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ruptured tubular g-C3N4 for enhanced photoelectrochemical water oxidation
Solar Energy 193 (2019) 403–412
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Synthesis and characterization of Z-scheme α-Fe2O3 NTs/ruptured tubular g-C3N4 for enhanced photoelectrochemical water oxidation
T
Ahmed Esmail A. Bakra,b, Waleed M.A. El Roubya, Malik D. Khanb, Ahmed A. Farghalia, B. Xuluc, Neerish Revaprasadub a
Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, 62511 Beni-Suef, Egypt Department of Chemistry, University of Zululand, Private Bag X1001, Kwadlangezwa 3886, South Africa c Department of Chemistry, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa b
A R T I C LE I N FO
A B S T R A C T
Keywords: α-Fe2O3 NTs Photoelectrochemical water splitting RT g-C3N4 Z-scheme heterostructure
A composite of tubular g-C3N4 with α-Fe2O3 has been synthesized to improve the photocatalytic performance of tubular g-C3N4 by the formation of a Z-scheme heterostructure. The composites were fabricated by the combination of varying amounts of hydrothermally synthesized α-Fe2O3 nanotubes (NTs) and protonated ruptured tubular (RT) g-C3N4, by the electrostatic self-assembly method. The obtained composites were characterized by zeta potential, powdered-X-ray diffraction (p-XRD), scanning/transmission electron microscopy (S/TEM), photoluminescence (PL), and Fourier transform infrared spectroscopy (FTIR). PL and photocurrent measurements indicates a higher charge separation of the photo-generated electron-hole pair for Fe2O3 NTs/RT g-C3N4 composite with higher concentration of α-Fe2O3 NTs. UV–Vis diffuse reflectance spectroscopy (UV–Vis/DRS) shows the band gaps of pristine α-Fe2O3 NTs and RT g-C3N4 to be 1.86 eV and 2.72 eV respectively, while the band structures were determined by Mott-Schottky measurements. I-V curves obtained from photoelectrochemical water splitting shows that the formation of the composite decreased the oxidation overpotential as compared to pristine α-Fe2O3 NTs and pristine RT g-C3N4. Bode plots showed that the composite was able to increase the lifetime of the photo-generated electrons as compared to both RT g-C3N4 and α-Fe2O3 NTs.
1. Introduction The rapid growth of population, expanding industrialization accompanied with environmental issues, depletion of fossil fuels, and increase in global energy demands, requires the investigation of new approaches for applicable energy sources that are efficient, clean and environmentally friendly (Faber and Jin, 2014; Lund et al., 2011; Lewis and Nocera, 2006). A suitable alternative is to utilize solar light for energy generation, especially photoelectrochemical (PEC) water splitting (Hisatomi et al., 2014; Sayed et al., 2017; Ahmed et al., 2017; Khan et al., 2018; Khan et al., 2018). Hydrogen generation by water splitting is an efficient and renewable energy source, which has a lower carbon foot print on the environment (Maeda and Domen, 2010; Walter et al., 2010). The Gibbs free energy required for photoelectochemical (PEC) water splitting is significantly high i.e. 237 kJ per mol., hence it is crucial to synthesize a photocatalyst that has the potential to carry out this reaction at much lower energy requirements. A wide range of materials used for the PEC splitting of water have been reported. Amongst these materials, hematite (α-Fe2O3) (Shen et al., 2016; Mishra and Chun,
2015; Sivula et al., 2011; Tamirat et al., 2016) and graphite-like carbon nitride (g-C3N4) (Wen et al., 2017; Ong et al., 2016; Fu et al., 2017; Naseri et al., 2017; Zhao et al., 2015), are cost effective, vastly available and non-toxic materials with remarkable photocatalytic efficiencies. Hematite (α-Fe2O3), an earth abundant and stable oxide of iron under ambient conditions, is an n-type semiconductor having band gap in range of 1.9-2.2 eV (Kennedy and Frese, 1978; Marusak et al., 1980). It is a captivating photocatalyst and an attractive material for various energy applications (Sivula et al., 2011; Li et al., 2018). However, the short lifetime of charge carriers (< 10 PS) and the high rate of recombination as well as low electrical conductivity of α-Fe2O3 (∼10−14 Ω−1 cm−1) impedes its use in practical applications (Barroso et al., 2013). Graphite-like carbon nitride (g-C3N4), is considered as a metal-free semiconductor with photocatalytic activity towards hydrogen production (Cao and Yu, 2014). It shows high chemical stability and can be easily synthesized at a scalable level with large surface area (Wang et al., 2012). However, its photocatalytic performance suffers from the fast recombination of electron-hole pairs. Several strategies have been adopted to overcome the drawbacks and enhance the performance of α-hematite and g-C3N4 photocatalysts.
E-mail addresses: [email protected] (W.M.A. El Rouby), [email protected] (N. Revaprasadu). https://doi.org/10.1016/j.solener.2019.09.052 Received 17 March 2019; Received in revised form 2 September 2019; Accepted 13 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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600 mL of deionized water, at room temperature, and stirred vigorously. After complete dissolution, the solution was transferred to a Teflon-lined stainless-steel autoclave (800 mL volume), which was heated to 220 °C in the furnace for 48 h. The precipitate formed at the end of the reaction, was separated by decanting the water after which it was washed several times with deionized water/ethanol, and dried overnight at 60 °C. The reddish precipitate was calcinated at 550 °C for two hours and at 800 °C for 20 min then left to cool naturally for further usage.
The recently adopted strategy for enhancing photocatalytic performance is the formation of a Z-scheme composite, which is generally composed of an O2-evolving photocatalyst, with a relatively low conduction band and reduction potential, and a H2-evolving photocatalyst, of a relatively high valence band and low oxidation potential. Hence, under solar light illumination, electrons with lower reduction potential present in the O2-evolving photocatalyst, transfer to holes of lower oxidation potential in the H2-evolving photocatalyst. Therefore, holes with high oxidation potential and electrons with high reduction potential are kept in the O2-evolving photocatalyst and H2-evolving photocatalyst respectively. In this way, the charge separation can be retained, which promotes the redox reactions. The interaction of αFe2O3 with g-C3N4 is supposed to generate Z-scheme composite, as the valence (EVB) and the conduction band edge potential (ECB) of α-Fe2O3 are 2.48 eV and 0.28 eV respectively and those of g-C3N4 are 1.57 eV and −1.13 eV respectively (Xu and Schoonen, 2000; Dong et al., 2011; Liu et al., 2012). There are various reports on the synthesis of iron oxide and carbon nitride heterostructures, which are used for photocatalysis (Shen et al., 2016; Huang et al., 2018). For instance, hierarchical structured, metalorganic framework-derived and 2D/2D g-C3N4/α-Fe2O3 composites (Jiang et al., 2018; Shi et al., 2018; Xu et al., 2018), direct solid-state Zscheme g-C3N4/Fe2O3 (Wang et al., 2018), g-C3N4@α-Fe2O3/C or Fe2O3/C-C3N4 based photocatalysts (Wu et al., 2018; Kong et al., 2018), Fe3O4 nanorods/g-C3N4 composite (Wu et al., 2017), g-C3N4/Ti- Fe2O3 (Liu et al., 2016), Ag-Fe3O4/ g-C3N4 (Pant et al., 2017), visible-light active α-Fe2O3/g-C3N4 (Theerthagiri et al., 2014), have been prepared in order to enhance the catalytic efficiency. However, compared to the bulk g-C3N4, most of the aforementioned work did not achieve a very high separation for the photogenerated charge carriers. It is a wellknown observation that a higher charge separation can be achieved in nanotubes as compared to sheet-like morphology, therefore a higher concentration of photogenerated charge carriers can be highly suitable for photocatalysis. However, until the present, the photocatalytic potential of composites of tubular structured α-Fe2O3 NTs and g-C3N4 has not been explored. Herein, we have prepared α-Fe2O3 NTs/RT g-C3N4 Z-scheme structure by a self-assembly technique to enhance the water splitting capability of the composite. We postulated that the presence of nanotubes in the prepared nanocomposite can enhance the photocatalytic activity by the creation of directional paths for the mobility of charge carriers which in turn leads to higher photoactivity of the nanotubes as compared to nanoparticles (Mohapatra et al., 2009). Likewise, one-dimensional materials are of low density and high aspect ratio that result in high specific surface area and provides more active sites for catalysis. Besides, the formation of hydrogen bonds in the synthesized composite may also contribute to a higher photocatalytic activity (Xu et al., 2018; Liu et al., 2018; Jin et al., 2018; Xie et al., 2018).
