Sieve-Like CNT Film Coupled with TiO2 Nanowire for High-Performance Continuous-Flow Photodegradation of Rhodamine B under Visible Light Irradiation.

Continuous-flow photoreactors hold great promise for the highly efficient photodegradation of pollutants due to their continuity and sustainability. However, how to enable a continuous-flow photoreactor with the combined features of high photodegradation efficiency and durability as well as broad-wavelength light absorption and large-scale processing remains a significant challenge. Herein, we demonstrate a facile and effective strategy to construct a sieve-like carbon nanotube (CNT)/TiO2 nanowire film (SCTF) with superior flexibility (180° bending), high tensile strength (75-82 MPa), good surface wettability, essential light penetration and convenient visible light absorption. Significantly, the unique architecture, featuring abundant, well-ordered and uniform mesopores with ca. 70 µm in diameter, as well as a homogenous distribution of TiO2 nanowires with an average diameter of ca. 500 nm, could act as a "waterway" for efficient solution infiltration through the SCTF, thereby, enabling the photocatalytic degradation of polluted water in a continuous-flow mode. The optimized SCTF-2.5 displayed favorable photocatalytic behavior with 96% degradation of rhodamine B (RhB) within 80 min and a rate constant of 0.0394 min-1. The continuous-flow photodegradation device made using SCTF-2.5 featured exceptional photocatalytic behavior for the continuous degradation of RhB under simulated solar irradiation with a high degradation ratio (99.6%) and long-term stability (99.2% retention after working continuously for 72 h). This work sheds light on new strategies for designing and fabricating high-performance continuous-flow photoreactors toward future uses.

1. Introduction

The emergence of continuous-flow devices opens an avenue to boost the prosperous development of successive batch manufacturing and has triggered a tremendous upsurge for the potential application in a variety of fields, such as chemical synthesis [1], mixture separation [2], pollutant treatment [3] and fuel cells [4], owing to the advantageous features of high efficiency, easy scalability, simple operation and low cost [5,6]. Of particular interest, the developing continuous-flow photoreactors, which use light for energy input, are increasingly appreciated as a reliable means to leap over the restrictions of traditional intermittent photocatalytic platforms and are regarded as a more promising strategy to carry out the photocatalytic removal of pollutants due to their continuity and sustainability [7,8].

To date, these materials with prominent photocatalytic properties, such as Sq-azo@PMO [9], pan> class="Chemical">ZnO-SnO2@C [10], anatase TiO2 [11], Bi2WO6/ZnFe2O4 [12], B/g-C3N4 [13], Ag3PO4/g-C3N4 [14] and UiO-66(Ti)-Fe3O4-WO3 [15], have been intensively explored for constructing continuous-flow photoreactors. Despite the exciting progress made in continuous-flow photoreactors for the photodegradation of pollutants, several issues remain to be addressed to improve the photodegradation efficiency, broad-wavelength light absorption, durability and large-scale processing to meet the rapidly growing demand in practical applications. Consequently, exploring and achieving photocatalytic materials and their rational structures that are well-matched to the continuous-flow mode for acquiring better photodegradation performances is highly desirable yet greatly challenging thus far.

CNT film, a macroscopic assembly of nanpan>oscale inpan>dividual Cpan> class="Chemical">NTs, can be fabricated by various approaches, such as floating catalyst chemical vapor deposition (FCCVD) [16,17,18], dry spinning [19], solution-based coating [20], printing [21] and vacuum filtration [22], with different types of architectures, including non-woven cloth, vertical alignment, aligned networks, transparent film, random planar film and compressible foams [23].

Owing to its prominent flexibility, high mechanical strength, electrical conductivity, ease of processing, good optical transparency, prominent durability and stability, superior abrasion resistance, good adhesion properties with many substrates and low density [24,25], CNT films have beenpan> extenpan>sively applied to various fields, inpan>cludinpan>g tranpan>sparenpan>t conductor [26], senpan>sor [27], enpan>ergy storage anpan>d conversion [28,29], filtration membranpan>e [30], tranpan>sistor [31], inpan>tegrated circuits [32], etc. Significanpan>tly, the multifunctionalization of Cpan> class="Chemical">NT film can be readily realized through combinations with various active and passive components (e.g., metal nanoparticles and nanowires, metal oxides, conductive polymers and graphene) [33].

Additionally, the flexible and robust CNT film fabricated by the FCpan> class="Disease">CVD method could be processed into porous architecture with ample well-ordered and uniform mesopores via a facile and effective laser drilling, which was used as a smart and flexible supercapacitor [34]. Thus, after its functionalization with photoactive materials, the mesoporous CNT film can be converted into a hybrid film that combines the best features of both components, i.e., superior flexibility, mechanical strength, photocatalytic capacity, stability and processibility, which might serve as suitable scaffolds for the continuous-flow photodegradation of polluted water.

