-700 catalyst showed a superior photocatalytic ability compared with the other catalysts; the new catalysts had a unique porous structure, high specific surface area, and mixed crystalline phase. Additionally, the synergistic photocatalytic effect of the TiO2 and activated carbon nanofiber resulted in the efficient degradation of organic pollutants in water or air.
Keywords: Titanium oxide (TiO2); Nanocrystalline cellulose; Nanofiber catalyst; Synergistic effect; Photocatalytic activity
Contact information: a: National Engineering Lab of Textile Fiber Materials & Processing Technology, Zhejiang Sci-Tech University, Hangzhou 310018, PR China; b: State Key Laboratory of Advanced Textiles Materials and Manufacture Technology, MOE, Zhejiang Sci-Tech University, Hangzhou 310018, PR China; *Corresponding authors: wwjm2001@126.com
INTRODUCTION
Organic/inorganic (hybrid) nanocomposites have attracted increasing attention because of their remarkable synergistic effects and applications in different fields (Alexandre and Dubois 2000; Sinha Ray and Okamoto 2003; Jia et al. 2012; Stoica-Guzun et al. 2013). With the strong awareness of recycled, sustainable, and biodegradable polymeric materials, biopolymers are considered to be prime material for nanocomposites. Silk (Foo et al. 2006; Kharlampieva et al. 2010), cellulose (Eichhorn et al. 2010; Klemm et al. 2011; Bi et al. 2013), chitin (Alonso and Belamie 2010; Ifuku et al. 2010), collagen (Marandi et al. 2013), starch (Matsui et al. 2004), and alginates (Rhim et al. 2006) have all been utilized to prepare organic/inorganic hybrid composites. Different methods have been utilized to fabricate the hybrid materials, which combined the biomass substances with inorganic materials. For example, Cook and Timms designed an exact replication of biological structure by using chemical vapor deposition (CVD) of silica (Cook and Timms 2003). Atomic layer deposition (ALD) was used for the preparation of photocatalytic TiO2/cellulose composites in nanometer-level replication of cellulosic substances (Kemell et al. 2005).
With regard to these biopolymers, nanocrystalline cellulose (NCC), which is nanosized cellulose fiber, has a number of fascinating characteristics, such as containing a large number of active hydroxyl groups on its surface, environmentally friendly nature, low cost, nanoscale dimension, and unique optical properties and utilities (de Souza Lima et al. 2004; Beck-Candanedo et al. 2005; Klemm et al. 2005; Foo et al. 2006; Klemm et al. 2006; Thakur and Thakur 2014; Thakur and Voicu 2016). Recently, to explore multifunctional nanocellulose hybrid composites, many different nanocellulose-inorganic hybrid composite materials have been prepared for applications in various fields. Nanocellulose-silver hybrids have been prepared as potential antibacterial agents (Díez et al. 2011). Homogeneous hybrids from NCC and spherical nanoparticles of amorphous calcium carbonate (ACC) were obtained by a simple chemical approach (Gebauer et al. 2011). Titanium dioxide (TiO2) coated nanocellulose aerogels showed photoswitchable hydrophobicity and photooxidative decomposition from a chemical vapor deposition (CVD) approach (Kettunen et al. 2011). Nanocellulose aerogel with a hydrophobic and oleophilic coat was achieved by functionalizing the native cellulose nanofibrils with a nanoscopic layer of TiO2 (Korhonen et al. 2011). Among these composites, as should be noted that nanocellulose-titania hybrid composite composites with well-defined structures could be acquired. Furthermore, carbonization of the natural cellulose substances under proper temperature and atmosphere could acquire unique nanostructured carbon materials because of the rigidity and thermal stability of the nanocrystalline cellulose.