2.2.2. Synthesis of tubular (RT) g-C3N4 Tubular g-C3N4 was prepared by the literature method (Tahir et al., 2013; Dong et al., 2017) with a slight modification. In brief, 1.0 g of melamine was dissolved in 30.0 mL of ethylene glycol under vigorous stirring then appropriate amount of nitric acid was added with continuous stirring. The as-prepared white precipitate was separated, washed with absolute ethanol several times to remove the excess nitric acid and ethylene glycol, and dried at 60 °C overnight. The dried white product was placed in a ceramic crucible covered with aluminum foil and annealed in a muffle furnace for 2 h at 450 °C, at a heating rate of 10 °C/min. For comparison, bulk g-C3N4 was prepared by direct pyrolysis of melamine at 520 °C for 2 h with a heating rate of 10 °C/min (Wang et al., 2009). 2.2.3. Synthesis of tubular α-Fe2O3/g-C3N4 composite α-Fe2O3/g-C3N4 composite was fabricated as previously adopted (Xu et al., 2018) with a slight modification. Typically, specific amounts of g-C3N4 and α-Fe2O3 NT were separately ultrasonicated for 2 h in 100 mL mixture (2:1 by volume) of ISP: H2O. G-C3N4 was protonated by the addition of specific amount of nitric acid (≈5 mL of 1 M HNO3) until the pH reached 4. Under continuous stirring the suspended αFe2O3 NTs were added slowly to the suspension of protonated g-C3N4. The as-obtained suspension was kept under stirring in a fume-hood at 60 °C, and the suspension was allowed to dry. Different samples were prepared by keeping the ratio of α-Fe2O3 NT: g-C3N4 wt% to be (1: X wt %); (where X = 1, 2, 3.75). The obtained powders were collected for further investigation. 2.3. Characterization The dynamic light-scattering analysis was used to investigate the surface Zeta potential of the samples by the Zetasizer Nano ZS90 (Malvern instruments). The phase of the samples was identified by powder X-ray diffraction (p-XRD) patterns using a Bruker AXS D8Advance instrument, (Cu Kα radiation, λ = 1.5406 Å). The accelerating voltage and applied current was 40 kV and 40 mA respectively. IR spectra were obtained from Bruker FT-IR Tensor 27 spectrophotometer, in 500–4000 cm−1 range. The microstructural analysis of the prepared samples was observed by Philips XL30 FEG SEM and transmission electron microscope (Talos F200X at 200 kV using a FEI ceta camera). Edwards coating system E306A was used for carbon coating of the samples prior to SEM analysis. Shimadzu UV-3600, Japan with Ba2SO4 as the reflectance standard was used for UV–Vis and DRS analysis. Photoluminescence (PL) spectra were measured using A Perkin-Elmer, LS 55 Luminescence spectrophotometer.
2. Experimental section 2.1. Materials Anhydrous ferric chloride (FeCl3), ammonium dihydrogen phosphate (NH4H2PO4), sodium sulphate (Na2SO4), melamine, ethylene glycol, nitric acid, isopropanol (ISP), ethanol, Nafion (5 wt%) and fluorine-doped tin oxide (FTO) substrates were purchased from Sigma Aldrich.
2.4. Photoelectrochemical measurements A potentiostat including a three-electrode cell system, where the synthesized materials were used as working electrode, reference electrode (SCE) and a counter electrode (Pt wire). The three electrodes were immersed in a solution of 0.1 M Na2SO4 (pH ∼ 6.5), which was used as an electrolyte, in photoelectrochemical cell modified with a window for illumination. Argon was used to purge the electrolyte for 20 min before measuring and was kept over the surface during all experiments. A solar simulator comprising of a xenon lamp (150 W) and coupled with an air
2.2. Synthesis of photocatalyst 2.2.1. Synthesis of α-Fe2O3 NTs α-Fe2O3 NTs were prepared as reported previously (Liu et al., 2015; Nathan and Boby, 2017; Jia et al., 2005), with slight modification. Typically, 1.945 g (0.0149 M) of FeCl3, 0.0495 g (5.3 × 10−4 M) of NH4H2PO4, and 0.0474 g (4.17 × 10−4 M) of Na2SO4 were dissolved in 404
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Fig. 1. Zeta potential values of the prepared solid materials of RT g-C3N4, protonated RT g-C3N4 and α-Fe2O3 NT dispersed in deionized water.
Fig. 2. p-XRD patterns of α-Fe2O3 NTs, bulk g-C3N4, RT g-C3N4 and α-Fe2O3 NTs/RT g-C3N4 composites with different ratios of RT g-C3N4.
mass of 1.5 global was used for illumination where the intensity of the simulated light was adjusted to 1 sun (100 mW cm−2).
the fabricated composite contains α-Fe2O3 NTs well distributed over the g-C3N4 and both materials maintained their own crystal structures. The peaks of α-Fe2O3 NT as well as g-C3N4 appear at the same 2θ values, without any shift, whereas a gradual disappearance of the g-C3N4 strong peak at 2θ = ∼27.4° was observed with increasing α-Fe2O3 NTs concentration. Fourier transform infrared spectroscopy (FT-IR) was used to supplement confirmation of the composite formation. Fig. 3 displays the FT-IR spectra of pristine α-Fe2O3 NTs, RT g-C3N4 and a series of αFe2O3 NTs/RT g-C3N4 composites. The presence of a strong characteristic peak around 520 cm−1, which originates from the stretching vibration of Fe-O bond in hematite nanoparticles was observed clearly (Wang et al., 1998). Likewise, for pristine RT g-C3N4, a broad and intense absorption band which extends from 3500 cm−1 to 3000 cm−1 was observed, that can be attributed to eOeH stretching vibration of adsorbed H2O molecules and the stretching vibration of the terminal amino groups of g-C3N4. In addition to a series of strong peaks in the range of 1650–1200 cm−1 originates from the stretching modes of CeN in the aromatic heterocyclic rings of g-C3N4. Another strong peak was observed around 808 cm−1 which corresponds to the characteristic modes of the triazine units of g-C3N4 (Bojdys et al., 2008; Yan et al., 2009). The α-Fe2O3 NTs/RT g-C3N4 hybrids show that the composite contains the characteristic peaks of both graphitic-C3N4 and α-Fe2O3, which confirms the presence of both components in every prepared composite. Notably, the peak intensity, associated with the triazine units, reduces with decreasing the content of g-C3N4 and synchronously
2.5. Fabrication of the working electrode 5.0 mg of the synthesized material was sonicated in 400.0 µL isopropanol for 10.0 min, then 10.0 µL Nafion (5.0 wt%) was added to the suspension. After that 30.0 µL of this suspension was casted (10.0 µL by 10.0 µL) on the surface of a well cleaned 1 cm2 FTO glass, and dried at 80 °C for 30 min. 3. Results and discussion A facile and inexpensive electrostatic self-assembly approach was used to generate heterostructured α-Fe2O3 NT/RT g-C3N4 composites. As the isoelectric point of g-C3N4 is around pH = 5 (Zhu et al., 2015), a lower pH will protonate and make it positively charged, due to the presence of abundant -C-N- motifs, by the addition of HNO3. Fig. 1 shows the zeta potential of solid RT g-C3N4, solid protonated RT g-C3N4 (which was prepared by sonication of specific amount of solid RT gC3N4 in isopropanol (ISP) and deionized (DI) H2O, then few drops of 1 M HNO3 were added until the pH = 4, and finally the solvent was allowed to evaporate), and solid α-Fe2O3 NT dispersed in DI H2O. It is clear that surface charge of the unprotonated RT g-C3N4 and protonated RT g-C3N4 were −28.2 and +25.8 mv, respectively. However, α-Fe2O3 NTs exhibited a negatively charged surface (−35.9 mv) owing to the existence of hydroxyl moieties over its surface. The α-Fe2O3 NT/RT gC3N4 composite was formed due to the combination of protonated RT gC3N4 (positively charged) and the negatively charged α-Fe2O3 NTs spontaneously through a self-assembly interaction when the two components were mixed. 3.1. Crystal phase and microstructure analysis Fig. 2 depicts the diffraction patterns of pristine α-Fe2O3 NT, g-C3N4 and α-Fe2O3 NT/g-C3N4 nanocomposites with varying compositions. The diffraction peaks for α-Fe2O3 were observed at 2θ values of 24.1°, 33.1°, 35.6°, 40.9°, 49.5°, 54.1°, 62.4°, 64.0° which can be indexed to the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) planes of α-Fe2O3 (JCPDS 33-0664). Two diffraction peaks for pure gC3N4, centered at 2θ = ∼13.1° and ∼27.4° were observed which corresponds to the (1 0 0) plane of in-planar motif of tri-s-triazine units and (0 0 2) plane of interlayer stacking of conjugated carbonaceous rings for graphitic materials, respectively (Wang et al., 2009). It is noteworthy that the α-Fe2O3 NT sample is pure and of high crystallinity as the obtained peaks was highly intense with no other diffraction peaks. The obtained patterns of α-Fe2O3 NT/g-C3N4 nanocomposites reflect that
Fig. 3. FT-IR spectra of α-Fe2O3 NTs, RT g-C3N4 and α-Fe2O3 NTs/RT g-C3N4 composites with different contents of RT g-C3N4. 405
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Fig. 4. (a & b) are SEM & TEM images respectively for RT g-C3N4, (c & d) are SEM images of α-Fe2O3 NTs, (e & f) are SEM images of 1 α-Fe2O3 NTs/1 RT g-C3N4 composite, (g) is TEM image of 1 α-Fe2O3 NTs/1 RT g-C3N4 composite, and (h) is TEM image of bulk g-C3N4.
