TiO2 nanoparticles loaded on graphene/carbon composite ...

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TiO2 nanoparticles loaded on graphene/carbon composite nanofibers by electrospinning for increased photocatalysis Chang Hyo Kim a, Bo-Hye Kim a b c

b,* ,

Kap Seung Yang

b,c,**

Department of Advanced Chemicals & Engineering, Chonnam National University, 300 Yongbong-dong, Gwangju, Republic of Korea Alan G. MacDiarmid Energy Research Institute, Chonnam National University, 300 Yongbong-dong, Gwangju, Republic of Korea Department of Polymer & Fiber System Engineering, Chonnam National University, 300 Yongbong-dong, Gwangju, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

Graphene/carbon composite nanofibers (CCNFs) with attached TiO2 nanoparticles (TiO2–

Received 29 October 2011

CCNF) were prepared, and their photocatalytic degradation ability under visible light irra-

Accepted 26 January 2012

diation was assessed. They were characterized using scanning and transmission electron

Available online 4 February 2012

microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible diffuse spectroscopy. The results suggest that the presence of graphene embedded in the composite fibers prevents TiO2 particle agglomeration and aids the uniform dispersion of TiO2 on the fibers. In the photodegradation of methylene blue, a significant increase in the reaction rate was observed with TiO2–CCNF materials under visible light. This increase is due to the high migration efficiency of photoinduced electrons and the inhibition of charge–carrier recombination due to the electronic interaction between TiO2 and graphene. The TiO2–CCNF materials could be used for multiple degradation cycles without a decrease in photocatalytic activity.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

TiO2 is one of the most promising catalysts because of its superior photocatalytic performance, easy availability, long-term stability, and nontoxicity [1,2]. Typically, photoexcited electron–hole pairs can be generated by irradiation with light with an energy greater than the band gap energy of TiO2 (Ebg = 3.2 eV for anatase). However, several problems can arise when TiO2 is applied as a photocatalyst: (1) photogenerated electron–hole pairs can recombine quickly, which affects the photocatalytic efficiency [3–7], and (2) TiO2 can only be excited with ultraviolet (UV) light, which is less than 5% of solar light, because of its wide band gap [7–9]. These disadvantages of TiO2 result in a low photocatalytic activity in practical application. Therefore, a number of recent studies have focused on the

preparation and modification of TiO2 to narrow its band gap and enhancethephotocatalyticactivityundervisiblelightradiation[10–30]. Among the carbon nanostructures (e.g., C60, carbon nanotubes, and graphenes), graphenes offer new opportunities in photovoltaic conversion and photocatalysis by the hybrid structures with a variety of nanomaterials, due to their excellent charge carrier mobility, a large specific surface area, and good electrical conductivity [31–36]. For example, TiO2 combined with graphene acts as an electron trap, which promotes electron–hole separation and facilitates interfacial electron transfer [14,24–26,30]. Furthermore, graphene can help control the morphology of TiO2 nanoparticles because it controls nucleation and growth of TiO2 nanoparticles and allows for optimal chemical interactions and bonding between nanoparticles and graphene.

* Corresponding author at : Alan G. MacDiarmid Energy Research Institute, Chonnam National University, Yong-Bong dong 300, 500-757 Gwangju, Republic of Korea. Tel.: +82 62 530 0774; fax: +82 62 530 1779. ** Corresponding author at : Department of Polymer&Fiber System Engineering, Chonnam National University, Yong-Bong dong 300, 500-757 Gwangju, Republic of Korea. Tel.: +82 62 530 0774; fax: +82 62 530 1779. E-mail addresses: [email protected] (B.-H. Kim), [email protected] (K.S. Yang). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2012.01.069

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In this work, carbon nanofibers (CNFs) containing micropores were used to support TiO2 because CNFs exhibit a high adsorption capability and adsorption rate due to the shallow and uniform pore structure. Shallow micropores open directly to the surface, which results in a large adsorption capacity and fast adsorption/desorption [37,38]. In addition, electrospinning has been widely used as a versatile technique to fabricate various hybrid nanofibers [39,40]. However, reports have seldom been published on the fabrication and evaluation of TiO2 nanoparticles loaded on graphene/carbon composite nanofibers (TiO2–CCNF) produced by electrospinning techniques. In the present work, TiO2–CCNF catalysts were prepared by electrospinning to determine if they exhibit improved photocatalytic efficiency in the visible spectrum, as illustrated in Fig. 1. By increasing the photocatalytic activity of the TiO2– CCNF materials, the current study aims to (i) extend the light absorption spectrum into the visible region, (ii) reduce electron/hole pair recombination, and (iii) enhance the adsorption capability and high adsorption/desorption rate of organic pollutants, which increases the reaction efficiency because of the high specific surface area of the CNFs.

