Growth of TiO₂ nanorod bundles on carbon fibers as flexible and weaveable photocatalyst/photoelectrode

Abstract

Semiconductor photocatalysis technology has great potential to become one of the most promising solutions for energy shortages and environmental pollution, and a prerequisite for the photocatalytic application is to obtain efficient, easily recyclable and large-area photocatalysts with nanostructures. In the present work, we have prepared TiO₂ nanorod bundles on carbon fibers as flexible and weaveable photocatalyst/photoelectrode. The growth of TiO₂ nanorod bundles is realized by using a dip-coating and hydrothermal growth method. TiO₂ nanorod-bundles exhibit square-column appearance with size of about 340–400 nm and length of ∼6 μm, and they are in fact composed of small nanorods with diameters of ∼30 nm. Subsequently, 16 bunches of CFs with TiO₂ nanorod bundles can be weaved to be a macroscale CFs/TiO₂ cloth (weight: 0.2 g, total area: ∼35 cm²) with excellent conductivity and flexibility. With CFs/TiO₂ cloth as the working electrode, photoelectrochemical measurements demonstrate that the separation of photo-induced charge carriers can be improved by increasing the applied voltage bias. Furthermore, under the illumination of simulated solar light, CFs/TiO₂ cloth can degrade 94.0% Rhodamine B (RhB) in 100 min by photoelectrocatalytic degradation process (bias: 0.6 V vs. SCE), which is higher than the efficiency from single photocatalysis (60.8% RhB) or electrocatalysis (5.6% RhB) process. In addition, CFs/TiO₂ cloth can be easily recycled with good performance stability. Therefore, this kind of CFs/TiO₂ cloth can be used as a promising, easily recyclable and large-area photocatalyst/photoelectrode in practical application (such as degrading organic pollutants in lake and/or river). More importantly, this work provides some insights into the design of efficient and macroscale photocatalyst/photoelectrode with other nanosized semiconductor for enhancing visible-light-driven photocatalytic activity.

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1. Introduction

In recent years, energy shortages and environmental pollution have become the bottleneck of social and economic development in the world. As one of the most promising solutions for both problems, semiconductor photocatalysis has attracted increasing interest, since it is a “green” and energy-saving technology for degrading organic contaminants and inactivating viruses in water/air as well as decomposing water into hydrogen and oxygen, under the irradiation of solar light at ambient conditions.^1–3 A key for the photocatalysis technology is to develop semiconductor photocatalysts with excellent performances.^1–3 Taking the macroscale shape of photocatalysts into account, three kinds of photocatalysts have been chiefly developed, resulting in great contributions to the progress in photocatalysis fields. The first kind is the nanosized semiconductors as powdery photocatalysts, including nanoparticles,^4,5 nanobelts,^6,7 nanosheets,^8–10 and nanocomposites.^11–13 We have also prepared some nanosized heterojunction photocatalysts (such as AgBr–Ag–Bi₂WO₆,¹⁴ Bi₂WO₆–Bi₂O₃,¹⁵ and Bi₂S₃/Bi₂MoO₆ (ref. 16)) and summarized their recent progress.¹ Usually, nanosized photocatalysts exhibit relatively high photocatalytic activity, due to their small size and large specific surface area. Unfortunately, their small size and powdery appearance also result in some problems. For example, it is difficult to recycle them in practical application (such as degrading organic pollutants in lake and/or river) and/or to enhance the photocatalytic activity by external electric field. The second kind is semiconductor films on flat substrates, such as Bi₂WO₆ film¹⁷/ZnO nanorods¹⁸ on ITO glass, and TiO₂ nanotubes on Ti foil.^19,20 These film-shaped photocatalysts can be easily recycled, and their photocatalytic activities can also be further improved by external electric field. However, there are also some limitations, such as relatively limited surface area and/or high cost. The last kind is semiconductor nonwoven cloth with nanostructure, such as Bi₂WO₆,²¹ TiO₂–SnO₂,²² TiO₂–CuO.²³ Recently, our group has developed Ta₃N₅/Pt²⁴ and Fe₂O₃/AgBr²⁵ nonwoven cloths (area: ∼9 or 1 cm²) as efficient and easily recyclable macroscale photocatalysts with nanostructure. It should be noted that these semiconductor nonwoven cloths are relatively easily fragile and thus it is difficult to prepare cloth with big-area (>1 m²). In addition, they also have poor electrical conductivity and thus cannot be directly used as photoelectrodes for the enhanced photoelectrocatalytic degradation of pollutants in water. Therefore, it is quite necessary to develop efficient, easily recyclable and large-area photocatalysts with nanostructures and good conductivity.

