A novel photocatalytic optical hollow-fiber with high photocatalytic activity for enhancement of 4-chlorophenol degradation

Accepted Manuscript A novel photocatalytic optical hollow-fiber with high photocatalytic activity for enhancement of 4-chlorophenol degradation Nianbing Zhong, Ming Chen, Yihao Luo, Zhengkun Wang, Xin Xin, Bruce E. Rittmann PII: DOI: Reference:

S1385-8947(18)31641-3 https://doi.org/10.1016/j.cej.2018.08.167 CEJ 19781

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

6 June 2018 21 August 2018 24 August 2018

Please cite this article as: N. Zhong, M. Chen, Y. Luo, Z. Wang, X. Xin, B.E. Rittmann, A novel photocatalytic optical hollow-fiber with high photocatalytic activity for enhancement of 4-chlorophenol degradation, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.08.167

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A novel photocatalytic optical hollow-fiber with high photocatalytic

activity

for

enhancement

of

4-chlorophenol degradation Nianbing Zhonga,b,*, Ming Chena, Yihao Luob, Zhengkun Wanga, Xin Xina, Bruce E. Rittmann b,*

a

Chongqing Key Laboratory of Modern Photoelectric Detection Technology and

Instrument, Chongqing Key Laboratory of Fiber Optic Sensor and Photodetector, Chongqing University of Technology, Chongqing 400054, China

b

Biodesign Swette Center for Environmental Biotechnology, Arizona State University,

Tempe, AZ 85271-5701, USA

*Corresponding

authors.

E-mail

addresses:

[email protected]

[email protected] (N.B. Zhong), [email protected] (B.E. Rittmann)

1

or

Abstract: We present a novel UV-visible-light photocatalytic optical hollow-fiber in which a hollow optical fiber (HOF) and TiO2-based composite are coupled for enhancement of photocatalytic activity toward degradation of 4-chlorophenol (4-CP). A HOF was coated with a TiO2-based composite composed of Er3+:YAlO3/SiO2/TiO2 (EYST) and acting as the photocatalytic element. The EYST coating showed good light-inducing capacity at the fiber-coating interface and high photocatalytic activity driven by UV-visible light. The photocatalytic activity of the fabricated photocatalytic HOFs was investigated for degradation of 4-CP in aqueous solution in the wavelength spectrum of 360–780 nm. Examined were the effects of the EYST coating gradient and reactor operating conditions (pH and temperature) on the degradation of 4-CP. The EYST-coated HOF significantly enhanced the 4-CP degradation, dechlorination, and mineralization, compared to pure TiO2-coated HOF and EYST-coated solid optical fibers. Specifically, high photocatalytic activity was obtained when the EYST coating thicknesses at the HOF’s incident end and HOF’s hemispherical end were 10 and 160 μm, the pH was 6.0, and temperature was 55 oC.

Keywords: Hollow optical fiber; Visible light photocatalysis; TiO2; 4-chlorophenol; Photocatalytic activity

2

1. Introduction Titanium dioxide (TiO2) is a low-cost, high-activity, non-toxic, and chemically stable semi-conductor that can be used for the transformation and mineralization of organic pollutants in the presence of suitable light [1,2]. Thus, TiO2 particles have been widely applied as photocatalysts for detoxification of water and air. Although TiO2 photocatalysis is promising, several drawbacks prevent it from gaining widespread practical application. One drawback stems from the cost of recovering TiO2 particles when they are used in suspension [3–5]. To overcome this drawback, Marinangeli and Ollis [6] proposed that optical fibers could be used for light transmission and as a support for TiO2 photocatalysts. Experimental application of the novel idea was demonstrated by Hofstadler et al. [7], who designed a TiO2-coated, solid-quartz fiber fixed in a tubular reactor in which photodegradation of 4-CP was carried out in water. Thereafter, many papers reported success with TiO2 coating on solid optical fibers (SOFs), including solid quartz/plastic optical fibers [6–12]. SOFs with normal core-clad structures were for the transmission of information in telecommunication, not luminescence at the surface of fiber [13–15]. In particular, photocatalytic layer received low illumination, because the penetration depth of the evanescent field ranges from ten to approximately two hundred nanometers in UV-visible light [16]. To enhance the excitation light intensity of photocatalysts at the SOF surface, the fiber cladding is removed by grinding or etching. Although the fibers without cladding by coating TiO2 nanoparticles can enhance the local excitation light intensity, the whole excitation light intensity and unevenness of light distribution along the fiber length are undesirable, making it difficult to achieve a high photocatalytic degradation

3

of organic water contaminants. Furthermore, the photocatalytic devices based on the plastic optical fibers without cladding age easily and etch because the fiber core (PMMA material) is difficult to operate in a high concentration (such as 200 mg/L phenol solution) of phenolic solutions for a long time. Thus, developing a photocatalytic quartz optical fiber that shows a high luminous intensity at the fiber surface and works in UV-visible light is very important in practice. Quartz hollow optical fiber (HOF) can serve as a good candidate for enhancing the luminous intensity on the surface because the refractive index (RI) of the core (air) is lower than that of the cladding (SiO2) and the SiO2 material has good UV-visible light transmission. In particular, when a semi-conductor material whose RI is higher than that of the cladding was coated on the surface of the HOFs, the luminous intensity of the HOFs can be further enhanced according to our previous studies [14,15,17]. Although the HOF plus coating can improve the luminous intensity at the fiber surface, an UV-visible-light photocatalytic optical hollow-fiber has not been reported previously. In this work, to obtain high excitation light intensity and extend visible light response of TiO2-based photocatalytic films, we explore a novel HOF that is coated with a gradient Er3+:YAlO3/SiO2/TiO2 (EYST) coating. The photocatalytic HOF consists of three layers: fiber core (air), cladding (SiO2), and EYST coating; EYST coating is activated by UV and visible light. The prepared photocatalytic HOFs are characterized using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), fluorescence spectra (FS), UV–vis spectroscopy (UV–vis S), optical microscopy (OM), and optical power meter (OPM). The photocatalytic activity of a single photocatalytic HOF toward degradation of

4

4-CP in aqueous solution is investigated for the UV-visible spectrum (360–780 nm). The effects of the thickness distribution of the EYST film on the HOF surface, pH, and temperature are examined for their impacts on for the degradation and mineralization of 4-CP.

