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Amine-assisted synthesis of FeWO4 nanorodg-C3N4 for enhanced visible light-driven Z-scheme photocatalysis
Accepted Manuscript Amine-assisted synthesis of FeWO4 nanorod g-C3N4 for enhanced visible lightdriven Z-scheme photocatalysis Devi Prashad Ojha, Hem Prakash Karki, Jun Hee Song, Han Joo Kim PII:
S1359-8368(18)32682-9
DOI:
10.1016/j.compositesb.2018.10.039
Reference:
JCOMB 6118
To appear in:
Composites Part B
Received Date: 20 August 2018 Revised Date:
1 October 2018
Accepted Date: 13 October 2018
Please cite this article as: Ojha DP, Karki HP, Song JH, Kim HJ, Amine-assisted synthesis of FeWO4 nanorod g-C3N4 for enhanced visible light-driven Z-scheme photocatalysis, Composites Part B (2018), doi: https://doi.org/10.1016/j.compositesb.2018.10.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Amine-assisted synthesis of FeWO4 nanorod g-C3N4 for enhanced visible light-driven Zscheme photocatalysis
a
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Devi Prashad Ojhaa, Hem Prakash Karkia, Jun Hee Songa* , Han Joo Kima,b* Department of Convergence Technology Engineering, Chonbuk National University, Jeonju
561-756, Republic of Korea b
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Eco-friendly Machine Parts Design Center, Chonbuk National University, Jeonju 561-756,
Republic of Korea
Han Joo Kim Email: [email protected],
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*Corresponding author:
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Telephone number: +82634722897, Fax number +82632704226
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Abstract: Highly crystalline FeWO4 nanorods (FWO NRs) were prepared using an amine in a
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hydrothermal reaction. A photocatalyst active to visible light was designed by preparing 5FWO/g-CN heterostructures via in-situ hydrothermal methods. Fabricated heterostructures were analyzed using X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), BET
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measurements, transmission electron microscopy (TEM), X-ray photoelectron microscopy (XPS), and Fourier transform spectroscopy (FTIR). The photocatalytic activity toward the degradation
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of salicylic acid (SA) was investigated under visible light irradiation. The active species trapping experiments showed that the holes, as well as the electrons, exhibited an obvious influence on the photocatalytic degradation process. Examinations of the mechanism showed that the enhanced photocatalytic activity was mainly ascribed to reduced recombination rate and band
potential of the holes.
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gap broadening in a Z-scheme mechanism, which enhance the efficient transfer and the oxidation
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Keywords: Iron tungstate, photocatalyst, graphitic carbon nitride, salicylic acid
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1.
Introduction:
Semiconductor metal oxides such as TiO2 [1], ZnO [2], WO3 [3], Fe2O3 [4] etc are among the
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most studied heterogeneous photocatalysts because of their low-cost, chemical stability, nontoxicity and appropriate energy band position. Despite many advantages, heterogeneous photocatalysis has significant limitations due to moderate photocatalytic reaction rates and
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consequently overall low energy conversion [5]. Low photoactivity of these single-phase semiconductors is linked to the limited light absorption and rapid recombination of charge
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carriers [6]. Alternatively, an impressive number of ternary metal oxides like titanates [7, 8], phosphates [9], vanadates [10] , tungstates [11] etc have also been developed as potential substitutes.
Iron tungstate (FeWO4) is a fascinating wolframite-type metal tungstate with several technological applications in areas like scintillation detectors, optical fibres, humidity sensors,
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photoanodes, catalysts, and pigments [12-14]. In addition, many other studies have found that FeWO4 has a narrow band gap (< 2.0 eV) to absorb a broad range of visible light in solar
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spectrum [15]. However, the visible light photocatalytic efficiency of FeWO4 is not promising because of the rapid electron-hole recombination ultimately resulting low quantum yield, as well
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as the poor electron and ion transport capacity [16]. Consequently, developing an efficient strategy is considerably significant to promote the separation of charge carriers for FeWO4 photocatalysts. Numerous studies have been carried out to confine the electrons by combining it with large band-gap semiconductors like TiO2, ZnO, WO3 and zero band gap graphene and has been found to exhibit high photodegradation efficiency, mainly attributed to the effective electron-hole separation [17, 18]. Next, graphitic carbon nitride (g-C3N4 or g-CN) could be a potential material for yet another strategy to increase the electron availability of FeWO4. The g-
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C3N4 is a stable, metal-free, two-dimensional (2D) polymeric semiconductor with a band gap of 2.7eV and can be easily synthesised by heating melamine [19]. Moreover, visible light active g-
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CN has a sufficiently negative conduction band potential whereas FeWO4 possesses a band gap of 1.98 eV with more negative valence band (VB) potential [20]. Therefore, this kind of heterostructure combining an oxidation photocatalyst (FeWO4) possessing the strong oxidation
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ability in the VB and a reduction photocatalyst (g-CN) with the strong reduction ability in the CB could be an excellent photocatalyst. In this kind of direct Z-scheme photocatalyst, the
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recombination of electron-holes is greatly minimised without losing the redox ability of the semiconductors yet efficiently utilizing enough visible light [5]. Similar Z-scheme photocatalysts like g-C3N4/TiO2 [21, 22] , g-C3N4/WO3 [23], g-C3N4/ZnO [24], g-C3N4/BiOI [25] etc. have been reported with enhanced photoactivity due to efficient charge separation and high redox ability. Nevertheless, the exfoliated 2D flat-structure of g-CN additionally extends its active
harvesting [26].
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surface area which provides more reactive sites, reduces e-h recombination, and enhances light
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There are several reports on the preparation of FeWO4 nanoparticles of varied shape and size using various synthetic methods [27, 28]. The morphologies alter the physical and chemical as
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well as optical properties of the metal oxides, which allow them to be used in various applications [29]. Previous studies have established that 1D nanostructures, like nanowires (NWs) and nanorods (NRs) increase the overall electron mobility and offer enhanced photocatalytic efficiency due to the higher surface to volume ratio, lower crystallinity and greater number of defects [30]. Thus, we synthesized a p-type FeWO4 NRs by s simple hydrothermal method in which the crystal growth is controlled by an amine, Triethanolamine (TEOA) that has equally been used for tuning the shape and size of metal oxides for a long time [31]. The NR is then
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coupled with g-CN to utilize the intrinsic properties of g-CN that increase the electron lifetime and decrease recombination to enhance photoactivity. The mechanism by which the
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photoactivity is proposed with respect to the electrochemical performances of the heterostructure and role of the active groups during photocatalytic reaction was emphasized. 2.
Materials
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2.1.
Experimental
All the reagents, Melamine, sodium tungstate (Na2WO4·2H2O), ferrous ammonium sulfate
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(Mohr salt, (NH4)2Fe(SO4)2·6H2O), triethanolamine (TEOA), sodium hydroxide (NaOH), nitric acid (HNO3), and ammonia solution (NH4OH) were were purchased from Sigma-Aldrich and used directly for experiments without any further purification. 2.2.
Preparation of the g-CN nanosheet
The graphitic carbon nitride (g-CN) nanosheet was prepared by the thermal polymerization of
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melamine [32]. The melamine was put in a crucible with a lid under ambient pressure. After thermal treatment at 550 °C for 3 hours, yellow-colored g-CN powder was crushed into fine
melamine.
Synthesis of FeWO4 NR and FWO/gCN heterostructures
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2.3.
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powder and again heated at 550 °C for 3 hr to complete the polymerization of the remaining
For preparation of the FeWO4 NRs, Na2WO4 .2H2O (2 mmol) and Mohr salt (2 mmol) were dissolved separately in 20 mL water. The Mohr salt solution was vigorously added to the sodium tungstate solution to produce a dark brown mixture. TEOA was added to the mixture, the pH of which was adjusted by 1M NaOH solution. The hydrothermal treatment of the resulting mixture was conducted in a 100 mL Teflon-lined autoclave at 200 °C for 12 hours and was allowed to cool naturally. The dark brown colored suspension was recovered, washed with ethanol and
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water for three times and finally dried at 80 °C for 12 h. The FeWO4 nanoparticles were also prepared without using TEOA under similar hydrothermal conditions.
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For the preparation of FWO/gCN heterostructures, 50ml of the 0.5 gm FeWO4 NR and the calculated amount of g-CN (1 wt%, 5 wt%, and 10 wt%) were sonicated separately in a 40 ml ethanol: water solution. After 30 min sonication, the two suspensions were mixed and further
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sonicated for 30 min. Finally, all the mixture was transferred into the Teflon lined hydrothermal bomb and kept at 200 °C for 12 hours. Thus, the obtained 1FWO/gCN, 5FWO/gCN, and
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10FWO/gCN heterostructures containing 1 wt% g-CN, 5 wt% g-CN, and 10 wt% g-CN, respectively, were centrifuged, washed several times, and dried. 2.4.