Recently, the growth/coating of nanosized semiconductors on conductive fibers has drawn much attention,^26–34 since the resulting materials combine many advantages from these materials, such as good conductivity, flexibility, stitchability and macroscale size from fibers, and large surface area and tuned functionalities from nanosized semiconductors. These conductive fibers decorated with nanosized semiconductors have been used to construct many kinds of fiber-shaped devices, including Li-ions batteries,^26,27 supercapacitors,^28,29 solar cells,^30,31 nanogenerators,^32,33 and photocatalysts.³⁴ Furthermore, these fiber-shaped devices can be weaved to fabric devices with controlled structures, resulting in great potential in the applications to the development of wearable devices.^27,28,31 These features trigger our interest in the novel concept of developing flexible fiber-shaped and weaveable photocatalyst/photoelectrode with macroscale appearance but nanostructures.

It is well known that semiconductor TiO₂ has undoubtedly proven to be one of the most excellent photocatalysts,^2,3 and carbon fiber (CF) is the most famous flexible fiber with excellent conductivity, high strength, corrosion-resistance, and etc.³⁵ The growth of TiO₂ nanorods on CFs can be realized by a “dissolve-grow”³⁰ and in situ growth^33,34,36 hydrothermal methods, part of which exhibited improved photocatalytic activities.^33,34 To further optimize TiO₂ performances and to construct the flexible and weaveable photocatalyst/photoelectrode with nanostructures, herein we report the growth of TiO₂ nanorod bundles on CFs by a modified dip-coating and hydrothermal growth two-step method. TiO₂ nanorod bundles have the square-column appearance with size of about 340–400 nm and length of ∼6 μm, and they are in fact composed of small nanorods with diameters of ∼30 nm. Furthermore, CFs with TiO₂ nanorods can be weaved to CFs/TiO₂ cloth (weight: 0.2 g, area: ∼35 cm²). The resulting macroscale cloth with nanostructure can photocatalytically degrade 60.8% Rhodamine B (RhB) in 100 min under the illumination of simulated solar light. More importantly, the photocatalytic activity of CFs/TiO₂ cloth can be easily improved by applying external electric field (bias: 0.6 V vs. SCE) through the more efficient photoelectrocatalytic degradation route, resulting in the degradation of 94.0% RhB in 100 min under the other identical conditions.

2. Experimental section

2.1 Materials

All chemicals were of analytical grade and commercially available. Tetrabutyl titanate (TBT) and titanium tetrachloride (TiCl₄) were purchased from Aladdin Industrial Corporation. Sodium sulfide (Na₂SO₄), hydrochloric acid (HCl), acetone, ethanol and Rhodamine B (RhB) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). The carbon fibers (CFs) were purchased from TOHO (UTS50-12K).