2. Experimental Methods 2.1. Synthesis of photocatalytic composites In this work, the Er3+:YAlO3 upconversion luminescence agent and EYST sample were prepared by following the route described by Zhong et al [18,19]. A SiO2/TiO2 (ST) sample with 0.5000 g of SiO2 and 4.500g of TiO2 was synthesized using the same method described for the synthesis of the EYST solution [18,19]. A TiO2 (T) solution is prepared using the method with 5.000 g of TiO2 particles. 2.2. Fabrication and characterization of photocatalytic HOFs To obtain a high-performance photocatalytic HOF, hollow quartz optical fibers (length, outer diameter, and inner diameter of 180, 5, and 3 mm, respectively) with a hemispherical end were the basis material for immobilizing the photocatalyst. First, the HOFs were washed, in turn, with acetone (Sigma-Aldrich), isopropyl alcohol (Sigma-Aldrich), and ethanol (≥99.8%, Aladdin, China). Then, the washed HOFs were dried under N2 gas. Thereafter, T-, ST-, and EYST-coated HOFs were fabricated by layer-by-layer coating of the prepared T, ST, or EYST solution on a thoroughly washed optical fiber using a dip coater (SYDC-500, Shanghai SAN-YAN Technology Co., Ltd., China) and dried at 250°C for 6 h. The thicknesses of the T-, ST-, and EYST films were controlled by the number of deposition cycles. Two types of EYST films on the HOF surface were prepared. The first type had a film with a uniform coating thickness of 100 μm. The second type had a gradient 5

coating thickness across the fiber surface: i.e., the coating thickness of all the samples increased along the fiber, with the thickness at the fiber’s incident end being 10 µm and the thickness at the fiber’s hemispherical end in the range of 100–200 μm. The fabrication method for gradient coating is given in Zhong et al [17]. 2.3. Characterization of photocatalytic HOFs An optical microscope system (IX81, Olympus, Japan) with a resolution of ±1 μm was used to measure the diameter of the HOFs. The fiber length was monitored using a digital caliper (111-253-20G, Guilin Guanglu Measuring Instrument Co., Ltd., China) with a resolution of 10 µm. The surface morphology of the photocatalytic HOFs was examined by SEM (JSM-7800F, JEOL Ltd., Japan), and its chemical composition was detected by EDS attached to the SEM and XPS (XSAM800, Kratos Co., UK). The TEM (JSM 2000-F, Jeol Company, Japan) was used to observe the crystal shape and particle size of the TiO2-based catalysts. The crystallographic structure of the products was determined by XRD using a diffractometer with Cu Kα radiation (RINT 2500, Rigaku Corporation, Japan). The FS of the EYS (Er3+:YAlO3/SiO2) powders were detected by a Horiba FluoroLog-3 spectrofluorometer equipped (HORIBA Scientific, Edison, NJ, USA). To investigate light absorption by the coating film, the surface spectra of the HOFs were recorded by an optical spectrometer (QE65 Pro, Ocean Optics, Florida, USA) and a light source (DH-2000, Ocean Optics, USA) with a deuterium tungsten halogen operated over the spectral range of 190–2000 nm. Specifically, to record the luminous intensities at the fiber surface and to perform photocatalytic degradation of 4-CP, the UV-visible light source was fabricated using a cooler LED with a power of 50 W in the spectral range of 360–780 nm and a condenser with a hole of 5.5 mm as shown in Fig. S1. To evaluate surface emission light intensity and excitation light intensity of 6

the TiO2 and Er3+:YAlO3 in the HOF film, the light intensity in the spectral range of 750–780 nm was measured by a power meter (36R, Newport Corporation, USA, obtained from the NBET Group Corp., China) with a self-prepared wedge-shaped UV fiber-optic probe (cladding and core diameters are 1000 and 1100 μm, respectively, with inclination at 30°). The emission intensity at the fiber surface could be used to evaluate the excitation light intensity of TiO2 and Er3+:YAlO3, because the emitted light at the fiber surface is from the fiber coating. Thus, higher surface-emission intensity means higher light intensity in the coating layer, i.e., photocatalytic film obtains higher excitation light intensity. 2.4. Operation and analysis of the photocatalytic reaction A cylindrical photocatalytic reactor was fabricated from glass with a working volume of 20 mL and a length of 10.5 cm, as shown in Fig. S1. The total volume of aqueous solutions was approximately 30 mL. A single prepared photocatalytic HOF was deployed as the photocatalytic element in the reactor. A container with a working volume of 100 mL was used to store the 4-CP solution, which had an initial concentration at 100 mg/L and was recycled with a flow rate of 600 mL/h. The initial pH of the 4-CP solutions was adjusted using nitric acid and NaOH solutions and was monitored by a Mettler Toledo 320-S pH meter (Mettler Toledo Instruments, China). The temperature of the 4-CP solutions was controlled by using a water bath (DCW-0530, Shunmatech, China) with a temperature variation of less than 0.1 oC. To investigate the photocatalytic activity of the photocatalytic HOFs toward degradation of 4-CP aqueous solution in the UV-visible spectrum (360–780 nm), the 4-CP degradation (Ct/C0), TOC removal (Ct/C0), and chloride ion (Cl−) concentration are investigated, where Ct and Co denote the 4-CP and TOC remaining concentrations at time t and initial concentrations, respectively. In particular, the remaining 7

concentration of 4-CP was determined by measuring the decrease of the absorbance at 225 nm on the QE65 Pro spectrometer. Total organic carbon (TOC) removal was determined in the 4-CP degradation processes using a TOC analyzer (TOC-VCPH, Shimadzu, Japan) with a 0.45-µm Millipore membrane filter. In addition, chloride-ion release from 4-CP was measured using a chloride-selective electrode (Cole Parmer, USA).

