Hematite microcube decorated TiO2 nanorods as heterojunction photocatalyst with in-situ carbon doping derived from polysaccharides bio-templates hydrothermal carbonization

Journal of Alloys and Compounds xxx (xxxx) xxx

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Hematite microcube decorated TiO2 nanorods as heterojunction photocatalyst with in-situ carbon doping derived from polysaccharides bio-templates hydrothermal carbonization Mohamad Azuwa Mohamed a, b, *, Nurashina Abdul Rahman b, M.F. M. Zain c, **, Lorna Jeffery Minggu a, Mohammad B. Kassim a, b, Juhana Jaafar d, Shuaiba Samad a, Mohd Sufri Mastuli e, f, Roong Jien Wong g a

Fuel Cell Institute (SELFUEL), Universiti Kebangsaan Malaysia, 43600, UKM Bangi, Selangor, Malaysia Department of Chemical Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600, UKM Bangi, Selangor, Malaysia c Sustainable Construction Materials and Building Systems (SUCOMBS) Research Group, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600, Bangi, Malaysia d Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310, Skudai, Johor Bahru, Malaysia e Centre for Nanomaterial Research, Institute of Science, Universiti Teknologi MARA, 40450, Shah Alam, Selangor, Malaysia f School of Chemistry and Environment, Faculty of Applied Sciences, Universiti Teknologi MARA, 40450, Shah Alam, Selangor, Malaysia g Particles and Catalysis Research Group, School of Chemical Engineering, The University of New South Wales, Sydney, NSW, 2052, Australia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 March 2019 Received in revised form 7 November 2019 Accepted 21 November 2019 Available online xxx

The novel in-situ formation of a heterojunction photocatalyst consisting of C-doped TiO2 nanorods decorated on the surface of C-doped a-Fe2O3 microcubes was successfully achieved using a one-pot hydrothermal carbonization synthesis. In this work, the treated kapok fibers (t-KF) used as a polysaccharide bio-template provided a dual function for crystal growth control and in-situ carbon doping of the heterojunction photocatalyst. It was found that the a-Fe2O3 precursor concentration plays an essential role in the unique and well-developed C-doped TiO2/a-Fe2O3 heterojunction formation. Assessment of photocatalytic activity of all samples indicated that the sample prepared with 0.25 M of aFe2O3 precursor concentration (BT-TF-0.25) exhibited the most efficient bisphenol A photodegradation in aqueous solution. The highest photocatalytic activity of BT-TF-0.25 under simulated solar irradiation was mainly associated with C-doping and favorable heterojunction formation between C-doped TiO2 and Cdoped a-Fe2O3. Excellent charge carrier and separation were confirmed from the photocurrent response and photoluminescence spectroscopy analysis. Overall, this study is expected to contribute to the development of more efficient visible light active heterojunction photocatalyst systems, as well as demonstrating the versatility of polysaccharide materials as a green and low-cost bio-templates. © 2019 Elsevier B.V. All rights reserved.

Keywords: Bio-templates In-situ heterojunction formation In-situ carbon doping Charge separation Bisphenol A TiO2/a-Fe2O3

1. Introduction Photocatalysis technology is undoubtedly a green and clean technology, as it ultimately depends on light irradiation to generate renewable energy and overcome environmental contamination. Titanium dioxide (TiO2) is a popular photocatalyst material that is

* Corresponding author. Department of Chemical Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600, UKM Bangi, Selangor, Malaysia. ** Corresponding author. E-mail addresses: [email protected] (M.A. Mohamed), [email protected]. my (M.F. M. Zain).

actively utilized for photocatalytic hydrogen production via water splitting, CO2 photoreduction for the generation of value-added products, and wastewater treatment [1]. However, its limitation to absorb visible light irradiation (400 nme700 nm), which consists of ~43% of solar light, is a disadvantage for its practical and efficient application. A promising approach to enhance its visible light absorption capability is to incorporate metal and non-metal doping. It has been reported that non-metal doping, such as with carbon and nitrogen, is an effective means to improve the visible light absorption capability by narrowing the band gap due to their comparable atomic size with oxygen [2e6]. Until now, various methods were suggested to introduce the

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dopant inside the lattice structure of TiO2, in which the most common approaches are sol-gel and hydrothermal syntheses due to their simplicity and low-cost [2,7e10]. Recently, there has been considerable progress in the utilization of polysaccharides as biotemplates in the preparation of photocatalysts since the biotemplates can control the growth of nanoparticles, while inducing a unique structure, and acting as a photocatalyst support [11e20]. The availability of the most abundant and renewable natural resources of polysaccharide materials have also motivated the utilization of this template strategy. For instance, the hierarchical urchin and carpet-like needle TiO2 superstructures have been successfully prepared under hydrothermal conditions using cotton wools as the bio-template [21], resulting in remarkable improvement for phenol photodegradation in aqueous solution. Moreover, bio-templates based on polysaccharide materials were also considered as in-situ C-doping sources for metal oxides to enhance visible light absorption [12,22,23]. For instance, Shao and co-workers successfully prepared a visible light-active, carbon decorated, and nitrogen-doped TiO2 photocatalyst following a hydrothermal synthesis using chitosan as the bio-template [23]. Another aspect of highly efficient photocatalyst preparation is that it must possess excellent charge carrier and separation ability during photocatalytic reactions. In this case, the charge carrier and separation characteristics of pristine TiO2 are inefficient requiring enhancement strategies. The construction of a TiO2-based heterojunction photocatalyst prepared through creating an close contact between two or more semiconductors demonstrated a promising approach to overcome this drawback [24,25]. The heterojunction structures can promote charge carrier mobility between different components of the samples, and thus it was reported that coupling TiO2 with other low band gap photocatalysts enhanced not only the charge separation but also improved visible light harvesting [26,27]. For instance, hematite photocatalyst (a-Fe2O3) was utilized in order to overcome issues related to pristine TiO2 [28e32], as aFe2O3 which is a semiconductor with a narrow-band gap of approximately 2.1 eVe2.3 eV, with demonstrated promise for industrial applications due to its easy fabrication, chemical stability, nontoxicity, abundance, and low cost [33,34]. Several studies have been reported on the bio-template preparations of a-Fe2O3, through utilization of regenerated cellulose fibers [35], cellulose nanocrystals [36,37], and extracts of green tea (Camellia sinensis) leaves [38]. To the best of our knowledge, in-situ preparation of a C-doped TiO2/a-Fe2O3 heterojunction photocatalyst employing treated kapok fibers (t-KF) as a bio-template has not been reported in the literature. Herein, the facile synthesis of a C-doped TiO2/a-Fe2O3 heterojunction photocatalyst following a hydrothermal biotemplate carbonization approach is reported for the first time. The impregnated Ti4þ and Fe3þ metal cations were bound to the tKF via a strong electrostatic interaction due to the presence of cellulose polymer structures containing electron-rich oxygen atoms of polar hydroxyl groups on the surface of t-KF. The effect of the a-Fe2O3 precursor concentration on the formation of the heterojunction structure between TiO2 and a-Fe2O3 was elucidated in detail. A plausible mechanism for C-doped TiO2/a-Fe2O3 heterojunction photocatalyst formation in the presence of t-KF biotemplate for hydrothermal synthesis is also discussed. The resulting heterojunction photocatalyst containing an appropriate amount of a-Fe2O3 exhibited excellent interfacial charge transfer and separation characteristics, ultimately promoting high photocatalytic performance. This study therefore provides a new conceptual scheme for designing highly efficient photocatalysts while utilizing polysaccharide materials as a green and low-cost biotemplate in various new applications.