TiO2 is an ideal and stable photocatalyst, and it is capable of degrading organic molecules via electrons and holes formed under ultraviolet (UV) light irradiation. There is tremendous amount of interest for TiO2 in photocatalytic applications because of the increasing demand for environmental remediation and energy conservation. TiO2-based materials are promising photocatalysts that have many advantages, such as non-toxicity, low cost, good photocatalytic activity, and chemical stability (Khan et al. 2002; Zhang et al. 2008). Unfortunately, the low electrical conversion efficiency has caused serious challenges because of the narrow light-response range of UV (only about 3% to 5% of total sunlight) (Liu et al. 2008), and the electrons and holes can easily recombine before the electrons emigrate to the surface of the photocatalysts. Considerable efforts have been exerted to design a special structural material of TiO2 to improve the photoelectric conversion efficiency. There are a number of possibilities for the synthesis of structural materials of TiO2 by using biological macromolecules as carriers or composites. It is worth noting that titania-carbon composite materials, which possess synergistic effects from the TiO2 and carbon, have a better photocatalytic performance than pure TiO2 materials (Shanmugasundaram and Horst 2003; Yang and Tian 2015). Both the nanostructure and high surface area are very important for enhancing the photocatalytic efficiency of TiO2. Many studies have been reported that cellulosic substances have been used as support template of titania films for developing the novel catalytic hybrids. However, the photocatalytic activity achieved was relatively poor due to some faulty structure or relatively low surface area of only 4 m2g-1 (Kemell et al. 2005). To the best of our knowledge, there has been a lack of work to investigate the influence of the different calcination temperature for the fabrication of TiO2/activated carbon fiber hybrids materials under the nitrogen atmosphere. Therefore, it is very important to investigate the optimum conditions of TiO2 wrapped activated carbon fiber hybrids materials systematically. As shown in this paper, these issues can be solved.
In this paper, the mesoporous and nanorod-like TiO2-wrapped nanofiber composite catalysts were acquired by a facile method, which could be applied for degradation of the wastewater. The morphology, crystalline phase, specific surface area, and chemical composition of the prepared photocatalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) spectroscopy, thermogravimetric and differential thermal analysis (TG-DTA), Brunauer-Emmett-Teller (BET) method, and X-ray photoelectron spectroscopy (XPS). Additionally, the photocatalytic activity of the prepared photocatalysts was evaluated by degrading methylene blue (MB) solution under UV irradiation. The results showed that the photocatalytic activity of the new photocatalysts was improved because of its highly unique porous structure, mixed crystalline phase, and synergistic effect between TiO2 and porous activated carbon nanofibres. Thus, the prepared photocatalysts could be used potentially to degrade organic pollutants in water or air.
EXPERIMENTAL
Preparation of NCC Cotton Fiber
All chemical reagents were of analytical grade and used without further purification. The NCC was prepared by sulfuric acid hydrolysis of CP20 cotton linters (Aladdin, China). First, the cotton fibers were washed with deionized water and dried at 60 °C. A 220-mL portion of 64% H2SO4 (109 mL of concentrated sulfuric acid with 111 mL of deionized water) was added to a round bottom flask and heated at 45 °C in a water bath for 1 h. A total of 20 g of cotton fiber was added to the round bottom flask, which was vigorously stirred at 45 °C for 45 min. The solution became white when the cotton paddle was completely dissolved, and then became pale yellow when heated at a constant temperature. The suspension was diluted 10-fold with deionized water, and the NCC-containing fraction was left in the bottom sediment over-night. The upper liquid phase was decanted. Afterwards, the NCC-rich phase was washed three times with deionized water and centrifuged at 8500 rpm. A finely dispersed NCC fiber thick solution was then obtained with a mass fraction of approximately 6 wt.%.
Synthesis of TiO2–Wrapped Nanofiber Composite Catalysts
An appropriate amount of crystalline nanocellulose solution was added to 180 mL of absolute ethanol, and homogeneously dispersed solution A was obtained after ultrasonic treatment. Solution B was obtained by adding 6 mL of butyl titanate dropwise into 180 mL of absolute ethanol solution isolated from the air, and then stirred vigorously. Solution B was slowly poured into solution A, and the mixed solution was vigorously stirred for 30 min to obtain solution C. A mixed solution of absolute ethanol: water (1:10) was added into solution C. The final solution was continuously stirred for 24 h in the air. The TiO2-wrapped nanofiber composites were collected by centrifugation, washed several times with deionized water and absolute ethanol, and air-dried at 60 °C for 3 h. The samples were divided and calcined at 500, 600, 700, 800, and 900 °C in a N2 atmosphere for 2 h. The original TiO2-wrapped nanofiber composites were coded as TNC, and the TiO2-wrapped nanofiber composites that were calcined at 500, 600, 700, 800, and 900 °C in a N2 atmosphere were coded as TNC-500, TNC-600, TNC-700, TNC-800, and TNC-900, respectively.