3. Results and discussion 3.1. Optical properties of photocatalytic HOFs with uniform coating Fig. 1a shows that the luminous intensity of solid plastic optical fiber (SPOF), solid plastic quartz optical fiber (SQOF), and the uncoated HOF at the fiber surface (The measurement points are shown in Fig. S2). Close to the light source, the luminous intensity of the HOF is significantly higher than that of the SPOF and SQOF, which can be explained as follows. First, the HOFs can capture a bundle of rays in any direction, whereas the light rays entering the SPOF and SQOF are restricted by their numerical aperture; the coupled light rays, which do not satisfy the total reflection condition, are refracted out of the fiber along the axial direction. Second, the light rays, Ii,1, in the solid optical fibers including SPOFs and SQOFs, are confined in the fiber core via total reflection at the core-cladding interface (Point A, see Fig. 2a), thus the luminous intensity at the solid optical fiber surface is a small part of the whole evanescent field. However, in the HOFs, the light rays, Ii,2, are refracted into the cladding (Point C, see Fig. 2b) because the RI of the cladding is higher than that of the core, which changes the light transmission modes in the fiber, thus the luminous intensity of at the HOF surface is based on the whole evanescent field. Third, the RIs increase from the core to cladding, increases number of attenuated total reflection

8

points [14,15].

Fig. 1. (a) Luminous intensity in the spectral range of 750–780 nm at the fiber surface along the axial direction; the SPOF is with a polystyrene core surrounded by a PMMA cladding (cladding diameter and NA of 5 mm and 0.5, respectively; the SQOF is with a ultra-pure quartz core surrounded by a PMMA cladding (cladding diameter and NA of 5 mm and 0.22, respectively.). (b) Surface luminous spectrum of the HOFs at an axial distance of 4 cm. Luminous intensity in the spectral range of 750–780 nm at the HOF surface along the axial direction (c) and along the radial direction at an axial distance of 4 cm (d) (coating thickness of 100 µm). In Fig. 1a, the HOF also exhibits an abnormal high attenuation coefficient and even the luminous intensity is slightly lower than the SPOF and SQOF after 60 mm. These facts are because of a large propagation loss caused by the scattering and refraction when the fiber length is before 60 mm; however, when the length is after 60 mm, the light is confined in the core by total internal reflection, and the luminous intensity at 9

the fiber surface is based on the evanescent field. Furthermore, we observe also that, close to the fiber end, the luminous intensity of the prepared fibers increases because of internal light reflection at the hemispherical tip of the fiber end [17]. These facts indicate that although the luminous intensity at the HOF surface exhibits an uneven distribution, the total emitted intensity from the uncoated HOF surface is higher than the emitted intensity of the SQOF and SPOF. Thus, the HOF can serve as a replacement material for fabricating the photocatalytic element. In Fig. 1b, the quality of the surface luminous spectrum of the HOFs decreased in the spectral range of 200–750 nm when the TiO2-based composites (i.e., T, ST, and EYST materials) were coated on the surface of the HOFs. In particular, the quality of the surface luminous spectrum of the HOFs is ordered as follows: HOFs > ST-coated HOFs > T-coated HOFs > EYST-coated HOFs. These facts can be explained as follows. First, the RI of the coatings is higher than that of the HOF cladding (RIs of SiO2, TiO2 and YAlO3 are, respectively, 1.548, 2.548 and 1.957), the increase in the RIs of the fiber core to that of the coating improves the light transmission modes and induces light beams into the fiber coating via refraction at the core-cladding and cladding-coating interfaces, as shown in Fig. 2c; although the UV light in the fiber coating was absorbed by TiO2 to drive photocatalytic reactions, the SiO2 improved the transmission of light in the fiber coating; thus the quality of the surface luminous spectrum of ST-coated HOFs is better than that of the T-coated HOFs. Second, the visible light in the fiber coating can be absorbed by Er3+:YAlO3 and emit the UV light to drive photocatalytic reactions, thus the quality of the surface luminous spectrum of EYST-coated HOFs was at the lowest level. In Fig. 1b, one can also see that the surface luminous spectrum of the HOFs with TiO2-based coating increased when the wavelength is above 750 nm. In particular, the transmitted spectrum of the HOFs is

10

ordered as follows: EYST-coated HOFs > T-coated HOFs > ST-coated HOFs > HOFs. The reasons are as follows. First, the T (ST and EYST) coating hardly absorbs light when the wavelength is greater than 750 nm. Second, the unattenuated visible light in the coating will be scattered by the, Er3+:YAlO3, TiO2, and SiO2; in particular, the Rayleigh scattering increases with the increase in light wavelength. Third, the Er3+:YAlO3 with large diameter compared with SiO2 and TiO2 (see Table S1), results in high light-scattering efficiencies in the fiber coating and high luminous power.