2.

Experimental

2.1.

Materials and methods

The graphenes used in this study were xGNP-C750-grade materials produced by XG Science, USA. The elementary analysis of graphene was characterized as 88.68% carbon, 0.79% hydrogen, 1.11% nitrogen, and 7.65% oxygen using the Mettler method (Metler-Toledo AG, Switzerland). The graphene with functional group can be dispersed in organic solvent. The 3 wt.% grpahene (3 wt.% relative to PAN) sample was immersed in dimethylformamide (DMF) and sonicated in a bath-type sonicator. The PAN (Mw = 150,000, Aldrich Chemical Co.) was dissolved in the homogeneous graphene-DMF solution. This mixture was continuously stirred at 60 C until a homogeneous solution formed, and it was then cooled to room temperature. This solution was then spun into nanofiber webs using an electrospinning apparatus (NTPS-35K, Ntsse Co., Korea) operating at 25 kV. Spinning solutions were fed through a capillary tip (diameter = 0.5 mm) using a syringe (30 ml). The anode of the high voltage power supply was clamped to a syringe needle tip and the cathode was connected to a metal collector. During electrospinning, the dis-

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tance between the tip and collector was 17 cm, and the flow rate of the spinning solution was 3 ml/h. The electrospun fibers were collected on aluminum foil wrapped on a metal drum rotating at approximately 300 rpm. The electrospun fiber webs were stabilized in flowing air at 280 C with a heating rate of 1 C/min for 1 h. A 30 mL ethanol solution containing 2.04 g of titanium n-butoxide (Ti(OnBu)4, STREM CHEMICALS) was diluted with 30 mL of toluene. The stabilized nanofibers (SNF) were dipped in the prepared sol solution, washed sufficiently with ethanol and water, dried in air, and carbonized at 800 C with a heating rate of 5 C/min in N2 atmosphere with a flow rate of 200 mL/min. This sample was identified as TiO2–CCNF. To compare the photocatalytic activity under visible light irradiation, the TiO2 loaded on PAN based carbon nanofiber (TiO2–CNF), Graphene/carbon composite nanofibers (CCNF), and PAN based carbon nanofiber (CNF) samples were synthesized.

2.2.

Characterization

The surface structures of the TiO2–CCNF materials were characterized by a Hitachi S-4700 field-emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectroscopy (EDX). The transmission electron micrographs (TEM) and the selected area electron diffraction (SAED) micrographs were taken with a TECHNAI-F20 unit (KJ111, Phillips) operating at 200 kV. The crystalline structure was characterized with an X-ray diffraction unit (XRD, D-Max-2400 diffractometer) equipped with graphite monochromatized CuKa radiation (k = 0.15418 nm). Backscattering Raman measurements were carried out with a Renishaw in via-Reflex using 633 nm at room temperature. The chemical state of the surface was examined by X-ray photoelectron spectroscopy (XPS) using a VGScientific ESCALAB250 spectrometer equipped with a monochromatized AlK X-ray source (15 mA, 14 kV). The surface functionalities of the copolymer were examined by Fourier Transform Infrared spectroscopy (FT-IR, Nicolet 200 instrument). All the samples were analyzed using the KBr pellet technique and scanned in the range from 4000 to 400 cm 1 . Specific surface areas were analyzed by the Brunauer– Emmett–Teller (BET) method using a surface area analyzer (Micrometrics, ASPS2020, USA). To observe the optical properties of the samples, the photoluminescence (PL) emission spectra (Laser Raman and PL spectrometer, SPEX1403; 325 nm (55 mW) He–Cd Laser) were measured which give information about the recombination rate of photogenerated

Fig. 1 – TiO2–CCNF hybrids and its response under visible irradiation.

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carriers. The porosity was investigated from the nitrogen adsorption isotherm at 77 K (ASAP 2020, Micromeritics, USA). The specific surface area, the mesopore size distribution, and the micropore size distribution of the samples were evaluated using the Brunauer–Emmett–Teller (BET) method and the Barrett–Joyner–Halenda (BJH) method. Photocatalytic efficiencies were evaluated by decolorizing an aqueous solution of methylene blue (MB) under irradiation by a fluorescent lamp. A 0.1 g sample of the photocatalyst powder was dispersed in 100 mL of 15 ppm MB (Aldrich) solution by stirring with a magnetic stirrer. A 13 W fluorescent lamp (FRX 13EXD) with light output in the 400–800 nm spectrum range was used as the visible light source. The MB concentration was measured every 30 min using a UV–Visible spectrophotometer (Shimadzu 160-A).