Characterization
The functional group changes of the samples at different calcination temperatures were determined using FTIR spectra, which were measured on a Nicolet Nexus spectrometer (Thermo Scientific, USA) with the samples embedded in KBr pellets. For the microsurface analysis, the samples were observed with SEM and TEM. A ZEISS SUPRA 40 scanning electron microscope (Zeiss, Germany) was used to obtain the images. TEM was carried out with a Tecnai G20 instrument (FEI, USA) operated at 100 kV. The samples were mixed in ethanol and then underwent supersonic oscillations for 10 min. Drops of the suspension were placed on carbon coated copper or silicon wafer grids for analysis after desiccation. The crystalline structures were characterized by XRD (miniflex600, Rigaku, Japan) using Ni-filtered Cu Kα radiation (λ = 1.5406Å) at 40 kV and 44 mA. The samples were scanned within the 2θ range of 10° to 80° at a scanning speed of 1°/min. The specific surface area (SBET), pore volume (Vp), and crystal size of the samples were measured by the BET method at a relative pressure (p/p0) ratio of 0.05 to 0.2 based on the N2 adsorption at the liquid-nitrogen temperature using a Micro ASAP2460 unit (Micro, USA). To determine the thermal decomposition behavior of the samples, TG-DTA was performed at a heating rate of 10°C/min in air with a SHIMADZU-DTG60 thermogravimetric analyzer (Shimadzu, Japan). XPS (K-Alpha, Thermofisher Scientific Company, Thermo Scientific, USA) was performed to characterize the elemental composition of the prepared samples.
Evaluation of the Photocatalytic Activity
The photocatalytic activities of the samples were evaluated using MB as the target degradation molecule in a reactor under UV light irradiation. The irradiation source was a 1000 W mercury lamp (λ < 400 nm). The initial concentration of MB was 10 mg/L, and the MB solution was kept at 25 °C. The photocatalytic experiments were carried out without bubbling. The MB solution in the photoreactor was always open for air with magnetic stirring. The reaction was composed of 10 mg of catalyst and 30 mL of MB solution with an original concentration of 10 mg/L. Before turning the light on, the solution was stirred with a magnetic bar for 1 h to ensure the adsorption-desorption equilibrium between the dye and catalyst. A total of 4 mL of the suspension was analyzed at 0.5 h intervals by recording the dye absorption spectra.
RESULTS AND DISCUSSION
Synthesis Approach of the Composite Catalysts
The fabrication procedure of theTiO2-wrapped nanofiber composite catalysts is schematically illustrated in Fig. 1. The TiO2-wrapped nanofiber catalysts were fabricated with two steps. Firstly, pre-hydrolyzed butyl titanate (TBOT) was added to the aqueous suspension of cotton nanofiber, and then an opaque colloidal dispersion solution was acquired after continuous stirring. Finally, the samples were calcined at different temperatures under a nitrogen atmosphere (Fig. 1).
Fig. 1. Synthesis process of the TiO2-wrapped nanofiber composite catalysts
FTIR Spectroscopy Analysis of the Composite Catalysts
The FTIR spectra of the TNC, TNC-500, TNC-600, TNC-700, TNC-800, and TNC-900 samples are shown in Fig. 2. All samples had strong bands at 400 to 900 cm-1, which corresponded to Ti–O–Ti bridge stretching and Ti–O stretching modes. The strong broad band near 3436 and 1634 cm-1 appeared in Fig. 2 (a, b, c, and d), which were assigned to the stretching and deformation vibration of the hydroxyl groups present (C-OH and TiO2-OH) in the surface of active carbon nanofiber and TiO2 composite sample (Ao et al. 2008). The intensive band of OH-group asymmetrical and symmetrical stretching vibrations at 3436 cm-1 and H-O-H deformation vibration at 1634 cm-1 could confirm the large amount of water molecules. However, the OH-group vibration bands became much weaker with the increase of calcination temperatures (Chen et al. 2003; Bezrodna et al. 2004; Watson et al. 2004).