Fig. 2. Schematic diagram of light transmission models in a solid optical fiber (a), HOF (b), HOF with TiO2-based coating (c), and solid optical fiber with TiO2-based coating (d), respectively (nca,1 > nco,1 > ncl,1, nco,2 < ncl,2 < nca,2). In Fig. 1c and Fig. 1d, the HOFs further exhibit improved luminous light intensity in the spectral range of 750–780 nm when the photocatalytic materials are coated on the surface of the HOFs; in particular, the EYST-coated HOF shows the highest luminous intensity. These facts have the same reason as discussed above for the surface luminous spectrum of the HOFs, as shown in Fig. 1b. Furthermore, the luminous intensity at the fiber surface of the EYST-coated SPOF (SQOF) was investigated, as shown in Fig. S3. The emitted intensity from the EYST-coated HOF surface is higher than the emitted intensity of the EYST-coated SQOF (SPOF) with 11

the same EYST coating because the luminous intensity, at the EYST-coated SQOF (SPOF) surface, is based on the evanescent field which was not absorbed by the EYST film, as shown in Fig. 2d. These facts indicate that the EYST-coated HOF is a good photocatalytic material due to its high excitation light intensity to the TiO2 and Er3+:YAlO3 particles. 3.2. Surface morphology and composition of the coating material The surface morphology and composition of photocatalytic HOFs are the important parameters that will significantly impact the photocatalytic activity. Thus, the SEM, TEM (see Fig. S4), EDS, XPS, and XRD have been used to investigate the surface morphology and composition of the TiO2-based catalysts, and the experimental results are shown in Fig. 3 (the photographs of HOFs are shown in Fig. S5).

Fig. 3. SEM, EDS, XPS, and XRD images of photocatalytic HOFs (Int. and BE, are respectively, of Intensity and Binding Energy; the SEM, EDS and XRD of EYST sample were adapted from our earlier published works [18,19]). SEM images (a), EDS images (b), XPS images (c), and XRD images (d) of the photocatalytic coatings. 12

Fig. 3a shows that the prepared T and ST coatings exhibit a similar, uniform, and porous structure, Fig. S4 also shows a similar structure of the T and ST composite, because the SiO2 and TiO2 nanoparticles have comparable diameters. Nevertheless, in Fig. 3a, the EYST sample shows a slightly different structure because of the doping of Er3+:YAlO3 particles, which have a larger diameter, as shown in Table S1, and the SiO2 and TiO2 nanoparticles are attached to the Er3+:YAlO3 particles (see Fig. S4). The EDS results shown in Fig. 3b demonstrate that the element of Ti and O, elements of O, Si, and Ti, and elements of O, Si, Ti, Y, Er and Al are, respectively, presented in the T, ST, and EYST coating (EDS data are given in Table S2). In Fig. 3c, all coatings exhibit three strong peaks at 286, 458.4, and 534 eV corresponding to C 1s, Ti 2p3/2 (TiO2), and O 1s (lattice oxygen of TiO2), respectively, revealing that the T, ST, and EYST coatings contain the TiO2 nanoparticles. Compared with the XPS spectrum of the T coating, new absorption bands at 102.9 and 154.6 eV appear; they are assigned to the Si 2p and Si 2s, implying that SiO2 nanoparticles must exist in the ST and EYST coatings. Furthermore, two weak peaks at 157.6 and 159.6 eV are revealed in the EYST coating spectrum; they are ascribed to the Y 3d5/2 and Y 3d3/2, verifying that Y3+ is in the chemical state of Er3+:YAlO3 in the EYST. In Fig. 3d, the nine primary peaks at 2θ = 25, 38, 48, 54, 55, 62.5, 69, 70, and 75 were assigned to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) reflection planes of tetragonal crystals of anatase TiO2, respectively, further confirming that all coatings contain the TiO2 nanocatalyst. Importantly, the additional characteristic diffraction peaks at 2θ = 34.4, 41.9, and 42.7 were discovered on the surface of the EYST coating and were assigned to the (400), (220), and (022) of the Er3+:YAlO3 particles, proving that the 45 nm Er3+:YAlO3 (the average particle diameter was calculated using Scherrer's formula, the results are shown in Table S1) has been synthesized

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successfully. Thus, Fig. 3 illustrates that the T, ST, and EYST coating is composed of TiO2, SiO2/TiO2, and Er3+:YAlO3/SiO2/TiO2, respectively. 3.3. Photocatalytic activity of photocatalytic HOFs with uniform coating To

evaluate

photocatalytic

activity

of

the

prepared

HOFs

with

uniform photocatalytic coating (100-µm-thick coating), the 4-CP degradation (Ct/C0), TOC removal (Ct/C0), and Cl− concentration are investigated, as shown in Fig. 4.

Fig. 4. Change in 4-CP concentration (a) and TOC (b) and the formation of Cl− (c), respectively (pH 7, and temperature of 30 oC). Fig. 4 shows that the reactor-employed uncoated HOFs does not produce a significant change in the 4-CP, TOC, and Cl−. However, when the T-, ST-, and EYST-coated HOFs are employed, the 4-CP, TOC, and Cl− show a significant change because the photo-mineralization of 4-CP occurs. Particularly, the reactor-employed EYST-coated HOFs exhibits the highest photocatalytic activity than other samples, including 4-CP degradation of 45.20 %, TOC removal of 21.5%, and dechlorination of 41.32% (total Cl- theoretical quantity of 777.83 µmol/L, the theoretical Cl- released quantity of 351.58 µmol/L within 10 h). The highest photocatalytic activity of the EYST-coated HOFs can be explained as follows: 1) The EYST-coated HOFs have the high excitation light intensity of TiO2 and Er3+:YAlO3 because the whole light rays in fiber core are transmitted into EYST coating (see Fig. 2c). The increased excitation 14