2.2 Growth of TiO₂ nanorod bundles on CFs

The growth of TiO₂ nanorod bundles on CFs was realized by using a modified dip-coating and hydrothermal growth two-step method.^30,33,34,36 To obtain TiO₂ seed layer, in the first dip-coating step, one bundle of CFs (length: 5–7 cm, weight: 95–103 mg) were ultrasonic cleaned in a mixture solution of deionized water, ethanol and acetone (v/v/v, 1 : 1 : 1) for 20 min. The cleaned and dried CFs were immersed in 0.1 M TiCl₄ aqueous solution for 10 min and then dried at ∼80 °C for 30 min. This step should be repeated twice to ensure the efficient coating. The dried as-prepared CFs were annealed in a muffle at 350 °C for 15 min. In the second step, TBT liquid (0.5 mL) was added into the mixture solution of concentrated HCl (15 mL) and deionized water (15 mL) under magnetically stirring. After stirring for 30 minutes, the above solution along with CFs coated with TiO₂ seed layer were moved into a 50 mL Teflon-lined stainless steel autoclave. Hydrothermal reaction was performed at 160 °C for 16 h, and the system was cooled to room temperature naturally. CFs coated with TiO₂ were taken out and washed by deionized water to remove the redundant ions. Then, they were dried in the oven at 50 °C for 12 h. With clean CFs as the substrate, this seed-growth/hydrothermal two-step process could be repeated several times to get more bundles of CFs with TiO₂. At last, these bundles of CFs with TiO₂ were reorganized and then weaved into a piece of cloth which contained 16 small bunches (8 × 8) of CFs with TiO₂.

2.3 Physical characterizations

The sizes and morphologies of the as-prepared samples were analyzed by a field emission scanning electron microscope (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM-2100F). The elemental composition and distribution were also investigated by using energy dispersive X-ray spectroscopy (EDS) on a Bruker Quantax 400 EDS system during SEM characterization. X-Ray diffraction (XRD) patterns were recorded on a Bruker D4 X-ray diffractometer by using Cu Kα radiation source (λ = 1.5418 Å). The optical diffuse reflectance spectrum was conducted on a UV-vis-NIR scanning spectrophotometer (UV3600, Shimadzu) using an integrating sphere accessory. The electric resistance of one bundle fiber was determined by using a source-meter.

2.4 Measurements of photocatalytic/photoelectrocatalytic activity

Photoelectrochemical measurements were performed on a standard three-electrode cell using a CHI660D electrochemical system (Chenhua Instruments, Co., Shanghai), in which CFs/TiO₂ cloth (weight: 0.2 g, total area: ∼35 cm²), a Pt sheet, and a standard calomel electrode (SCE) were served as the working electrode, counter electrode, and reference electrode, respectively. Na₂SO₄ aqueous solution (0.5 M, 60 mL) was used as the electrolyte and a 500 W Xe lamp (Model PLS-SXE300) was used at the light source. The current–voltage curves from the linear sweep voltammetry were recorded with the scan rate of 10 mV s⁻¹ in the absence or presence light illumination. In addition, the photoresponse was also measured at 0 V (vs. SCE) with a light switch (off: 0–40 s, and then on-50 s/off-50 s; 4 cycles). The effects of the applied voltage were investigated by recording the current with a light switch (off: 0–40 s, and then on-50 s/off-50 s) at different potentials (0, 0.1, 0.3 and 0.6 V vs. SCE).

Photoelectrocatalytic (PEC) activity of CFs/TiO₂ cloth was measured by the degradation of RhB as the model of the pollutant. RhB was added into Na₂SO₄ electrolyte (0.5 M, 60 mL) at a concentration of 5 × 10⁻⁶ mol L⁻¹. CFs/TiO₂ cloth (weight: ∼0.2 g, total area: ∼35 cm²), a Pt sheet, and a SCE were put into the electrolyte as the working electrode, counter electrode, and reference electrode, respectively. Prior to irradiation, the solution was magnetically stirred for 30 min in the dark to ensure adsorption/desorption equilibrium between target organic contaminant and the surface of the catalyst at room temperature. Subsequently, an external potential of 0.6 V was applied on the CFs/TiO₂ cloth. After that, a 500 W xenon lamp as the light source was turned on. When the remaining RhB concentration needed to be tested at given irradiation time intervals (per 20 min), 3 mL RhB solution was transferred to a quartz colorimetric utensil for measuring the absorbance by using a UV-vis spectrophotometer (Shimadzu UV-2550). For comparison, the photocatalytic (PC) activity was investigated by switching off the external potential, while the electrocatalytic (EC) activity was measured by turning off the xenon lamp, under the other identical conditions. In addition, EC, PC and PEC degradation efficiency of RhB by pure CF cloth (weight: ∼0.2 g, area: ∼35 cm²) were also measured under the other identical conditions.