2. Experimental 2.1. Chemicals and materials Kapok fiber bio-template was obtained from Perusahaan Bonda, at Kati in Kuala Kangsar, Perak, Malaysia. The kapok fibers were pre-treated with NaClO2 according to a previous study [39,40]. Iron chloride hexahydrate (FeCl3$6H2O), isopropanol (CH3)2CHOH) and nitric acid (HNO3) were provided by QReC Malaysia. Titanium-nbutoxide (Ti(OBu)4) was procured from Sigma Aldrich. 2.2. Synthesis of bio-template TiO2/a-Fe2O3 heterojunction photocatalyst Approximately 25 mL of Ti(OBu)4 as the TiO2 precursor was added to 25 mL of isopropanol to avoid vigorous precursor hydrolysis. Treated kapok fibers (t-KF, ~0.4 g) were simultaneously immersed in 75 mL of distilled water, and the Ti(OBu)4/isopropanol solution was added dropwise to the t-KF suspension, followed by addition of a specific amount of FeCl3$6H2O (for concentrations of 0.25, 0.50, or 0.75 M) as the a-Fe2O3 precursor, and stirring for 30 min at room temperature. The mixture was then sonicated for an additional 30 min, and subsequently transferred to a 200 mL Teflon-lined autoclave. The mixture was sealed and kept in the oven at 150  C for 2 h. After cooling to room temperature, the resulting product was calcined at 500  C for 2 h at a heating rate of 5  C min1. The resulting samples were denoted as BT-TF-0.25, BTTF-0.50, and BT-TF-0.75 corresponding to the FeCl3$6H2O concentrations of 0.25, 0.50, and 0.75 M, respectively. The TiO2 biotemplate was prepared following the same procedure, but without addition of the a-Fe2O3 precursor, denoted as BT-TiO2. Pristine TiO2, as a control sample, was also prepared in the same manner but without the addition of t-KF bio-templates or a-Fe2O3 precursor. 2.3. Characterization The crystallinity and phase of the prepared samples were analyzed using a Bruker D8 Advance X-ray diffractometer. Functional group determination was carried out with a PerkinElmer infrared spectrometer with an attenuated total reflection (ATR) accessory. The morphology and structure analysis were evaluated using a field-emission scanning electron microscope (SEM Quanta Fei 400f) and transmission electron microscope (JEOL JEM2010HR). The texture, surface area, and porosity of the prepared samples were analyzed using a BELSORP-mini II instrument from BEL Japan Inc. Details of the chemical composition and binding were obtained with an X-ray photoelectron spectrometer (Fourier Kratos Analytical Axis Ultra DLD/2009). Optical absorption of the prepared samples was analyzed using a UVeviseNIR spectrophotometer (PerkinElmer, Lambda 950). The photoluminescence spectra of prepared samples were obtained with a photoluminescence spectrophotometer (Horiba Scientific). Photocurrent response of the prepared samples was analyzed by using an electrochemical workstation with a standard three-electrode setup consisting of the working electrode, Ag/AgCl reference electrode, Pt counter electrode, and 0.5 M Na2SO4 electrolyte. The working electrode was prepared by drop-casting a sample on a clean FTO substrate. A xenon lamp with solar AM 1.5 illumination (100 mW cm2) was used as the light source. 2.4. Photocatalytic measurement Photodegradation of 10 ppm of bisphenol A (BPA) as a model pollutant was conducted under xenon lamp irradiation (500 W

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with light intensity of ~150 mW/cm2). The sample (50 mg) was dispersed in 100 mL of BPA and stirred for ~ 60 min in the dark to reach the adsorption-desorption equilibrium. The BPA concentration was monitored every 30 min after irradiation at lmax ¼ 277 nm for 270 min using a UVevis spectrophotometer (PerkinElmer Lambda 35). The degradation percentage of BPA was calculated as follows: degradation (%) ¼ (C0 e Ct)/C0, where C0 is the initial concentration and Ct is the concentration of BPA at a specific time. 3. Results and discussion 3.1. Phase and crystallinity The phase and crystallinity of the prepared samples with different Fe2O3 precursor concentrations (0.25, 0.50, and 0.75 M) were obtained using powder X-ray diffractometry (XRD) technique as shown in Fig. 1 (a). Both pure TiO2 and BT-TiO2 exhibit a similar XRD pattern with a mixed anatase/rutile phase as the dominant phase in both samples, whereby the main peak of anatase TiO2 (101) and rutile TiO2 were located at 2q ¼ 25.3 and 27.3 , respectively. Small traces of diffraction peak signals that belong to the brookite phase were observed at 2q ¼ 30.8 in both samples. The in-situ formation of TiO2/a-Fe2O3 heterojunction photocatalyst significantly suppressed the formation of the brookite phase trace, in which its characteristic XRD peak was not observed in BT-TF0.25, BT-TF-0.50, and BT-TF-0.75 samples. The in-situ formation of TiO2/a-Fe2O3 heterojunction photocatalyst in BT-TF-0.25, BT-TF0.50, and BT-TF-0.75 samples was confirmed from the hematite phase of a-Fe2O3 found in the samples. The presence of the hematite phase of a-Fe2O3 is observed at 2q ¼ 24.1, 33.1, 35.6 , 40.8 , 49.4 , 54.0 , 56.2 , 62.4 , 63.9 , 66.0 , and 75.2 , which correspond to the crystal planes of (012), (104), (110), (113), (024), (116), (211), (214), (300), (125) and (220), respectively. It was also observed that the intensity of characteristic TiO2 peaks, especially the peak corresponding to (101) of the anatase phase, were decreased while the peak intensity of hematite increased with increasing a-Fe2O3 precursor concentration. In addition, the main XRD peak of anatase TiO2 (101) in all heterojunction samples was broad compared to the main XRD peak of hematite (104). This suggests that the crystallite size of anatase TiO2 is smaller than the hematite particles.