facilitates the ruptured structure of the g-C3N4 nanotubes. Representative images of the α-Fe2O3 NTs/RT g-C3N4 composite (Fig. 4(e–g)) imply that α-Fe2O3 NTs are anchored in a random way to the RT g-C3N4 to form agglomerates of different sizes that confirm the followed approach (self-assembly) in the composite fabrication. The small-size agglomerations resulting from the combination of small sized crystallites of α-Fe2O3 NTs and RT g-C3N4 and the large crystallites combined together to form large-sized agglomerations. Also, for comparison, TEM image was obtained for bulk g-C3N4 (Fig. 4h) which shows the sheet-like morphology as previously reported (Sun et al., 2017). Moreover, the intimate combination between α-Fe2O3 NTs and RT g-C3N4 was further confirmed by EDS and elemental mapping analysis for 1 α-Fe2O3 NTs/ 1 RT g-C3N4hybrid (Fig. 5). It shows that the components (C, N, Fe and O atoms) present and homogeneously distributed in the formed composite.
the peak intensity accompanying the FeeO bond increases with higher ratio of α-Fe2O3, in the synthesized composites. The results are in good agreement with the obtained p-XRD patterns. Also, the peak for the FeeO bond was slightly shifted to a higher wavenumber, which might be due to the interactions, present between α-Fe2O3 NTs and RT g-C3N4 in the obtained composites. Microstructural analysis of the products can be shown by SEM and TEM images. Fig. 4(c–d) shows that the obtained α-Fe2O3 NTs are similar to what previously reported by the same method we followed, except that the average length of the obtained tubes is lower (∼200–400 nm) as well as the thickness of the wall is higher (∼50 nm), which might be due to the calcination at higher temperature of 800 °C for 20 min. SEM images confirm the formation of α-Fe2O3 NTs with good morphology. However, from the SEM & TEM images of the obtained RT g-C3N4, (Fig. 4(a–b)) it can be seen that it does not have a well-defined tubular morphology; a wrapped and curved shaped nanoflake- like morphology was observed. The indefinite structure of RT g-C3N4 can be ascribed to the high heating rate (10 °C/min), before maintaining the temperature at 450 °C for 2 h (Dong et al., 2017). In addition, the covering of the crucible, during the pyrolysis process, may in turn decelerate the fast escape of NH3 gas and consequently
3.2. Optical properties The optical behavior of the synthesized samples was investigated by UV–Vis diffuse reflectance spectroscopy (Fig. 6). Interestingly, in comparison to the RT g-C3N4, the absorption edges of the prepared 406
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Fig. 5. Elemental mapping images of C, N, Fe and O and EDS spectrum of 1 α-Fe2O3 NTs/1 RT g-C3N4 composite.
series of α-Fe2O3 NTs/RT g-C3N4 composites gradually shifts toward the longer wavelength and synchronously the intensity of the visible light absorption increases with increasing α-Fe2O3 ratio in the synthesized hybrids (Fig. 6a). As shown in Fig. 6b, by using the Kubelka − Munk function, the band gap energies can be calculated by a plot of (αhv)2 versus the photo energy (hv) for the direct transition semiconductor. For pristine RT g-C3N4, the absorption edge was observed at ∼460 nm, which corresponds to a band gap of ca. 2.72 eV, as reported previously (Wang et al., 2009). It can be seen that pristine α-Fe2O3 NTs is considered as a potential photosensitizer because it can absorb light over a wide region with (Eg) of almost 1.86 eV (Fig. 6b). The band gap (∼1.93 eV) of the composite (1 α-Fe2O3 NTs: 1 RT g-C3N4) lies in between that of the pristine α-Fe2O3 NTs and RT g-C3N4, which infers a good interaction between both components that results in band gap alignment. These results imply that α-Fe2O3 NTs can improve the PEC water splitting activity, by decreasing the band gap of g-C3N4 and enhancing the visible light absorption. The photo image (Fig. 6c) of the synthesized compounds infers that the composite was formed successfully. Likewise, a change in the color of the obtained composite was also observed, from yellowish white for RT g-C3N4 to more reddish, as the concentration of α-Fe2O3 NTs was increased in the composite.
recombination, in comparison to RT-C3N4. With increase in content of α-Fe2O3 NTs, the PL intensity decreases, which means better electronhole separation due to the formation of a Z-scheme structures as previously reported (Jiang et al., 2018; Xu et al., 2018; Wang et al., 2018; She et al., 2017). Also, it was observed that the formation of α-Fe2O3 NT/RT g-C3N4 hetero-structures, results in a slight shifting of peaks towards lower wavelength that might be due to the interaction of αFe2O3 NT with RT-C3N4 which leads to the quantum confinement effects of the nanostructure (Liu et al., 2017). 3.4. Photoelectrochemical water splitting measurements PEC water splitting requires a semiconductor (photo-electrode) material to absorb the solar light photons with appropriate energy to initiate the transfer of electrons between valence and conduction band, to generate the electron-hole pairs. The holes tend to oxidize water molecules on the semiconductor surface, while the electrons are percolated, via the external circuit, to reduce the water molecules at the counter electrode (van de Krol et al., 2008). Hence, the effectiveness of a photocatalyst toward PEC splitting can be determined by examining its ability to absorb solar light and generate charge carriers, prevent the recombination of the excited charges within its bulk, allow the migration of electrons to its surface and holes to the counter electrode, and enhance the surface reactions for O2 and H2-evolution. Herein, the activity of pristine α-Fe2O3 NTs, RT g-C3N4 and their composites were tested for photoelectrochemical performance. Fig. 8 demonstrates that α-Fe2O3 NTs/RT g-C3N4 composite formation successfully decreases the onset potential and enhances the PEC water splitting activity. From Fig. 8, compared with pristine RT g-C3N4, it is clear that the α-Fe2O3 NTs/RT g-C3N4 combination was effective in enhancing the PEC water splitting oxidation of water. The onset potential of 1 α-Fe2O3 NTs: 1 RT g-C3N4 composite was negatively shifted to lower value from that of pristine α-Fe2O3 NTs and pristine RT gC3N4. Also, the obtained current density (Fig. 8), which represents the PEC water splitting efficiency (Maeda, 2013), indicates that the current density of the composite photoanode (1 α-Fe2O3 NTs: 1 RT g-C3N4) is the highest as compared to the pristine α-Fe2O3 NTs and pristine RT g-
3.3. PL measurements Generally, the efficiency of a photocatalytic material can be improved by the formation of a hetero-structure between two semiconductors that results in better separation of the photo-generated electron-hole pairs (Zhou et al., 2014). PL is an effective tool to reveal the ability of the prepared composites towards the separation of the charge carriers. Theoretically, the higher intensity of the peak indicates a lower charge separation due to the higher rate of electron-hole recombination. From Fig. 7, it can be clearly seen that the peak intensity for RT g-C3N4 was decreased to almost half, as compared to the peak intensity for bulk g-C3N4, indicating a better suppression of recombination by RT-C3N4. Similarly, it was observed that the compositions of α-Fe2O3 NT/RT g-C3N4 composites, with higher amount of αFe2O3 NT content showed even better inhibition of electron-hole 407
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Fig. 7. Photoluminescence (PL) spectra of pristine and composite materials, excited at 325 nm.
Fig. 8. Linear sweeps voltammograms (LSV) curves of an anodic scan from 0.0 to 1.2 V (vs SCE) in 0.1 M Na2SO4 solution and at a scan rate of 0.1 V/s. vs. SCE.
carriers, as from the results of the Bode-phase plots. Consequently, the oxidation of the water molecules which is represented by the obtained current density is enhanced. The properties of charge carrier transfer at the electrode/electrolyte interface can be further examined by electrochemical impedance spectroscopy (EIS) measurements. Generally, the arc diameter of (EIS) Nyquist plot signifies the resistance value of the charge transfer (Rct) and consequently the efficiency of the photogenerated charge separation and their transport. The small diameter of the arc shows a lower resistance to charge transfer (Elbakkay et al., 2018). So, from Fig. 9a it is revealed that 1 α-Fe2O3 NTs: 1 RT g-C3N4 composite has the lowest arc diameter which suggests the highest charge separation and an effective charge-transfer, resulting in enhanced PEC water splitting performance. Fig. 9b shows the Bode plot of electrochemical impedance spectroscopy which can be used for the calculation of the photoelectron lifetime (τ) generated at a photoanode as shown by the following equation (Kern et al., 2002; Sahu et al., 2018; Rafieh et al., 2017):
Fig. 6. (a) UV–Vis Diffuse reflectance spectra of α-Fe2O3 NTs (I), 1 α-Fe2O3 NTs/1 RT g-C3N4 (II), 1 α-Fe2O3 NTs/2 RT g-C3N4 (III), 1 α-Fe2O3 NTs/3.75 RT g-C3N4 (IV), and RT g-C3N4 (V), (b) Tauc plots of α-Fe2O3 NTs, 1 α-Fe2O3 NTs: 1 RT g-C3N4 composite, and RT g-C3N4, and (c) photo image of the prepared materials.