Fig. 2. FTIR spectra of the samples calcined at different temperatures
The aforementioned bands at 3436 and 1634 cm-1 were assigned to the bending vibration absorption of OH-group; both bands completely vanished when the calcination temperature increased above 700 °C. In other words, the spectra of the TNC-800 and TNC-900 samples did not show the two broad peaks (Jia et al. 2009). These results indicated that a mass of OH-group and absorbed water molecules in TNC-800 and TNC-900 had been removed with the rising of the temperature above 700°C. In some extent, the disappearance of both bands at 3436 and 1634 cm-1 of the TNC-800 and TNC-900 resulted in the reduction of the photocatalytic activities. The bands from the range of 1000 to 1400 cm-1 in the spectra of a, b, c, and d in Fig. 2 were caused by O-H bending vibration and C-OH stretching, which indicated the existence of many residual surface OH-group in these samples (Sun and Li 2004; Zhu et al. 2007). In the photocatalytic reaction, the surface OH-group, which can react with photogenerated holes on the surface of the catalyst, played a major role in enhancing the photocatalytic activity. Because the surface OH-group resulted in better charge transfer, the recombination of the electron-hole pairs was inhibited, and there were higher concentrations of hydroxyl radicals, which were vital oxidants for the photocatalytic reaction. Therefore, the surface area and pore volume were reduced during high temperature calcination (Table 1).
SEM and TEM Analysis of the Composite Catalysts
The morphological changes of the TiO2-wrapped nanofiber composite catalysts calcined at different temperatures were analyzed by SEM and TEM, and the images are shown in Figs. 3 and 4, respectively.
Fig. 3. SEM images of (a) TNC, (b) TNC-500, (c) TNC-600, (d) TNC-700, (e) TNC-800, and (f) TNC-900
The TiO2-wrapped nanofiber composite catalysts showed a nanorod morphology with different pore volumes at different calcination temperatures. The image of the original TiO2-wrapped nanofiber composite catalyst (Fig. 3a) showed a nanofiber core-shell structure, which was further confirmed by the TEM (Fig. 4a). The TEM image of the original samples revealed that the coating of TiO2 was uniform and dense on the surface. Figures 3 b, c, d, e, and f present the low magnification SEM images of the calcined products. The TNC-500 samples (Fig. 3b) showed some apparent changes in the morphology, and had more irregular nanopores. When the calcination temperature was increased up to 600 °C (TNC-600), more agglomerates of TiO2-wrapped nanofiber composite catalysts were observed (Fig. 3c). The interaction effect between the small TiO2 nanoparticles, which deposited on the surface of the nanofiber, might have caused agglomeration, and led to a decreased surface area (24.07 m²/g) compared with TNC-500 (48.46 m²/g), as shown in Table 1. The samples calcined at and below 700 °C remained nanorod-like in shape. For theTNC-800 and TNC-900 samples, only TiO2 nanoparticles were observed because the high temperature degraded the nanocellulose.