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Accordingly, the crystallite sizes of TiO2 and a-Fe2O3 in all samples were estimated using the Scherrer equation (tabulated in Table 1). It was found that the estimated crystallite size of BT-TiO2 was smaller than pure TiO2, suggesting that t-KF bio-template serves as a nano-caster to significantly control nanoparticle growth [22]. During the concurrent growth of TiO2 and a-Fe2O3, it was found that the TiO2 particle sizes were relatively comparable between 28.5 and 29.1 nm, as the concentration of a-Fe2O3 precursor increased from 0.25 M to 0.75 M. The a-Fe2O3 precursor concentration during TiO2/a-Fe2O3 heterojunction formation led to controlled of a-Fe2O3 crystallite size growth from 240.7 nm to 1650.7 nm. Higher precursor concentration significantly increased the growth of species in the fixed reaction volume. As a result, the mass transfer of species grown in this way decreased in the reaction medium and induced the formation of larger crystallite [42]. The Fourier-transform infrared (FTIR) spectra of bio-templated TiO2/a-Fe2O3 heterojunction photocatalysts (a-Fe2O3 precursor concentrations of 0.25, 0.50, and 0.75 M) are shown in Fig. 1 (b). As compared to the FTIR spectra of t-KF, all prepared bio-template samples do not exhibit cellulose peaks characteristic of t-KF, in the range from 700 cm1 to 2000 cm1 [39]. The disappearance of these FTIR peaks is expected, as t-KF was decomposes upon calcination at 500  C [43]. Moreover, the presence of the eOH bending mode of adsorbed water on the surface of the prepared biotemplate samples was evident from the presence of a small absorbance peak located at ~1625 cm1. The presence of superficial hydroxyl groups on the surface of the prepared samples could be beneficial for the formation of hydroxyl radicals for higher photocatalytic degradation of organic pollutants. Surprisingly, the FTIR spectra of prepared bio-templated TiO2/a-Fe2O3 heterojunction photocatalysts revealed the emergence of a new absorbance peak located between 2000 cm1 and 2500 cm1. According to a previous study, the emergence of this new absorbance peak could correspond to C]O and graphitic-like carbon [44]. The presence of C]O in the prepared samples most likely is due to the adsorption of CO2 on the sample surface, as the calcination treatment was performed under air. Meanwhile, the presence of graphitic-like carbon in the samples suggests the existence of carbonaceous material or C-doping in the prepared samples, which occurred during decomposition of the t-KF bio-template.

Fig. 1. (a) Powder XRD diffraction pattern and (b) FTIR spectra of bio-templated C-doped TiO2/a-Fe2O3 heterojunction photocatalysts synthesized with different a-Fe2O3 precursor concentrations (0.25, 0.50, to 0.75 M). Powder XRD patterns and FTIR spectra of TiO2 and bio-templated TiO2 are shown for comparison (along with the FTIR spectrum for t-KF).

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Table 1 Detailed BET surface areas, BJH pore volumes, pore size, and weight fraction of the as-prepared samples. Sample

TiO2 BT-TiO2 BT-TF-0.25 BT-TF-0.50 BT-TF-0.75

Crystallite size (nm)a TiO2

a-Fe2O3

35.4 29.2 28.5 28.8 29.1

e e 240.7 966.2 1650.7

BET surface area (m2 g1)

BJH Pore volume (cm3 g1)

Pore size (nm)

81.53 90.22 83.41 71.79 44.23

0.20 0.20 0.15 0.07 0.10

9.63 9.04 7.18 3.69 8.73

Note. a Data obtained from XRD Scherrer equation.

3.2. Morphological and textural analysis The morphology of the prepared samples was analyzed using FESEM and TEM/HRTEM (Fig. 2). The t-KF bio-template exhibited a microtubular structure is shown in Fig. 2(a), unfortunately, was not preserved in bio-templated samples (BT-TiO2, BT-TF-0.25, BT-TF0.50, and BT-TF-0.75) due to being very brittle and possessing high crystallinity metal oxides. As a result, the microtubular structure collapses during calcination treatment. It is interesting to note that the preparation of TiO2 without a bio-template exhibited irregular particle shapes and larger crystallite size ((Fig. 2(b)) as compared to BT-TiO2, which exhibited a relatively uniform size and shape (Fig. 2(c)). It was difficult to distinguish the shape of TiO2 nanoparticles in BT-TF-0.25, whereas the microcube-like structures of aFe2O3 with a rough surface were clearly observed (Fig. 2(d)). The increase in a-Fe2O3 precursor concentration from 0.50 M to 0.75 M led to larger crystallite growth of a-Fe2O3 microcubes with smooth surfaces, followed by severe agglomeration of TiO2 nanoparticles in both samples BT-TF-0.50 and BT-TF-0.75 (Fig. 2(e) and (f), respectively). TiO2 nanorods were distributed on the surface of a-Fe2O3, especially for the sample prepared with a low concentration of aFe2O3 precursor (BT-TF-0.25), as shown in its TEM image. In addition, the surface of a-Fe2O3 in BT-TF-0.25 was coarser than those of BT-TF-0.50 and BT-TF-0.75. The coarser surface could be due to the decoration of TiO2 nanoparticles on the surface of a-Fe2O3 in the BT-TF-0.25 sample, which led to the well-developed interfacial heterojunction between a-Fe2O3 microcubes and TiO2 nanorods. Contradictorily, the surface of a-Fe2O3 became smoother with increasing Fe2O3 precursor, which was also accompanied by larger a-Fe2O3. The smoother surface indicates an inefficient interfacial heterojunction formation between a-Fe2O3 microcubes and TiO2 nanorods. This inefficient interfacial heterojunction formation could also be explained by the huge lattice mismatch between both components. In a heterostructure, the interface between different components can be strained due to the different lattice parameters and symmetries of the two adjacent phases [45]. Moreover, it has been suggested that the increase in particle sizes also increases the lattice constants [46,47]. In this case, the relatively large lattice constant differences promote a huge lattice mismatch, causing an unfavorable heterojunction formation between a-Fe2O3 and TiO2. The agglomeration of TiO2 nanoparticles can be explained by surface energy. It is commonly accepted that smaller particles inherently possess higher surface/interfacial energy. Therefore, to minimize excess surface energy, the TiO2 nanoparticles strive to reduce the higher surface energies by clumping, and thus the agglomeration phenomenon occurs. In addition, it was suspected that a large surface/interfacial energy difference between TiO2 and a-Fe2O3 due to the significant difference in crystallite size between TiO2 and a-Fe2O3 exits. As a result, well-distributed TiO2 nanoparticles decorated on the surface of a-Fe2O3 were not observed in the BT-TF-0.50 and BT-TF-0.75 samples. Moreover, agglomeration