Evaluation of Photocatalytic Activity
The photocatalytic samples were utilized to estimate the photocatalytic activity by monitoring the decomposition of a typical organic pollutant, salicylic acid (SA). In the photocatalytic
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degradation process, 10 mg FWO/gCN samples were mixed in 50 mL of 50µM aqueous SA solution and stirred in the dark for 30 minutes. A 300 W Xe lamp (DY. Tech., Korea) equipped
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with a UV and an IR cut-off filters (Edmund optics, USA) was used for the sample irradiation after adsorption-desorption equilibrium is ensured. A 2 mL aliquot of the test solution was
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withdrawn in different time interval and used to analyze the absorbance of the remnant SA using a UV–vis spectrophotometer. The progress of the reaction was studied from its characteristic absorption peak at 297 nm in the absorption spectrum. Photodegradation with FWO and g-CN, and blank tests were also carried out to compare the degradation efficiency of the heterostructures. 2.5.
Characterization
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X-ray diffraction (XRD) graphs were characterized using Rigaku x-ray diffractometer using light intensity of Cu Kα radiation (λ = 1.540 Å) in the range of 2θ range of 20 – 80° with 2°/min scan
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rate. UV-Visible diffuse reflectance spectra were evaluated with Perkin-Elmer Lambda 40 spectrophotometer. Transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) images were analyzed using JEM-2200, JEOL Japan. X-ray photoelectron spectra (XPS) of the
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catalysts were recorded on a Kα X-ray photoelectron spectrometer (Thermo Scientific) using Al Kα (1486.6 eV) radiation. The N2 adsorption-desorption isotherms were recorded in
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Micromeritics ASAP 2420. The surface area was determined by BET method whereas the pore size distribution was derived from the desorption isotherm using BJH method. 3.
Results and discussion
The crystal structures of the FeWO4 nanostructures were analyzed through XRD. Fig. 1 showed XRD pattern of the highly crystallized FeWO4 NRs prepared via a hydrothermal reaction by
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using triethanolamine. All the diffraction peaks were indexed as a highly crystallized wolframitelike monoclinic structure (JCPDS file 74-1130) with space group P2/c. Impurity phases weren’t
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detected from the XRD patterns, indicating the formation of a single phase FeWO4. Fig. 1 also showed the XRD pattern of the pristine g-C3N4 prepared by the thermal treatment of melamine at
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550 °C. The characteristic peak at 27.3° was indexed to (002) interspacing of the conjugated aromatic systems at a distance d=0.33 and indicates that the g-C3N4 sheets were stacked (JCPDS87-1526) [33]. However, the peak of g-CN at 27.3° wasn’t detected in the FWO/gCN heterostructures possibly due to the relatively low graphitic carbon nitride content in the final catalysts [18]. FeWO4 NR was also analyzed by TEM to study the mechanism of crystal growth. Fig. 2(a) showed the FeWO4 nanoparticles synthesized by hydrothermal growth method at pH 8 controlled
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by NaOH. However, highly crystalline FeWO4 nanorods were formed when TEOA, a tertiary amine, was used under similar reaction conditions (Fig. 2(b)). This change of the crystal shape
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clearly indicated the amine played key role for the formation of the nanorods. Low magnification TEM in Fig. 2(c) revealed the nanorods had an average length between 150-200 nm whereas the diameter ranging from 30-40 nm. The NRs were further characterized by HRTEM and SAED
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patterns (Fig. 2(d)). The spacing of the fringe patterns was measured to be 0.473 nm and 0.363 nm, interplanar lattice spacing of which corresponded to the (100) and (110) planes of the
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monoclinic FeWO4 phase. The well-defined spots in the SAED patterns (inset, Fig. 2(d)) confirmed that an individual FeWO4 NR is a single crystal in the monoclinic phase [34]. Likewise, Fig. 2(e) showed the pristine g-CN sheet with multiple stacked layers. The TEM image of a 5FWO/gCN (5 wt% g-CN) heterostructure in Fig. 2(f) prepared by the one-pot hydrothermal method showed that the shape and size of a NR were unaffected by g-CN. Several of the FeWO4
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NRs were embedded in the g-CN suggesting the FeWO4 NRs were tightly bound into the wellexfoliated g-CN sheets [35].
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The EDX mapping of the 5FWO/gCN heterostructure in Fig. 3 confirmed the presence of Fe, C, N, O and W. X-ray photoelectron spectroscopy was utilized to the information concerning the
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binding energy of the elements in the near surface region and their respective oxidation states. Since the electron binding energy differs for each element, a full-scan spectrum was produced for an overall understanding of the elements on surface of the tested sample (Fig. 4). The XPS spectrum (Fig. 4(a)) of the FeWO4 NRs peaks correspond to Fe2p3 (711.2 eV), O1s (531.1 eV), and W4f (35.6 eV) [36]. The C1s peak at 285.3 eV is identified to the adventitious carbon adsorbed on the surface. Similarly, in the spectrum of 5FWO/gCN, there are peaks corresponding to Fe2p3 (711.3 eV), O1s (531.1 eV), W4f (35.6 eV), and C1s (285.3 eV). The
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survey XPS spectrum of the g-C3N4 sample reveals the predominant presence of N and C elements, and the intensity of the N 1s core level line is very high [37]. No detectable difference
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in the binding energies of Fe element between FeWO4 and 5FWO/gCN has been found. In Fig. 4(b), the binding energies of Fe 2p3/2 and Fe 2p1/2 are indicated at 711.2 and 724.8 eV respectively. In Fig. 4(c), two peaks, located at 36.5 and 38.2 eV, are assigned to W4f, with
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binding energies corresponding to the tungsten in the formal valence of +6. From Fig. 4(d), the N 1s XPS spectrum can be deconvoluted into one distinct peak and three small peaks with BEs of
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398.6, 399.1, 400.7, and 405.1 eV, which are assigned to C=N-C (398.6 eV), tertiary nitrogen (C)3-N (399.1 eV), N-H (400.7 eV), and π-excitation (405.1 eV), respectively. All the above results further confirm the formation of a 5FWO/gCN heterostructure [38, 39]. UV-visible diffuse reflectance spectra were also recorded to investigate the light harvesting ability of the heterostructures and respective band gap calculations (Fig. 5). As shown in Fig.
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5(a), pure g-CN shows absorption the UV and visible absorption only up to 460 nm where as pure FeWO4 showed the absorption in the entire visible light region that suggested that enough
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solar light can be utilized. Thus, the photoabsorption ability of the FeWO4 is well retained in FeWO4/gCN heterostructures indicating the strengthened electronic coupling between two
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semiconductors. Furthermore, the band gaps of the photocatalysts can be calculated by the equation [10]:
αhv = A(hv-Eg)n/2
where α, A, hv, and Eg are the optical absorption coefficient, a proportionality constant, a photon energy and the band gap energy respectively. The value of n depends upon the type of optical transition of the semiconductors: for direct transition, value of n is 1 where as for the indirect transition the value of n is 4. Taking the reference of previous reports, the value of n for bulk g-
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CN and FeWO4 semiconductors used is 4 [40, 41] . Thus, the band gap of g-CN and FeWO4 calculated from Tauc plot was estimated to be about 2.68 eV and 1.68 eV respectively (Fig. 5(b)).
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Nitrogen adsorption-desorption isotherms were checked to observe the surface area and pore size distribution of the FeWO4, g-CN and 5FWO/gCN nanocomposite as shown in Fig. 6 and Table 1. Specific surface area of FeWO4, g-CN and 5FWO/gCN are 23.30, 12.60 and 42.70 m2/g
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respectively. The shape of the hysteresis loops of the FeWO4 and 5FWO/gCN are of H3 type in IV isotherm indicating the presence of slit-like pores. The isotherm of the bulk g-CN represents
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type II isotherm indicating the presence of the nonporous surface [42]. The inset of Fig. 6 shows the pore size distribution of FeWO4, g-CN and 5FWO/gCN. The broad pore distribution of 5FWO/gCN at higher pore size shows the presence of the mesopores and macropores whereas micropores in lower value represent the presence of g-CN sheets [42]. The pore size and pore
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volume of the FeWO4, g-CN and 5FWO/gCN are shown in Table 1.