light intensity, that is, the UV light in the spectral range of 360–387 nm, can be used directly to generate more electron−hole pairs by the TiO2 particles. The generated holes can directly decompose 4-CP and can oxidize water molecules to form hydroxyl radicals (HO·), and the injected electron can then be scavenged by adsorbed oxygen for producing oxygen superoxide radicals (·O-2 ). The HO· and ·O-2 that are produced can mineralize the 4-CP, as shown in Fig. 5. 2) The upconversion of a luminescence agent (Er3+: YAlO3) continuously absorbs visible light and converts it to UV light. In particular, the absorbed light at 455 nm, 486 nm, 533 nm, 542 nm, 652 nm could emit UV light with a wavelength of 360 nm, 318 nm, 320 and 360 nm, 320 nm, 338 and 357 nm, respectively [20,21]; representative luminescence spectra of the EYS material are shown in Fig. S6. The converted UV light is directly transferred to the TiO2 particles and performs the 4-CP degradation. 3) The Er3+: YAlO3 nanoparticle heterojunctions can induce formation of oxygen vacancies and enhance charge transfer from 4-CP [22]. 4) The SiO2 doping with a high bandgap (8–9 eV) can act as a potential trap for the photogenerated electrons and as a result will inhibit the recombination rate and significantly enhance the quantum and photocatalytic efficiency [23]. Of course, the doping of SiO2 improves the uniformity of the photocatalytic activity of the prepared HOFs because the SiO2 optimizes the distribution of excitation light intensity in the photocatalytic film. Thus the photocatalytic activity of the EYST-coated HOF is higher than that of the Er3+: YAlO3/TiO2 (EYT)-coated HOF, because the 4-CP degradation of 42.08%, TOC removal of 19.12%, and dechlorination of 36.27% of the EYT-coated HOF is lower than that of the EYST-coated HOF, respectively, as shown in Fig. 4 and Fig. S7. Furthermore, the photocatalytic activity of ESYT-coated SQOF and ESYT-coated SPOF has been investigated and was present at a very low level (see Fig. S8) because

15

of the low excitation light intensity of TiO2. These facts further verify that enhancement of the excitation light intensity of TiO2-based composite film is very important for promoting the photocatalytic activity.

Fig. 5. 4-CP degradation mechanism by using EYST-coated HOF. 3.4. Optical and photocatalytic properties of HOFs with gradient EYST coating Although the ESYT-coated HOF shows improved photocatalytic activity, the 4-CP degradation, TOC removal, and dechlorination are still undesirable because the unreasonable EYST coating thickness distribution on the HOF surface. To further enhance the photocatalytic activity, we prepared a gradient distribution of the EYST coating on the HOFs. The effects of the thickness distribution of the EYST coating on the optical and photocatalytic activity of HOFs are shown in Fig. 6. In Fig. 6, the coating thickness of the EYST-coated HOF_ i (i=I, II, III, …, V) close to the 2-cm-length fiber-incident end (Fig. S2) is 10 μm; the coating thickness close to the fiber hemispherical tip end are 100, 120, 140, 160, and 180 μm, respectively.

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Fig. 6. Luminous intensity along the axial direction (a) and along the radial direction at an axial length of 4 cm (b). (c) Structure and picture of a HOF with the gradient EYST coating. (d) 4-CP degradation employing the HOFs with the gradient EYST coating (pH 7.0, temperature 30oC). Figs. 6a–b show that the unevenness of the emission intensity of the EYST-coated HOF_i along the axial and radial direction is significantly improved by increasing the coating thickness near the fiber hemispherical tip end (the thickness of 100 μm is increased to 180 μm). These results can be attributed to the density distributions of TiO2 and Er3+: YAlO3 increasing with increasing fiber length, as shown in Fig. 6c; thus, the induction ability of the coating for the light at the coating-cladding interface increases, which leads to the increases on the light intensity in the coating and transmitted scattered intensity at the interface between the coating and external environment. However, when the coating thickness is more than 160 μm near the fiber

17

hemispherical tip end, the luminous intensity of the EYST-coated HOFs is hardly increased, as shown in Figs. 6a–b (EYST-coated HOF_IV and HOF_V). The reasons are as follows. Although the thick EYST coating increased the amount of TiO2 and Er3+: YAlO3, they are distributed on the outer photocatalytic layer and do not contribute to enhancing the light intensity in the coating. Unfortunately, the light attenuation in the coating increases with increasing of coating thickness. Accurately, the luminous intensity of the EYST-coated HOF will be further decreased with an increase in the coating thickness near the fiber hemispherical tip end (see Fig. S9). In Fig. 6d, one can see that the 4-CP degradation first increased with increasing EYST coating thickness near the fiber hemispherical tip end in the thickness rage of 100 to 160 μm, and then the degradation performance decreased when the coating thickness further increased. Particularly, the reactor-employed EYST-coated HOF_IV shows the highest 4-CP degradation of 43.53%, which is 1.38 and 1.25 times that of the EYST-coated HOF_I and EYST-coated HOF, respectively. These facts can be attributed to the following. First, the excitation light intensity of TiO2 and Er3+: YAlO3 along the fiber length increased with increasing coating thickness in the range of 100 to 160 μm. The gradient EYST coating improves the distribution of the excitation light intensity along the fiber length, helping to enhance the photocatalytic activity of the EYST-coated HOF and improving its uniformity. Second, the amounts of the HO· and ·O-2 at the fiber surface increase with increasing EYST coating thickness because of the optimized excitation light. Third, the degeneration of the 4-CP degradation is because the thicker EYST coating results in the light limitation of the outer photocatalytic layer and increases the recombination of electrons and holes.

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3.5. Effects of pH and temperature on the photocatalytic activity To realize a new level of the 4-CP degradation, we investigated the effects of the temperature and initial pH on the photocatalytic activity of the EYST-coated HOF_IV, as shown in Fig. 7.