3.

Results and discussion

Fig. 2 presents SEM images of TiO2–CCNF and TiO2–CNF hybrids prepared with and without graphene. The electrospun PAN based- and PAN/graphene based nanofibers showed a smooth surface with an average diameter of 180 and 270 nm, respectively, before coating of TiO2. It was observed that the nanoparticles were uniformly distributed across the surface of the TiO2–CCNF materials (Fig. 2a) without aggregation, whereas the TiO2 on pure PAN/based fibers (TiO2–CNF) consisted of large, aggregated nanoparticles with diameters in the range of 80–110 nm (Fig. 2b). The graphene, functionalized with carboxylic acid groups, is capable of binding to metal oxide particles, such as TiO2, without aggregation effects and yields small nanparticles [14]. Goncalves et al. [41]. have shown that oxygen functionalities at the graphene surface

act as reactive sites for the nucleation and growth of gold nanoparticles. They observed that the nucleation and growth of these metal nanoparticles were dependent on the density of oxygen functional groups in the graphene surface sheets [42]. TiO2 nanoparticles directly grown on graphene appeared to exhibit strong interactions, which should lead to advanced hybrid materials for various applications, including photocatalysis. TEM images, depicted in Fig. 3a, were collected to obtain microstructural information for the TiO2–CCNF materials. The images indicated that TiO2 nanoparticles, with an average particle size of 10–30 nm, had been successfully grown on the surface of CNFs. SAED measurements (Fig. 3b) confirmed the nanocrystalline nature of the investigated TiO2 samples. Based on the SAED ring pattern shown in Fig. 3b, the clear lattice fringes indicate that the nanoparticles have some crystallinity. In addition, the SAED patterns indicate that the nanoparticles, identified as TiO2 rutile structures, reflected from the (1 1 0), (2 1 1), and (1 0 1) lattice planes as indexed in Fig. 3b. The corresponding elemental mapping of an individual TiO2–CCNF materials provided information on the distribution of Ti, O, and C atoms in the fiber (Fig. 3c). Fig. 4 presents the XRD patterns of the TiO2–CCNF and TiO2/CNF materials. The broad peak located between 20 and 30 was attributed to the (0 0 2) plane of the carbon structure. The diffraction peaks of TiO2 were also clearly observed in Fig. 4a and b. The peak locations for TiO2 are cited from the Joint Committee on Powder Diffraction Standards (JCPDS) database. The peaks located at 25 Correspond to the (1 0 1) plane of the anatase phase (JCPDS 21-1272), and the peaks located at 28o, 36o, 41o, and 54 Correspond to the (1 1 0), (1 0 1), (1 1 1), and (2 1 1) planes of the rutile phase (JCPDS 21-1276),

Fig. 2 – SEM images of (a) electrospun PAN based nanofiber, (b) electrospun PAN/graphene based nanofiber, (c) TiO2–CNF, (d) TiO2–CCNF.

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Fig. 3 – (a) TEM image of the TiO2–CCNF hybrids. (b) SAED patterns of the TiO2 nanostructures. (c) Elemental mapping of the individual TiO2–CCNF hybrids.

Fig. 4 – XRD patterns of the (a) TiO2–CCNF and (b) TiO2/CNF.

respectively [43]. All products were confirmed to be a mixture of the anatase and rutile phases. The anatase phase content for all products was calculated from the XRD patterns using the following equation: Xa = [1 + 1.26(Ir/Ia)] 1, where Xa is the share of anatase in the mixture and Ir and Ia are the integrated intensities of the (1 0 1) reflection of anatase and the (1 1 0) reflection of rutile, respectively. TiO2–CCNF has a 1:1 ratio of anatase to rutile, whereas the ratio for TiO2–CNF is 1:4. Therefore, this result indicates that the addition of graphene to TiO2 can effectively suppress the transformation of TiO2 from an anatase to a rutile-type phase and prevents the growth of TiO2 particles. The Raman spectrum (Fig. 5) indicates the presence of TiO2 crystalline nanoparticles and showed three Raman bands: 144(B1g) for anatase and 431(Eg)/612(A1g) for the rutile structure ranging between 100 and 800 cm 1 [44]. Fig. 5 shows that the