C3N4. It shows a higher separation of charge carriers within the composite and a facile and an efficient charge transfer to the sites of the redox reactions. It can be attributed to high absorption potential of the composite for solar light due to the presence of α-Fe2O3 with a suitable band gap (1.86 eV). It can hinder the electron-hole recombination within the composite due to the development of hydrogen bonds between α-Fe2O3 NTs and protonated RT g-C3N4, facilitating the movement of the excited charges to the semiconductor surface (Jin et al., 2018). Besides, the lower onset potential value of the composite infers that the required external potential for overall water splitting has been decreased, which may be due to the presence of α-Fe2O3 NTs, that facilitates the transfer of the photo-generated charge carriers to the electrolyte (El Rouby and Farghali, 2018). Also, the formation of the Zscheme structure enhances the lifetime of the photo-generated charge
τ = 1/(2π fpeak)
(1)
where fpeak is the maximum frequency peak. So, by using the values of fpeak obtained from the Bode plot it is found that the values of τ are 6 ms, 12.6 ms, and 16 ms for RT g-C3N4, α-Fe2O3 NTs, and 1 α-Fe2O3 NTs : 1 RT g-C3N4 composite respectively. Accordingly, the charge separation can be improved in the composite by formation of the zscheme structure which results in enhanced current density. The photocurrent-time (I-t) curves of the prepared samples were plotted for several on-off illumination cycles (Fig. 10a), to examine the 408
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Fig. 9. (a) Electrochemical impedance spectroscopy and (b) Bode-phase plots of the EIS spectra for RT g-C3N4, α-Fe2O3 NTs, and 1 α-Fe2O3 NTs: 1 RT g-C3N4.
photocatalyst. From chronoamperometry measurements, it can be seen that the photo-stability of the synthesized composite is much better as compared to the parent components. For the pristine α-Fe2O3 NTs, the photostability is lower which might be ascribed to the photocorrosion from which many semiconducting materials experience (Han et al., 2014), but after the formation of the composite, the photostability increases as a result of the presence of RT g-C3N4 which might inhibit the photocorrosion (Fan et al., 2016). Mott-Schottky studies were performed to investigate the band structure, and semiconductor nature of the synthesized materials. An ntype semiconductor has a slope with a positive value in Mott-Schottky plot, while a p-type semiconductor has Mott-Schottky plot with a negative slope value. Fig. 11 shows positive slopes for both RT g-C3N4 and α-Fe2O3 NTs, indicating n-type nature of both materials (Xu et al., 2015; Luo et al., 2016). The flat band potential of n-type semiconductors can be determined by extrapolating the tangent of the MottSchottky plot to intercept with x-axis. As a result, the flat band potentials of RT g-C3N4 and α-Fe2O3 NTs are −1.427 and −0.12 V vs. SCE respectively. It is widely known that, the potential of the conduction band of the n-type semiconductor is close to its flat band potential. So, by combining the results obtained from Mott-Schottky plots with band gaps determined from the Uv–vis spectra, the valence band potential of RT g-C3N4 and α-Fe2O3 NTs are 1.293 V vs. SCE and 1.74 V vs. SCE respectively. The electron spin resonance (ESR) technique was used to confirm the Z-scheme mechanism. It was carried out in the photocatalytic system for reactive species determination. 5,5-dimethyl-l pyrroline Noxide (DMPO) was used for the trapping of the superoxide anion radicals (%O2−) and hydroxyl radicals (%OH), producing the adducts of
charge separation and transfer properties of the charge carriers. As shown from the measurements, the obtained photo-current was enhanced after the incorporation of α-Fe2O3 NTs with RT g-C3N4 inferring that the charge separation was enhanced which is in accordance with the peaks obtained in the PL spectra. It is clear that the transient photocurrent of the three tested samples has an instant increase of current once the light is switched on as well as instant decrease when the light is switched off. It shows a fast response for light absorption and high ability of the prepared materials towards the separation of charge carriers. The observed photocurrent spikes at the beginning of illumination, indicates that a recombination occurs between the photogenerated holes at the electrode surface and electrons in the conduction band, rather than transfer of holes to the electrolyte and consequently occupied by the electrons from the reduced species in the electrolyte (Yu et al., 2014; Xiang et al., 2011). The photoelectrochemical results are also supplemented by the results obtained from PL measurements, confirming that the formed composite can effectively suppress the recombination of photogenerated charge carriers. Moreover, as the transient photo-current response measurement refers to the ability of the prepared material to separate the photo-produced charge carriers (Ansari et al., 2019; Abdalla et al., 2019). Table 1 summarizes the ability of some previously prepared g-C3N4/α-Fe2O3 composites to suppress the electron-hole recombination. It can be said that the asprepared composite (1 RT g-C3N4:1 α-Fe2O3 NTs) in this work has the highest activity to separate the photo-generated electron-hole pairs. Fig. 10b shows the stability of pristine RT g-C3N4, α-Fe2O3 NTs and 1 RT g-C3N4:1 α-Fe2O3 NTs composite photoanodes at 1 V vs. SCE in 0.1 M Na2SO4 electrolyte. The stability of the photoactive materials toward the sun light is an essential feature for the effective
Fig. 10. (a) Photo-current measurement of catalysts in 0.1 M Na2SO4 solution under 1 V (vs. SCE), and (b) Chronoamperometry stability test of the as-prepared photoanodes of RT g-C3N4, α-Fe2O3 NTs, and 1 RT g-C3N4:1 α-Fe2O3 NTs. 409
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Table 1 G-C3N4/α-Fe2O3-based prepared composites and its comparative photo-generated electron-hole separation and transfer ability. Composite
Morphology of the components
Transient Photo-current density approximately
Electrolyte
References
2.55 µA
Hao et al. (2018)
G-C3N4
α-Fe2O3
0D/2D Fe2O3/g-C3N4
2-Dimensional
Quantum dots
Fe2O3/C-C3N4 g-C3N4@α-Fe2O3/C
Layered structure 2-Dimensional Sheet-like structure of g-C3N4@α-Fe2O3/C
0.7 µA 2 µA
α-Fe2O3/g-C3N4 α-Fe2O3-g-C3N4
Nanosheets Irregular crystals
Nanoparticles Nanoparticles
0.55 µA 3.5 µA
0.1 M potassium chloride solution with 5 Mm ferricyanide/ferrocyanide 0.5 M Na2SO4 solution 0.1 M Na2SO4 with 0.2 mM rhodium complex 0.1 M Na2SO4 0.1 M Na2SO4
Fe2O3/g-C3N4 α-Fe2O3/g-C3N4
Layered structure Irregular lamellar shape Ruptured tubular structure
Regularly round particles Nanotubes
0.65 µA 0.5 µA
0.5 M Na2SO4 0.2 M Na2SO4
Xiao et al. (2015) Theerthagiri et al. (2014) Hu et al. (2014) Li et al. (2017)
5.5 µA
0.1 M Na2SO4
This work
α-Fe2O3 NTs/RT gC3N4
Kong et al. (2018) Wu et al. (2018)
Fig. 11. Mott–Schottky plots of RT g-C3N4 (a), and α-Fe2O3 NTs (b) measured in 0.1 M Na2SO4 solution. Fig. 12. ESR spectra of DMPO-%O2− spin adduct with irradiation for 10 min in methanol dispersion (a) and DMPO-%OH spin adduct with irradiation for 10 min in aqueous dispersion (b) in the presence of α-Fe2O3 NTs, RT g-C3N4 and1 RT g-C3N4:1 α-Fe2O3 NTs nanocomposite.
DMPO-%OH and DMPO-%O2− in aqueous and methanol solutions, respectively. From Fig. 12a, it is clear that the DMPO-%O2− adduct was detected in case of both pristine RT g-C3N4 and1 RT g-C3N4:1 α-Fe2O3 NTs composite while it was not detected in case of neat α-Fe2O3 NTs, suggesting the formation of a Z-scheme heterostructure. A type II would settle the photogenerated electrons in the conduction band of α-Fe2O3 NTs rather than that of RT g-C3N4 after combination and would not show peaks in the composite. The photoinduced electrons in Type II would be positioned at a potential more positive than that of O2/%O2− (∼−0.955 V vs. SCE in pH ∼ 6.5). Likewise, from Fig. 12b, it is obvious that the DMPO-%OH adduct was detected in the case of both pristine α-Fe2O3 NTs and 1 RT g-C3N4:1 α-Fe2O3 NTs composite
however it was not detected in case of neat RT g-C3N4, suggesting the formation of Z-scheme mechanism. Type II heterostructure would settle the photogenerated holes in the valence band of RT g-C3N4 rather than that of α-Fe2O3 NTs after combination between them and would not show DMPO-%OH peaks for the composite. The photoinduced holes in heterostructure Type II would be positioned at potential lower positive than that of %OH/H2O (∼1.365 V vs. SCE in pH ∼ 6.5). Additionally, the intensity of the peaks is higher in case of the composite than for each separate component indicating the long-lived photogenerated electrons and holes due to the formation of Z-scheme structure (Xiao et al., 2018).