In the stability and reusability test of CFs/TiO₂ cloth, four consecutive cycles of photoelectrocatalysis were tested under the simulated solar light illumination with the applied potential of 0.6 V. After each cycle, CFs/TiO₂ cloth was washed thoroughly with deionized water and then dried at 70 °C for 2 h. Then CFs/TiO₂ cloth was immersed in the same volume (60 mL) of fresh RhB solution again, under the other identical conditions.

3. Results and discussion

3.1 Fabrication and characterization of CFs/TiO₂ cloth

With flexible and conductive carbon fibers (CFs) as the substrate, CFs/TiO₂ cloth was prepared by using a three-step method, including dip-coating, hydrothermal growth and weaving steps, as demonstrated in Fig. 1. To facilitate the subsequent hydrothermal reaction in 50 mL autoclave, the black CFs were tailored to have the length of 5–7 cm (Fig. 2a), and they had smooth surface and the diameter of about 7 μm (Fig. 2b). In the first step (step 1 in Fig. 1), the clean CFs were immersed in TiCl₄ aqueous solution and then annealed in air at 350 °C for 15 min, resulting in the growth of dense TiO₂ seed layer on CF surface. SEM image (Fig. 2c) indicates that CFs with TiO₂ seed layer have the coarse surface, accompanying the formation of some carbon fragments during the anneal process.

Fig. 1 Schematic illustration of the preparation of CFs/TiO₂ cloth.

Fig. 2 (a and b) The photo and SEM image of pure and clean CFs, and (c) SEM image of CFs with TiO₂ seed layer.

In the second step (step 2 in Fig. 1), the growth of TiO₂ nanorod bundle array was performed by hydrothermally treating CFs with TiO₂ seed layer in a TBT aqueous acidic solution at 160 °C for 16 h. After this hydrothermal process, the surface color of CFs turned from black to grey-white, as vividly shown in Fig. 3a. Their surface and cross-sectional morphologies were investigated by SEM images (Fig. 3b–f). SEM image with low magnification (Fig. 3b) reveals that CFs have the coarse surface and there is an obvious and uniform coating on their surface. This uniform coating consists of densely nanorod-bundles (Fig. 3c). Furthermore, nanorod-bundles exhibit square-column appearance with size of about 340–400 nm, and they are in fact composed of small nanorods with diameters of ∼30 nm (Fig. 3d and e). Cross-sectional SEM image (Fig. 3f) further confirms that there is a high-density and orderly nanorod-bundle array on the surface of CF, and these nanorod-bundles have the length of about 6 μm. In addition, some nanorod bundles were peeled off from CFs, and their size and structure were further investigated by TEM images (Fig. 4a and b). TEM image (Fig. 4a) confirms that nanorod-bundles have the diameter of about 350 nm, and they consist of small nanorods with the diameter of about 30 nm as the building block (Fig. 4b), which agrees well with SEM images (Fig. 3). In addition, the high-resolution TEM image (Fig. 4c) reveals that the nanorod should be single crystal, and the lattice fringes are indicative of the good crystallinity. The distance between lattice fringes is measured to be 0.323 Å, which corresponds to the d spacing of (110) crystal plane of rutile TiO₂ (JCPDS Card no. 21-1276). The corresponding selected-area electron diffraction (SAED) pattern (Fig. 4d) further confirms the single-crystalline nature and can be indexed to the pure rutile TiO₂ phase.³⁰

Fig. 3 The photo (a) and surface/cross-section SEM images (b–f) of CFs with TiO₂ nanorod bundles.

Fig. 4 TEM image (a and b), HR-TEM image (c) and SAED pattern (d) of TiO₂ nanorods from CFs/TiO₂.