Fig. 7. (a) The 4-CP degradation under different initial pH within the first 4 h. (b) The 4-CP degradation, TOC removal, and dechlorination at initial pH of 6.0 and temperature of 30 oC. (c) The 4-CP degradation under different temperature within the first 4 h. (d) The 4-CP degradation, TOC removal, and dechlorination at initial pH of 6.0 and temperature of 55 oC. In Fig. 7a, the 4-CP degradation shows a complex dependence on the initial pH of the 4-CP solution, which can be explained as follows. First, the photodegradation mechanisms of the 4-CP include three aspects, as shown in Fig. 5, namely, hydroxyl radical attack, direct oxidation by the positive hole and direct reduction by the electron in the conducting band. Second, the photocatalytic degradation process is

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related to the ionization state of the TiO2 surface in EYST photocatalytic film [24]. The photocatalytic degradation process is significantly affected by the initial pH and the pore structure of the photocatalytic film because the adsorption of 4-CP molecules onto the surface of the photocatalytic film depends on the pH and its structure and is an important step for the photocatalytic reactions to take place [24]. Third, the zero point of charge (pHzpc) of TiO2 is around 6.25 and shows a positive charge in the acidic medium (pH < 6.25) and negative charge under alkaline conditions (pH > 6.25). In acidic aqueous solution with a pH < 6.25, the photocatalytic activity of the TiO2 is controlled by the positive holes because more oxidation species are formed and more 4-CP molecules can interact with TiO2 surface sites via hydrogen bonding to be adsorbed on TiO2 [25]. In particular, the contents of the hydroxyl group continuously increase when the pH closes to the TiO2 pHzpc in an acidic medium [24]. Thus, the 4-CP degradation increases with increasing pH in the range of 3.0 to 6.0, and the 4-CP degradation reached a maximum at pH 6.0 because the pH is close to the pHzpc of TiO2. However, the activity is controlled by OH· at neutral or high pH levels [24], because OH· is more easily generated by oxidizing more hydroxide ions available on the TiO2 surface in an alkaline solution. However, a Coulombic repulsion forms between the negatively charged surface of the photocatalyst and the hydroxide anions exposed when the photocatalyst is immersed in an alkaline solution. This repulsion could prevent the formation of OH·, and thus, the 4-CP degradation rate decreases with increasing pH in the range of 6.0 to 8.0. Furthermore, a very high pH has been found to be favorable, that is, the degradation performance increases in the range of 8.0 to 10.0, this fact can be explained as follows. The pKa of 4-CP is 9.25 [26,27], thus, when the pH value of the solution is close to the pKa of 4-CP but higher than the pHzpc of TiO2, photocatalyst can produce more OH· that can make up for the 4-CP

20

molecules hampering the adsorption on the negatively charged surface. Unfortunately, an unlimited increase in the pH is not desirable because the 4-CP degradation decreases again with increasing pH above 10.0, the reasons are as follows. The 4-CP is present in its ionized form and does not readily accumulate at the TiO2 surface in high pH solution, and the contribution of increased OH· to the 4-CP degradation is not being enough to make up for the Coulombic repulsion, Fourth, the 4-CP degradation in the acidic medium is higher than in the alkaline solution, which can be explained as follows. The pHzpc of TiO2 is lower than the pKa values of 4-CP, as a result, the content of the positive holes produced in the acidic medium is higher than the content of the hydroxyl radicals in the alkaline solution; the surface of the photocatalyst can absorb more 4-CP molecules in the acidic medium than in the alkaline solution. Fig. 7b shows a high photocatalytic activity of the EYST-coated HOF_IV at pH 6.0 and temperature of 30 oC. The results of 4-CP reduction of 61.91%, TOC removal of 35.18%, and dechlorination of 56.74% were achieved employing the EYST-coated HOF_IV within 10 h (the theoretical Cl- released quantity of 481.55 µmol/L within 10 h). These observations prove that pH plays an important role in the enhancement of the photocatalytic activity of the EYST-coated HOFs. Fig. 7c shows the 4-CP degradation increases with increasing reaction temperature in the range of 20 to 55 oC, while temperatures higher than 55 °C lead to a decrease in the degradation. The results can be explained as follows. First, although the photocatalytic reaction is less influenced by the temperature less that 100 °C in water because of the low activation energy of photocatalytic reactions, for the degradation of 4-CP 1.3 kJ mol−1, an increase in temperature helps the degradation of 4-CP to compete more effectively with eCB− − hVB+ recombination [28]; second, the transfer rates of the photocatalytic substrate and product in the EYST photocatalytic film

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increase with increasing temperature; third, the rising temperature then generates higher amounts of photogenerated carriers reaching the surface of the EYST film [29], and the TiO2 produced ·O -2 more quickly with the increasing temperature [30]. However, a possible explanation in the case of the temperatures above 55 °C is due to the lower saturation value of oxygen in the solution. The dissolved oxygen concentration is an important factor because it regulates the photocatalytic mechanism by capturing the photogenerated electrons and increases the desorption of reactants from the EYST surface [29]. In Fig. 7d, the reactor-employed EYST-coated HOF_IV realizes a new level of the 4-CP degradation at pH of 6.0 and temperature of 55 oC, and shows 69.82% 4-CP degradation (Fig. S10 shows the detailed results of the 4-CP degradation), 42.57% TOC removal, and 65.21% dechlorination within 10 h (the theoretical Cl- released quantity of 543.08 µmol/Lwithin 10 h); these results are better than those obtained in Fig. 7b. These facts illustrate that the temperature plays a significant role to enhance the photocatalytic activity of the EYST-coated HOF_IV. The 4-CP degradation using the EYST-coated HOF_IV at a temperature of 55 oC and pH 6.0 is 2.28,1.56, 2.65, and 3.83 times that of the T-coated HOF (Fig. 4), EYST-coated HOF (Fig. 4), EYST-coated SQOF (Fig. S8), and EYST-coated SPOF (Fig. S8) at a temperature of 30 oC and pH 7.0, respectively. Furthermore, when the photocatalytic activity of the fibers is evaluated using the total surface area of the photocatalyst film, the photocatalytic activity (4-CP degradation rate of 0.037 mg/cm2/h) of the EYST-coated HOF_IV is higher than that of TiO2-coated plastic optical fiber (numerical aperture of 0.5) with a 4-CP degradation rate of 0.017 mg/cm2/h [31], a coupling light emitting diodes with photocatalyst (TiO2)-coated silica optical fiber

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(numerical aperture of 0.39) with a methylene blue degradation rate of 0.015 mg/cm2/h [32].