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4. Conclusion
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Synthesis and characterization of Z-scheme α-Fe2O3 NTs/ruptured tubular g-C3N4 for enhanced photoelectrochemical water oxidation
T
Ahmed Esmail A. Bakra,b, Waleed M.A. El Roubya, Malik D. Khanb, Ahmed A. Farghalia, B. Xuluc, Neerish Revaprasadub a
Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, 62511 Beni-Suef, Egypt Department of Chemistry, University of Zululand, Private Bag X1001, Kwadlangezwa 3886, South Africa c Department of Chemistry, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa b
A R T I C LE I N FO
A B S T R A C T
Keywords: α-Fe2O3 NTs Photoelectrochemical water splitting RT g-C3N4 Z-scheme heterostructure
A composite of tubular g-C3N4 with α-Fe2O3 has been synthesized to improve the photocatalytic performance of tubular g-C3N4 by the formation of a Z-scheme heterostructure. The composites were fabricated by the combination of varying amounts of hydrothermally synthesized α-Fe2O3 nanotubes (NTs) and protonated ruptured tubular (RT) g-C3N4, by the electrostatic self-assembly method. The obtained composites were characterized by zeta potential, powdered-X-ray diffraction (p-XRD), scanning/transmission electron microscopy (S/TEM), photoluminescence (PL), and Fourier transform infrared spectroscopy (FTIR). PL and photocurrent measurements indicates a higher charge separation of the photo-generated electron-hole pair for Fe2O3 NTs/RT g-C3N4 composite with higher concentration of α-Fe2O3 NTs. UV–Vis diffuse reflectance spectroscopy (UV–Vis/DRS) shows the band gaps of pristine α-Fe2O3 NTs and RT g-C3N4 to be 1.86 eV and 2.72 eV respectively, while the band structures were determined by Mott-Schottky measurements. I-V curves obtained from photoelectrochemical water splitting shows that the formation of the composite decreased the oxidation overpotential as compared to pristine α-Fe2O3 NTs and pristine RT g-C3N4. Bode plots showed that the composite was able to increase the lifetime of the photo-generated electrons as compared to both RT g-C3N4 and α-Fe2O3 NTs.
1. Introduction The rapid growth of population, expanding industrialization accompanied with environmental issues, depletion of fossil fuels, and increase in global energy demands, requires the investigation of new approaches for applicable energy sources that are efficient, clean and environmentally friendly (Faber and Jin, 2014; Lund et al., 2011; Lewis and Nocera, 2006). A suitable alternative is to utilize solar light for energy generation, especially photoelectrochemical (PEC) water splitting (Hisatomi et al., 2014; Sayed et al., 2017; Ahmed et al., 2017; Khan et al., 2018; Khan et al., 2018). Hydrogen generation by water splitting is an efficient and renewable energy source, which has a lower carbon foot print on the environment (Maeda and Domen, 2010; Walter et al., 2010). The Gibbs free energy required for photoelectochemical (PEC) water splitting is significantly high i.e. 237 kJ per mol., hence it is crucial to synthesize a photocatalyst that has the potential to carry out this reaction at much lower energy requirements. A wide range of materials used for the PEC splitting of water have been reported. Amongst these materials, hematite (α-Fe2O3) (Shen et al., 2016; Mishra and Chun,
2015; Sivula et al., 2011; Tamirat et al., 2016) and graphite-like carbon nitride (g-C3N4) (Wen et al., 2017; Ong et al., 2016; Fu et al., 2017; Naseri et al., 2017; Zhao et al., 2015), are cost effective, vastly available and non-toxic materials with remarkable photocatalytic efficiencies. Hematite (α-Fe2O3), an earth abundant and stable oxide of iron under ambient conditions, is an n-type semiconductor having band gap in range of 1.9-2.2 eV (Kennedy and Frese, 1978; Marusak et al., 1980). It is a captivating photocatalyst and an attractive material for various energy applications (Sivula et al., 2011; Li et al., 2018). However, the short lifetime of charge carriers (< 10 PS) and the high rate of recombination as well as low electrical conductivity of α-Fe2O3 (∼10−14 Ω−1 cm−1) impedes its use in practical applications (Barroso et al., 2013). Graphite-like carbon nitride (g-C3N4), is considered as a metal-free semiconductor with photocatalytic activity towards hydrogen production (Cao and Yu, 2014). It shows high chemical stability and can be easily synthesized at a scalable level with large surface area (Wang et al., 2012). However, its photocatalytic performance suffers from the fast recombination of electron-hole pairs. Several strategies have been adopted to overcome the drawbacks and enhance the performance of α-hematite and g-C3N4 photocatalysts.
E-mail addresses: [email protected] (W.M.A. El Rouby), [email protected] (N. Revaprasadu). https://doi.org/10.1016/j.solener.2019.09.052 Received 17 March 2019; Received in revised form 2 September 2019; Accepted 13 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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600 mL of deionized water, at room temperature, and stirred vigorously. After complete dissolution, the solution was transferred to a Teflon-lined stainless-steel autoclave (800 mL volume), which was heated to 220 °C in the furnace for 48 h. The precipitate formed at the end of the reaction, was separated by decanting the water after which it was washed several times with deionized water/ethanol, and dried overnight at 60 °C. The reddish precipitate was calcinated at 550 °C for two hours and at 800 °C for 20 min then left to cool naturally for further usage.
The recently adopted strategy for enhancing photocatalytic performance is the formation of a Z-scheme composite, which is generally composed of an O2-evolving photocatalyst, with a relatively low conduction band and reduction potential, and a H2-evolving photocatalyst, of a relatively high valence band and low oxidation potential. Hence, under solar light illumination, electrons with lower reduction potential present in the O2-evolving photocatalyst, transfer to holes of lower oxidation potential in the H2-evolving photocatalyst. Therefore, holes with high oxidation potential and electrons with high reduction potential are kept in the O2-evolving photocatalyst and H2-evolving photocatalyst respectively. In this way, the charge separation can be retained, which promotes the redox reactions. The interaction of αFe2O3 with g-C3N4 is supposed to generate Z-scheme composite, as the valence (EVB) and the conduction band edge potential (ECB) of α-Fe2O3 are 2.48 eV and 0.28 eV respectively and those of g-C3N4 are 1.57 eV and −1.13 eV respectively (Xu and Schoonen, 2000; Dong et al., 2011; Liu et al., 2012). There are various reports on the synthesis of iron oxide and carbon nitride heterostructures, which are used for photocatalysis (Shen et al., 2016; Huang et al., 2018). For instance, hierarchical structured, metalorganic framework-derived and 2D/2D g-C3N4/α-Fe2O3 composites (Jiang et al., 2018; Shi et al., 2018; Xu et al., 2018), direct solid-state Zscheme g-C3N4/Fe2O3 (Wang et al., 2018), g-C3N4@α-Fe2O3/C or Fe2O3/C-C3N4 based photocatalysts (Wu et al., 2018; Kong et al., 2018), Fe3O4 nanorods/g-C3N4 composite (Wu et al., 2017), g-C3N4/Ti- Fe2O3 (Liu et al., 2016), Ag-Fe3O4/ g-C3N4 (Pant et al., 2017), visible-light active α-Fe2O3/g-C3N4 (Theerthagiri et al., 2014), have been prepared in order to enhance the catalytic efficiency. However, compared to the bulk g-C3N4, most of the aforementioned work did not achieve a very high separation for the photogenerated charge carriers. It is a wellknown observation that a higher charge separation can be achieved in nanotubes as compared to sheet-like morphology, therefore a higher concentration of photogenerated charge carriers can be highly suitable for photocatalysis. However, until the present, the photocatalytic potential of composites of tubular structured α-Fe2O3 NTs and g-C3N4 has not been explored. Herein, we have prepared α-Fe2O3 NTs/RT g-C3N4 Z-scheme structure by a self-assembly technique to enhance the water splitting capability of the composite. We postulated that the presence of nanotubes in the prepared nanocomposite can enhance the photocatalytic activity by the creation of directional paths for the mobility of charge carriers which in turn leads to higher photoactivity of the nanotubes as compared to nanoparticles (Mohapatra et al., 2009). Likewise, one-dimensional materials are of low density and high aspect ratio that result in high specific surface area and provides more active sites for catalysis. Besides, the formation of hydrogen bonds in the synthesized composite may also contribute to a higher photocatalytic activity (Xu et al., 2018; Liu et al., 2018; Jin et al., 2018; Xie et al., 2018).