Subsequently, the composition, phase and optical property of these nanorod-bundles on CFs were further investigated. EDS pattern (Fig. 5a) reveals that besides C element from CFs, there are only O and Ti elements in the sample. Elemental mappings (Fig. 5b–e) further confirm that elemental O and Ti are also homogeneously distributed among nanorod-bundles on the fiber. In addition, XRD pattern (Fig. 6) indicates that the pure CFs have a weak and broad diffraction peak at 25°, which should be associated with (002) plane of the graphite structure. Compared with pure CFs, CFs with TiO₂ nanorod bundles exhibit strong and well-defined diffraction peaks. Six diffraction peaks (signed with ▼) at 27.48°, 36.12°, 41.28°, 54.36°, 56.68°, 69.04° are respectively corresponding to (110), (101), (111), (211), (220) and (301) crystal planes of rutile phase TiO₂ (JCPDS Card no. 21-1276), while diffraction peaks (signed with *) at 25.28° and 48.05° can be respectively assigned to (101) and (200) planes of anatase phase TiO₂ (JCPDS Card no. 21-1272). In addition, no characteristic peaks peculiar to other impurities are observed. Additionally, the optical property of the CFs with TiO₂ nanorod-bundles was studied by using UV-vis spectroscopy (Fig. 7). Its UV-visible diffuse reflection spectrum exhibits the short-wavelength absorption with the edge at ∼390 nm, which agrees with the reported value for the band gap (E_g = 3.2 eV) of bulk TiO₂.³⁷ Based on the above results, one can conclude that TiO₂ nanorod bundles have been well grown on the surface of CFs, and they are the mixture of rutile phase and anatase phase TiO₂ with typical UV photoabsorption.

Fig. 5 EDS pattern (a) and elemental mappings (b–d) and of CFs with TiO₂ nanorod bundles.

Fig. 6 XRD patterns of pure CFs as well as CFs with TiO₂ nanorod bundles.

Fig. 7 UV-vis diffuse reflection spectrum of CFs with TiO₂ nanorod-bundles.

The last step was to weave cloth with these CFs with TiO₂ nanorod bundles (step 3 in Fig. 1). In fact, the size and area of cloth can be well controlled by adjusting the weaving method. In the present work, we weaved a CFs/TiO₂ cloth, as shown in the photos (Fig. 8a and b). This cloth consists of 16 bunches (8 × 8) of CFs with TiO₂, with the weight of ∼0.2 g and total area of ∼35 cm² (Fig. 8a). TiO₂ content in CFs/TiO₂ cloth can be determined to be about 18.9 mg, by weighting CFs before and after the growth of TiO₂ nanorod bundle. In particular, this CFs/TiO₂ cloth can be reversibly bent at large angles (0–360°) (Fig. 8b), resulting from the excellent flexibility of CFs. Meanwhile, the electric property of CFs/TiO₂ cloth was evaluated by comparing the electric resistance of one bundle of CFs/TiO₂ fibers from the cloth as well as one bundle of pure CFs. Usually, pure CFs have very good conductivity, and herein one bundle of pure CFs (length: ∼7 cm, fiber number: 50) exhibit a low electric resistance of ∼35 Ω. After the growth of TiO₂ nanorods, CFs/TiO₂ fibers (length: ∼7 cm, fiber number: 50) remain low electric resistance of ∼40 Ω, indicating that the present preparation method and presence of TiO₂ have no obvious adverse effects on the electric property along axial direction.

Fig. 8 Photos of CFs/TiO₂ cloth.

3.2 Performances of CFs/TiO₂ cloth as photocatalyst/photoelectrode

Obviously, CFs/TiO₂ cloth has a macroscale appearance and size (area: ∼35 cm²) as well as the excellent conductivity and flexibility (Fig. 8a and b). Simultaneously, it also contains plenty of small TiO₂ nanorods with diameters of ∼30 nm (Fig. 3c–f). These features make them to have great potential as efficient, easily recyclable and large-area photocatalyst/photoelectrode with nanostructures.