of TiO2 in BT-TF-0.50 and BT-TF-0.75 results in inefficient heterojunction formation for excellent interfacial charge transfer. TEM and HRTEM were employed to gain more insight into the structure of the prepared samples. The TEM image of BT-TiO2 reveals the relatively uniform size of nanorod structures with an average width and length of ~15 nm and ~29.5 nm, respectively (Fig. 2(g)). The HRTEM also shows a clear lattice fringe spacing of 0.35 nm, which is associated with the anatase phase (101) crystal planes (Fig. 2(g1)). This observation is consistent with XRD analysis, whereby the anatase (101) was the dominant phase among the BTTiO2 samples. The TEM image of BT-TF-0.25 revealed that the TiO2 nanorods decorate the surface of a-Fe2O3 microcubes and promote well-developed heterojunction structures [Fig. 2(h)]. HRTEM image of BT-TF-0.25 (Fig. 2(h1)) display lattice fringes of 0.36 nm, 0.27 nm, 0.25 nm, and 0.35 nm corresponded to a-Fe2O3 (012), (104), (110), and anatase TiO2 (101) crystal planes, respectively. The textural analysis of the prepared samples was employed by means of N2 adsorption-desorption analysis, as shown in Fig. 3. The effect of different a-Fe2O3 precursor concentrations on the N2 adsorption-desorption isotherm characteristics of the prepared samples is depicted in Fig. 3(a). It was found that the N2 adsorptiondesorption isotherms of all samples contained hysteresis loop that satisfied the type IV isotherms based on the IUPAC classification [48], indicating that all samples possessed mesoporous structures. Accordingly, the type H2(a) hysteresis loop with a very steep desorption branch was observed for TiO2, and BT-TiO2 suggesting that the isotherm behavior of both TiO2 and BT-TiO2 samples can be attributed to the percolation of liquid nitrogen in a narrow range of cage-like pore networks [49] or ink-bottle-like pores [50]. As compared to pure TiO2, the desorption branch of BT-TiO2 was observed at a lower relative pressure, which indicated that desorption occurred via cavitation [51]. This phenomenon suggests that the pore neck diameter of BT-TiO2 was smaller than pure TiO2, which is consistent with Barrett-Joyner-Halenda (BJH) pore size distribution, as shown in Fig. 3(b). The prepared heterojunction photocatalysts (BT-TF-0.25, BT-TF-0.50, and BT-TF-0.75) exhibited a type H1 hysteresis loop, which is associated with a narrow range of uniform mesopore structures in the samples. The specific surface area, pore-volume, and average pore size of all samples are tabulated in Table 1. It was found that the specific surface area value of heterojunction photocatalysts (BT-TF-0.25, BT-TF-0.50, and BTTF0.75) drastically decreased as compared to the BT-TiO2 sample. In addition, as the a-Fe2O3 precursor concentration increased, the overall specific Brunauer-Emmett-Teller (BET) surface area decreased. The decrease in the specific surface area was associated with the existence of larger particle sizes of a-Fe2O3 relative to TiO2, which is in accordance with the XRD results and morphological analysis. In addition, a higher specific surface area and pore volume would promote greater adsorption capacity for organic pollutants. As a result, higher photocatalytic activity could be expected.

Please cite this article as: M.A. Mohamed et al., Hematite microcube decorated TiO2 nanorods as heterojunction photocatalyst with in-situ carbon doping derived from polysaccharides bio-templates hydrothermal carbonization, Journal of Alloys and Compounds, https://doi.org/ 10.1016/j.jallcom.2019.153143

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Fig. 2. FESEM images of (a) treated kapok fiber, (b) Pristine TiO2, (c) BT-TiO2, (d) BT-TF-0.25, (e) BT-TF-0.50, and (f) BT-TF-0.75. TEM and HRTEM of (g-g1) BT-TiO2 and (h-h1) BT-TF0.25.

3.3. X-ray photoelectron spectroscopy analysis The chemical composition and expected interactions between different components in the BT-TiO2 and BT-TF-0.25 samples were further confirmed by XPS analysis. The high resolution (HR) XPS spectra of C 1s, Ti 2p, O 1s, and Fe 2p are depicted in Fig. 4. In Fig. 4(a), the HR XPS spectrum of C 1s of the sample BT-TiO2 can be deconvoluted into three main peaks located at 283.3 eV, 284.8 eV, and 288.6 eV, while the BT-TF-0.25 sample can be deconvoluted into five peaks at 279.7 eV, 282.1 eV, 284.8 eV, 287.2 eV, and 289.1 eV. The intense peak located at 284.8 eV in both samples was assigned to adventitious carbon species. The binding energy

located at 283.3 eV corresponds to the existence of the carbon dopant in the presence of TieCeO bonds in the BT-TiO2 sample [52]. The formation of TiO2/a-Fe2O3 heterojunction via the biotemplate approach in the BT-TF-0.25 sample produced two small peaks at lower binding energies of 279.7 eV and 282.1 eV. It could also describe the presence of C dopant in the sample [53e55]. In addition, the peaks located from 288.6 to 289.1 eV in the BT-TiO2 and BT-TF-0.25 samples are associated with C]O bonds of carbonate-like species due to the interstitial carbon doping. This is in good agreement with the FTIR analysis. Indeed, carbon doping could be introduced in the lattice structure of both TiO2 and aFe2O3 after decomposition of the t-KF bio-template during

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Fig. 3. (a) N2 adsorption-desorption isotherms and (b) pore size distribution curves of the as-prepared photocatalysts.