2.2.2. Synthesis of tubular (RT) g-C3N4 Tubular g-C3N4 was prepared by the literature method (Tahir et al., 2013; Dong et al., 2017) with a slight modification. In brief, 1.0 g of melamine was dissolved in 30.0 mL of ethylene glycol under vigorous stirring then appropriate amount of nitric acid was added with continuous stirring. The as-prepared white precipitate was separated, washed with absolute ethanol several times to remove the excess nitric acid and ethylene glycol, and dried at 60 °C overnight. The dried white product was placed in a ceramic crucible covered with aluminum foil and annealed in a muffle furnace for 2 h at 450 °C, at a heating rate of 10 °C/min. For comparison, bulk g-C3N4 was prepared by direct pyrolysis of melamine at 520 °C for 2 h with a heating rate of 10 °C/min (Wang et al., 2009). 2.2.3. Synthesis of tubular α-Fe2O3/g-C3N4 composite α-Fe2O3/g-C3N4 composite was fabricated as previously adopted (Xu et al., 2018) with a slight modification. Typically, specific amounts of g-C3N4 and α-Fe2O3 NT were separately ultrasonicated for 2 h in 100 mL mixture (2:1 by volume) of ISP: H2O. G-C3N4 was protonated by the addition of specific amount of nitric acid (≈5 mL of 1 M HNO3) until the pH reached 4. Under continuous stirring the suspended αFe2O3 NTs were added slowly to the suspension of protonated g-C3N4. The as-obtained suspension was kept under stirring in a fume-hood at 60 °C, and the suspension was allowed to dry. Different samples were prepared by keeping the ratio of α-Fe2O3 NT: g-C3N4 wt% to be (1: X wt %); (where X = 1, 2, 3.75). The obtained powders were collected for further investigation. 2.3. Characterization The dynamic light-scattering analysis was used to investigate the surface Zeta potential of the samples by the Zetasizer Nano ZS90 (Malvern instruments). The phase of the samples was identified by powder X-ray diffraction (p-XRD) patterns using a Bruker AXS D8Advance instrument, (Cu Kα radiation, λ = 1.5406 Å). The accelerating voltage and applied current was 40 kV and 40 mA respectively. IR spectra were obtained from Bruker FT-IR Tensor 27 spectrophotometer, in 500–4000 cm−1 range. The microstructural analysis of the prepared samples was observed by Philips XL30 FEG SEM and transmission electron microscope (Talos F200X at 200 kV using a FEI ceta camera). Edwards coating system E306A was used for carbon coating of the samples prior to SEM analysis. Shimadzu UV-3600, Japan with Ba2SO4 as the reflectance standard was used for UV–Vis and DRS analysis. Photoluminescence (PL) spectra were measured using A Perkin-Elmer, LS 55 Luminescence spectrophotometer.
2. Experimental section 2.1. Materials Anhydrous ferric chloride (FeCl3), ammonium dihydrogen phosphate (NH4H2PO4), sodium sulphate (Na2SO4), melamine, ethylene glycol, nitric acid, isopropanol (ISP), ethanol, Nafion (5 wt%) and fluorine-doped tin oxide (FTO) substrates were purchased from Sigma Aldrich.
2.4. Photoelectrochemical measurements A potentiostat including a three-electrode cell system, where the synthesized materials were used as working electrode, reference electrode (SCE) and a counter electrode (Pt wire). The three electrodes were immersed in a solution of 0.1 M Na2SO4 (pH ∼ 6.5), which was used as an electrolyte, in photoelectrochemical cell modified with a window for illumination. Argon was used to purge the electrolyte for 20 min before measuring and was kept over the surface during all experiments. A solar simulator comprising of a xenon lamp (150 W) and coupled with an air
2.2. Synthesis of photocatalyst 2.2.1. Synthesis of α-Fe2O3 NTs α-Fe2O3 NTs were prepared as reported previously (Liu et al., 2015; Nathan and Boby, 2017; Jia et al., 2005), with slight modification. Typically, 1.945 g (0.0149 M) of FeCl3, 0.0495 g (5.3 × 10−4 M) of NH4H2PO4, and 0.0474 g (4.17 × 10−4 M) of Na2SO4 were dissolved in 404
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Fig. 1. Zeta potential values of the prepared solid materials of RT g-C3N4, protonated RT g-C3N4 and α-Fe2O3 NT dispersed in deionized water.
Fig. 2. p-XRD patterns of α-Fe2O3 NTs, bulk g-C3N4, RT g-C3N4 and α-Fe2O3 NTs/RT g-C3N4 composites with different ratios of RT g-C3N4.
mass of 1.5 global was used for illumination where the intensity of the simulated light was adjusted to 1 sun (100 mW cm−2).
the fabricated composite contains α-Fe2O3 NTs well distributed over the g-C3N4 and both materials maintained their own crystal structures. The peaks of α-Fe2O3 NT as well as g-C3N4 appear at the same 2θ values, without any shift, whereas a gradual disappearance of the g-C3N4 strong peak at 2θ = ∼27.4° was observed with increasing α-Fe2O3 NTs concentration. Fourier transform infrared spectroscopy (FT-IR) was used to supplement confirmation of the composite formation. Fig. 3 displays the FT-IR spectra of pristine α-Fe2O3 NTs, RT g-C3N4 and a series of αFe2O3 NTs/RT g-C3N4 composites. The presence of a strong characteristic peak around 520 cm−1, which originates from the stretching vibration of Fe-O bond in hematite nanoparticles was observed clearly (Wang et al., 1998). Likewise, for pristine RT g-C3N4, a broad and intense absorption band which extends from 3500 cm−1 to 3000 cm−1 was observed, that can be attributed to eOeH stretching vibration of adsorbed H2O molecules and the stretching vibration of the terminal amino groups of g-C3N4. In addition to a series of strong peaks in the range of 1650–1200 cm−1 originates from the stretching modes of CeN in the aromatic heterocyclic rings of g-C3N4. Another strong peak was observed around 808 cm−1 which corresponds to the characteristic modes of the triazine units of g-C3N4 (Bojdys et al., 2008; Yan et al., 2009). The α-Fe2O3 NTs/RT g-C3N4 hybrids show that the composite contains the characteristic peaks of both graphitic-C3N4 and α-Fe2O3, which confirms the presence of both components in every prepared composite. Notably, the peak intensity, associated with the triazine units, reduces with decreasing the content of g-C3N4 and synchronously
2.5. Fabrication of the working electrode 5.0 mg of the synthesized material was sonicated in 400.0 µL isopropanol for 10.0 min, then 10.0 µL Nafion (5.0 wt%) was added to the suspension. After that 30.0 µL of this suspension was casted (10.0 µL by 10.0 µL) on the surface of a well cleaned 1 cm2 FTO glass, and dried at 80 °C for 30 min. 3. Results and discussion A facile and inexpensive electrostatic self-assembly approach was used to generate heterostructured α-Fe2O3 NT/RT g-C3N4 composites. As the isoelectric point of g-C3N4 is around pH = 5 (Zhu et al., 2015), a lower pH will protonate and make it positively charged, due to the presence of abundant -C-N- motifs, by the addition of HNO3. Fig. 1 shows the zeta potential of solid RT g-C3N4, solid protonated RT g-C3N4 (which was prepared by sonication of specific amount of solid RT gC3N4 in isopropanol (ISP) and deionized (DI) H2O, then few drops of 1 M HNO3 were added until the pH = 4, and finally the solvent was allowed to evaporate), and solid α-Fe2O3 NT dispersed in DI H2O. It is clear that surface charge of the unprotonated RT g-C3N4 and protonated RT g-C3N4 were −28.2 and +25.8 mv, respectively. However, α-Fe2O3 NTs exhibited a negatively charged surface (−35.9 mv) owing to the existence of hydroxyl moieties over its surface. The α-Fe2O3 NT/RT gC3N4 composite was formed due to the combination of protonated RT gC3N4 (positively charged) and the negatively charged α-Fe2O3 NTs spontaneously through a self-assembly interaction when the two components were mixed. 3.1. Crystal phase and microstructure analysis Fig. 2 depicts the diffraction patterns of pristine α-Fe2O3 NT, g-C3N4 and α-Fe2O3 NT/g-C3N4 nanocomposites with varying compositions. The diffraction peaks for α-Fe2O3 were observed at 2θ values of 24.1°, 33.1°, 35.6°, 40.9°, 49.5°, 54.1°, 62.4°, 64.0° which can be indexed to the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) planes of α-Fe2O3 (JCPDS 33-0664). Two diffraction peaks for pure gC3N4, centered at 2θ = ∼13.1° and ∼27.4° were observed which corresponds to the (1 0 0) plane of in-planar motif of tri-s-triazine units and (0 0 2) plane of interlayer stacking of conjugated carbonaceous rings for graphitic materials, respectively (Wang et al., 2009). It is noteworthy that the α-Fe2O3 NT sample is pure and of high crystallinity as the obtained peaks was highly intense with no other diffraction peaks. The obtained patterns of α-Fe2O3 NT/g-C3N4 nanocomposites reflect that
Fig. 3. FT-IR spectra of α-Fe2O3 NTs, RT g-C3N4 and α-Fe2O3 NTs/RT g-C3N4 composites with different contents of RT g-C3N4. 405
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Fig. 4. (a & b) are SEM & TEM images respectively for RT g-C3N4, (c & d) are SEM images of α-Fe2O3 NTs, (e & f) are SEM images of 1 α-Fe2O3 NTs/1 RT g-C3N4 composite, (g) is TEM image of 1 α-Fe2O3 NTs/1 RT g-C3N4 composite, and (h) is TEM image of bulk g-C3N4.