To investigate its photoelectrochemical behaviors, CFs/TiO₂ cloth as the working electrode was lying flat in the electrolyte with the depth of ∼1 cm, as shown in Fig. 9. The electrolyte was 0.5 M Na₂SO₄ aqueous solution, while Pt sheet and SCE were served as the counter electrode and reference electrode. At first, current–voltage (I–V) curves were measured from the linear sweep voltammetry with the scan rate of 10 mV s⁻¹ in the absence or presence light illumination, as shown in Fig. 10a. For both curves, the currents go up slowly with the increase of the potential from 0 to 0.8 V. Importantly, in the entire scan range (0–0.8 V), the current in the presence of light is higher than that in the dark, indicating the efficient formation of photocurrent. Subsequently, we further investigated the effects of on–off light illumination by recording photocurrent–time (I–t) characteristics of CFs/TiO₂ cloth at 0 V (vs. SCE) with a 50 s light pulse generated by a Xe lamp (Fig. 10b). Without the light irradiation, the current in dark remains about −0.040 mA. When the light is turned on, the photocurrent jumps immediately to about 0.180 mA, indicating an excellent photoelectric response. Interestingly, the current in dark can recover to be −0.040 mA when the light is off. This photocurrent generation is prompt and reproducible during four on–off light illumination cycles, which suggests the high stability of CFs/TiO₂ cloth. At last, the dependence of photocurrent on the applied potential was also studied by recording the on–off current at different potential (0, 0.1, 0.3, 0.6 V vs. SCE) (Fig. 10c). With the increase of the applied potential from 0 to 0.6 V, the photocurrent goes up from 0.180 to 0.725 mA. This fact may result from that higher potential facilitates the more efficient separation and transfer of photo-generated electron–holes in CFs/TiO₂ cloth.

Fig. 9 Schematic illustration of experimental setups and the mechanism for the photoelectrocatalytic (PEC) degradation by CFs/TiO₂ cloth.

Fig. 10 (a) I–V curves in dark and under light illumination, (b) on–off I–t curves at 0 V potential, (c) on–off I–t curves at different potentials (0, 0.1, 0.3, 0.6 V vs. SCE).

To determine its photoelectrocatalytic (PEC) activity, CFs/TiO₂ cloth (total weight: 0.2 g, area: ∼35 cm²; TiO₂ content: 18.9 mg) was directly lying flat in the electrolyte containing the organic pollutant with the depth of ∼1 cm, and then used as the working electrode (Fig. 9). It is well known that TiO₂ as the photocatalyst and/or photoelectrocatalyst can degrade many kinds of organic contaminants (such as, color dye: RhB, methyl orange; colorless: p-chlorophenol) and inactivating viruses in water/air.^1–4,33 To facilitate the analysis, herein RhB was used as the model of the pollutant. When dissolved in the electrolyte (0.5 M Na₂SO₄ aqueous solution), RhB dye displayed a major absorption band centered at 554 nm, which was used to monitor the PEC degradation. The photoelectrocatalysis was carried out at the applied potential of 0.6 V under the illumination of simulated solar light. As the photoelectrocatalysis proceeds, the color of RhB solution gradually changes from initially pink to nearly transparent (Fig. 11a), which reveals the efficient degradation of RhB by CFs/TiO₂ cloth. Meanwhile, the temporal evolution of the absorption spectra is shown in Fig. 11b. Obviously, there is a rapid decrease of RhB absorption at wavelength of 554 nm with the increased illumination time, accompanying with the absorption band shift to shorter wavelengths (∼498 nm). This hypsochromic shift should be attributed to the formation of some intermediates in a stepwise manner, which has been observed in our previous reports^15,16,25 and other reports.^10,20 After 100 min, the curve is nearly a straight line besides a small wave at the wavelength of 498 nm, which reveals that almost all RhB molecules have been degraded due to the excellent PEC activity of CFs/TiO₂ cloth.

Fig. 11 Photoelectrocatalytic (PEC) degradation of RhB by CFs/TiO₂ cloth: (a) photos of the color change of RhB solution with time, (b) UV-vis spectra of RhB solution with time variation.