Fig. 4. High resolution XPS spectra of (a) C 1s, (b) Ti 2p, (c) O 1s, and (d) Fe 2p for BT-TiO2 and BT-TF.

calcination treatment. This is because both TiO2 and a-Fe2O3 precursors can concurrently interact with the t-KF bio-template via strong covalent or coordinate bonds between the positive charge of

Ti/Fe ion and the electron-rich hydroxyl group of the t-KF biotemplate. As shown in Fig. 4(b), deconvolution of the Ti 2p HR XPS spectra

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afforded two peaks located at 458.5 eV and 464.2 eV for BT-TiO2, whereas the peak located at 459.1 eV and 464.7 eV for the BT-TF0.25 sample. The fitted peaks at 458.5 eVe459.1 eV and 464.2 eVe464.7 eV correspond to Ti 2p3/2 and Ti 2p1/2, respectively, for both samples. It was found that the splitting binding energy between Ti 2p3/2 and Ti 2p1/2 was approximately 5.6 eVe5.7 eV, which is consistent with the standard value of common TiO2 species in both samples. Another important observation was that the formation of TiO2/a-Fe2O3 heterojunction led to a shift of the Ti 2p3/ 2 and Ti 2p1/2 peaks in BT-TF-0.25 to a higher binding energy. This phenomenon suggests that there is a strong interaction between TiO2 and a-Fe2O3 in BT-TF-0.25, indicating that the heterojunction interface between a-Fe2O3 and TiO2 in the BT-TF-0.25 sample is formed well. This strong interaction could possibly led to the decrement of electron density around Ti in the TiO2 lattice structure in BT-TF-0.25 [56,57]. On the other hand, the electron-deficient TiO2 nanoparticles in BT-TF-0.25 could also lead to migration of electrons from a-Fe2O3 to TiO2 since the work function of a-Fe2O3 (5.88 eV) is higher than TiO2 (4.308 eV) [58]. The HR XPS O1s spectrum of BT-TiO2 was deconvoluted into three main peaks located at 530.1 eV, 531.0 eV, and 532.5 eV, which could be assigned to the TieOeTi linkage, TieOH species [59], and CeOeC species resulting from decomposition of the t-KF biotemplate [60,61]. The fitted peak at 530.4 eV of sample BT-TF0.25 was broader and more intense than that of BT-TiO2 due to the overlapping of TieOeTi and FeeOeFe linkage characteristics from TiO2 and a-Fe2O3 in the samples. As compared to BT-TiO2, the slight shift from 530.1 eV to 530.4 eV in BT-TF-0.25 sample is also likely be due to the electron migration phenomenon from a-Fe2O3 to TiO2, which is similar to the Ti 2p HR XPS spectra. On the other hand, the disappearance of the peak characteristic of hydroxyl species present in BT-TF-0.25 might be due to its merging with the CeOeC species peak at ~532.5 eV. As a result, the appearance of the CeOeC peak broadens compared with that of the BT-TiO2 sample. Moreover, the presence of CeOeC is consistent with the O 1s HR XPS spectra, which suggests the existence of carbonate-like species associated with interstitial carbon doping in the BT-TF-0.25 sample. The weak fitted peak observed around 536.5 eV could be related to adsorbed CO2 resulting from the calcination treatment under air [62]. The formation of well-developed a-Fe2O3 in BT-TF-0.25 was further confirmed with the HR XPS spectrum of Fe 2p (Fig. 4(d)). The peaks located at 711.9 eV and 726.1 eV correspond to the Fe 2p2/ 3þ 3 and Fe 2p1/2 of a-Fe2O3. Furthermore, two satellite peaks of Fe located at 718.2 eV and 734.2 eV were also observed in accordance with the electronic state of a-Fe2O3. It is important to note that the binding energies of Fe 2p2/3 and Fe 2p1/2 for a-Fe2O3 in the BT-TF0.25 were located at slightly higher binding energy than the previously reported a-Fe2O3/TiO2 heterojunction system [32,58,63]. The presence of Fe 2p2/3 and Fe 2p1/2 at higher binding energy could result from carbon doping in a-Fe2O3 in the BT-TF-0.25 samples. The electron density around the Fe3þ of a-Fe2O3 decrease due to the presence of carbon doping in the lattice structure, which is more electronegative than Fe3þ of a-Fe2O3. Therefore, the binding energy around Fe3þ in a-Fe2O3 increased, causing a shift to higher binding energies. Ultimately, XPS analysis convincingly indicates that C atoms from t-KF decomposition were in-situ doped in both the TiO2 and a-Fe2O3 lattice structures in the BT-TF-0.25 sample. 3.4. Mechanism for formation of bio-templated C-doped TiO2/aFe2O3 heterojunction photocatalyst derived from treated kapok fibers Based on the aforementioned findings, the concurrent growth of C-doped bio-template TiO2 and a-Fe2O3, as well as its