Fig. 4. TEM images of (a) TNC, (b) TNC-500, (c) TNC-600, (d) TNC-700, (e) TNC-800, and (f) TNC-900; (g) HRTEM of TNC-700; (h) the SAED pattern image of TNC-700
The TEM images showed much finer morphological changes and crystal structure of the calcined TNC samples. TNC-700 (Fig. 4d) showed a nanofiber-like morphology of TiO2 with a porous structure, which led to an improvement of the pore volume (0.2083 cm3/g). Additionally, the TiO2 wrapped nanofibers mainly accumulated in the direction of the cotton cellulose nanofibers. The high resolution TEM (HRTEM) image of TNC-700 (Fig. 4g) showed an interplanar spacing (d) of 0.365 and 0.251 nm, which corresponded to the (101) plane of the anatase and rutile phases, respectively. The selected area electron diffraction (SAED) pattern of TNC-700 (Fig. 4h) indicated that there were mixed polycrystalline phases, including anatase and rutile, which was in accordance with the results of the XRD patterns, as shown in Fig. 5. The crystalline phase structure of TiO2 played an important role in the photoelectrochemical properties and photocatalytic activity. High crystallinity could diminish the recombination of the photogenerated charges and holes in the bulk and on the surface of the photocatalysts. So, high temperature calcination was a simple and effective approach to improve the crystallinity.
XRD Analysis of the Composite Catalysts
Figure 5 showed the XRD patterns of the samples calcined at different temperatures. There were no apparent diffraction peaks in the pure sample (Fig. 5 a). However, strong diffraction peaks were observed for the other calcination samples, which indicated the formation of new crystal structures at high temperatures. In Fig. 5 b, the diffraction peaks at 25.2°, 36.9°, 37.8°, 38.6°, 48.0°, 53.9°, and 55.1° were related to the diffractions of the (101), (103), (004), (112), (200), (105), and (211) planes of anatase TiO2 (JCPDS No. 21-1272), respectively (Zhang et al. 1999; Zhang et al. 2000; Bae et al. 2009; Li and Zeng 2011). The crystalline phase of the calcined samples showed a slightly stronger diffractive intensity with the increase of temperature, i.e., the higher temperature, then the stronger diffractive intensity. However, when the calcination temperature was 700 °C or higher, the diffraction features of rutile TiO2(JCPDS No. 21-1276) were observed (Fig. 3 d). Additionally, the TNC-800 and TNC-900 samples clearly showed the sharper diffractive peaks that are typical of rutile TiO2 (Fig. 3 e and f).
As shown in the XRD patterns of the TiO2-wrapped nanofiber composite catalysts calcined at different temperatures (500, 600, 700, 800, and 900 °C), the transformation from the anatase to rutile phase was accompanied with the carbonization of cellulose. TG-DTA was performed to explore the process of this phase transformation.
Fig. 5. Typical XRD patterns of the pure sample and samples calcined at different temperatures
Fig. 6. TG-DTA curves of the TiO2-wrapped nanofiber hybrid materials
Figure 6 represented the TG-DTA of the TNC composites, including the transformation process of the titania/cotton nanofiber composites to TiO2-wrapped nanofiber hybrid materials under nitrogen atmosphere. In the TG-DTA curves, the weight loss of the samples was mainly showed in three steps. The first mass loss ranged from about 25 to 180 °C and was attributed to absorbed water, which was determined from the endothermic peak in the DTA curve. The DTA curve showed two exothermic peaks at 275 and 450 °C. The first exothermic peak began at 180 °C and finished at 350 °C, which was assigned to the carbonization process of the cellulose. The results were in accordance with previously reported values (Wielageet al. 1999; Moltó et al. 2006). The exothermic peak at 450 °C indicated the phase transformation of TiO2 from the anatase to rutile phase.
Fig. 7(a & b). N2 adsorption-desorption isotherms of the TiO2-wrapped nanofiber hybrid materials after being calcined at (a) 500, (b) 600, (c) 700, (d) 800, and (e) 900 °C
Fig. 7(c, d, & e). N2 adsorption-desorption isotherms of the TiO2-wrapped nanofiber hybrid materials after being calcined at (a) 500, (b) 600, (c) 700, (d) 800, and (e) 900 °C
Table 1. Impact of the Calcination Temperature on the BET Specific Surface Area (SBET), Pore Volume (Vp), and Pore Diameter (D) of the TiO2-Wrapped Nanofiber Hybrid Materials
a: BJH adsorption cumulative pore volume of pores between 1.7 and 300 nm in diameter;
b: Adsorption average pore diameter (4V/A by BET); c: BJH desorption average pore diameter (4V/A).