The photocatalytic activity of several composites was evaluated by monitoring the decomposition of aqueous salicylic acid under visible-light irradiation (λ≥400 nm). Fig. 7(a)
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depicts the absorption spectra of SA as a function of irradiation time at various time intervals in presence of 5FWO/gCN heterostructure. The intensity of the SA peak at 298 nm decreases as the
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SA degrades with the progress of the photocatalytic reaction. It is revealed from Fig. 7(b) that the photolysis of SA is negligible whereas FeWO4 NR and g-CN samples exhibit low photocatalytic degradation of SA under visible light. Fig. 7(b) also showed the significantly enhanced photocatalytic activity with various FWO/gCN heterostructures. The decomposition reaction of SA obeys the pseudo-first order kinetics which can be represented by -d[A]/dt = k[A] where [A] denotes the concentration of SA and k denotes the degradation rate constant. The percentage of the remnant SA, the respective kinetics of the various photocatalysts and the calculated rate
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constants are shown in Fig. 7(b-d). The FWO/gCN composites showed remarkably higher photocatalytic activity than the bare FeWO4 or g-CN sample: rate constant k of the 5FWO/gCN
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was 40.0 and 7.1 times that of the bare FeWO4 and g-CN, respectively. Furthermore, in 2.5 h, ~95% SA was removed with FWO/gCN, whereas only 36% was removed with g-CN. Compared to the FeWO4 and g-CN, the heterostructures displayed significant enhancement in the
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photodegradation of SA. The gradual increase in the degradation rate from 1% g-CN to 5% g-CN is attributed to the simultaneous effect of the increased light absorption (red shift in Fig. 5) and
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enhanced adsorption of the pollutant (SA) due to increased specific surface area. Thus, more molecules are prone to photodegradation on the catalyst surface. This phenomenon is supported from Fig. 7(b) which shows the adsorption of the pollutant in dark is higher in the heterostructures. Furthermore, on increasing the g-CN content beyond 5%, the photoactivity is decreased which might be due to the less availability of the slit-like channels on the
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heterostructures [43]. This clearly indicates that g-CN has significant role in the photoactivity. The stability of the 5FWO/g-CN heterostructure was evaluated by reusing the sample for the 5
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consecutive cycles (Fig. 8(d)). In each cycle, the sample was collected by centrifugation, washed with ethanol and water and dried completely at 80 °C. The photocatalytic activity of the
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5FWO/gCN is retained without significant deactivation. Similarly, the interfacial charge separation efficiency of the photoinduced electron-hole pairs is also studied by measuring the photocurrent response with or without visible light as shown in Fig. 8(a). 5FWO/gCN displays higher photocurrent density of 0.031 mA/cm2 compared to the bare FeWO4 (0.14 mA/cm2) which indicates the heterostructure has the better interfacial charge transfer ability [44, 45]. This phenomenon is further supported in the photoluminescence (PL) spectra in Fig. 8(b). The higher
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PL intensity of 5FWO/gCN over FeWO4 shows that the recombination of photogenerated electron-hole is reduced and further verifies the effective formation of the heterostructure [46] .
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Thus, based on above analysis, a series of radical trapping experiments using various scavengers were conducted to explore the main active species (•OH, •O2-, h+) responsible for the photocatalytic degradation of SA. During the radical trapping experiments, 1 mmol of
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isopropanol (IPA, a scavenger of •OH), 1,4-benzoquinone (p-BQ, a scavenger of •O2-) and 1 mmol of TEOA (AO, a scavenger of h+) were used with SA solution under visible light
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irradiation, respectively [47]. The effects of different scavengers on the visible light photocatalytic reaction of SA by FeWO4, g-CN and 5FWO/gCN heterostructure were presented in Fig. 8(c). For FeWO4, when TEOA was added into the reaction solution, the photodegradation is quenched from 8 % to 2 % however with p-BQ and IPA, the photodegradation is unchanged (~7%) indicating the hole as a major reactive species. Similarly, for g-CN, the photodegradation
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rate of SA is un32affected with TEOA but significantly inhibited with p-BQ (21%) and IPA (24.5%). This implies in g-CN, the •O2- and the •OH radicals are the major reactive species. On the other hand, the photoactivity of SA is significantly inhibited with p-BQ (48 %), TEOA
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(54 %), and IPA (30 %) indicating that •O2-, •OH, and h+ are obviously the active species during
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the photodegradation process. The photocatalytic degradation rate is significantly inhibited with TEOA indicating h+ is the predominant radical responsible for the SA degradation. These results indicate the oxidation potential of the hole and the reduction potential of the electron are sufficiently high for photodegradation process [48]. In order to explain the enhanced photoactivity of the FWO/gCN heterostructures over individual semiconductors, conduction band (CB) and valance band (VB) potentials of FeWO4 and g-C3N4 were calculated using following equations [49]:
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EVB = X - Eo + 0.5 Eg
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ECB = EVB - Eg
(2)
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Where χ denotes absolute electronegativity and the value of X for FeWO4 and g-CN are 6.23 and 4.73 respectively [40, 41]. EVB and ECB are the valence band edge potential and conduction band edge potential, E° (=4.5eV) is the energy of the free electrons in hydrogen scale and Eg is the
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band gap energy. From the above equations the ECB of the FeWO4 and g-CN are estimated to be +0.80 V and -1.05 V vs. NHE, and the EVB of FeWO4 and g-CN are estimated to be +2.72 V
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+1.58 respectively. Thus, it is evident both the semiconductors are excited in visible light and charge transfer in a simple “photosensitizer” mechanism is insufficient to explain the enhanced photoactivity. This is because the electrons from the highly negative CB of g-CN if transferred to the CB of FeWO4, the reduction of O2 may not take place to produce superoxide radicals that have already been detected in the radical trapping experiment in fig. The redox potential of
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O2/•O2- (-0.28 V, NHE) is more negative than the CB position of FeWO4. Based on the above trapping experiments and band edge analysis, a simple Z-scheme mechanism
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(Fig. 9) was suggested. In this FWO/gCN heterostructure, the electron-hole charge separation is induced in FeWO4 and g-CN simultaneously upon the illumination in visible light. The electrons
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from the CB of FeWO4 can easily shift to the VB g-CN by Shottky barrier and photogenerated electron-hole recombination is effectively reduced compared to the individual components. Meanwhile, the well separated holes in the VB of FeWO4 and the electrons in CB of g-CN take part in the redox reaction after the interfacial charge transfer. The reduction of O2 at the CB of gCN of produce the active species superoxide (•O2-) and hydroxyl (•OH) radical [50]. Similarly, holes generated in the FeWO4 VB resultantly degrade SA. Thus, superoxide (•O2-) and h+ are the
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two predominant active species responsible for the enhanced photodegradation of SA as elaborated in the following equations:
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Photo-excitation: FWO/gCN + hv →FWO/gCN (e− + h+) Photo-reduction:
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O2 + e- → •O2-, •
O2- + e- + 2H+ → H2O2,
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H2O2 + e- → •OH + OHPhoto-oxidation: 2h+ + H2O → 2H+ + •OH Photo-degradation:
O2-, •OH, + SA → Degradation products.
4. Conclusion:
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Highly crystallized FeWO4 nanorods were successfully prepared by amine controlled
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hydrothermal synthesis at 200 OC for 12 h. These nanorods were combined with g-CN to prepare a visible-light active Z-scheme FWO/gCN photocatalyst which exhibited higher photocatalytic
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activity than both pure g-CN and FeWO4. The catalytic activity of the heterostructure was found 40 times higher than that of FeWO4. The mechanism was further confirmed by observing the active species participation in the photodegradation process by using electron and hole scavengers. The enhanced photocatalytic activity attributed to the efficient and prolonged electron/hole separation and effective interfacial charge transport between FeWO4 and g-CN. Thus, the photocatalyst is easily synthesized in a cost-effective way.
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Acknowledgement: This paper was supported by a grant through the national Research Foundation (NRF, Project no.
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2017-R1C1B2011968) provided by Ministry of Education, Science, Technology (MEST), Republic of Korea. We would also like to thank the staff at the Center for Chonbuk University
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Research Facility (CURF) for providing the facilities for analysis.
