Full Length ArticleBand gap engineering of nanotubular Fe2O3-TiO2 photoanodes by wet impregnation
Graphical abstract
Introduction
Recent studies in the field of material science are focused on nanostructured transition metal oxides because of their extraordinary chemical, optical, and electrical properties, which can be tuned by the structure, material composition, size, and specific shape [1], [2]. Anodization technique today allows obtaining nanostructured oxides on the surface of variety of metallic substrates such as Ti [3], W [4], Fe [5], Sn [6], Zn [7], and Zr [8]. Depending on operating conditions, different oxides morphologies including nanowires [9], nanotubes [10], [11], and nanoporous layers [12] can be synthesized. Such materials are characterized by a high surface to volume ratio, quantum confinement effect, and greater mobility of charge carriers compared to macroscopic materials [13]. In addition, an advantage of oxide layers obtained by anodization is good adhesion between the nanostructured film and conductive metal, and perpendicular orientation of pores/tubes in the oxide layer to the metallic substrate, which provides a facile electron transfer pathway [14]. Some morphological features of anodic oxides (pore diameter, wall thickness, and oxide thickness) can be controlled by adjusting anodization conditions (type and concentration of electrolyte, applied voltage, time, and temperature) [15], [16], [17], [18], [19], [20], [21], [22].
Nanostructured TiO2 is a widely used semiconducting material due to a good chemical stability, biocompatibility, and non-toxicity [23], which allow its use in photoelectrochemical water splitting, photocatalysis, and even medicine [24], [25], [26]. However, considering photoelectrochemical applications of such a material, the main disadvantage is a relatively high-energy gap (3.2 eV for anatase), which causes that only ultraviolet light can be absorbed [27], [28]. A lot of effort has been made so far to synthesize nanostructured titania-based composite semiconductors, and consequently, extend the absorption range of anodic TiO2 layers using different approaches like dye-sensibilization [29], deposition of metal and metal oxide nanoparticles [30], [31], [32], filling nanotubes with different metals [33], and impregnation with a solution containing desired transition metal ions [34]. Among various methods used for the preparation of composite semiconductor nanostructures, such as dip coating [30], electrodeposition [31], [32], layer by layer assembly [35], atomic layer deposition [36], impregnation is commonly used for the preparation of co-catalyst [37]. The latter technique involves dipping TiO2 samples in a suitably selected solution, typically followed by annealing at the appropriate conditions in order to obtain a desired crystal structure of both oxides. In this way, transition metal oxide can be deposited on walls and surface of the porous structure. A very attractive strategy involves formation of iron(III) oxide which is one of the most promising semiconductors that can be used as a visible light photocatalyst in different applications due to its narrow band gap of 2.2 eV, low cost, non-toxicity, and high chemical stability [38]. When such a composite is irradiated with light, excited electrons from the valence band of Fe2O3 can be easily injected into the conduction band of TiO2 [39].
As it was already reported [40], [41], [42], [43], [44], [45], [46], a morphological, structural, optical, and photoelectrochemical properties of FeOx-TiO2 materials can significantly differ based on an impregnation procedure. Table 1 presents some details on the preparation and properties of FeOx-TiO2 nanotubular materials obtained by impregnation.
Kuang et al. [40], reported modification of anodic TiO2 nanotubes with Fe2O3 by simple immersion of nanotubes in an aqueous solution containing FeCl3, and NaOH. After four immersion cycles, samples were annealed at 550 °C for 4 h to obtain a crystalline anatase and hematite phases. Photoelectrochemical performance of synthesized materials under the white light illumination was improved about 4 times compared to unmodified TiO2 nanotubes. Other approaches were proposed by Xu et al. [41] and Wu et al. [42], in which TiO2 nanotubes were covered with Fe2O3 nanoparticles by dip-coating in a suspension of previously synthesized nanoparticles and ultrasound-assisted impregnation in a Fe(NO3)3 solution followed by calcination, respectively. X-ray diffraction measurements for modified samples showed a shift in the peak position of anatase (1 0 1) toward lower 2θ angles when compared with unmodified TiO2 nanotubes. This phenomenon was attributed to diffusion of Fe3+ ions into the crystal structure of anodic TiO2, and their location at interstices or substitution some of Ti4+ ions in the lattice [41], [42]. It was shown that photocatalytic, and photoelectrochemical properties of FeOx-TiO2 materials are better than unmodified TiO2 nanotubes [47]. For instance, Xu et al. [41] studied photoelectrocatalytic (PEC) degradation of methylene orange (MO) and 4-chlorophenol in water under visible-light irradiation. It was found that both, the value of the generated photocurrent and degradation rate of organic compounds have been enhanced several times as compared to unmodified materials. Zhang et al. [43] made similar observations for nitrobenzene (NB) degradation. What is more, reduction in the band gap energy was observed from 3.2 eV for anodic TiO2 nanotubes to 2.5 eV for modified layers [43]. As can be seen from the examples collected in Table 1, an important semiconductor characteristics of Fe2O3-TiO2 nanotubular materials (e.g., band gap) was not provided or completely different procedures of their synthesis were used.