4. Conclusions We developed a novel, high photocatalytic activity three-layer photocatalytic HOF employing a gradient Er3+:YAlO3/SiO2/TiO2. The advantages of the created photocatalytic HOF overcome the fundamental problem of low excitation light intensity to the photocatalyst associated with SOFs with core-clad structure; the Er3+:YAlO3 dopant in TiO2 film extends its visible light-driven photocatalytic activity; the SiO2 dopant inhibits the recombination of electrons and holes and improves the light distribution in EYST coating; optimized pH and temperature enhance the oxidation species and mass transfer rates, respectively. The experimental results show that the high photocatalytic activity of the EYST-coated HOF_IV toward degradation of 4-CP aqueous solution in the spectral range of 360–780 nm was obtained at initial pH of 6.0 and temperature of 55 oC. The high activity of the reported HOFs under optimized conditions is 1.56, 2.65, and 3.83 times that of the EYST-coated HOF, EYST-coated SQOF, and EYST-coated SPOF employed under unoptimized conditions (temperature of 30 oC and pH of 7.0), respectively. Our work presents a new way to design a high-performance photocatalytic reaction system for environmental applications. However, in this work, we investigated only the photocatalytic activity of the single photocatalytic HOF toward degradation of 4-CP aqueous solution in the spectral range of 360–780 nm. Further research is needed to explore the mechanisms facilitating phenolic degradation, improvement of the photocatalyst and structure of the photocatalytic fiber to enhance photocatalytic activity, and fabrication of the

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photocatalytic fiber bundle to degradation of dioxane, perfluorooctanesulfonate, and polychlorinated biphenyls.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2018.##.###

Notes The authors declare no competing financial interest.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51876018), the Foundation and Frontier Research Project of Chongqing of China (cstc2016jcyjA0311, cstc2017jcyjAX0268), Postgraduate Research Innovation Project of Chongqing (CYS18309), and the University Innovation Team Building Program of Chongqing (grant No. CXTDX201601030).

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References [1] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop, P.S.M. Dunlop, J. A. Byrne, K. O'Shea, M.H. Entezari, M.H. Entezari, D.D. Dionysiou, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B Environ. 125 (2012) 331–349. [2] R. Molinari, C. Lavorato, P. Argurio, Photocatalytic reduction of acetophenone in membrane reactors under UV and visible light using TiO2 and Pd/TiO2 catalysts, Chem. Eng. J. 274 (2015) 307–316. [3] C. Yu, W. Zhou, H. Liu, Y. Liu, D.D. Dionysiou, Design and fabrication of microsphere photocatalysts for environmental purification and energy conversion, Chem. Eng. J. 287 (2016) 117–129. [4] M.N. Subramaniam, P.S. Goh, W.J. Lau, Y.H. Tan, B.C. Ng, A.F. Ismail, Hydrophilic hollow fiber PVDF ultrafiltration membrane incorporated with titanate nanotubes for decolourization of aerobically-treated palm oil mill effluent, Chem. Eng. J. 316 (2017) 101–110. [5] L. Li, D. Tang, Y. Song, B. Jiang, Dual-film optofluidic microreactor with enhanced light-harvesting for photocatalytic applications, Chem. Eng. J. 339 (2018) 71–77. [6] R.E. Marinangeli, D.F. Ollis, Photoassisted heterogeneous catalysis with optical fibers: I. Isolated single fiber, AIChE J. 23 (1977) 415−426. [7] K. Hofstadler, R. Bauer, S. Novalic, G. Heisler, New reactor design for photocatalytic wastewater treatment with TiO2 immobilized on fused-silica glass fibers: photomineralization of 4-chlorophenol, Environ. Sci. Technol. 28 (1994) 670–674. 25

[8] A. Danion, J. Disdier, C. Guillard, O. Païssé, N. Jaffrezic-Renault, Photocatalytic degradation of imidazolinone fungicide in TiO2-coated optical fiber reactor, Appl. Catal. B Environ. 62 (2006) 274–281. [9] K. Natarajan, T.S. Natarajan, H.C. Bajaj, R.J. Tayade, Photocatalytic reactor based on UV-LED/TiO2 coated quartz tube for degradation of dyes, Chem. Eng. J. 178 (2011) 40–49. [10] S. Zhang, L. Chen, H. Liu, W. Guo, Y. Yang, Y. Guo, M. Huo, Design of H3PW12O40/TiO2

and

Ag/H3PW12O40/TiO2

film-coated

optical

fiber

photoreactor for the degradation of aqueous rhodamine B and 4-nitrophenol under simulated sunlight irradiation, Chem. Eng. J. 200 (2012) 300–309. [11] K. Baba, S. Bulou, M. Quesada-Gonzalez, S. Bonot, D. Collard, N.D. Boscher, P. Choquet, Significance of a Noble Metal Nanolayer on the UV and Visible Light Photocatalytic Activity of Anatase TiO2 Thin Films Grown from a Scalable PECVD/PVD Approach, ACS Appl. Mater. Inter. 9 (2017) 41200–41209. [12] L. Lin, H. Wang, P. Xu, Immobilized TiO2-reduced graphene oxide nanocomposites on optical fibers as high performance photocatalysts for degradation of pharmaceuticals, Chem. Eng. J. 310 (2017) 389–398. [13] M. Ibanescu, Y. Fink, S. Fan, E.L. Thomas, J.D. Joannopoulos, An all-dielectric coaxial waveguide, Science 289 (2000) 415–419. [14] X. Xin, N. Zhong, Q. Liao, Y. Cen, R. Wu, Z. Wang, High-sensitivity four-layer

polymer

fiber-optic

evanescent

wave

sensor, Biosens.