Proving this hypothesis, we designed and fabricated a flexible and robust sieve-like carbon nanpan>otube (Cpan> class="Chemical">NT)/TiO2 nanowire film (SCTF) for the continuous-flow removal of RhB used as a model pollutant with a laser-drilling CNT film coupled with TiO2 nanowire. The resulting SCTF contained abundant, well-ordered and uniform mesopores, thereby, enabling channels for efficient solution infiltration through the film. The good transmittance of the as-prepared hybrid film not only facilitated light penetration into the film interior but also favored effective visible light absorption. Benefiting from these unique features, the assembled continuous-flow photodegradation device based on SCTF displayed superior photocatalytic behavior for the continuous degradation of RhB under simulated solar irradiation in terms of the degradation ratio and long-term stability.

2. Experimental

2.1. Materials

n class="Chemical">Ethyl alcohol, pan> class="Chemical">ferrocene, thiophene, sulfuric acid (H2SO4), hydrochloric acid (HCl) and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Shanghai Co., Ltd. (Shanghai, China). Titanium butoxide (Ti(OH9C4)4), n-hexane and RhB were purchased from Aladdin Reagent Database Inc. (Shanghai, China). All chemicals were of analytical grade and utilized without further purification.

2.2. Fabrication of SCTF

Cn class="Chemical">NT films with anpan> area of meter level anpan>d a thicknpan>ess of ca. 8 μm were manpan>ufactured by the FCpan> class="Disease">CVD method [18,35]. TiO2 nanowires were coupled facilely onto porous CNT films via a hydrothermal strategy as reported in previous works [36,37]. Specifically, the pristine CNT films were cut into identical circles (Ø 5.5 cm) and then engraved by laser drilling. The porous CNT films (10 mg) were eluted with 1 M H2SO4 to remove the iron catalysts, followed by washing with deionized (DI) water until reaching a neutral solution. Subsequently, the treated porous CNT films were immersed into a 50 mL solution of Ti(OH9C4)4 and n-hexane with a volume ratio of 1:9 for 5 min at room temperature, followed by taking out and exposure in air for 10 min to enable the n-hexane volatilization and hydrolysis of Ti(OH9C4)4 into seed crystals.

Afterward, the obtained composite films were mixed with 80 mL NaOH aqueous solution (10 M) anpan>d 0.5 mL pan> class="Chemical">Ti(OH9C4)4. The mixture was, then, transferred into a 100 mL Teflon-line stainless steel reactor and maintained at 200 °C, and the amount of TiO2 nanowires generated on porous CNT films could be regulated by changing the reaction time. After cooling naturally to room temperature, the resultant films were washed with 0.1 M HCl and DI water in sequence for several times, followed by annealing at 500 °C for 3 h, thus, eventually achieving SCTFs. The mass fractions of CNTs in the SCTFs were 2%, 2.5%, 3.5% and 5%, respectively, and the final products were named SCTF-2, SCTF-2.5, SCTF-3.5 and SCTF-5.

2.3. Characterization

Scanning electron microscope (SEM) images were collected using a field emission scanning electron microanalyzer (Apreo C HiVac, Thermo Fisher Scientific, Waltham, MA, USA) at an acceleration voltage of 10 kV. X-ray diffraction (XRD, Bruker Inc., karlsruhe, Germany) patterns were recorded on a Bruker AXS X-ray diffractometer (Bruker Inc., karlsruhe, Germany) using a Cu Kɑ radiation generator (1.5406 Å, 40 kV and 40 mA). Raman spectra were obtained by a Renishaw RM3000 Raman microscope (Renishaw Inc., London, UK) with a 633 nm laser source. UV-vis diffuse reflectance spectra (DRS) were measured using a Hitachi U-4100 spectrophotometer (Hitachi Limited, Tokyo, Japan) with BaSO4 as a reflectanpan>ce referenpan>ce. The conpan>tact anpan>gle was determinpan>ed by a Drop Shape Anpan>alyzer (DSA100, KRÜSS, Hamburg, GmbH). The pan> class="Disease">tensile stress–strain tests were carried out on an Instron 3365 equipped with a fine force detector at a tensile rate of 0.5 mm per minute.