two structural types of TiO2 coexist, which is consistent with the reported XRD pattern. In addition to different TiO2 modes, the broad D-band (defect-induced mode) at 1340–1360 cm 1 and G band (E2g2 graphite mode) at 1570–1590 cm 1 were observed. The intensity ratio of D band to G band (ID/IG) is proposed to be an indication of disorder in TiO2–CCNF, TiO2–CNF, and CNF, originating from defects associated with vacancies, grain boundaries, and amorphous carbons. It is seen that there is little change in the ID/IG of TiO2–CCNF (1.0547) compared with TiO2–CNF (1.0551) and CNF (1.0522). This result seems to indicate that graphitic (sp2) nature of the carbon in the photocatalysts remained the same after loading with TiO2. XPS analysis can provide valuable insight into the surface structure of the CCNF-supported TiO2 photocatalyst. The Ti2p photoelectron peaks of TiO2–CCNF materials are presented in Fig. 6a. The peaks at 465.23 and 459.62 eV represent the Ti2p1/2

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Fig. 5 – Raman spectrum of TiO2–CCNF hybrids.

and Ti2p3/2 of Ti4+, respectively, and the Ti3+ peaks at 461.90 eV (Ti2p1/2) and 458.58 eV (Ti2p3/2) were also observed. However, in the case of TiO2–CNF materials (Fig. 6b), the peaks of Ti2p1/2 and Ti2p3/2 of Ti4+ were present, whereas the peaks of Ti3+ in Ti2O3 or TiO were barely observed. The Ti3+ species were generated for the TiO2/CCNFs, which might be due to the carboxyl groups by graphene. Some of these groups combine with titanium through chelation, and then the positions of the oxygen in TiO2 were occupied by carboxyl groups. The chelation effect reduce the valence of titanium, whereas increase the content of Ti3+ [46]. Generally, the Ti3+ state on the TiO2 surface plays a dominant role in the photocatalytic activity because it can trap photogenerated electrons and leave behind unpaired charges to promote photoactivity. Therefore, the increasing Ti3+ density promotes effective segregation of electron and cavity, interface charge transfer as well as lowered the probability of compounding cavity, and then increases the photocatalysis performance. Fig.7 shows the high-resolution XPS spectra of the C1s and O1s region on the surface of TiO2–CCNF. The deconvolution of the C1s XPS spectrum (Fig. 7a) revealed three Gaussian curves

centered at 285.0, 286.3, 287.4, and 289.1 eV, which can be assigned to graphitic carbon (C–C and CHn), alcohol or ether carbon (C–OH, C–O–C), carbonyl groups (C@O), and carboxyl or ester carbon (O@C–O), respectively. The O1s core level spectrum (Fig. 7b) was deconvoluted into two peaks, indicating the presence of C@O or Ti–O bond of TiO2 (530.5 eV) and C–O in ether (O2 , 532.2 eV) [46]. Atomic ratios of titanium to oxygen for TiO2–CCNF and TiO2–CNF can be calculated through the rough stoichiometry using the XPS data. The Ti:O ratios of TiO2–CCNF and TiO2–CNF were determined to be 1:2.63 and 1:2.50, respectively. However, it has been supposed that the titanium-oxygen ratio is less than 1:2. There is an overall increase in the Ti:O ratio, which is consistent with the increase in the proportion of oxygen in the photocatalysts. It means that there may be more oxygen species such as carboxylic acid group of graphene in the TiO2–CCNF photocatalyst surface besides the crystal lattice oxygen [46,47]. The FT-IR spectrum of the TiO2–CCNF was reported in Fig. 8. We can observe absorption at 3439.08 cm 1 (OH broadened band of either alcoholic or hydroxyl groups), 1612.49/ 1454.33 cm 1 (C@C and C–C stretching bands in the aromatic range, carbonyl), and 1000–1300 cm 1 (C–O stretching band of ether). In particular, the intensity of SNF is larger than that of TiO2–CCNF, which represent indirectly that TiO2 nanoparticles are grown on surface of CCNF with the aid of the oxygen functionalities as reactive sites at the graphene surface. Hence, the XPS and FT-IR spectra show the presence and decrease of the oxygen functional groups, such as –C–O–, –C@O, and –O–C@O, on the TiO2–CCNF catalyst after carbonization. The photoluminescence (PL) emission spectroscopy has been widely used to study the transfer behavior of the photogenerated electrons and holes in semiconductor materials, so that it can reflect the separation and recombination of photogenerated carries. [48,49]. The measured PL-emission spectra of TiO2–CCNF, TiO2–CNF, and CNF in the range of 300–850 nm are presented in Fig. 9. The spectra are broad (extending from 400 to 700 nm) and centered around 485 nm (2.56 eV) for TiO2– CCNF and 497 nm (2.50 eV) for TiO2–CNF. The intensity of the PL-spectra decreased in the following order for the photocatalysts: CNF, TiO2–CNF, and TiO2–CCNF; this result indicates that the reduction of the PL-intensity shows the diminution

