Preparation of porous α-Fe2O3 thin films for efficient photoelectrocatalytic degradation of basic blue 41 dye
Graphical Abstract
Introduction
The photoelectrochemical (PEC) approach has emerged as a promising advanced oxidation process to eliminate a broad range of organic contaminants from wastewaters and indoor air [1], [2]. The synergistic effect of electro- and photocatalytic processes in PEC has been shown to yield excellent performance for organic pollutant degradation [3], [4]. It has been demonstrated that oxidation of organics using PEC can proceed via direct or indirect mechanisms [5]. The direct oxidation occurs with electron transfer between the organic molecules and the electrode. On the other side, the indirect oxidation involves strong oxidant species such as reactive chlorine species (RCS), hydroxyl radicals (•OH), hydrogen peroxide (H2O2), or sulfate radicals (SO4•−) [1]. Especially, indirect electro-oxidation using RCS such as chlorine radicals (Cl•, Cl2•−), chlorine (Cl2), hypochlorite ion (ClO−), and hypochlorous acid (HOCl) showed as an efficient species for degradation of alcohols and carboxylic acids [6]. The most common source of Cl− ions include an electrolyte such as sodium chloride (NaCl). It is well-known that the Cl− ions oxidation using the electrocatalytic (EC) or PEC approach yields Cl2 gas. Depending on the solution pH, the obtained Cl2 may be stable under acidic condition (pH < 3.3), hydrolyze to either HOCl (3.3 < pH < 7.5) or ClO− ion (pH > 7.5) [7]. The relative oxidation power of HOCl is 80 times higher than that of ClO− and usually, PEC oxidation is more efficient in an acidic medium [8], [9].
Titanium dioxide (TiO2), as one of the most studied semiconductors in PEC studies, has been applied for degradation of various contaminants such as sulfamethoxazole, fulvic acid, sodium p-cumenesulfonate, etc [10], [11], [12], [13]. However, the large band-gap of TiO2 (~3.2 eV) limits its light-harvesting ability only in the UV region [14]. In this context, researchers have focused on semiconductor materials such as tungsten trioxide (WO3), bismuth vanadate (BiVO4), or Fe2O3, which absorb in the visible range [15]. Koo et al. demonstrated that WO3 with a band-gap of ~2.8 eV prepared by electrodeposition is efficient for PEC degradation of organics using in-situ generated RCS [7]. Other semiconductors that possess a small band-gap include α-Fe2O3 (~2.1 eV). The use of α-Fe2O3 in PEC degradation of organics is mainly limited to Na2SO4 and HClO4 electrolytes. Zhang et al. have used α-Fe2O3 films for PEC degradation of methyl orange and p-nitrophenol in Na2SO4 electrolyte [16]. In the presence of Na2SO4 electrolytes and after applying sufficient voltage PEC produces reactive SO4•− radicals that take part in organics degradation [1]. Other reports where the main oxidant is the SO4•− radical include α-Fe2O3 thin films for degradation of methylene blue (MB) [17], [18] or composites made of α-Fe2O3/ZnO for removal of 4–chlorophenol [19] and α-Fe2O3/Cu2O for degradation of oxytetracycline [20]. In similar ways, Suryavanshi et al. and Mahadik et al. have used α-Fe2O3 thin films to degrade MB and rhodamine B dyes in HClO4 electrolyte [21], [22]. To the best of author's knowledge, in the case of α-Fe2O3 PEC systems the main oxidants are in-situ generated SO4•− and •OH radicals [23]. However, standard potentials for the generation of •OH (2.80 V vs. NHE) and SO4•− (2.60 V vs. NHE) are quite high, and their use presents a drawback in PEC oxidation of organics [1].
α-Fe2O3 is considered a promising material for PEC wastewater treatment because it is earth-abundant, non-toxic, and exhibits a small band-gap [24]. Besides its several advantages, Fe2O3 suffers from low absorption coefficient, low carrier mobility (<1 cm2 V−1 s−1), short hole diffusion length (2–4 nm), and shorter excited state lifetime (~10 ps) [25]. To overcome these limitations, various strategies are employed, such as doping with various elements (Ti4+, Pt4+, or Sn4+) or the use of nanoparticles are convenient ways to achieve an enhancement in photocurrent response [26], [27], [28]. The approaches used for preparation of α-Fe2O3 thin films include hydrothermal [29], electrodeposition [30], sol-gel [31], atomic layer deposition (ALD) [32], chemical vapor deposition (CVD) [33], [34], etc. One very common technique for the growth of Fe2O3 thin film is the CVD approach. This process involves the reaction of precursors in the vapor phase within a deposition chamber on a substrate surface [35]. In this way, the obtained films are typically non-conformal and granular [36], [37]. On the other side, ALD produces very uniform Fe2O3 thin films and allows precise control over the thickness of the films. However, this technique usually uses expensive precursors like bis(2,4-methylpentadienyl) iron (II) [38], bis(N,N′-di-t-butylacetamidinato)iron(II) [32], tris(2,2,6,6-tetramethyl-3,5-heptanedionate) iron (III) [39], etc. Another drawback for ALD is the slow growth rate of materials [40]. Hydrothermal technique can grow directly thin films on substrates [41]. However, the small sizes of the vessels often used can be a limitation to grow large area metal oxide thin films. Therefore, it is important to employ an approach that avoids the use of expensive chemicals and apparatuses, toxic precursors, high vacuum systems, etc [42].