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[16] Anderson C, Crowell C. Threshold energies for electron-hole pair production by impact ionization in semiconductors. Physical Review B. 1972;5(6):2267. [17] Rothenberger G, Moser J, Graetzel M, Serpone N, Sharma DK. Charge carrier trapping and recombination dynamics in small semiconductor particles. Journal of the American Chemical Society. 1985;107(26):8054-9. [18] He G-L, Chen M-J, Liu Y-Q, Li X, Liu Y-J, Xu Y-H. Hydrothermal synthesis of FeWO4graphene composites and their photocatalytic activities under visible light. Applied Surface Science. 2015;351:474-9. [19] Huang H, Xiao K, Tian N, Dong F, Zhang T, Du X, et al. Template-free precursorsurface-etching route to porous, thin g-C3N4 nanosheets for enhancing photocatalytic reduction and oxidation activity. Journal of Materials Chemistry A. 2017;5(33):17452-63. [20] Cao X, Chen Y, Jiao S, Fang Z, Xu M, Liu X, et al. Magnetic photocatalysts with ap–n junction: Fe 3 O 4 nanoparticle and FeWO 4 nanowire heterostructures. Nanoscale. 2014;6(21):12366-70. [21] Yu J, Wang S, Low J, Xiao W. Enhanced photocatalytic performance of direct Z-scheme g-C3N4–TiO2 photocatalysts for the decomposition of formaldehyde in air. Physical Chemistry Chemical Physics. 2013;15(39):16883-90. [22] Liu J, Cheng B, Yu J. A new understanding of the photocatalytic mechanism of the direct Z-scheme g-C3N4/TiO2 heterostructure. Physical Chemistry Chemical Physics. 2016;18(45):31175-83. [23] Yu W, Chen J, Shang T, Chen L, Gu L, Peng T. Direct Z-scheme g-C3N4/WO3 photocatalyst with atomically defined junction for H2 production. Applied Catalysis B: Environmental. 2017;219:693-704. [24] Yu W, Xu D, Peng T. Enhanced photocatalytic activity of gC 3 N 4 for selective CO 2 reduction to CH 3 OH via facile coupling of ZnO: a direct Z-scheme mechanism. Journal of Materials Chemistry A. 2015;3(39):19936-47. [25] Wang J-C, Yao H-C, Fan Z-Y, Zhang L, Wang J-S, Zang S-Q, et al. Indirect Z-scheme BiOI/g-C3N4 photocatalysts with enhanced photoreduction CO2 activity under visible light irradiation. ACS applied materials & interfaces. 2016;8(6):3765-75. [26] Cao S, Low J, Yu J, Jaroniec M. Polymeric Photocatalysts Based on Graphitic Carbon Nitride. Advanced Materials. 2015;27(13):2150-76. [27] M Zawawi SM, Yahya R, Hassan A, Mahmud HNME, Daud MN. Structural and optical characterization of metal tungstates (MWO(4); M=Ni, Ba, Bi) synthesized by a sucrosetemplated method. Chemistry Central Journal. 2013;7:80-. [28] Abhijit B, Alice C, Arunkumar K, Yingyot I, Arkady Y, Villy S, et al. Control of the size and shape of TiO 2 nanoparticles in restricted media. Nanotechnology. 2013;24(19):195601. [29] Garcia SP, Semancik S. Controlling the Morphology of Zinc Oxide Nanorods Crystallized from Aqueous Solutions: The Effect of Crystal Growth Modifiers on Aspect Ratio. Chemistry of Materials. 2007;19(16):4016-22. [30] Yan SC, Li ZS, Zou ZG. Photodegradation of Rhodamine B and Methyl Orange over Boron-Doped g-C3N4 under Visible Light Irradiation. Langmuir. 2010;26(6):3894-901. [31] Sugimoto T, Zhou X, Muramatsu A. Synthesis of uniform anatase TiO2 nanoparticles by gel–sol method: 4. Shape control. Journal of Colloid and Interface Science. 2003;259(1):53-61. [32] Mao J, Zhang L, Wang H, Zhang Q, Zhang W, Li P. Facile fabrication of nanosized graphitic carbon nitride sheets with efficient charge separation for mitigation of toxic pollutant. Chemical Engineering Journal. 2018;342:30-40.
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[33] Yan S, Li Z, Zou Z. Photodegradation performance of g-C3N4 fabricated by directly heating melamine. Langmuir. 2009;25(17):10397-401. [34] Almeida M, Cavalcante L, Morilla-Santos C, Dalmaschio C, Rajagopal S, Li MS, et al. Effect of partial preferential orientation and distortions in octahedral clusters on the photoluminescence properties of FeWO 4 nanocrystals. CrystEngComm. 2012;14(21):7127-32. [35] Ge L, Han C, Liu J. Novel visible light-induced g-C3N4/Bi2WO6 composite photocatalysts for efficient degradation of methyl orange. Applied Catalysis B: Environmental. 2011;108:100-7. [36] Qian J, Peng Z, Wu D, Fu X. FeWO4/FeS core/shell nanorods fabricated by thermal evaporation. Materials Letters. 2014;122:86-9. [37] Khyzhun OY. XPS, XES and XAS studies of the electronic structure of tungsten oxides. Journal of Alloys and Compounds. 2000;305(1):1-6. [38] Rajagopal S, Nataraj D, Khyzhun OY, Djaoued Y, Robichaud J, Mangalaraj D. Hydrothermal synthesis and electronic properties of FeWO4 and CoWO4 nanostructures. Journal of Alloys and Compounds. 2010;493(1):340-5. [39] Zhang Q, Chen X, Zhou Y, Zhang G, Yu S-H. Synthesis of ZnWO4@MWO4 (M = Mn, Fe) Core−Shell Nanorods with Optical and Antiferromagnetic Property by Oriented Attachment Mechanism. The Journal of Physical Chemistry C. 2007;111(10):3927-33. [40] Ma Y, Guo Y, Jiang H, Qu D, Liu J, Kang W, et al. Preparation of network-like ZnOFeWO4 mesoporous heterojunctions with tunable band gaps and their enhanced visible light photocatalytic performance. New Journal of Chemistry. 2015;39(7):5612-20. [41] Ma X, Lv Y, Xu J, Liu Y, Zhang R, Zhu Y. A Strategy of Enhancing the Photoactivity of g-C3N4 via Doping of Nonmetal Elements: A First-Principles Study. The Journal of Physical Chemistry C. 2012;116(44):23485-93. [42] Huang H, Reshak AH, Auluck S, Jin S, Tian N, Guo Y, et al. Visible-Light-Responsive Sillén-Structured Mixed-Cationic CdBiO2Br Nanosheets: Layer Structure Design Promoting Charge Separation and Oxygen Activation Reactions. The Journal of Physical Chemistry C. 2018;122(5):2661-72. [43] Hong Y, Jiang Y, Li C, Fan W, Yan X, Yan M, et al. In-situ synthesis of direct solid-state Z-scheme V2O5/g-C3N4 heterojunctions with enhanced visible light efficiency in photocatalytic degradation of pollutants. Applied Catalysis B: Environmental. 2016;180:663-73. [44] Huang H, Li X, Wang J, Dong F, Chu PK, Zhang T, et al. Anionic Group Self-Doping as a Promising Strategy: Band-Gap Engineering and Multi-Functional Applications of HighPerformance CO32–-Doped Bi2O2CO3. ACS Catalysis. 2015;5(7):4094-103. [45] Huang H, Tu S, Zeng C, Zhang T, Reshak AH, Zhang Y. Macroscopic Polarization Enhancement Promoting Photo- and Piezoelectric-Induced Charge Separation and Molecular Oxygen Activation. Angewandte Chemie International Edition. 2017;56(39):11860-4. [46] Huang H, He Y, Li X, Li M, Zeng C, Dong F, et al. Bi2O2(OH)(NO3) as a desirable [Bi2O2]2+ layered photocatalyst: strong intrinsic polarity, rational band structure and {001} active facets co-beneficial for robust photooxidation capability. Journal of Materials Chemistry A. 2015;3(48):24547-56. [47] Wang H, Yuan X, Wu Y, Zeng G, Chen X, Leng L, et al. Synthesis and applications of novel graphitic carbon nitride/metal-organic frameworks mesoporous photocatalyst for dyes removal. Applied Catalysis B: Environmental. 2015;174-175:445-54.
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[48] Zeng Q, Xie X, Wang X, Wang Y, Lu G, Pui DYH, et al. Enhanced photocatalytic performance of Ag@TiO2 for the gaseous acetaldehyde photodegradation under fluorescent lamp. Chemical Engineering Journal. 2018;341:83-92. [49] Yang J, Hu R, Meng W, Du Y. A novel p-LaFeO3/n-Ag3PO4 heterojunction photocatalyst for phenol degradation under visible light irradiation. Chemical Communications. 2016;52(12):2620-3. [50] Zhang J, Nosaka Y. Mechanism of the OH radical generation in photocatalysis with TiO2 of different crystalline types. The Journal of Physical Chemistry C. 2014;118(20):10824-32.