Fig. 6 – XPS spectra of the deconvoluted Ti(2p) peaks for the (a)TiO2–CCNF and (b) TiO2–CNF materials.

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Fig. 7 – Deconvolution of (a) C1s, (b) O1s core levels of TiO2–CCNF according to XPS spectra.

Fig. 8 – FT-IR spectra of the TiO2–CCNF and SNF.

of the electron/hole pairs recombination process. Generally, the lower PL intensity suggests the lower the recombination rate of photogenerated electron–hole pairs, which leads to the high the photocatalytic activity of semiconductor

photocatalysts [50]. Therefore, the low PL intensity for TiO2– CCNF indicates that the photocatalytic activities of TiO2 may be improved due to the interactions between the excited electron of TiO2 particles and the graphene. The photocatalytic activities of the TiO2–CCNF, TiO2–CNF, CCNF, and CNFs were evaluated by measuring the decoloration rate of MB under visible light irradiation (Fig. 10) at 18 C. The experimental results showed that the MB decomposition rate in the samples occurred in the following order: TiO2–CCNF > TiO2–CNF  CCNF > CNF. The concentration reduced rapidly over the first 30 min, and then the decomposition rate slowed down or stopped after the amount of time for all of the samples. As seen in Fig. 10a, the TiO2–CCNF showed the best photocatalytic activity under visible light irradiation. The use of the TiO2–CCNF catalyst yielded nearly a 100% reduction in the first 30 min of irradiation. The MB decomposition reaction followed pseudo-first-order kinetics, and the kinetic constants (k, min 1) were calculated and are presented in the inset in Fig. 10a. The superior photocatalytic activity of TiO2–CCNF materials was also demonstrated in the degradation of MB. The average rate constant for the TiO2–CCNF (k = 0.16 min 1) is much larger than those for TiO2–CNF (k = 0.012 min 1), CCNF (k = 0.011 min 1), and CNFs

Fig. 9 – Photoluminescence spectra of (a) TiO2–CCNF, (b) TiO2–CNF, (c) CNF.

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Fig. 10 – (a) Photocatalytic degradation of MB monitored as the concentration change versus irradiation time (inset: average reaction rate constant (min 1) for the photodegradation of MB with free TiO2). (b) Photocatalytic activity of the TiO2–CCNF hybrids for MB degradation over five degradation cycles.

(k = 0.003 min 1). Additionally, the stability of the TiO2–CCNF materials was examined by following the degradation of MB during a five-cycle experiment. After each run, the TiO2–CCNF photocatalyst was evacuated for 30 min and was reused in the next run. As shown in Fig. 10b, the photocatalytic degradation of MB with TiO2–CCNF phtocatalysts under visible light irradiation was consistently effective over five degradation cycles. In general, the efficiency of the photodegradation by TiO2 depends on the illuminated catalyst surface area in contact with the solution and on the mass transfer to the catalyst surface [37,38]. Nitrogen adsorption isotherms and pore size distributions for TiO2–CCNF, TiO2–CNF, and CNF are shown in Fig. 11. The adsorption isotherms (Fig. 11a) of TiO2–CNF and CNF show typical type I behavior representing the microporous adsorption, and the adsorption of nitrogen was nearly complete at a low relative pressure (P/P0 < 0.1). The adsorption isotherms of TiO2–CCNF exhibit combined type I and type II characteristics. Hysteresis at a relative pressure higher than