BET Analysis of the Composite Catalysts
The BET surface area, pore volumes, and pore sizes of the samples are summarized in Fig. 7 and Table 1. Figure 7 depicted the N2 adsorption-desorption isotherms of the samples. According to the IUPAC classification, the N2 isotherm results of all samples were typical of type IV with an H2-type hysteresis loop, which confirmed that most of the pores were mesopores (2 to 50nm). Furthermore, the hysteresis loop was moved to a higher relative pressure range with the rising of the calcination temperature, and simultaneously, the area of the hysteresis loop decreased. The multi-layer adsorption and capillary condensation steps mainly appeared at the high relative pressure range of 0.6 to 0.98, which further established that the relatively massive and uniform mesopores were constituted on the surface of the TiO2-wrapped nanofiber hybrid materials. Moreover, the results indicated that the samples possessed multifarious roller distributions of porous materials because of the presence of H2 hysteresis loops, which are observed in the high relative pressure range of 0.6 to 0.98. Compared with the N2 adsorption-desorption isotherm curves of the other five samples, the TNC-700 sample showed the highest adsorption capacity. As shown in Table 1, the TNC-700 sample had the highest pore volume (0.2083 cm3/g). That was because of the decrease of pores volumes at higher calcination temperatures. These were due to the destruction of catalyst morphology for the high calcination temperature above 700℃. The structure of rod-like nanofiber composite catalysts could be maintained below 700℃during the carbonization process of cellulose. This was one of the reason why TNC-800 and TNC-900 showed the relatively poor photocatalytic activities. From Table 1, the surface area, which is one of the most important factors for enhancing the photocatalytic activity, varied as the calcination temperature increased. The BET method was implemented to measure the surface area and porosity of the TiO2-wrapped nanofiber hybrid materials. The BET specific surface area of the TNC sample was 448.47 m3/g, and the pore volume was 0.625 cm3/g.
XPS Analysis of the Composite Catalysts
To characterize the chemical composition and chemical states of the samples at different temperatures, the samples calcined at different temperatures were investigated by XPS analysis (Fig. 8). The TNC-700 samples had a higher binding energy for the C 1s, O 1s, and Ti 2p XPS spectra than the other samples. The results indicated that the strong integration between activated carbon fiber and TiO2 be calcined at the temperature of 700 C under nitrogen atmosphere. Results also confirmed the temperature of 700 C was the optimal choice for fabrication of TiO2-wrapped nanofiber composite catalysts. By calcination treatment in this temperature, catalysts had the strongest interaction between the Ti, O, and C compared with other samples. Therefore, it also could be understood that the TNC-700 samples showed the fascinating properties for degrading the MB, which is shown in Fig. 10. The peak of C 1s, O 1s, and Ti 2p in the survey spectra of the TNC-700 samples are shown in Fig. 9a. Two intense peaks for Ti 2p3/2 at 458.6 eV and Ti 2p1/2 at 464.3 eV are shown in Fig. 9b, and there was a peak separation of 5.7 eV between those two peaks. Liu and Wang (2002). The results both indicated the existence of Ti4+ ions (TiO2). Figure 9c displays the spectra of C 1s, and information of the different carbon-carbon bonding characteristics found from the raw data. Three intense peaks at 284.9, 286.5, and 288.2 eV were observed for the TiO2-wrapped nanofiber hybrid materials that corresponded to the C 1s binding energy region. Furthermore, the oxygen-containing functional groups, which were affiliated with carbon, showed high peaks for C-O (286.5 eV) and C=O (288.2 eV) at the higher binding energy region. Additionally, the peak at 284.9 eV was assigned to the C-C, C=C, and C-H bonds (sp2), which was in accordance with other literature (Takahagi and Ishitani 1984; Paiva et al. 2000). The O 1s spectra of the TNC-700 hybrid materials are presented in Fig. 9 d. The O 1s region of the samples revealed the existence of two similar peaks corresponding to Ti-O of TiO2 (530.5 eV) and C-O bonds of carbon fibers (532.5 eV) (Paiva et al. 2000; Yu et al. 2000; Liu and Wang 2002). This strong evidence confirmed that TiO2 was successfully loaded on the nanocellulose fiber through chemical bonds. Therefore, the TNC-700 sample displayed higher photocatalytic performance.