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Table Caption: Specific surface area, pore volume and pore diameter of FeWO4, g-CN and 5FWO/gCN
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heterostructure.
Figures Captions:
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Fig. 1 XRD spectra of g-CN, FeWO4 NR and 5FWO/gCN heterostructure.
Fig. 2 TEM images of (a) FeWO4 nanoparticles with NaOH, (b) FeWO4 nanorods with NaOH
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and TEOA, (c) a single FeWO4 NR (Inset: SAED pattern), (d) HRTEM of the FeWO4 NR, (e) gCN sheet and (f) 5FWO/gCN heterostructure.
Fig. 3 EDX map of the 5FWO/gCN heterostructure showing C, N, O, Fe and W. Fig. 4 XPS survey spectra and high-resolution XPS spectra of (b) Fe 2p (c)W 4f, and (c) C 1s for 5FWO/gCN heterostructure.
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Fig. 5 (a) UV-Visible DRS spectra of as prepared photocatalysts and (b) Touc plots of (αhν)1/2 versus photo energy (hν) for FeWO4 and g-CN.
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Fig. 6 Nitrogen adsorption-desorption isotherms of FeWO4, g-CN and 5FWO/gCN heterostructure (Inset: the pore size distribution curves).
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Fig. 7 (a) UV-visible absorbance curve for the photodegradation of the aqueous SA solution over 5FWO/gCN heterostructure, (b) photodegradation efficiency of SA under visible light irradiation, (c) kinetic curves of the studied photocatalysts and (d) degradation rate constant for SA over as-prepared heterostructures. Fig. 8 (a) Transient photocurrent densities of FeWO4 and 5FWO/g-CN heterostructure, (b) photoluminescence spectra of FeWO4 and 5FWO/gCN heterostructure, (c) active species
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trapping experiments for SA degradation over FeWO4, g-CN and 5FWO/gCN heterostructure and (d) cyclic photocatalytic experiment for the 5FWO/gCN heterostructure.
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Fig. 9 Suggested mechanism of the photocatalytic activity of 5FWO/gCN heterostructure.
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Table: Specific surface area, pore volume and pore diameter of FeWO4, g-CN and 5FWO/gCN
BET (m2/g)
Mean pore diameter (nm)
23.30
39.10
g-C3N4
12.60
5.35
5FWO/g-CN
42.70
32.85
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(cm3/g)
0.181
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FeWO4
Pore volume
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Photocatalyst
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heterostructure.
0.0345
0.175
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Figures:
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Fig. 1 XRD spectra of g-CN, FeWO4 NR and FWO/gCN heterostructure.
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Fig. 2 TEM images of (a) FeWO4 nanoparticles with NaOH, (b) FeWO4 nanorods with NaOH and TEOA, (c) a single FeWO4 nanorod (Inset: SAED pattern), (d) HRTEM of the FeWO4 NR,
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(e) g-CN sheet and (f) 5FWO/gCN heterostructure.
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Fig. 3 EDX map of the 5FWO/gCN heterostructure showing C, N, O, Fe and W.
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Fig. 4 XPS survey spectra and high-resolution XPS spectra of (b) Fe 2p (c)W 4f, and (c) C 1s for
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5FWO/gCN heterostructure.
Fig. 5 (a) UV-Visible DRS spectra of as prepared photocatalysts and (b) Touc plots of (αhν)1/2 versus photo energy (hν) for FeWO4 and g-CN.
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Fig. 6 Nitrogen adsorption-desorption isotherms of FeWO4, g-CN and 5FWO/gCN
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heterostructure (Inset: the pore size distribution curves).
Fig. 7 (a) UV-visible absorbance curve for the photodegradation of the aqueous SA solution over 5FWO/gCN heterostructure, (b) photodegradation efficiency of SA under visible light
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irradiation, (c) kinetic curves of the studied photocatalysts and (d) degradation rate constant for
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SA over as-prepared heterostructures.
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Fig. 8 (a) Transient photocurrent densities of FeWO4 and 5FWO/g-CN heterostructure, (b) photoluminescence spectra of FeWO4 and 5FWO/gCN heterostructure, (c) active species trapping experiments for SA degradation over FeWO4, g-CN and 5FWO/gCN heterostructure
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and (d) cyclic photocatalytic experiment for the 5FWO/gCN heterostructure.
Fig. 9 Suggested mechanism of the photocatalytic activity of 5FWO/gCN heterostructure.
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Highlights: Wolframite-type FeWO4 nanorod was synthesized hydrothermally using triethanolamine.
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Photodegradation of salicylic acid was enhanced by FeWO4/g-C3N4 heterostructure. Predominant active species were identified by using scavengers.
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Z-scheme photodegradation mechanism was suggested.
S1359-8368(18)32682-9
DOI:
10.1016/j.compositesb.2018.10.039
Reference:
JCOMB 6118
To appear in:
Composites Part B
Received Date: 20 August 2018 Revised Date:
1 October 2018
Accepted Date: 13 October 2018
Please cite this article as: Ojha DP, Karki HP, Song JH, Kim HJ, Amine-assisted synthesis of FeWO4 nanorod g-C3N4 for enhanced visible light-driven Z-scheme photocatalysis, Composites Part B (2018), doi: https://doi.org/10.1016/j.compositesb.2018.10.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Amine-assisted synthesis of FeWO4 nanorod g-C3N4 for enhanced visible light-driven Zscheme photocatalysis
a
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Devi Prashad Ojhaa, Hem Prakash Karkia, Jun Hee Songa* , Han Joo Kima,b* Department of Convergence Technology Engineering, Chonbuk National University, Jeonju
561-756, Republic of Korea b
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Eco-friendly Machine Parts Design Center, Chonbuk National University, Jeonju 561-756,
Republic of Korea
Han Joo Kim Email: [email protected],
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*Corresponding author:
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Telephone number: +82634722897, Fax number +82632704226
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Abstract: Highly crystalline FeWO4 nanorods (FWO NRs) were prepared using an amine in a
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hydrothermal reaction. A photocatalyst active to visible light was designed by preparing 5FWO/g-CN heterostructures via in-situ hydrothermal methods. Fabricated heterostructures were analyzed using X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), BET
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measurements, transmission electron microscopy (TEM), X-ray photoelectron microscopy (XPS), and Fourier transform spectroscopy (FTIR). The photocatalytic activity toward the degradation
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of salicylic acid (SA) was investigated under visible light irradiation. The active species trapping experiments showed that the holes, as well as the electrons, exhibited an obvious influence on the photocatalytic degradation process. Examinations of the mechanism showed that the enhanced photocatalytic activity was mainly ascribed to reduced recombination rate and band
potential of the holes.
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gap broadening in a Z-scheme mechanism, which enhance the efficient transfer and the oxidation
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Keywords: Iron tungstate, photocatalyst, graphitic carbon nitride, salicylic acid
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1.
Introduction:
Semiconductor metal oxides such as TiO2 [1], ZnO [2], WO3 [3], Fe2O3 [4] etc are among the
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most studied heterogeneous photocatalysts because of their low-cost, chemical stability, nontoxicity and appropriate energy band position. Despite many advantages, heterogeneous photocatalysis has significant limitations due to moderate photocatalytic reaction rates and
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consequently overall low energy conversion [5]. Low photoactivity of these single-phase semiconductors is linked to the limited light absorption and rapid recombination of charge
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carriers [6]. Alternatively, an impressive number of ternary metal oxides like titanates [7, 8], phosphates [9], vanadates [10] , tungstates [11] etc have also been developed as potential substitutes.