To further evaluate the superiority of PEC process, we investigated the degradation efficiency of RhB by CFs/TiO₂ cloth through photocatalytic (PC) and electrocatalytic (EC) route, where PC route was performed by switching off the external potential, while the EC route was performed by turning off the xenon lamp, under the other identical conditions. For comparison, we also measured the EC, PC and PEC degradation efficiency of RhB by pure CF cloth (weight: ∼0.2 g, area: ∼35 cm²) under the other identical conditions. Obviously, when pure CF cloth was used, the degradation of RhB is extremely slow, and the degradation efficiency is very low (<6%) after 100 min through EC, PC and PEC process (Fig. 12). These facts indicate that pure CF cloth has very poor EC, PC and PEC activity for the degradation of organic pollutant. With CFs/TiO₂ cloth as the electrocatalyst, the degradation efficiency is also low (5.6%) after 100 min, indicating that the growth of TiO₂ on CFs cannot remarkably improve their EC activity. Importantly, with CFs/TiO₂ cloth as the photocatalyst, 60.8% RhB can be degraded after 100 min of light irradiation, indicating an obvious PC activity from TiO₂. Furthermore, 94.0% RhB can be degraded through the PEC process with CFs/TiO₂ cloth, indicating the highest degradation efficiency. It is clear that the PEC degradation efficiency (94.0%) by CFs/TiO₂ cloth is even much higher than the sum (66.4%) of that (5.6%) from EC process and that (60.8%) from PC process. These results strongly confirm the synergic effects in PEC process, since the separation and transfer of photo-generated electron–holes in CFs/TiO₂ cloth from PC process can be significantly improved by the external electric field from EC process, as demonstrated in Fig. 9 and 10.

Fig. 12 The electrocatalytic (EC), photocatalytic (PC), photoelectrocatalytic (PEC) degradation efficiency of RhB solution (5 × 10⁻⁶ mol L⁻¹) by pure CF cloth or CFs/TiO₂ cloth, as a function of time.

Most importantly, the macroscale CFs/TiO₂ cloth can be easily transferred and/or recycled in practical application. To investigate the stability and reusability of CFs/TiO₂ cloth, a recycling test was performed, as shown in Fig. 13. The PEC degradation of RhB was measured for four consecutive cycles (each cycle lasted 100 min). After each cycle, CFs/TiO₂ cloth was washed thoroughly with deionized water and then dried at 70 °C for 2 h. Then CFs/TiO₂ cloth was immersed in the same volume (60 mL) of fresh electrolyte with RhB again, under the other identical conditions. The degradation efficiency of RhB remains very high (>85%) during the four cycle, indicating no obvious loss of activity. This fact reveals that CFs/TiO₂ cloth has excellent PEC stability and recyclability.

Fig. 13 Cycling tests in the PEC degradation of RhB by CFs/TiO₂ cloth.

4. Conclusions

In summary, CFs/TiO₂ cloth has been prepared by using a dip-coating, hydrothermal growth and weaving three-step method, and it consists of CFs with TiO₂ nanorod bundles. These TiO₂ nanorod bundles have square-column appearance with size of about 340–400 nm and length of ∼6 μm, and they are composed of small nanorods with diameters of ∼30 nm. CFs/TiO₂ cloth can degrade 94.0% RhB in 100 min by PEC degradation process (bias: 0.6 V vs. SCE), which is higher than the efficiency from single photocatalytic (60.8% RhB) or electrocatalytic (5.6% RhB) route. Therefore, this kind of CFs/TiO₂ cloth has great potential as an efficient, easily recyclable and large-area photocatalyst/photoelectrode with nanostructures for future practical photocatalytic application, for example, degrading organic pollutants in lake and/or river. Furthermore, the present TiO₂ can also be replaced with other nanosized visible-light-driven photocatalysts, facilitating the design and construction of visible-light-driven and macroscale photocatalyst/photoelectrode with nanostructures.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21477019, 51272299, and 51473033), Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT1221 and T2011079), project of the Shanghai Committee of Science and Technology (13JC1400300), Shanghai Leading Academic Discipline Project (Grant No. B603), the Fundamental Research Funds for the Central Universities, and DHU Distinguished Young Professor Program.

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