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heterostructured formation obtained from the hydrothermal carbonization of t-KF, and subsequent calcination treatment can be deliberated as follows. The t-KF mainly consists of cellulose, which possesses a high density of polar OH-groups on its surface, which is beneficial to control the growth and induce the unique shape of metal oxides. Addition of the TiO2 precursor (Ti(OBu)4) and a-Fe2O3 precursor (FeCl3$6H2O) to an aqueous suspension t-KF allowed the Ti4þ and Fe3þ ions to impregnate the three-dimensional (3D) network structure of cellulose until an adsorption equilibrium was established. At this moment both Ti4þ and Fe3þ were competing to interact with the electron-rich hydroxyl groups of t-KF via strong electrostatic interactions to induce the formation of coordination complexes of metal cations and t-KF OH groups [64]. As Fe3þ is more electronegative than Ti4þ, it is more favorable to coordinate with six oxygen atoms between the cellulose polymeric chains and forced Ti4þ to be coordinated to four oxygen atoms at the edge of the cellulose polymeric chains, as illustrated in Fig. 5. This kind of coordination stabilization could be responsible for the decoration of TiO2 on the a-Fe2O3 surface, as shown in the TEM image. Under the hydrothermal conditions, both cellulose polymer-metal ion complexes (Ti4þ and Fe3þ) were initially hydrolyzed with water to form cellulose-Ti(OH)4 and cellulose-Fe(OH)3. This water hydrolysis continues with the polycondensation of both cellulose-metal hydroxides to promote nucleation as well as the formation of 3D metal oxide skeleton within the cellulose polymeric chain. The amorphous 3D metal oxide skeleton continues to grow, and the close-packed skeleton induces formation of TiO2 and a-Fe2O3 colloidal particles. During the formation of colloidal particles of TiO2 and a-Fe2O3, the cellulose polymer can act as a steric stabilizer to control particle growth and shape, as well as prevent the aggregation of particles [65]. At this stage, the polymeric cellulose can also play the dual roles of nanoparticle bio-generator and protecting agent. However, it must be noted that the phenomenon where all coordination sites are fully occupied could possibly occur when an excessive concentration of Fe3þ from the a-Fe2O3 precursor was combined with a limited number of t-KF-OH groups during synthesis. Therefore, the excess Fe3þ is unable to be stabilized by coordination and steric stabilization, resulting in uncontrolled aFe2O3 growth and larger particle sizes of a-Fe2O3, as confirmed by FESEM. As the hydrothermal condition reaches a high pressure and temperature, the destruction of cellulose polymeric structures by hydronium ions via hydrolysis occurs [66]. The hydronium ion is produced under hydrothermal conditions, whereby water dissociates into acidic hydronium ions (H3Oþ) and basic hydroxide ions (OH) [67]. Initially, the b-1,4 glycosidic linkage of cellulose is attacked by the hydronium ions [68]. It was suggested that the hydrothermal conditions successfully hydrolyze the cellulose polymer into various kinds of soluble and insoluble products such as cellobiose, cellohexaose, cellopentaose, cellotetraose, cellotriose, glucose, fructose, and various carboxylic acid (acetic, lactic, propenoic, levulinic, and formic) [66]. In addition, the presence of the FeCl3 precursor might lead to gluconic acid production during cellulose hydrolysis [69]. All hydrolyzed products are carbon sources for formation of the carbon-doped heterojunction photocatalyst. For instance, the soluble hydrolyzed products could possibly adsorb on the surface and diffuse inside the TiO2 and aFe2O3 nanoparticles. Dehydration and fragmentation into several hydrolyzed products could be realized in subsequent reaction stages until complete cellulose carbonization into the carbonaceous material completed. The complete rearrangement and crystallization of TiO2 and a-Fe2O3 nanoparticles proceed upon calcination treatment at 500  C. At this stage, the complete removal of the t-KF bio-template and incorporation of carbon doping into both TiO2 and a-Fe2O3 lattice occurs, as confirmed by XPS analysis.

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Fig. 5. Plausible mechanism for formation of bio-templated C-doped TiO2/a-Fe2O3 heterojunction photocatalyst derived from treated kapok fibers under hydrothermal condition.

3.5. Optical analysis The effects of C-doping and heterojunction formation on optical properties of the prepared samples were analyzed using diffuse

reflectance spectroscopy as depicted in Fig. 6. All prepared samples exhibited excellent optical absorption capability with noticeable broad-spectrum absorption curve from the UV to near-infrared, except for pristine TiO2, which only absorbs UV light irradiation.

Fig. 6. (a) Kubelka-Munk optical absorption curve and (b) band gap determination of bio-templated C-doped TiO2/a-Fe2O3.

Please cite this article as: M.A. Mohamed et al., Hematite microcube decorated TiO2 nanorods as heterojunction photocatalyst with in-situ carbon doping derived from polysaccharides bio-templates hydrothermal carbonization, Journal of Alloys and Compounds, https://doi.org/ 10.1016/j.jallcom.2019.153143

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The effect of carbon doping in BT-TiO2 significantly enhanced the visible light absorption capability, confirming that t-KF acted as a bio-template and provided a carbon dopant source for the BT-TiO2 sample. It is commonly accepted that carbon doping results in band gap narrowing by inducing local states above the valence band edges. As can be seen in Fig. 6(b), the band gap of BT-TiO2 is 2.85 eV, which is lower than that of pristine TiO2 (3.15 eV), further verifying band gap narrowing due to C-doping. In addition, a higher a-Fe2O3 precursor concentration led to apparent band gap narrowing. The band gap for pure a-Fe2O3 was determined as approximately 2.0 eVe2.2 eV [29,30,70]. It is believed that the low band gap of aFe2O3 could extend the absorption band edge of a-Fe2O3/TiO2 heterojunction system towards the visible light region [70]. The band gap values for BT-T-0.25, BT-TF-0.50, and BT-TF-0.75 samples were 2.15 eV, 1.92 eV, and 1.52 eV, respectively. A similar trend could also be found in the same a-Fe2O3/TiO2 heterojunction system, as reported in a previous study, in which the presence of more a-Fe2O3 in the sample led to a lower band gap [32]. However, the band gap value of all TiO2/a-Fe2O3 heterojunction photocatalysts in this work was much lower than several previous studies with similar heterojunction systems (2.95 eV [28], 2.05 eVe2.10 eV [71], and 2.51 eV [72]). It should be noted that the band gap of a a-Fe2O3/ TiO2 heterojunction system in a previous study was not affected by elemental doping. Therefore, as compared to the previously reported band gap, the lower band gap value in the current work is associated with the in-situ C-doping in both TiO2 and a-Fe2O3 under bio-templated hydrothermal synthesis. In addition, as discussed in section 3.4, the TiO2 and a-Fe2O3 precursors were impregnated onto the t-KF surface due to strong interaction between Ti4þ and Fe3þ ions with the OH-group of t-KF. At higher concentrations of a-Fe2O3 precursor, more Fe3þ species would interact with OH-group of t-KF bio-template as compared to Ti4þ species. As a result, a higher concentration of carbon could be doped into the a-Fe2O3 lattice structure during bio-template carbonization, decomposition, atom rearrangement, and crystallization of a-Fe2O3. Therefore, significant band gap narrowing in the prepared samples could be expected, whereby a high concentration of C-doping in a-Fe2O3 induced an upward shift of the a-Fe2O3 valence band in the heterojunction sample. All of the prepared samples could be utilized as visible light-driven heterojunction photocatalysts. 3.6. Photocatalytic properties The photocatalytic activity of the prepared samples was evaluated by photodegradation of bisphenol A (BPA) under simulated solar irradiation using a xenon lamp. As can be seen in Fig. 7(a), in the absence of a photocatalyst, the photolysis of BPA experienced no significant degradation, indicating that it was stable and resistant under light irradiation. Meanwhile, the photodegradation of BPA over TiO2 P25 and pristine TiO2 was considerably low as compared to the carbon-doped heterojunction photocatalyst. It could be expected that the photocatalytic activity of TiO2 P25 and pristine TiO2 was the lowest because it could only be activated under UV light irradiation resulting from its large band gap. The BTTiO2 showed enhanced in photocatalytic activity with 52% BPA degradation which is 1.8 times higher than pristine TiO2. This is mainly due to the BT-TiO2 sample that possessed a larger surface area than pristine TiO2 and its ability to absorb visible light irradiation for photoinduced electron-hole pair formation. The photocatalytic activity of BT-TiO2 was further improved after the formation of the heterojunction with carbon-doped a-Fe2O3. The highest photocatalytic activity in the degradation of BPA was ~79.0% for BT-TF-0.25, followed by 71.0% and 63.0% for BT-TF-0.50 and BT-TF-0.75, respectively. The overall improvement of