Iron tungstate (FeWO4) is a fascinating wolframite-type metal tungstate with several technological applications in areas like scintillation detectors, optical fibres, humidity sensors,
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photoanodes, catalysts, and pigments [12-14]. In addition, many other studies have found that FeWO4 has a narrow band gap (< 2.0 eV) to absorb a broad range of visible light in solar
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spectrum [15]. However, the visible light photocatalytic efficiency of FeWO4 is not promising because of the rapid electron-hole recombination ultimately resulting low quantum yield, as well
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as the poor electron and ion transport capacity [16]. Consequently, developing an efficient strategy is considerably significant to promote the separation of charge carriers for FeWO4 photocatalysts. Numerous studies have been carried out to confine the electrons by combining it with large band-gap semiconductors like TiO2, ZnO, WO3 and zero band gap graphene and has been found to exhibit high photodegradation efficiency, mainly attributed to the effective electron-hole separation [17, 18]. Next, graphitic carbon nitride (g-C3N4 or g-CN) could be a potential material for yet another strategy to increase the electron availability of FeWO4. The g-
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C3N4 is a stable, metal-free, two-dimensional (2D) polymeric semiconductor with a band gap of 2.7eV and can be easily synthesised by heating melamine [19]. Moreover, visible light active g-
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CN has a sufficiently negative conduction band potential whereas FeWO4 possesses a band gap of 1.98 eV with more negative valence band (VB) potential [20]. Therefore, this kind of heterostructure combining an oxidation photocatalyst (FeWO4) possessing the strong oxidation
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ability in the VB and a reduction photocatalyst (g-CN) with the strong reduction ability in the CB could be an excellent photocatalyst. In this kind of direct Z-scheme photocatalyst, the
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recombination of electron-holes is greatly minimised without losing the redox ability of the semiconductors yet efficiently utilizing enough visible light [5]. Similar Z-scheme photocatalysts like g-C3N4/TiO2 [21, 22] , g-C3N4/WO3 [23], g-C3N4/ZnO [24], g-C3N4/BiOI [25] etc. have been reported with enhanced photoactivity due to efficient charge separation and high redox ability. Nevertheless, the exfoliated 2D flat-structure of g-CN additionally extends its active
harvesting [26].
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surface area which provides more reactive sites, reduces e-h recombination, and enhances light
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There are several reports on the preparation of FeWO4 nanoparticles of varied shape and size using various synthetic methods [27, 28]. The morphologies alter the physical and chemical as
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well as optical properties of the metal oxides, which allow them to be used in various applications [29]. Previous studies have established that 1D nanostructures, like nanowires (NWs) and nanorods (NRs) increase the overall electron mobility and offer enhanced photocatalytic efficiency due to the higher surface to volume ratio, lower crystallinity and greater number of defects [30]. Thus, we synthesized a p-type FeWO4 NRs by s simple hydrothermal method in which the crystal growth is controlled by an amine, Triethanolamine (TEOA) that has equally been used for tuning the shape and size of metal oxides for a long time [31]. The NR is then
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coupled with g-CN to utilize the intrinsic properties of g-CN that increase the electron lifetime and decrease recombination to enhance photoactivity. The mechanism by which the
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photoactivity is proposed with respect to the electrochemical performances of the heterostructure and role of the active groups during photocatalytic reaction was emphasized. 2.
Materials
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2.1.
Experimental
All the reagents, Melamine, sodium tungstate (Na2WO4·2H2O), ferrous ammonium sulfate
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(Mohr salt, (NH4)2Fe(SO4)2·6H2O), triethanolamine (TEOA), sodium hydroxide (NaOH), nitric acid (HNO3), and ammonia solution (NH4OH) were were purchased from Sigma-Aldrich and used directly for experiments without any further purification. 2.2.
Preparation of the g-CN nanosheet
The graphitic carbon nitride (g-CN) nanosheet was prepared by the thermal polymerization of
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melamine [32]. The melamine was put in a crucible with a lid under ambient pressure. After thermal treatment at 550 °C for 3 hours, yellow-colored g-CN powder was crushed into fine
melamine.
Synthesis of FeWO4 NR and FWO/gCN heterostructures
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2.3.
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powder and again heated at 550 °C for 3 hr to complete the polymerization of the remaining
For preparation of the FeWO4 NRs, Na2WO4 .2H2O (2 mmol) and Mohr salt (2 mmol) were dissolved separately in 20 mL water. The Mohr salt solution was vigorously added to the sodium tungstate solution to produce a dark brown mixture. TEOA was added to the mixture, the pH of which was adjusted by 1M NaOH solution. The hydrothermal treatment of the resulting mixture was conducted in a 100 mL Teflon-lined autoclave at 200 °C for 12 hours and was allowed to cool naturally. The dark brown colored suspension was recovered, washed with ethanol and
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water for three times and finally dried at 80 °C for 12 h. The FeWO4 nanoparticles were also prepared without using TEOA under similar hydrothermal conditions.
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For the preparation of FWO/gCN heterostructures, 50ml of the 0.5 gm FeWO4 NR and the calculated amount of g-CN (1 wt%, 5 wt%, and 10 wt%) were sonicated separately in a 40 ml ethanol: water solution. After 30 min sonication, the two suspensions were mixed and further
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sonicated for 30 min. Finally, all the mixture was transferred into the Teflon lined hydrothermal bomb and kept at 200 °C for 12 hours. Thus, the obtained 1FWO/gCN, 5FWO/gCN, and
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10FWO/gCN heterostructures containing 1 wt% g-CN, 5 wt% g-CN, and 10 wt% g-CN, respectively, were centrifuged, washed several times, and dried. 2.4.
Evaluation of Photocatalytic Activity
The photocatalytic samples were utilized to estimate the photocatalytic activity by monitoring the decomposition of a typical organic pollutant, salicylic acid (SA). In the photocatalytic
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degradation process, 10 mg FWO/gCN samples were mixed in 50 mL of 50µM aqueous SA solution and stirred in the dark for 30 minutes. A 300 W Xe lamp (DY. Tech., Korea) equipped
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with a UV and an IR cut-off filters (Edmund optics, USA) was used for the sample irradiation after adsorption-desorption equilibrium is ensured. A 2 mL aliquot of the test solution was
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withdrawn in different time interval and used to analyze the absorbance of the remnant SA using a UV–vis spectrophotometer. The progress of the reaction was studied from its characteristic absorption peak at 297 nm in the absorption spectrum. Photodegradation with FWO and g-CN, and blank tests were also carried out to compare the degradation efficiency of the heterostructures. 2.5.
Characterization
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X-ray diffraction (XRD) graphs were characterized using Rigaku x-ray diffractometer using light intensity of Cu Kα radiation (λ = 1.540 Å) in the range of 2θ range of 20 – 80° with 2°/min scan
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rate. UV-Visible diffuse reflectance spectra were evaluated with Perkin-Elmer Lambda 40 spectrophotometer. Transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) images were analyzed using JEM-2200, JEOL Japan. X-ray photoelectron spectra (XPS) of the
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catalysts were recorded on a Kα X-ray photoelectron spectrometer (Thermo Scientific) using Al Kα (1486.6 eV) radiation. The N2 adsorption-desorption isotherms were recorded in
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Micromeritics ASAP 2420. The surface area was determined by BET method whereas the pore size distribution was derived from the desorption isotherm using BJH method. 3.
Results and discussion
The crystal structures of the FeWO4 nanostructures were analyzed through XRD. Fig. 1 showed XRD pattern of the highly crystallized FeWO4 NRs prepared via a hydrothermal reaction by
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using triethanolamine. All the diffraction peaks were indexed as a highly crystallized wolframitelike monoclinic structure (JCPDS file 74-1130) with space group P2/c. Impurity phases weren’t
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detected from the XRD patterns, indicating the formation of a single phase FeWO4. Fig. 1 also showed the XRD pattern of the pristine g-C3N4 prepared by the thermal treatment of melamine at
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550 °C. The characteristic peak at 27.3° was indexed to (002) interspacing of the conjugated aromatic systems at a distance d=0.33 and indicates that the g-C3N4 sheets were stacked (JCPDS87-1526) [33]. However, the peak of g-CN at 27.3° wasn’t detected in the FWO/gCN heterostructures possibly due to the relatively low graphitic carbon nitride content in the final catalysts [18]. FeWO4 NR was also analyzed by TEM to study the mechanism of crystal growth. Fig. 2(a) showed the FeWO4 nanoparticles synthesized by hydrothermal growth method at pH 8 controlled
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by NaOH. However, highly crystalline FeWO4 nanorods were formed when TEOA, a tertiary amine, was used under similar reaction conditions (Fig. 2(b)). This change of the crystal shape
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clearly indicated the amine played key role for the formation of the nanorods. Low magnification TEM in Fig. 2(c) revealed the nanorods had an average length between 150-200 nm whereas the diameter ranging from 30-40 nm. The NRs were further characterized by HRTEM and SAED
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patterns (Fig. 2(d)). The spacing of the fringe patterns was measured to be 0.473 nm and 0.363 nm, interplanar lattice spacing of which corresponded to the (100) and (110) planes of the
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monoclinic FeWO4 phase. The well-defined spots in the SAED patterns (inset, Fig. 2(d)) confirmed that an individual FeWO4 NR is a single crystal in the monoclinic phase [34]. Likewise, Fig. 2(e) showed the pristine g-CN sheet with multiple stacked layers. The TEM image of a 5FWO/gCN (5 wt% g-CN) heterostructure in Fig. 2(f) prepared by the one-pot hydrothermal method showed that the shape and size of a NR were unaffected by g-CN. Several of the FeWO4
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NRs were embedded in the g-CN suggesting the FeWO4 NRs were tightly bound into the wellexfoliated g-CN sheets [35].