2.4. Photocatalytic Degradation Procedure

The photocatalytic activity of as-fabricated SCTFs was evaluated by the degradation of RhB. A 300 W Xe lamp (Shaoxinpan>g Chanpan>gtuo Imp. Exp. Co., Shaoxinpan>g, Chinpan>a) with a wavelenpan>gth ranpan>ge of 420–800 nm anpan>d light inpan>tenpan>sity of 100 mW cm−2 was used as the simulated solar light source. The light inpan>tenpan>sity of sunlight was 45 mW cm−2. Briefly, 12 mg of photocatalyst was immersed inpan>to 100 mL of pan> class="Chemical">RhB aqueous solution (10 mg L−1). The obtained suspension was placed for 30 min in the dark to attain the adsorption–desorption equilibrium and then exposed to visible light. At a given interval of 10 min, 3 mL of RhB aqueous solution was taken out, and its concentration was measured with a UV-vis spectrophotometer (Model lambda 35, PerkinElmer) at a maximum absorption wavelength of 553 nm.

3. Results and Discussion

3.1. Fabrication and Characterization of SCTF

The procedure for the SCTF fabrication is schematically illustrated in Figure 1. The flexible CNT film was treated by laser drillinpan>g to form well-organpan>ized holes with a pore size onpan> the micrometer level. Subsequenpan>tly, pan> class="Chemical">TiO2 nanowires were coupled with the resultant sieve-like CNT film via hydrothermal treatment in an aqueous solution of Ti(OH9C4)4 at 200 °C, eventually forming a porous SCTF. From the optical images of films, the film featured no significant destruction in the structure (e.g., fractures and cracks) and its color changed from black to gray owing to the pore formation and TiO2 modification on the CNT film during the process of laser drilling and hydrothermal treatment.

Figure 1

The schematic procedure of fabricating sieve-like carbon nanotube (CNT)/TiO2 nanowire film (SCTF) and the corresponding films obtained at each step.

Figure 2a presents the optical photograph of an SCTF placed on a piece of paper with certain words. A high definition of these words below the film was observed, implying excellent transparency of the SCTF, which was propitious to the light penetration into the interior of film when exposed to solar irradiation. As envisioned in Figure 2b, even under bending with an angle up to 180°, SCTF displayed exceptional flexibility without any local damage generated, thus, confirming that the inner structure of film was well maintained. The light harvesting and absorption ability of the n class="Chemical">TiO2 nanpan>owire anpan>d SCTF was examinpan>ed usinpan>g UV-vis DRS.

Figure 2

(a,b) Digital photographs of SCTF. (c) UV-vis DRS of a TiO2 nanowire and SCTF. (d) Photograph of a water droplet standing on a sieve-like CNT film and SCTF. (e) Tensile stress–strain curves of SCTFs.

In contrast to the neat TiO2 nanpan>owire, SCTF featured significanpan>tly enpan>hanpan>ced UV-vis light absorptionpan>, especially withinpan> the visible light ranpan>ge (Figure 2c), signifyinpan>g that couplinpan>g a pan> class="Chemical">TiO2 nanowire to a sieve-like CNT film would be favorable for improving both the visible light absorption and photocatalytic activity. Static water-droplet contact angle measurements were used to characterize the surface wettability.

As illuminated in Figure 2d, the contact angle at the surface of the sieve-like CNT film was 90°, anpan>d this was drastically reduced to 0° at the surface of the SCTF, thereby, provinpan>g that a significanpan>tly enpan>hanpan>ced surface wettability of SCTF originpan>ated from the inpan>troduction of pan> class="Chemical">TiO2 nanowire with abundant hydrophilic hydroxyl groups, which would be beneficial to the polluted water diffusion throughout the entire film when served as a photocatalyst. Additionally, the as-prepared SCTFs featured a satisfactory mechanical property with tensile strength of 75–82 MPa and fracture elongation of 1.79–1.86% (Figure 2e), which was critical and essential to their use in the continuous-flow photodegradation of polluted water.

The microstructure of sieve-like Cn class="Chemical">NT films anpan>d SCTFs was characterized through SEM. The sieve-like Cpan> class="Chemical">NT film displayed a relatively flat surface and consisted of abundant, well-ordered and uniform mesopores with pore diameter of ca. 100 µm and pore interval of ca. 70 µm (Figure 3a). In contrast, SCTF displayed an obvious difference (Figure 3b). The massive needle-like substances were homogeneously attached to the surface of SCTF. Additionally, note that the pore size was reduced from 100 to 70 µm, however, they maintained a nearly circular shape.

Figure 3

SEM images of a sieve-like CNT film (a) and SCTF at low and high magnifications (b–d). XRD patterns (e) and Raman spectra (f) of CNT, TiO2 and SCTF.