Therefore, in this work, we present for the first time, a systematic study focusing on the influence of Fe3+ ion concentration in the impregnation solution on the morphology, structure, optical, semiconducting, and photoelectrochemical properties of anodic Fe2O3-TiO2 materials. To accomplish this, iron-modified anodic TiO2 layers were prepared by immersion in solutions with different concentrations of FeCl3 and subjected to annealing at 400 °C for 2 h. An important part of this investigation was an attempt to establish correlations between the concentration of iron ions and resulting materials properties. A complex characterization of all studied samples was performed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Raman spectroscopy, X-ray diffraction (XRD), reflectance measurements, Mott-Schottky analysis, and photocurrent vs. wavelength measurements.
Section snippets
Synthesis of nanostructured Fe2O3-TiO2
Titanium foil (95.5% purity, thickness 0.25 mm, Alfa Aesar) was polished electrochemically and chemically [48], and then used as an anode in the anodization process. Nanoporous anodic titanium oxide layers were synthesized in an ethylene glycol-based solution containing NH4F (0.38 wt%) (Sigma Aldrich, St. Louis, USA) and H2O (1.79 wt%) [15]. A three-step anodization was carried out at the constant voltage of 40 V at 20 °C. The first and second anodizing steps lasted 3 h. After each step, an
Influence of the Fe3+ ion concentration in the impregnation solution on the morphology and crystallinity of Fe2O3-TiO2 nanostructured materials
Anodic TiO2 layers were impregnated in a ferric chloride solution of different concentrations (5 – 100 mM), and then annealed at 400 °C for 2 h. In addition, anodic titanium oxide impregnated according to the same procedure in distilled water and then annealed served as a reference sample. The top morphology of obtained Fe2O3-TiO2 composite materials was investigated on macro and micro scales. Optical images of the studied samples are presented in Fig. 1. The anodic TiO2 layer soaked in
Conclusions
To sum up, band gap engineering based on introduction of the Fe2O3 phase into TiO2 by wet impregnation has been proposed. As-received anodic TiO2 layers were soaked in ferric chloride solutions (5, 10, 25, 50, and 100 mM) followed by their air annealing at 400 °C. The morphology and properties of resulting nanotubular Fe2O3-TiO2 materials were strongly connected with the iron content. With increasing the concentration of ferric chloride in the impregnation solution the iron content increase
CRediT authorship contribution statement
Monika Sołtys-Mróz: Investigation, Conceptualization, Methodology, Visualization, Writing - original draft. Karolina Syrek: Investigation, Conceptualization, Methodology, Visualization, Writing - original draft, Writing - review & editing. Joanna Pierzchała: Investigation. Ewelina Wiercigroch: Investigation. Kamilla Malek: Conceptualization, Methodology, Writing - original draft. Grzegorz D. Sulka: Conceptualization, Methodology, Writing - review & editing, Supervision.
Declaration of Competing Interest
None.
Acknowledgements
The research was partially supported by the National Science Centre, Poland (Project No. 2016/23/B/ST5/00790). The SEM imaging was performed in the Laboratory of Field Emission Scanning Electron Microscopy and Microanalysis at the Institute of Geological Sciences, Jagiellonian University, Poland.
References (68)
- et al.
TiO2 nanotubes: self-organized electrochemical formation, properties and applications
Curr. Opin. Solid State Mater. Sci.
(2007) - et al.
Influence of annealing conditions on anodic tungsten oxide layers and their photoelectrochemical activity
Electrochim. Acta
(2017) - et al.
Fabrication of iron oxide nanotube arrays by electrochemical anodization
Corros. Sci.
(2014) - et al.