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photocatalytic activity was due to the formation of a heterojunction between TiO2 and a-Fe2O3, which led to an efficient charge transfer and suppressed recombination of the photogenerated holeelectron [31]. The photodegradation of BPA over the heterojunction photocatalyst was found to be a pseudo-first order kinetics reaction process according to the following equation:

lnð

C Þ ¼  kKt ¼ kapp t C0

(1)

where kapp is the apparent rate constant (min1), C is the concentration of pollutants (mg L1) at a specific time (t) is the reaction time (min), and C0 is the initial concentration of contaminants (mg L1). The value of the rate constant (kapp) is equal to the corresponding slope of the fitting line kinetics of the disappearance of BPA degradation, as shown in Fig. 7 (b). As expected, BT-TF-0.25 possessed the highest rate constant (kapp ¼ 4.86  103), followed by BT-TF-0.50 (kapp ¼ 3.82  103), BT-TF-0.75 (kapp ¼ 3.23  103), BT-TiO2 (kapp ¼ 2.12  103), and pristine TiO2 (kapp ¼ 0.79  103). Based on the findings in Fig. 7(a) and (b), increasing the amount of a-Fe2O3 significantly reduced the photocatalytic activity of the prepared samples. There are several factors that could lead to this phenomenon. As discussed earlier, increasing a-Fe2O3 precursor concentration led to formation of larger particle sizes of a-Fe2O3, which was followed by severe agglomeration of TiO2 in the samples. These situations result in a smaller surface area, which reduces the adsorption capability for BPA on the surface, as shown in Fig. 7(a) under dark conditions. Low adsorption capability decreases the photocatalytic reactivity, which presumably occurs on the photocatalyst surface. On the other hand, as observed with FESEM, the larger a-Fe2O3 size and severe agglomeration of TiO2 led to an inefficient heterojunction interface for photo-generated electron migration. Therefore, the higher photodegradation of BPA over BT-TF-0.25 was expected due to a unique and compact heterojunction formation, whereby TiO2 nanorods were decorated on the a-Fe2O3 surface via the hydrothermal bio-template approach. A previous study also suggested that the presence of high a-Fe2O3 content in the heterojunction sample promoted the excessive Fe3þ ions formation, where it could act as a recombination centre of the photogenerated electron-holes [31]. As a result, the photocatalytic activity of the sample with higher a-Fe2O3 content significantly decreased. Excellent stability is crucial for practical applications. The stability test of BT-TF-0.25 for BPA photodegradation under the same conditions is shown in Fig. 7(c). Comparable photodegradation with a slight decrease from 79.0% to 75.9% after four consecutive reaction cycles was observed, indicating the high stability of BT-TF-0.25 in the photocatalytic process under simulated solar light irradiation. The improved in photocatalytic activity of the heterojunction photocatalyst can be further explained by investigating the charge carrier separation properties of the prepared samples by measuring the transient photocurrent response and photoluminescence. Theoretically, higher photocatalytic activity upon light irradiation is associated with excellent electron-hole pairs separation and charge transfer [73]. As shown in Fig. 7(d), BT-TF-0.25 shows the highest photocurrent response followed by BT-TF-0.50, BT-TF-0.75, BT-TiO2, TiO2, and P25. The high photocurrent intensity of BT-TF-0.25 suggests the high separation efficiency of electrons and holes in the sample under light irradiation [74]. The photoluminescence spectra of all prepared samples were obtained to further understand the charge transfer within the heterojunction of TiO2 and a-Fe2O3, as shown in Fig. 7(e). In principle, the semiconductor emits photons (photoluminescence, PL) of due to photo-induced electron-hole recombination. Therefore, a higher peak intensity of PL spectra is attributed to the higher recombination rate of photo-induced

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Fig. 7. (aeb) Photodegradation curve of bisphenol A (BPA) and the kinetics for disappearance of BPA over synthesized samples. (c) Photocatalytic stability and photocatalytic hydrogen evolution rate for four cycles over BT-TF-0.25. (d) Photocurrent response of synthesized samples. (e) Photoluminescence (PL) spectra of all prepared samples.