The magnified SEM image corresponding to the red frame-marked region in b was featured by numerous interlaced fibers with an average diameter of ca. 500 nm (Figure 3c), while the SEM image corresponding to the blue square region in b expressly presented a pore with a rough edge encircled by a network structure (Figure 3d). The homogeneous distribution of nanowires and high porosity on SCTF made it favorable for the continuous-flow photodegradation of organic pollutants when served as photocatalyst due to effective photoexcitation and convenient liquid penetration.

The composition of the SCTFs was examined using XRD and Raman spectra, and, for a comparison, the XRD and Raman spectra of the CNTs anpan>d pan> class="Chemical">TiO2 were also analyzed. As seen in Figure 3e, all characteristic peaks of the CNTs and TiO2 appeared in the XRD pattern of the SCTFs, as expected. For instance, these sharp and intensive peaks at 2θ of 25.23°, 37.91°, 48.04°, 53.81° and 55.14° corresponding to the reflections from the (101), (004), (200), (105) and (211) crystalline planes of anatase TiO2, respectively, as well as a relatively broad peak of the CNTs at 25.51°. The Raman analysis of SCTFs presented D and G bands corresponding to CNTs at 1338 and 1580 cm−1, as well as characteristic peaks of anatase TiO2 at 147, 397, 515 and 638 cm−1 (Figure 3f). Both the XRD and Raman spectra proved that anatase TiO2 nanowires were successfully combined with a porous CNT film to form the composite SCTF.

3.2. Photocatalytic Performance of SCTF

3.2.1. Optimization of SCTF

The photocatalytic activity of the as-fabricated samples was evaluated through the degradation of RhB under visible light irradiationpan>. Prior to irradiationpan>, the pan> class="Chemical">RhB solutions containing photocatalysts were kept in the dark under continuous stirring for 30 min to reach the adsorption–desorption equilibrium. Figure 4a presents the C/C0 profile as a function of the irradiation time, where C and C0 refer to RhB concentrations at a given time and the initial time before irradiation, respectively. For pure TiO2, the RhB was slowly degraded, with a degradation ratio of only 25% within 80 min, manifesting its poor photocatalytic performance.

Figure 4

Photocatalytic degradation curves (a), kinetic curves (b) and degradation ratios (c) of RhB for different photocatalysts under simulated solar irradiation. (d) Absorption spectra of RhB with SCTF-2.5 as photocatalyst in different reaction times under simulated solar irradiation.

In contrast, all the SCTF heterojunction photocatalysts displayed a significantly enhanced photocatalytic activity toward the removal of RhB as expected. The degradationpan> ratio of pan> class="Chemical">RhB reached 75%, 86%, 96% and 92% for SCTF-5, SCTF-3.5, SCTF-2.5 and SCTF-2, respectively (Figure 4c). SCTF-2.5 featured the most pronounced photocatalytic activity, which was approximately four-times higher than that of the pure TiO2 sample. At high TiO2 loading, a decline in the degradation ratio occurred owing to the shading effect and the aggregation of TiO2 nanowires, which could hinder the light transmission to the surface of catalysts and restrict the efficient light absorption.

As seen in Figure 4b, the time-dependent photodegradation behavior of RhB obeyed a pseudo-first-order kinpan>etic model: ln(C0/C) = kt (where t is the irradiation time anpan>d k is the rate constanpan>t) [38,39]. The calculated k value for SCTF-2.5 was 0.0394 minpan>−1, which was 10.10-, 2.24-, 1.71- anpan>d 1.29-times higher thanpan> pure pan> class="Chemical">TiO2, SCTF-5, SCTF-3.5 and SCTF-2, respectively, and was also substantially higher as compared to most of the previously reported values for other carbon-based photocatalysts [40,41,42]. The above findings demonstrate that the loading of TiO2 nanowires to a porous CNT film was beneficial for enhancing its photocatalytic performance toward RhB degradation, and SCTF-2.5 exhibited much better photocatalytic activity, which was used for the following photodegradation measurements.

Figure 4d shows the changes in the typical time-dependent absorption spectra of RhB durinpan>g the photodegradation process. The maximum absorption peak of pan> class="Chemical">RhB, centered at 553 nm, gradually decreased with the increasing irradiation time and almost disappeared after simulated solar irradiation for 80 min, implying that most of RhB could be degraded in the presence of SCTF-2.5. For comparison, the photodegradation of RhB was also conducted under the identical conditions with no photocatalyst and only a CNT film.