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The EDX mapping of the 5FWO/gCN heterostructure in Fig. 3 confirmed the presence of Fe, C, N, O and W. X-ray photoelectron spectroscopy was utilized to the information concerning the
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binding energy of the elements in the near surface region and their respective oxidation states. Since the electron binding energy differs for each element, a full-scan spectrum was produced for an overall understanding of the elements on surface of the tested sample (Fig. 4). The XPS spectrum (Fig. 4(a)) of the FeWO4 NRs peaks correspond to Fe2p3 (711.2 eV), O1s (531.1 eV), and W4f (35.6 eV) [36]. The C1s peak at 285.3 eV is identified to the adventitious carbon adsorbed on the surface. Similarly, in the spectrum of 5FWO/gCN, there are peaks corresponding to Fe2p3 (711.3 eV), O1s (531.1 eV), W4f (35.6 eV), and C1s (285.3 eV). The
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survey XPS spectrum of the g-C3N4 sample reveals the predominant presence of N and C elements, and the intensity of the N 1s core level line is very high [37]. No detectable difference
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in the binding energies of Fe element between FeWO4 and 5FWO/gCN has been found. In Fig. 4(b), the binding energies of Fe 2p3/2 and Fe 2p1/2 are indicated at 711.2 and 724.8 eV respectively. In Fig. 4(c), two peaks, located at 36.5 and 38.2 eV, are assigned to W4f, with
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binding energies corresponding to the tungsten in the formal valence of +6. From Fig. 4(d), the N 1s XPS spectrum can be deconvoluted into one distinct peak and three small peaks with BEs of
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398.6, 399.1, 400.7, and 405.1 eV, which are assigned to C=N-C (398.6 eV), tertiary nitrogen (C)3-N (399.1 eV), N-H (400.7 eV), and π-excitation (405.1 eV), respectively. All the above results further confirm the formation of a 5FWO/gCN heterostructure [38, 39]. UV-visible diffuse reflectance spectra were also recorded to investigate the light harvesting ability of the heterostructures and respective band gap calculations (Fig. 5). As shown in Fig.
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5(a), pure g-CN shows absorption the UV and visible absorption only up to 460 nm where as pure FeWO4 showed the absorption in the entire visible light region that suggested that enough
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solar light can be utilized. Thus, the photoabsorption ability of the FeWO4 is well retained in FeWO4/gCN heterostructures indicating the strengthened electronic coupling between two
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semiconductors. Furthermore, the band gaps of the photocatalysts can be calculated by the equation [10]:
αhv = A(hv-Eg)n/2
where α, A, hv, and Eg are the optical absorption coefficient, a proportionality constant, a photon energy and the band gap energy respectively. The value of n depends upon the type of optical transition of the semiconductors: for direct transition, value of n is 1 where as for the indirect transition the value of n is 4. Taking the reference of previous reports, the value of n for bulk g-
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CN and FeWO4 semiconductors used is 4 [40, 41] . Thus, the band gap of g-CN and FeWO4 calculated from Tauc plot was estimated to be about 2.68 eV and 1.68 eV respectively (Fig. 5(b)).
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Nitrogen adsorption-desorption isotherms were checked to observe the surface area and pore size distribution of the FeWO4, g-CN and 5FWO/gCN nanocomposite as shown in Fig. 6 and Table 1. Specific surface area of FeWO4, g-CN and 5FWO/gCN are 23.30, 12.60 and 42.70 m2/g
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respectively. The shape of the hysteresis loops of the FeWO4 and 5FWO/gCN are of H3 type in IV isotherm indicating the presence of slit-like pores. The isotherm of the bulk g-CN represents
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type II isotherm indicating the presence of the nonporous surface [42]. The inset of Fig. 6 shows the pore size distribution of FeWO4, g-CN and 5FWO/gCN. The broad pore distribution of 5FWO/gCN at higher pore size shows the presence of the mesopores and macropores whereas micropores in lower value represent the presence of g-CN sheets [42]. The pore size and pore
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volume of the FeWO4, g-CN and 5FWO/gCN are shown in Table 1.
The photocatalytic activity of several composites was evaluated by monitoring the decomposition of aqueous salicylic acid under visible-light irradiation (λ≥400 nm). Fig. 7(a)
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depicts the absorption spectra of SA as a function of irradiation time at various time intervals in presence of 5FWO/gCN heterostructure. The intensity of the SA peak at 298 nm decreases as the
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SA degrades with the progress of the photocatalytic reaction. It is revealed from Fig. 7(b) that the photolysis of SA is negligible whereas FeWO4 NR and g-CN samples exhibit low photocatalytic degradation of SA under visible light. Fig. 7(b) also showed the significantly enhanced photocatalytic activity with various FWO/gCN heterostructures. The decomposition reaction of SA obeys the pseudo-first order kinetics which can be represented by -d[A]/dt = k[A] where [A] denotes the concentration of SA and k denotes the degradation rate constant. The percentage of the remnant SA, the respective kinetics of the various photocatalysts and the calculated rate
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constants are shown in Fig. 7(b-d). The FWO/gCN composites showed remarkably higher photocatalytic activity than the bare FeWO4 or g-CN sample: rate constant k of the 5FWO/gCN
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was 40.0 and 7.1 times that of the bare FeWO4 and g-CN, respectively. Furthermore, in 2.5 h, ~95% SA was removed with FWO/gCN, whereas only 36% was removed with g-CN. Compared to the FeWO4 and g-CN, the heterostructures displayed significant enhancement in the
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photodegradation of SA. The gradual increase in the degradation rate from 1% g-CN to 5% g-CN is attributed to the simultaneous effect of the increased light absorption (red shift in Fig. 5) and
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enhanced adsorption of the pollutant (SA) due to increased specific surface area. Thus, more molecules are prone to photodegradation on the catalyst surface. This phenomenon is supported from Fig. 7(b) which shows the adsorption of the pollutant in dark is higher in the heterostructures. Furthermore, on increasing the g-CN content beyond 5%, the photoactivity is decreased which might be due to the less availability of the slit-like channels on the
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heterostructures [43]. This clearly indicates that g-CN has significant role in the photoactivity. The stability of the 5FWO/g-CN heterostructure was evaluated by reusing the sample for the 5
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consecutive cycles (Fig. 8(d)). In each cycle, the sample was collected by centrifugation, washed with ethanol and water and dried completely at 80 °C. The photocatalytic activity of the
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5FWO/gCN is retained without significant deactivation. Similarly, the interfacial charge separation efficiency of the photoinduced electron-hole pairs is also studied by measuring the photocurrent response with or without visible light as shown in Fig. 8(a). 5FWO/gCN displays higher photocurrent density of 0.031 mA/cm2 compared to the bare FeWO4 (0.14 mA/cm2) which indicates the heterostructure has the better interfacial charge transfer ability [44, 45]. This phenomenon is further supported in the photoluminescence (PL) spectra in Fig. 8(b). The higher
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PL intensity of 5FWO/gCN over FeWO4 shows that the recombination of photogenerated electron-hole is reduced and further verifies the effective formation of the heterostructure [46] .