electron-holes. As revealed in Fig. 7 (e), BT-TF-0.25 exhibited the lowest PL intensity, which suggests that the well-developed heterojunction formation between TiO2 and a-Fe2O3 enhanced the electron-hole separation with efficient charge transfer between the two components. Furthermore, the presence of interstitial carbons induced several localized occupied states in the gap to enhance charge separation and migration, which was able to restrain recombination of the photo-induced electron and hole carriers [58]. The charge transfer and photo-generated electron-hole separation during the photodegradation of BPA over BT-TF-0.25 are illustrated in Fig. 8. Under simulated solar irradiation, the BT-TF0.25 heterojunction photocatalyst absorbs more photon energy to induce more electron excitations that are more efficient from the valence band to the conduction band. Carbon doping in TiO2 and aFe2O3 led to excellent light-harvesting capabilities in the broad

light spectrum due to the upward shifting of the valence band in both components. Based on this, the band gap structures of carbondoped TiO2 and carbon-doped a-Fe2O3 (with mid-gap levels shown in Fig. 8) exhibit the effect of C-doping in the heterojunction sample. Several studies proposed that after the formation of the heterojunction, the photo-generated electron in the conduction band (CB) of a-Fe2O3 could be transferred to the CB of TiO2 because the work function (Ф) of a-Fe2O3 (5.88 eV) was higher than the work function of TiO2 (4.31 eV) [30,31,58]. Before two components come in contact, the band gap structure of both carbon-doped TiO2 and carbon-doped a-Fe2O3 is shown in Fig. 8(a). It was observed that the band gap structure of carbon-doped a-Fe2O3 was straddled between the band gap structure of carbon-doped TiO2. Subsequently, the decoration of C-doped TiO2 on the surface of carbondoped a-Fe2O3 microcubes formed the carbon-doped TiO2/aFe2O3 heterojunction structures. The work function of a-Fe2O3 was

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Fig. 8. Charge transfer mechanism during the degradation of BPA in aqueous solution over BT-TF-0.25.

higher than that of TiO2, which caused the electrons to move from carbon-doped TiO2 to carbon-doped a-Fe2O3 until the Fermi energy level (Ef) of the two semiconductors was aligned, as shown in Fig. 8(b) [75]. The alignment of Ef of two semiconductors resulted in a built-in electric field at their interface directed from TiO2 to the aFe2O3 concurrently, which could stop the charge diffusion from TiO2 and a-Fe2O3 [76]. As a result, the a-Fe2O3 surface accumulated negative charges while the TiO2 layer gathered positive charges. Therefore, during the photocatalytic reaction, the photo-induced electron in the conduction band (CB) of carbon-doped a-Fe2O3 could be transferred to the CB of carbon-doped TiO2. In addition, the presence of the built-in electric field near the interface could also facilitate the separation of photogenerated electron-hole pairs [76]. The accumulated photo-induced electrons in the CB of TiO2 react with the dissolved oxygen to produce superoxide radicals to degrade BPA. Spontaneously, the photo-generation of positive holes in the valence band (VB) of TiO2 transfer to the VB of a-Fe2O3. The accumulated positive holes then react with water and OH species that exist on the surface of the catalyst, producing reactive hydroxyl radicals (COH) that oxidize and degrade BPA.

4. Conclusion In conclusion, the t-KF that consisted mainly of cellulose was used as a bio-template and in-situ carbon dopant in the preparation of TiO2/a-Fe2O3 heterojunction photocatalysts. In addition, the t-KF bio-template promoted the uniform size of carbon-doped TiO2 nanoparticles by controlling the crystal growth via coordination and steric stabilization of Ti4þ cation and TiO2 colloidal particles. This kind of stabilization also occurred during the formation of aFe2O3 at lower precursor concentration. These findings indicate that higher concentration of a-Fe2O3 caused an undesirable heterojunction structure, whereby severe agglomeration of TiO2 was observed and accompanied by larger a-Fe2O3 particle size. Contradictorily, lower a-Fe2O3 precursor concentration promoted a well-developed and unique heterojunction formation, whereby the carbon-doped TiO2 nanorods were decorated on the surface of carbon-doped a-Fe2O3 microcubes. The resulting heterojunction photocatalyst showed remarkably improve on photocatalytic activity for the degradation of organic pollutants from aqueous solution. Therefore, the concentration of precursor can be considered

a critical factor for the preparation of a well-developed heterojunction following bio-templated hydrothermal synthesis. Most importantly, this work utilized polysaccharide materials as biotemplates by in-situ carbon-doping and controlled crystal growth for the development of highly active broad solar spectrum heterojunction photocatalysts. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors would like to acknowledge financial support from the Ministry of Education Malaysia under a Fundamental Research Grant Scheme (FRGS/1/2017/TK10/UKM/01/3). The authors would also like to acknowledge technical and management support from the Centre for Research and Instrumentation (CRIM) at the Universiti Kebangsaan Malaysia. The first author also would like to thank Universiti Kebangsaan Malaysia for a Ph.D. scholarship award under the Skim Zamalah Yayasan Canselor 2016. References [1] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891e2959, https:// doi.org/10.1021/cr0500535. [2] J. Shim, Y.-S. Seo, B.-T. Oh, M. Cho, Microbial inactivation kinetics and mechanisms of carbon-doped TiO2 (C-TiO2) under visible light, J. Hazard Mater. 306 (2016) 133e139, https://doi.org/10.1016/j.jhazmat.2015.12.013. [3] S. Delavari, N.A.S. Amin, M. Ghaedi, Photocatalytic conversion and kinetic study of CO2 and CH4 over nitrogen-doped titania nanotube arrays, J. Clean. Prod. 111 (2016) 143e154, https://doi.org/10.1016/j.jclepro.2015.07.077. [4] B. Qiu, C. Zhong, M. Xing, J. Zhang, Facile preparation of C-modified TiO2 supported on MCF for high visible-light-driven photocatalysis, RSC Adv. 5 (2015) 17802e17808, https://doi.org/10.1039/C4RA17151A. [5] C. Yu, J.C. Yu, A Simple way to prepare CeN-codoped TiO2 photocatalyst with visible-light activity, Catal. Lett. 129 (2009) 462e470, https://doi.org/10.1007/ s10562-008-9824-7. [6] M.A. Mohamed, M.F.M. Zain, L.J. Minggu, M.B. Kassim, J. Jaafar, N.A. Saidina Amin, Z.A. Mohd Hir, M.S. Rosmi, Enhancement of visible light photocatalytic hydrogen evolution by bio-mimetic C-doped graphitic carbon nitride, Int. J. Hydrogen Energy 44 (2019) 13098e13105, https://doi.org/10.1016/ j.ijhydene.2019.02.243.

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