Fig. 8. XPS results of the (a) survey scan spectra and (b) Ti 2p, (c) C 1s, and (d) O 1s regions of the TiO2-wrapped nanofiber hybrid materials
Photocatalytic Analysis of the Composite Catalysts
Photocatalytic experiments were implemented to investigate the photocatalytic activity of the samples. The catalysts were evaluated by decomposing 10 mg/L MB under UV light irradiation (wavelength = 365 nm, power = 1000 W). MB is a typical organic dye that often contaminates the environment. The dye molecules were quickly adsorbed onto the surface of the catalysts because of the unique porous structure, and then upon decomposition. The blank experiments with no catalysts were conducted under the same conditions in this study.
Fig. 9. XPS results for (a) survey scan spectra; (b) Ti 2p; (c) C 1s; (d) O 1s regions for TNC-700
Figure 10 shows that the photodegradation efficiencies of the TNC-500, TNC-600, and TNC-700 samples were beyond that of P25 (TiO2 nanoparticles). In particular, the photodegradation efficiency of the TNC-700 sample was approximately 35% higher than that of the P25 for the MB solution, while the TNC-800 and TNC-900 samples had relatively poor photocatalytic activities.
Fig. 10. Photocatalytic activity of (a) no catalyst, (b) TNC-500, (c) TNC-600, (d) TNC-700, e) TNC-800, (f) TNC-900, and (g) P25
The discrepancy of the photocatalytic properties between the different samples could be because of the following reasons. First, the BET surface area of these samples played a necessary role in catalytic decomposition, which strongly fluctuated with the different calcination temperatures. The SBET values of the TNC-500 samples were 48.46 m2/g, which were the highest of all of the samples, and the SBET of the TNC-700 sample was 26.03 m2/g. The SBET of the samples decreased as the calcination temperature increased above 700 °C. As Table 1 showed, the SBET values of the TNC-800 and TNC-900 samples were 21.26 and 17.52 m2/g, respectively. The different effect of the photocatalysts could be explained by the discrepancy of the SBET between the five samples. It should be noted that the TNC-700 samples showed the highest photocatalytic degradation rate for MB. However, the SBET values of the TNC-700 samples were the second highest among all of the samples. That was because the TNC-700 sample had a bicrystal phase of anatase and rutile, which was demonstrated in the XRD patterns (Fig. 5). As the calcination temperature increased, the mixed-phase photocatalysts also underwent cellulose carbonization. The difference between the conduction band (CB) edges of the two phases could have facilitated irreversible charge transfer from one phase to the other. The charge transfer contributed to the reduction of the chance for photogenerated electrons and holes recombination. Such as, the TNC-700 samples showed superior photoactivity. Usually, a decrease of the SBET was ascribed to the aggregation of TiO2 nanoparticles, which was confirmed by the SEM images (Fig. 4). Figure 4 shows that the TNC-700 samples were nanofiber-shaped and had superior dispersion. The TNC-700 samples showed the highest photocatalytic activity, which was due to the appropriate mixed phase of TiO2, photogenerated electrons quickly transforming from rutile to anatase (rutile antenna mechanism), and efficient electron-hole pairs separation. However, when the calcination temperature was up to 800 and 900 °C, the photocatalytic degradation rate was a little low. The results could be ascribed to the larger percentage of TiO2 rutile phase and lower SBET of the samples, and further confirmed that the TNC-700 samples possessed a superior photocatalytic activity compared with the other photocatalysts.