facilitates the ruptured structure of the g-C3N4 nanotubes. Representative images of the α-Fe2O3 NTs/RT g-C3N4 composite (Fig. 4(e–g)) imply that α-Fe2O3 NTs are anchored in a random way to the RT g-C3N4 to form agglomerates of different sizes that confirm the followed approach (self-assembly) in the composite fabrication. The small-size agglomerations resulting from the combination of small sized crystallites of α-Fe2O3 NTs and RT g-C3N4 and the large crystallites combined together to form large-sized agglomerations. Also, for comparison, TEM image was obtained for bulk g-C3N4 (Fig. 4h) which shows the sheet-like morphology as previously reported (Sun et al., 2017). Moreover, the intimate combination between α-Fe2O3 NTs and RT g-C3N4 was further confirmed by EDS and elemental mapping analysis for 1 α-Fe2O3 NTs/ 1 RT g-C3N4hybrid (Fig. 5). It shows that the components (C, N, Fe and O atoms) present and homogeneously distributed in the formed composite.
the peak intensity accompanying the FeeO bond increases with higher ratio of α-Fe2O3, in the synthesized composites. The results are in good agreement with the obtained p-XRD patterns. Also, the peak for the FeeO bond was slightly shifted to a higher wavenumber, which might be due to the interactions, present between α-Fe2O3 NTs and RT g-C3N4 in the obtained composites. Microstructural analysis of the products can be shown by SEM and TEM images. Fig. 4(c–d) shows that the obtained α-Fe2O3 NTs are similar to what previously reported by the same method we followed, except that the average length of the obtained tubes is lower (∼200–400 nm) as well as the thickness of the wall is higher (∼50 nm), which might be due to the calcination at higher temperature of 800 °C for 20 min. SEM images confirm the formation of α-Fe2O3 NTs with good morphology. However, from the SEM & TEM images of the obtained RT g-C3N4, (Fig. 4(a–b)) it can be seen that it does not have a well-defined tubular morphology; a wrapped and curved shaped nanoflake- like morphology was observed. The indefinite structure of RT g-C3N4 can be ascribed to the high heating rate (10 °C/min), before maintaining the temperature at 450 °C for 2 h (Dong et al., 2017). In addition, the covering of the crucible, during the pyrolysis process, may in turn decelerate the fast escape of NH3 gas and consequently
3.2. Optical properties The optical behavior of the synthesized samples was investigated by UV–Vis diffuse reflectance spectroscopy (Fig. 6). Interestingly, in comparison to the RT g-C3N4, the absorption edges of the prepared 406
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Fig. 5. Elemental mapping images of C, N, Fe and O and EDS spectrum of 1 α-Fe2O3 NTs/1 RT g-C3N4 composite.
series of α-Fe2O3 NTs/RT g-C3N4 composites gradually shifts toward the longer wavelength and synchronously the intensity of the visible light absorption increases with increasing α-Fe2O3 ratio in the synthesized hybrids (Fig. 6a). As shown in Fig. 6b, by using the Kubelka − Munk function, the band gap energies can be calculated by a plot of (αhv)2 versus the photo energy (hv) for the direct transition semiconductor. For pristine RT g-C3N4, the absorption edge was observed at ∼460 nm, which corresponds to a band gap of ca. 2.72 eV, as reported previously (Wang et al., 2009). It can be seen that pristine α-Fe2O3 NTs is considered as a potential photosensitizer because it can absorb light over a wide region with (Eg) of almost 1.86 eV (Fig. 6b). The band gap (∼1.93 eV) of the composite (1 α-Fe2O3 NTs: 1 RT g-C3N4) lies in between that of the pristine α-Fe2O3 NTs and RT g-C3N4, which infers a good interaction between both components that results in band gap alignment. These results imply that α-Fe2O3 NTs can improve the PEC water splitting activity, by decreasing the band gap of g-C3N4 and enhancing the visible light absorption. The photo image (Fig. 6c) of the synthesized compounds infers that the composite was formed successfully. Likewise, a change in the color of the obtained composite was also observed, from yellowish white for RT g-C3N4 to more reddish, as the concentration of α-Fe2O3 NTs was increased in the composite.
recombination, in comparison to RT-C3N4. With increase in content of α-Fe2O3 NTs, the PL intensity decreases, which means better electronhole separation due to the formation of a Z-scheme structures as previously reported (Jiang et al., 2018; Xu et al., 2018; Wang et al., 2018; She et al., 2017). Also, it was observed that the formation of α-Fe2O3 NT/RT g-C3N4 hetero-structures, results in a slight shifting of peaks towards lower wavelength that might be due to the interaction of αFe2O3 NT with RT-C3N4 which leads to the quantum confinement effects of the nanostructure (Liu et al., 2017). 3.4. Photoelectrochemical water splitting measurements PEC water splitting requires a semiconductor (photo-electrode) material to absorb the solar light photons with appropriate energy to initiate the transfer of electrons between valence and conduction band, to generate the electron-hole pairs. The holes tend to oxidize water molecules on the semiconductor surface, while the electrons are percolated, via the external circuit, to reduce the water molecules at the counter electrode (van de Krol et al., 2008). Hence, the effectiveness of a photocatalyst toward PEC splitting can be determined by examining its ability to absorb solar light and generate charge carriers, prevent the recombination of the excited charges within its bulk, allow the migration of electrons to its surface and holes to the counter electrode, and enhance the surface reactions for O2 and H2-evolution. Herein, the activity of pristine α-Fe2O3 NTs, RT g-C3N4 and their composites were tested for photoelectrochemical performance. Fig. 8 demonstrates that α-Fe2O3 NTs/RT g-C3N4 composite formation successfully decreases the onset potential and enhances the PEC water splitting activity. From Fig. 8, compared with pristine RT g-C3N4, it is clear that the α-Fe2O3 NTs/RT g-C3N4 combination was effective in enhancing the PEC water splitting oxidation of water. The onset potential of 1 α-Fe2O3 NTs: 1 RT g-C3N4 composite was negatively shifted to lower value from that of pristine α-Fe2O3 NTs and pristine RT gC3N4. Also, the obtained current density (Fig. 8), which represents the PEC water splitting efficiency (Maeda, 2013), indicates that the current density of the composite photoanode (1 α-Fe2O3 NTs: 1 RT g-C3N4) is the highest as compared to the pristine α-Fe2O3 NTs and pristine RT g-
3.3. PL measurements Generally, the efficiency of a photocatalytic material can be improved by the formation of a hetero-structure between two semiconductors that results in better separation of the photo-generated electron-hole pairs (Zhou et al., 2014). PL is an effective tool to reveal the ability of the prepared composites towards the separation of the charge carriers. Theoretically, the higher intensity of the peak indicates a lower charge separation due to the higher rate of electron-hole recombination. From Fig. 7, it can be clearly seen that the peak intensity for RT g-C3N4 was decreased to almost half, as compared to the peak intensity for bulk g-C3N4, indicating a better suppression of recombination by RT-C3N4. Similarly, it was observed that the compositions of α-Fe2O3 NT/RT g-C3N4 composites, with higher amount of αFe2O3 NT content showed even better inhibition of electron-hole 407
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Fig. 7. Photoluminescence (PL) spectra of pristine and composite materials, excited at 325 nm.
Fig. 8. Linear sweeps voltammograms (LSV) curves of an anodic scan from 0.0 to 1.2 V (vs SCE) in 0.1 M Na2SO4 solution and at a scan rate of 0.1 V/s. vs. SCE.
carriers, as from the results of the Bode-phase plots. Consequently, the oxidation of the water molecules which is represented by the obtained current density is enhanced. The properties of charge carrier transfer at the electrode/electrolyte interface can be further examined by electrochemical impedance spectroscopy (EIS) measurements. Generally, the arc diameter of (EIS) Nyquist plot signifies the resistance value of the charge transfer (Rct) and consequently the efficiency of the photogenerated charge separation and their transport. The small diameter of the arc shows a lower resistance to charge transfer (Elbakkay et al., 2018). So, from Fig. 9a it is revealed that 1 α-Fe2O3 NTs: 1 RT g-C3N4 composite has the lowest arc diameter which suggests the highest charge separation and an effective charge-transfer, resulting in enhanced PEC water splitting performance. Fig. 9b shows the Bode plot of electrochemical impedance spectroscopy which can be used for the calculation of the photoelectron lifetime (τ) generated at a photoanode as shown by the following equation (Kern et al., 2002; Sahu et al., 2018; Rafieh et al., 2017):
Fig. 6. (a) UV–Vis Diffuse reflectance spectra of α-Fe2O3 NTs (I), 1 α-Fe2O3 NTs/1 RT g-C3N4 (II), 1 α-Fe2O3 NTs/2 RT g-C3N4 (III), 1 α-Fe2O3 NTs/3.75 RT g-C3N4 (IV), and RT g-C3N4 (V), (b) Tauc plots of α-Fe2O3 NTs, 1 α-Fe2O3 NTs: 1 RT g-C3N4 composite, and RT g-C3N4, and (c) photo image of the prepared materials.