Bioelectron. 91 (2017) 623–628. [15] N. Zhong, M. Zhao, L. Zhong, Q. Liao, X. Zhu, B. Luo, Y. Li, A high-sensitivity fiber-optic evanescent wave sensor with a three-layer structure

26

composed of Canada balsam doped with GeO2, Biosens. Bioelectron. 85 (2016) 876–882. [16] R. Han, J.W. Ha, C. Xiao, Y. Pei, Z. Qi, B. Dong, N.L. Bormann, W.Y. Huang, N. Fang, Geometry-Assisted Three-Dimensional Superlocalization Imaging

of

Single-Molecule

Catalysis

on

Modular

Multilayer

Nanocatalysts, Angew. Chem. Int. Edit. 126 (2014) 13079–13083. [17] N.B.

Zhong,

X.

Zhu,

Q.

GeO2-SiO2-chitosan-medium-coated

Liao,

Y.Z.

hollow

Wang,

optical

fiber

R. for

Chen, cell

immobilization, Opt. Lett. 38 (2013) 3115–3118. [18] N. Zhong, M. Chen, Z. Wang, X. Xin, B. Li, Photochemical device for selective detection of phenol in aqueous solutions, Lab Chip 18 (2018) 1621–1632. [19] N. Zhong, M. Chen, H. Chang, T. Zhang, Z. Wang, X. Xin, Optic fiber with Er3+:YAlO3/SiO2/TiO2 coating and polymer membrane for selective detection of phenol in water, Sens. Actuators, B 273 (2018) 1744–1753. [20] H. Yang, Z. Dai, Z. Sun, Upconversion luminescence and kinetics in Er3+: YAlO3 under 652.2 nm excitation, J. Lumin. 124 (2007) 207–212. [21] D. Zhou, J. Shi, S. Dong, M. Huo, Response to Comment on “Intimate Coupling of Photocatalysis and Biodegradation for Degrading Phenol Using Different Light Types: Visible Light vs UV Light”, Environ. Sci. Technol. 49 (2015) 13077–13078. [22] S.P. Sahu, S.L. Cates, H.I. Kim, J.H. Kim, E.L.Cates, The Myth of Visible Light Photocatalysis Using Lanthanide Upconversion Materials, Environ. Sci. Technol. 52 (2018) 2973–2980.

27

[23] J. Lyu, L. Sun, J. Zhong, Q. Fu, M. Zhang, S. Wang, J. Li, Shielding of surface photogenerated charges by SiO2 coating for the photocatalytic degradation of air pollutants, Chem. Eng. J. 303 (2016) 314–321. [24] I.K. Konstantinou, T.A. Albanis, TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal. B Environ. 9 (2004) 1–14. [25] A. Sharma, B.K. Lee, Growth of TiO2 nano-wall on activated carbon fibers for enhancing the photocatalytic oxidation of benzene in aqueous phase, Catal. Today 287 (2017) 113–121. [26] O. Hamdaoui, E. Naffrechoux, Sonochemical and photosonochemical degradation of 4-chlorophenol in aqueous media, Ultrason. Sonochem. 15 (2008) 981–987. [27] S. Hejda, M. Drhova, J. Kristal, D. Buzek, P. Krystynik, P. Kluson, Microreactor as efficient tool for light induced oxidation reactions, Chem. Eng. J. 255 (2014) 178–184. [28] J. Saien, M. Asgari, A.R. Soleymani, N.Taghavinia, Photocatalytic decomposition of direct red 16 and kinetics analysis in a conic body packed bed reactor with nanostructure titania coated Raschig rings, Chem. Eng. J. 151 (2009) 295–301. [29] A. Chatzitakis, C. Berberidou, I. Paspaltsis, G. Kyriakou, T. Sklaviadis, I. Poulios,

Photocatalytic

degradation and drug activity reduction of

chloramphenicol, Water. Res. 42 (2008) 386–394.

28

[30] F. Meng, Y. Liu, J. Wang, X. Tan, H. Sun, S. Liu, S. Wang, Temperature dependent photocatalysis of g-C3N4, TiO2 and ZnO: differences in photoactive mechanism,. J. Colloid. Interf. Sci. 532 (2018) 321–330. [31] S. Kim, M. Kim, S.K. Lim, Y. Park, Titania-coated plastic optical fiber fabrics for remote photocatalytic degradation of aqueous pollutants, J. Environ. Chem. Eng. 5 (2017) 1899–1905. [32] L. Ling, H. Tugaoen, J. Brame, S. Sinha, C. Li, J. Schoepf, J. Jared Schoepf, K. Hristovski, J. Kim, C. Shang, P. Westerhoff, Coupling light emitting diodes with photocatalyst-coated optical fibers improves quantum yield of pollutant oxidation, Environ. Sci. Technol. 51 (2017) 13319–13326.

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Graphical Abstract

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Highlights 1. A novel optical hollow-fiber with high photocatalytic activity was developed. 2. A photocatalyst of Er3+:YAlO3/SiO2/TiO2 was graded on the surface of the fibers. 3. Photocatalytic activity of a single fiber toward degradation of 4-CP was tested. 4. Optimized operating conditions to enhance 4-CP degradation were investigated. 5. Photocatalytic fiber shows high activity toward degradation of 4-CP. [33]

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