The synergistic effect of the TiO2 and carbon fiber for the TiO2-wrapped nanofiber hybrid materials contributed to the noticeable enhancement of the photocatalytic activity, as well as its unique structure and properties. The unique porous structure of the TiO2-wrapped nanofiber hybrid materials had the exclusive advantage of preventing the clustering of TiO2nanoparticles, and guaranteed a high specific surface area and strong adsorption capability. Moreover, the hydroxyl groups on the surface of the TiO2, the existence of which were shown by FTIR spectroscopy, not only improved the adsorption of MB molecules (Anderson and Bard 1997; Ang et al. 2009), but also retarded the photogenerated electron-hole pairs recombination process by a hole-trapping effect, and enhanced photocatalytic oxidation. The porous active carbon nanofiber, which acted as a catalyst support, concentrated the target substrate onto its surface, enhanced the adsorption capacity, and provided a substrate-rich environment on the interface of the carbon/TiO2 nanocrystal composite catalysts. The adsorbed MB molecules were decomposed afterwards.
When the light energy was equal to or greater than the band gap energies (hʋ), the electrons could be excited from the valence band (VB) to the CB of the catalyst, and the electron-hole pairs (e–/h+) of the TiO2 photocatalyst were generated. The photogenerated positive holes (h+) reacted with hydroxyl groups (-OH) and/or adsorbed water molecules to generate hydroxyl radicals (•OH), which acted as strong oxidizing agents during the photocatalytic reaction, and then the potential hydroxyl radicals (•OH) rapidly reacted with MB. Moreover, the photogenerated electrons (e–) reacted with electron acceptors, such as O2 adsorbed onto the surface of the catalyst or dissolved in water, to produce the superoxide radical anions O2• and •HO2. The •OH radicals were also utilized for the oxidation of MB. Additionally, the •HO2 radicals reacted with each other, resulting in the formation of hydrogen peroxide and increasing the gaseous oxygen in the photocatalytic reaction. Because of the high oxidizing potential of the hydroxyl, superoxide, and •HO2 radicals, the radicals could react with organic pollutants to produce degradation products. An ordinary simplified scheme of the photocatalytic process was given by Eqs. 1 to 10. Both water and gaseous oxygen played an important part during the photocatalytic reaction, as could be seen by the following equations:
CONCLUSIONS
- TiO2-wrapped nanofiber hybrid materials, which possessed a unique porous structure and mixed crystalline phase, were synthesized by the combination of surface sol–gel and post-calcination processes. By means of the surface sol–gel process, TiO2 was deposited on the surface of each nanocellulose fiber, and then the TiO2-wrapped nanofiber hybrid materials were calcined at different temperatures under nitrogen atmosphere.
- The morphology, crystalline phase, specific surface area, and chemical composition of the novel photocatalysts were characterized by SEM, TEM, XRD, FTIR, TG-DTA, BET, and XPS. With the rising of the calcinations temperature, the original cellulose nanofibers were converted into porous carbon nanofibers, which were wrapped by TiO2 and accompanied by a crystal transformation.
- The photocatalytic activity of the new catalysts was evaluated by the degradation of MB under UV irradiation. The results demonstrated that the new catalysts showed better photocatalytic ability, and the photocatalysis degradation effect under UV light irradiation was in the order of TNC-700, TNC-600, TNC-500, P25, TNC-800 and TNC-900 samples from high to low. The TNC-700 catalysts showed a superior photocatalytic ability compared with the other catalysts. The high photocatalytic activity of the TNC-700 samples could be due to the three following crucial factors: the unique appropriate porous structure, high specific surface area and mixed crystalline phase with proper anatase and rutiles. The synergistic photocatalytic effect of the TiO2 and carbon nanofibers favored the degradation of organic pollutants in water or air.
ACKNOWLEDGMENTS
The authors acknowledge the financial support provided by the public technology research plan of Zhejiang Province (2015C33053), Natural Science Foundation of Zhejiang Province (No. LY13B030009), Science Foundation of Zhejiang Sci-Tech University (ZSTU) (No. 1101820-Y), and National Natural Science Foundation of China (No. 21271155 and 20703038).
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Article submitted: April 4, 2017; Peer review completed: June 10, 2017; Revised version received and accepted: June 24, 2017; Published: July 11, 2017.
DOI: 10.15376/biores.12.3.6062-6081