Electrochemical fabrication of tin nanowires: a short review
C. R. Chimie
(2008) - et al.
The effect off oil purity on morphology of anodized nanoporous ZrO2
Appl. Surf. Sci.
(2016) - et al.
Tailoring the surface morphology of TiO2 nanotube arrays connected with nanowires by anodization
Mater. Sci. Semicond. Process.
(2011) - et al.
Cobalt-doped titanium oxide nanotubes grown via one-step anodization for water splitting applications
Appl. Surf. Sci.
(2019) - et al.
Electrochemical and photoelectrical properties of titania nanotube arrays annealed in different gases
Sens. Actuat. B
(2008) - et al.
Fabrication of nanoporous TiO2 by electrochemical anodization
Electrochim. Acta
(2010) - et al.
Influence of electrolyte pH on TiO2 nanotube formation by Ti anodization
J. Alloys Compd.
(2009)
Effect of the previous usage of electrolyte on growth of anodic titanium dioxide (ATO) in a glycerol-based electrolyte
Electrochim. Acta
Effect of anodization voltage on performance of TiO2 nanotube arrays for hydrogen generation in a two-compartment photoelectrochemical cell
Int. J. Hydrogen Energy
Anodic growth of TiO2 nanopore arrays at various temperatures
Electrochim. Acta
Electrochemical growth of porous titanium dioxide in a glycerol-based electrolyte at different temperatures
Electrochim. Acta
Characterization of native and anodic oxide films formed on commercial pure titanium using electrochemical properties and morphology techniques
Appl. Surf. Sci.
The surface science of titanium dioxide
Surf. Sci. Rep.
TiO2 photocatalysis: design and applications
J. Photoch. Photobio. C
Effect of the anodization parameters on TiO2 nanotubes characteristics produced in aqueous electrolytes with CMC
Appl. Surf. Sci.
Electrodeposition of maghemite (γ-Fe2O3) nanoparticles
Chem. Eng. J.
Preparation, characterization and visible-light-driven photocatalytic activity of Fe-incorporated TiO2 microspheres photocatalysts
Appl. Surf. Sci.
Hematite doped magnetic TiO2 nanocomposites with improved photocatalytic activity
J. Alloys Compd.
Fabrication, characterization and photoelectrochemical properties of Fe2O3 modified TiO2 nanotube arrays
Appl. Surf. Sci.
Ultrasound-assisted synthesis and visible-light-driven photocatalytic activity of Fe-incorporated TiO2 nanotube array photocatalysts
J. Hazard. Mater.
Highly ordered Fe3+/TiO2 nanotube arrays for efficient photocataltyic degradation of nitrobenzene
Appl. Surf. Sci.
Effect of electrolyte agitation on anodic titanium dioxide (ATO) growth and its photoelectrochemical properties
Electrochim. Acta
Effects of anodizing conditions and annealing temperature on the morphology and crystalline structure of anodic oxide layers grown on iron
Appl. Surf. Sci.
Reactive and morphological trends on porous anodic TiO2 substrates obtained at different annealing temperatures
Int. J. Hydrogen Energy
Interfacial barrier layer properties of three generations of TiO2 nanotube arrays
Electrochim. Acta
Synthesis and photoelectrochemical water oxidation of (Y, Cu) codoped α-Fe2O3 nanostructure photoanode
J. Alloys Compd.
Nanoporous tin oxides synthesized via electrochemical anodization in oxalic acid and their photoelectrochemical activity
Electrochim. Acta
Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculations
J. Phys. Chem. Solids
Nanomaterials and its potential applications
Int. J. Chemtech Res.
Self-organized oxide nanotube layers on titanium and other transition metals
Self-organized ZnO nanorods prepared by anodization of zinc in NaOH electrolyte
RSC Adv.
Cited by (42)
Photocatalytic H<inf>2</inf> generation under blue and white LEDs by Fe<inf>2</inf>O<inf>3</inf> /KTLO/rGO S-scheme composite photocatalyst
2023, Journal of Alloys and CompoundsA facile synthesized robust catalyst for efficient regeneration of biphasic solvent in CO<inf>2</inf> capture: characterization, performance, and mechanism
2023, Separation and Purification TechnologyCuO decorated graphene TiO<inf>2</inf> derived MIL-125 nanocomposite with enhanced photo-response as a highly efficient indirect sunlight driven photocatalyst
2023, Journal of Photochemistry and Photobiology A: Chemistry