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Thus, based on above analysis, a series of radical trapping experiments using various scavengers were conducted to explore the main active species (•OH, •O2-, h+) responsible for the photocatalytic degradation of SA. During the radical trapping experiments, 1 mmol of
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isopropanol (IPA, a scavenger of •OH), 1,4-benzoquinone (p-BQ, a scavenger of •O2-) and 1 mmol of TEOA (AO, a scavenger of h+) were used with SA solution under visible light
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irradiation, respectively [47]. The effects of different scavengers on the visible light photocatalytic reaction of SA by FeWO4, g-CN and 5FWO/gCN heterostructure were presented in Fig. 8(c). For FeWO4, when TEOA was added into the reaction solution, the photodegradation is quenched from 8 % to 2 % however with p-BQ and IPA, the photodegradation is unchanged (~7%) indicating the hole as a major reactive species. Similarly, for g-CN, the photodegradation
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rate of SA is un32affected with TEOA but significantly inhibited with p-BQ (21%) and IPA (24.5%). This implies in g-CN, the •O2- and the •OH radicals are the major reactive species. On the other hand, the photoactivity of SA is significantly inhibited with p-BQ (48 %), TEOA
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(54 %), and IPA (30 %) indicating that •O2-, •OH, and h+ are obviously the active species during
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the photodegradation process. The photocatalytic degradation rate is significantly inhibited with TEOA indicating h+ is the predominant radical responsible for the SA degradation. These results indicate the oxidation potential of the hole and the reduction potential of the electron are sufficiently high for photodegradation process [48]. In order to explain the enhanced photoactivity of the FWO/gCN heterostructures over individual semiconductors, conduction band (CB) and valance band (VB) potentials of FeWO4 and g-C3N4 were calculated using following equations [49]:
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EVB = X - Eo + 0.5 Eg
(1)
ECB = EVB - Eg
(2)
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Where χ denotes absolute electronegativity and the value of X for FeWO4 and g-CN are 6.23 and 4.73 respectively [40, 41]. EVB and ECB are the valence band edge potential and conduction band edge potential, E° (=4.5eV) is the energy of the free electrons in hydrogen scale and Eg is the
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band gap energy. From the above equations the ECB of the FeWO4 and g-CN are estimated to be +0.80 V and -1.05 V vs. NHE, and the EVB of FeWO4 and g-CN are estimated to be +2.72 V
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+1.58 respectively. Thus, it is evident both the semiconductors are excited in visible light and charge transfer in a simple “photosensitizer” mechanism is insufficient to explain the enhanced photoactivity. This is because the electrons from the highly negative CB of g-CN if transferred to the CB of FeWO4, the reduction of O2 may not take place to produce superoxide radicals that have already been detected in the radical trapping experiment in fig. The redox potential of
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O2/•O2- (-0.28 V, NHE) is more negative than the CB position of FeWO4. Based on the above trapping experiments and band edge analysis, a simple Z-scheme mechanism
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(Fig. 9) was suggested. In this FWO/gCN heterostructure, the electron-hole charge separation is induced in FeWO4 and g-CN simultaneously upon the illumination in visible light. The electrons
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from the CB of FeWO4 can easily shift to the VB g-CN by Shottky barrier and photogenerated electron-hole recombination is effectively reduced compared to the individual components. Meanwhile, the well separated holes in the VB of FeWO4 and the electrons in CB of g-CN take part in the redox reaction after the interfacial charge transfer. The reduction of O2 at the CB of gCN of produce the active species superoxide (•O2-) and hydroxyl (•OH) radical [50]. Similarly, holes generated in the FeWO4 VB resultantly degrade SA. Thus, superoxide (•O2-) and h+ are the
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two predominant active species responsible for the enhanced photodegradation of SA as elaborated in the following equations:
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Photo-excitation: FWO/gCN + hv →FWO/gCN (e− + h+) Photo-reduction:
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O2 + e- → •O2-, •
O2- + e- + 2H+ → H2O2,
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H2O2 + e- → •OH + OHPhoto-oxidation: 2h+ + H2O → 2H+ + •OH Photo-degradation:
O2-, •OH, + SA → Degradation products.
4. Conclusion:
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Highly crystallized FeWO4 nanorods were successfully prepared by amine controlled
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hydrothermal synthesis at 200 OC for 12 h. These nanorods were combined with g-CN to prepare a visible-light active Z-scheme FWO/gCN photocatalyst which exhibited higher photocatalytic
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activity than both pure g-CN and FeWO4. The catalytic activity of the heterostructure was found 40 times higher than that of FeWO4. The mechanism was further confirmed by observing the active species participation in the photodegradation process by using electron and hole scavengers. The enhanced photocatalytic activity attributed to the efficient and prolonged electron/hole separation and effective interfacial charge transport between FeWO4 and g-CN. Thus, the photocatalyst is easily synthesized in a cost-effective way.
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Acknowledgement: This paper was supported by a grant through the national Research Foundation (NRF, Project no.
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2017-R1C1B2011968) provided by Ministry of Education, Science, Technology (MEST), Republic of Korea. We would also like to thank the staff at the Center for Chonbuk University
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Research Facility (CURF) for providing the facilities for analysis.
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References:
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Table Caption: Specific surface area, pore volume and pore diameter of FeWO4, g-CN and 5FWO/gCN
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heterostructure.
Figures Captions:
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Fig. 1 XRD spectra of g-CN, FeWO4 NR and 5FWO/gCN heterostructure.
Fig. 2 TEM images of (a) FeWO4 nanoparticles with NaOH, (b) FeWO4 nanorods with NaOH
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and TEOA, (c) a single FeWO4 NR (Inset: SAED pattern), (d) HRTEM of the FeWO4 NR, (e) gCN sheet and (f) 5FWO/gCN heterostructure.
Fig. 3 EDX map of the 5FWO/gCN heterostructure showing C, N, O, Fe and W. Fig. 4 XPS survey spectra and high-resolution XPS spectra of (b) Fe 2p (c)W 4f, and (c) C 1s for 5FWO/gCN heterostructure.
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Fig. 5 (a) UV-Visible DRS spectra of as prepared photocatalysts and (b) Touc plots of (αhν)1/2 versus photo energy (hν) for FeWO4 and g-CN.
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Fig. 6 Nitrogen adsorption-desorption isotherms of FeWO4, g-CN and 5FWO/gCN heterostructure (Inset: the pore size distribution curves).
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Fig. 7 (a) UV-visible absorbance curve for the photodegradation of the aqueous SA solution over 5FWO/gCN heterostructure, (b) photodegradation efficiency of SA under visible light irradiation, (c) kinetic curves of the studied photocatalysts and (d) degradation rate constant for SA over as-prepared heterostructures. Fig. 8 (a) Transient photocurrent densities of FeWO4 and 5FWO/g-CN heterostructure, (b) photoluminescence spectra of FeWO4 and 5FWO/gCN heterostructure, (c) active species
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trapping experiments for SA degradation over FeWO4, g-CN and 5FWO/gCN heterostructure and (d) cyclic photocatalytic experiment for the 5FWO/gCN heterostructure.
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Fig. 9 Suggested mechanism of the photocatalytic activity of 5FWO/gCN heterostructure.
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Table: Specific surface area, pore volume and pore diameter of FeWO4, g-CN and 5FWO/gCN
BET (m2/g)
Mean pore diameter (nm)
23.30
39.10
g-C3N4
12.60
5.35
5FWO/g-CN
42.70
32.85
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(cm3/g)
0.181
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FeWO4
Pore volume
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Photocatalyst
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heterostructure.
0.0345
0.175
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Figures:
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Fig. 1 XRD spectra of g-CN, FeWO4 NR and FWO/gCN heterostructure.
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Fig. 2 TEM images of (a) FeWO4 nanoparticles with NaOH, (b) FeWO4 nanorods with NaOH and TEOA, (c) a single FeWO4 nanorod (Inset: SAED pattern), (d) HRTEM of the FeWO4 NR,
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(e) g-CN sheet and (f) 5FWO/gCN heterostructure.
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Fig. 3 EDX map of the 5FWO/gCN heterostructure showing C, N, O, Fe and W.
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Fig. 4 XPS survey spectra and high-resolution XPS spectra of (b) Fe 2p (c)W 4f, and (c) C 1s for
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5FWO/gCN heterostructure.
Fig. 5 (a) UV-Visible DRS spectra of as prepared photocatalysts and (b) Touc plots of (αhν)1/2 versus photo energy (hν) for FeWO4 and g-CN.
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Fig. 6 Nitrogen adsorption-desorption isotherms of FeWO4, g-CN and 5FWO/gCN
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heterostructure (Inset: the pore size distribution curves).
Fig. 7 (a) UV-visible absorbance curve for the photodegradation of the aqueous SA solution over 5FWO/gCN heterostructure, (b) photodegradation efficiency of SA under visible light
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irradiation, (c) kinetic curves of the studied photocatalysts and (d) degradation rate constant for
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SA over as-prepared heterostructures.
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Fig. 8 (a) Transient photocurrent densities of FeWO4 and 5FWO/g-CN heterostructure, (b) photoluminescence spectra of FeWO4 and 5FWO/gCN heterostructure, (c) active species trapping experiments for SA degradation over FeWO4, g-CN and 5FWO/gCN heterostructure
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and (d) cyclic photocatalytic experiment for the 5FWO/gCN heterostructure.
Fig. 9 Suggested mechanism of the photocatalytic activity of 5FWO/gCN heterostructure.
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Highlights: Wolframite-type FeWO4 nanorod was synthesized hydrothermally using triethanolamine.
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Photodegradation of salicylic acid was enhanced by FeWO4/g-C3N4 heterostructure. Predominant active species were identified by using scavengers.
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Z-scheme photodegradation mechanism was suggested.