P/P0 = 0.5 was observed, which was typical type II behavior of mesoporous adsorption and micropore filling were observed at a low relative pressure of P/P0 < 0.1, which was typical type I behavior [51]. The pore size distribution in TiO2–CCNF, TiO2–CNF, and CNF samples based on the BJH method show a broad distribution of mesopores, with sizes ranging from 2–50 nm (Fig. 11b). In particular, TiO2–CCNF sample has a much larger pore volume around 3.8 nm than the other samples, suggesting that the presence of mesopores can lead to adsorption of MB molecular easily. Because the molecular size of MB is 1.36 · 0.47 · 0.24 nm [20], and it can access pores with diameters larger than 1.5 nm [52]. Furthermore, as shown in Table 1, the pore characteristics of the photocatalysts decreased in the following order: TiO2–CCNF, CNF, and TiO2–CNF; this result indicates that the exposed CNFs surface with high surface area will be functioned as centers of condensing substrates with a physical adsorption process. Therefore, rapid decomposition of MB in the

Fig. 11 – (a) Nitrogen adsorption isotherms at 77 K, (b) Differential pore volume of various photocatalysts as a function of the pore diameter.

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Table 1 – Pore characteristicsa of the photocatalysts used in this study. Samples

TiO2–CCNF TiO2–CNF CCNF CNF a

BET surface area (m2 g 1) 434 361 447 405

Total pore volume (cm3 g 1)

Average pore diameter (nm)

0.23 0.15 0.17 0.18

2.17 1.60 1.58 1.81

Pore parameters were obtained from N2 adsorption.

Fig. 12 – Adsorption behavior of MB on TiO2–CCNF and TiO2– CNF.

beginning is reasonably supposed to be due to further adsorption of MB into CNF layer, and the following gradual change in

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color could be due to consequential processes of adsorption into CNF layer and then decomposition on the surface of TiO2 particle. As CNFs and graphene of photocatalysts have some adsorption capacity for MB, as shown in Fig. 11 and Table 1, a comparison between adsorption and photocatalytic degradation of MB was carried out experimentally with and without visible light irradiation to evaluate the actual photocatalytic activity. Under the same conditions, adsorption of TiO2–CCNF and TiO2–CNF to MB in the dark was also tested and the results are shown in Fig. 12. When the catalyst is illuminated by visible light irradiation, the MB concentration is much higher decreased than that without visible light irradiation, indicating that MB is rapidly decomposed by the TiO2–CCNF catalysts photocatalytically with visible light irradiation. The results indicate that TiO2–CCNF has a higher efficiency for the decomposition of MB than the TiO2–CNF, CCNF, and CNFs. It was confirmed that the introduction of graphene and CNFs to the TiO2 photocatalyst could increase the decomposition rate of some organic compounds using a photocatalytic process. The reasons for these improvements are attributed to the following functions, as shown in Fig. 13. (i) The graphene with carboxylic acid groups is capable of binding TiO2 without aggregation effects, resulting in small nanoparticles. As the particle size of TiO2 becomes small, the photogenerated electrons and holes can easily migrate to reaction sites on the surface, thereby reducing the recombination probability [13,45]. (ii) In the TiO2–CCNF materials with a mixture of anatase and rutile structures, the graphene may act as an electron acceptor (Fig. 13a) for rutile (Ebg = 3.0 eV) and as a photosensitizer (Fig. 13b) for anatase (Ebg = 3.2 eV), which prevents the recombination of electron–hole

Fig. 13 – Schematic illustration of photocatalysis with TiO2–CCNF hybrids.

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pairs and promotes photodegradation [13]. This phenomenon may be responsible for extending photocatalytic activity into the visible light range. (iii) A large surface area is necessary for the photocatalytic degradation of organic compounds because adsorption of the organic compound is an important process. The exposed fibers with high-surface-area TiO2–CCNF materials will function as centers of substrate condensation in the physical adsorption process. There the peculiar properties of CNFs are expected to elevate the photocatalytic activities of TiO2.

[8]

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4.

Conclusion

Small TiO2 nanoparticles were successfully deposited on electrospun CCNFs by the sol–gel method, and these composites were very active photocatalysts in the photodegradation of MB under visible light irradiation. Furthermore, TiO2–CCNF materials could be recycled without a decrease in the photocatalytic activity. The results suggest that graphene acts as an electron acceptor and a photosensitizer, which causes an increase in the photodegradation rate and reduces electron– hole pair recombination. In addition, the CNFs’ high surface area synergistically improved the photocatalytic activity of TiO2 by enhancing the physical adsorption of the substrate. Therefore, TiO2–CCNF materials are expected to be a promising candidate for photocatalytic processes using sunlight irradiation.

Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2011-0005890) and the Ministry of Education, Science and Technology (MEST) (K20903002024-11E010004010).

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