C3N4. It shows a higher separation of charge carriers within the composite and a facile and an efficient charge transfer to the sites of the redox reactions. It can be attributed to high absorption potential of the composite for solar light due to the presence of α-Fe2O3 with a suitable band gap (1.86 eV). It can hinder the electron-hole recombination within the composite due to the development of hydrogen bonds between α-Fe2O3 NTs and protonated RT g-C3N4, facilitating the movement of the excited charges to the semiconductor surface (Jin et al., 2018). Besides, the lower onset potential value of the composite infers that the required external potential for overall water splitting has been decreased, which may be due to the presence of α-Fe2O3 NTs, that facilitates the transfer of the photo-generated charge carriers to the electrolyte (El Rouby and Farghali, 2018). Also, the formation of the Zscheme structure enhances the lifetime of the photo-generated charge
τ = 1/(2π fpeak)
(1)
where fpeak is the maximum frequency peak. So, by using the values of fpeak obtained from the Bode plot it is found that the values of τ are 6 ms, 12.6 ms, and 16 ms for RT g-C3N4, α-Fe2O3 NTs, and 1 α-Fe2O3 NTs : 1 RT g-C3N4 composite respectively. Accordingly, the charge separation can be improved in the composite by formation of the zscheme structure which results in enhanced current density. The photocurrent-time (I-t) curves of the prepared samples were plotted for several on-off illumination cycles (Fig. 10a), to examine the 408
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Fig. 9. (a) Electrochemical impedance spectroscopy and (b) Bode-phase plots of the EIS spectra for RT g-C3N4, α-Fe2O3 NTs, and 1 α-Fe2O3 NTs: 1 RT g-C3N4.
photocatalyst. From chronoamperometry measurements, it can be seen that the photo-stability of the synthesized composite is much better as compared to the parent components. For the pristine α-Fe2O3 NTs, the photostability is lower which might be ascribed to the photocorrosion from which many semiconducting materials experience (Han et al., 2014), but after the formation of the composite, the photostability increases as a result of the presence of RT g-C3N4 which might inhibit the photocorrosion (Fan et al., 2016). Mott-Schottky studies were performed to investigate the band structure, and semiconductor nature of the synthesized materials. An ntype semiconductor has a slope with a positive value in Mott-Schottky plot, while a p-type semiconductor has Mott-Schottky plot with a negative slope value. Fig. 11 shows positive slopes for both RT g-C3N4 and α-Fe2O3 NTs, indicating n-type nature of both materials (Xu et al., 2015; Luo et al., 2016). The flat band potential of n-type semiconductors can be determined by extrapolating the tangent of the MottSchottky plot to intercept with x-axis. As a result, the flat band potentials of RT g-C3N4 and α-Fe2O3 NTs are −1.427 and −0.12 V vs. SCE respectively. It is widely known that, the potential of the conduction band of the n-type semiconductor is close to its flat band potential. So, by combining the results obtained from Mott-Schottky plots with band gaps determined from the Uv–vis spectra, the valence band potential of RT g-C3N4 and α-Fe2O3 NTs are 1.293 V vs. SCE and 1.74 V vs. SCE respectively. The electron spin resonance (ESR) technique was used to confirm the Z-scheme mechanism. It was carried out in the photocatalytic system for reactive species determination. 5,5-dimethyl-l pyrroline Noxide (DMPO) was used for the trapping of the superoxide anion radicals (%O2−) and hydroxyl radicals (%OH), producing the adducts of
charge separation and transfer properties of the charge carriers. As shown from the measurements, the obtained photo-current was enhanced after the incorporation of α-Fe2O3 NTs with RT g-C3N4 inferring that the charge separation was enhanced which is in accordance with the peaks obtained in the PL spectra. It is clear that the transient photocurrent of the three tested samples has an instant increase of current once the light is switched on as well as instant decrease when the light is switched off. It shows a fast response for light absorption and high ability of the prepared materials towards the separation of charge carriers. The observed photocurrent spikes at the beginning of illumination, indicates that a recombination occurs between the photogenerated holes at the electrode surface and electrons in the conduction band, rather than transfer of holes to the electrolyte and consequently occupied by the electrons from the reduced species in the electrolyte (Yu et al., 2014; Xiang et al., 2011). The photoelectrochemical results are also supplemented by the results obtained from PL measurements, confirming that the formed composite can effectively suppress the recombination of photogenerated charge carriers. Moreover, as the transient photo-current response measurement refers to the ability of the prepared material to separate the photo-produced charge carriers (Ansari et al., 2019; Abdalla et al., 2019). Table 1 summarizes the ability of some previously prepared g-C3N4/α-Fe2O3 composites to suppress the electron-hole recombination. It can be said that the asprepared composite (1 RT g-C3N4:1 α-Fe2O3 NTs) in this work has the highest activity to separate the photo-generated electron-hole pairs. Fig. 10b shows the stability of pristine RT g-C3N4, α-Fe2O3 NTs and 1 RT g-C3N4:1 α-Fe2O3 NTs composite photoanodes at 1 V vs. SCE in 0.1 M Na2SO4 electrolyte. The stability of the photoactive materials toward the sun light is an essential feature for the effective
Fig. 10. (a) Photo-current measurement of catalysts in 0.1 M Na2SO4 solution under 1 V (vs. SCE), and (b) Chronoamperometry stability test of the as-prepared photoanodes of RT g-C3N4, α-Fe2O3 NTs, and 1 RT g-C3N4:1 α-Fe2O3 NTs. 409
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Table 1 G-C3N4/α-Fe2O3-based prepared composites and its comparative photo-generated electron-hole separation and transfer ability. Composite
Morphology of the components
Transient Photo-current density approximately
Electrolyte
References
2.55 µA
Hao et al. (2018)
G-C3N4
α-Fe2O3
0D/2D Fe2O3/g-C3N4
2-Dimensional
Quantum dots
Fe2O3/C-C3N4 g-C3N4@α-Fe2O3/C
Layered structure 2-Dimensional Sheet-like structure of g-C3N4@α-Fe2O3/C
0.7 µA 2 µA
α-Fe2O3/g-C3N4 α-Fe2O3-g-C3N4
Nanosheets Irregular crystals
Nanoparticles Nanoparticles
0.55 µA 3.5 µA
0.1 M potassium chloride solution with 5 Mm ferricyanide/ferrocyanide 0.5 M Na2SO4 solution 0.1 M Na2SO4 with 0.2 mM rhodium complex 0.1 M Na2SO4 0.1 M Na2SO4
Fe2O3/g-C3N4 α-Fe2O3/g-C3N4
Layered structure Irregular lamellar shape Ruptured tubular structure
Regularly round particles Nanotubes
0.65 µA 0.5 µA
0.5 M Na2SO4 0.2 M Na2SO4
Xiao et al. (2015) Theerthagiri et al. (2014) Hu et al. (2014) Li et al. (2017)
5.5 µA
0.1 M Na2SO4
This work
α-Fe2O3 NTs/RT gC3N4
Kong et al. (2018) Wu et al. (2018)
Fig. 11. Mott–Schottky plots of RT g-C3N4 (a), and α-Fe2O3 NTs (b) measured in 0.1 M Na2SO4 solution. Fig. 12. ESR spectra of DMPO-%O2− spin adduct with irradiation for 10 min in methanol dispersion (a) and DMPO-%OH spin adduct with irradiation for 10 min in aqueous dispersion (b) in the presence of α-Fe2O3 NTs, RT g-C3N4 and1 RT g-C3N4:1 α-Fe2O3 NTs nanocomposite.
DMPO-%OH and DMPO-%O2− in aqueous and methanol solutions, respectively. From Fig. 12a, it is clear that the DMPO-%O2− adduct was detected in case of both pristine RT g-C3N4 and1 RT g-C3N4:1 α-Fe2O3 NTs composite while it was not detected in case of neat α-Fe2O3 NTs, suggesting the formation of a Z-scheme heterostructure. A type II would settle the photogenerated electrons in the conduction band of α-Fe2O3 NTs rather than that of RT g-C3N4 after combination and would not show peaks in the composite. The photoinduced electrons in Type II would be positioned at a potential more positive than that of O2/%O2− (∼−0.955 V vs. SCE in pH ∼ 6.5). Likewise, from Fig. 12b, it is obvious that the DMPO-%OH adduct was detected in the case of both pristine α-Fe2O3 NTs and 1 RT g-C3N4:1 α-Fe2O3 NTs composite
however it was not detected in case of neat RT g-C3N4, suggesting the formation of Z-scheme mechanism. Type II heterostructure would settle the photogenerated holes in the valence band of RT g-C3N4 rather than that of α-Fe2O3 NTs after combination between them and would not show DMPO-%OH peaks for the composite. The photoinduced holes in heterostructure Type II would be positioned at potential lower positive than that of %OH/H2O (∼1.365 V vs. SCE in pH ∼ 6.5). Additionally, the intensity of the peaks is higher in case of the composite than for each separate component indicating the long-lived photogenerated electrons and holes due to the formation of Z-scheme structure (Xiao et al., 2018).
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4. Conclusion
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