This study proposes a novel method for preparation of photoactive α-Fe2O3 thin films using spin-coating of iron precursors on fluorine-doped tin oxide (F:SnO2, FTO) substrates. The approach includes a short heat treatment step at 750 °C. The advantage of spin-coating is the simplicity of the procedure, which allows getting uniform coatings with defined thicknesses [43], [44]. For the first time, α-Fe2O3 films were used in PEC degradation of a model B41 dye contaminant where the oxidative species include in-situ generated RCS. Reusability tests confirmed that the α-Fe2O3 films are stable and efficient during the degradation of the B41 dye. The selection of B41 dye was due the fact that it is a harmful effluent in the textiles industry [45]. High performance liquid chromatography coupled with UV–VIS detection (HPLC-UV/VIS) and gas chromatography-mass spectrometry (GC-MS) were used to monitor the degradation of B41 dye and to evaluate the by-products during degradation. Various operation parameters, including anodic potential, solution pH, electrolytes, and dye concentration, were investigated to determine the optimal conditions for B41 dye degradation.
Section snippets
Chemicals and materials
2-methoxyethanol (99%), ethyl acetoacetate, tetrabutylammonium perchlorate, iron (III) nitrate nonahydrate (Fe (NO3)3.9H2O, 98%), ethylenediaminetetraacetic acid (EDTA), Sodium thiosulfate (Na2S2O3), tert-butyl alcohol (TBA)), acetonitrile and methanol were purchased from Sigma-Aldrich. Polyvinylpyrrolidone (PVP, M.W. 55,000), sodium chloride (NaCl, 99%), sodium sulfate (Na2SO4, >99%), coumarin, sodium hypochlorite (NaOCl) and ferrocene (99%) were purchased from Alfa Aesar. Ammonium chloride (NH
Characterization of α-Fe2O3 photoanode
XRD patterns of the pure FTO/glass substrate and the α-Fe2O3 thin film deposited onto the FTO/glass are shown in Fig. 1. The intense diffraction peaks at 35.58° and 63.91° (2θ) correspond to (104) and (300) crystal planes of the hexagonal α-Fe2O3 (R3c (167); PDF# 2101169). The crystallite sizes of α-Fe2O3 calculated using the Scherrer’s gave an average size of 38 nm. Besides α-Fe2O3 peaks, intensive peaks are seen from the FTO substrate. These intense diffraction peaks were indexed to the SnO2
Conclusions
An elegant method is presented for preparing porous α-Fe2O3 thin films based on spin-coating and subsequent heat-treatment at elevated temperatures. The procedure allows precise control over α-Fe2O3 film thickness (~300 nm). Detailed electrochemical analyses of the α-Fe2O3 electrode were performed using techniques like flat-band measurements, EIS, and LSV. The prepared α-Fe2O3 is a promising dual-function catalyst for water oxidation and dye degradation. For the first time, the α-Fe2O3 thin
CRediT authorship contribution statement
M. Machreki: conducted the experiments, analyzed the data, and wrote the manuscript. T.Chouki: contributed to DPD and COD measurements. M. Martelanc: carried out the HPLC and GC-MS measurements. L. Butinar contributed to phytotoxicity tests, fund raising and manuscript editing. B.M. Vodopivec: contributed to fund raising and manuscript editing. S. Emin: supervised the work, contributed to data analysis, manuscript writing and editing, fund raising and Project Administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Slovenian Research Agency (ARRS) under the bilateral project for scientific cooperation between the Republic of Slovenia and the State of Israel (NI-0002). S. Emin acknowledge the ARRS program P2–0412. M. Machreki and T. Chouki acknowledge the scholarships provided by the Public Scholarship, Development, Disability and Maintenance Fund of the Republic of Slovenia (Ad futura program) for Ph.D. studies at the University of Nova Gorica. The authors acknowledge the
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