As envisioned in Figure S1, after irradiation for different times, the absorbance of RhB remainpan>ed almost unchanpan>ged inpan> the absenpan>ce of a photocatalyst anpan>d displayed a slight declinpan>e inpan> the presenpan>ce of the Cpan> class="Chemical">NT film, which revealed poor photodegradation of RhB, thus, further proving the prominent photocatalytic activity of SCTF-2.5 to degrade RhB. Figure S2 compares the photocatalytic behavior of RhB under natural and simulated solar irradiation. RhB was effectively degraded under natural solar irradiation, despite the relatively lower degradation ratio at the same irradiation time and rate constant as compared to that under simulated solar irradiation due to the low light intensity of the sun, which suggests a potential application of SCTF-2.5 to RhB degradation in real-world situations.

3.2.2. Continuous-Flow Photodegradation

The photocatalytic oxidation of organic pollutants to carbon dioxide (pan> class="Chemical">CO2) in a continuous and scalable way is becoming vital concern for the efficient treatment of polluted water. To verify the feasibility of SCTF for continuous-flow photodegradation, a single-inlet–outlet plate column-like photoreactor was elaborately designed and assembled to carry out the continuous-flow photooxidation of RhB. Figure 5 shows a schematic illustration of the ready-to-use continuous-flow device connected by three photoreactors in a series.

Figure 5

Schematic diagram of the continuous-flow photodegradation and photocatalytic degradation mechanism of RhB on SCTF-2.5.

The photoreactor was constructed by consecutively fixing SCTF-2.5 (Ø 5 cm) into a sealed transparent glass vessel with an ca. 3 cm distance between the SCTF-2.5 photoreactors. The aqueous n class="Chemical">RhB solution was sprayed inpan>to the photoreactor through the inpan>let, which simultanpan>eously allowed the inpan>put of air. The photodegradation was operated inpan> a continpan>uous-flow mode with a continpan>uous pan> class="Chemical">RhB inflow from the bottom of the front photoreactor to the top of the latter photoreactor via syringe pumps.

To realize the photodegradation of n class="Chemical">RhB to pan> class="Chemical">CO2 in the continuous-flow device, an optimized condition was used after multiple trial experiments. Specifically, the device was fed with aqueous RhB solution (<2 mg L−1) at a flow rate of 2 mL min−1, and air was introduced under one bar of pressure. The photooxidation reaction of RhB was performed at room temperature with simulated solar irradiation, and the products were collected from the outlet of the device for analysis at certain time intervals.

As seen in Figure 6, when the time interval of sampling varied from 6 to 72 h, very low absorbance and a slight change in absorbance were observed (Figure 6a), and the degradation ratio of RhB mainpan>tainpan>ed a high level ranpan>ginpan>g from 99.2–99.6% (Figure 6b), adequately demonstratinpan>g that the as-fabricated device was feasible for the continpan>uous-flow photodegradation of pan> class="Chemical">RhB under visible light irradiation with a high degradation ratio and long-term stability. Based on the obtained results, a possible mechanism for the photocatalytic degradation of RhB in the continuous-flow device was proposed.

Figure 6

UV-vis spectra (a) and the degradation ratios (b) of the RhB solution for different times during the continuous-flow photodegradation under simulated solar irradiation.

Under simulated solar irradiation, n class="Chemical">TiO2 nanpan>owires coupled inpan>to a porous Cpan> class="Chemical">NT film in the SCTF became photoexcited to produce electrons (e−) and holes (h+). The photogenerated electrons could be transferred to the CNT, which would be further captured by O2 to form •O2−. The positively charged holes in the valence band were able to react with H2O/OH− to generate •OH. •O2− and •OH as powerful oxidants effectively degraded RhB into CO2 and H2O.

4. Conclusions

In summary, to further extend and boost the photocatalytic application of CNT films, a flexible anpan>d robust SCTF with a tenpan>sile strenpan>gth of 75–82 MPa for the continpan>uous degradation of polluted pan> class="Chemical">water was fabricated via a facile coupling of a TiO2 nanowire onto a laser-drilling CNT film. The resulting hybrid film demonstrated the distinctive advantages of good transmittance and convenient visible light absorption, thus, guaranteeing high photosensitivity and effective photoexcitation. SCTF revealed unique structural features with abundant, well-ordered and uniform mesopores (ca. 70 µm in diameter) as well as the homogenous distribution of TiO2 nanowires (ca. 500 nm in diameter), thereby, enabling channels for efficient solution infiltration through the film during a continuous degradation of pollutants.

In contrast, SCTF-2.5 featured a high photocatalytic performance for RhB degradation with a degradation ratio of 96% anpan>d rate constanpan>t of 0.0394 min−1. The continuous-flow photodegradation device assembled with SCTF-2.5 achieved a high degradation ratio (99.6%) of pan> class="Chemical">RhB and long-term stability (99.2% retention after working continuously for 72 h) under simulated solar irradiation.