Full Length Article
Band gap engineering of nanotubular Fe2O3-TiO2 photoanodes by wet impregnation

https://doi.org/10.1016/j.apsusc.2020.146195Get rights and content

Highlights

  • Effect of Fe3+ ion concentration in the impregnation solution on material properties was investigated.

  • Morphology and chemical composition of synthesized materials were examined.

  • Correlation between the flat band potential and band gap was observed.

  • Photoelectrochemical performance of obtained photoanodes was studied.

Abstract

Due to many limitations of titanium oxide, its direct use in photoelectrochemical water splitting under solar radiation is ineffective. Band gap engineering based on introduction of the Fe2O3 phase into TiO2 by wet impregnation has been proposed. In particular, nanoporous anodic titanium oxide layers were soaked in solutions with different concentrations of iron ion (5–100 mM) followed by their air annealing at 400 °C. As-prepared nanotubular Fe2O3-TiO2 materials were characterized by using field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), Raman spectroscopy, X-ray diffraction (XRD), reflectance measurements, Mott-Schottky analysis, and photoelectrochemical tests. It was found that increasing the iron content in anodic materials strongly affects their resulting semiconducting and photoelectrochemical properties. The maximum of generated photocurrent shifts towards visible light as the band gap energy decreases (from 3.36 to 2.89 eV) with the respect to increasing the iron content in the anodic materials.

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)

  • M. Jarosz et al.

    Effect of the previous usage of electrolyte on growth of anodic titanium dioxide (ATO) in a glycerol-based electrolyte

    Electrochim. Acta

    (2014)
  • Y. Sun et al.

    Effect of anodization voltage on performance of TiO2 nanotube arrays for hydrogen generation in a two-compartment photoelectrochemical cell

    Int. J. Hydrogen Energy

    (2014)
  • G.D. Sulka et al.

    Anodic growth of TiO2 nanopore arrays at various temperatures

    Electrochim. Acta

    (2013)
  • J. Kapusta-Kołodziej et al.

    Electrochemical growth of porous titanium dioxide in a glycerol-based electrolyte at different temperatures

    Electrochim. Acta

    (2014)
  • S.A. Fadl-allah et al.

    Characterization of native and anodic oxide films formed on commercial pure titanium using electrochemical properties and morphology techniques

    Appl. Surf. Sci.

    (2010)
  • U. Diebold

    The surface science of titanium dioxide

    Surf. Sci. Rep.

    (2003)
  • K. Nakata et al.

    TiO2 photocatalysis: design and applications

    J. Photoch. Photobio. C

    (2012)
  • R. Aguirre Ocampo et al.

    Effect of the anodization parameters on TiO2 nanotubes characteristics produced in aqueous electrolytes with CMC

    Appl. Surf. Sci.

    (2019)
  • H. Park et al.

    Electrodeposition of maghemite (γ-Fe2O3) nanoparticles

    Chem. Eng. J.

    (2008)
  • J.Q. Li et al.

    Preparation, characterization and visible-light-driven photocatalytic activity of Fe-incorporated TiO2 microspheres photocatalysts

    Appl. Surf. Sci.

    (2012)
  • D. Liŭ et al.

    Hematite doped magnetic TiO2 nanocomposites with improved photocatalytic activity

    J. Alloys Compd.

    (2016)
  • S. Kuang et al.

    Fabrication, characterization and photoelectrochemical properties of Fe2O3 modified TiO2 nanotube arrays

    Appl. Surf. Sci.

    (2009)
  • Q. Wu et al.

    Ultrasound-assisted synthesis and visible-light-driven photocatalytic activity of Fe-incorporated TiO2 nanotube array photocatalysts

    J. Hazard. Mater.

    (2012)
  • Y. Zhang et al.

    Highly ordered Fe3+/TiO2 nanotube arrays for efficient photocataltyic degradation of nitrobenzene

    Appl. Surf. Sci.

    (2017)
  • K. Syrek et al.

    Effect of electrolyte agitation on anodic titanium dioxide (ATO) growth and its photoelectrochemical properties

    Electrochim. Acta

    (2015)
  • A. Pawlik et al.

    Effects of anodizing conditions and annealing temperature on the morphology and crystalline structure of anodic oxide layers grown on iron

    Appl. Surf. Sci.

    (2017)
  • K. Syrek et al.

    Reactive and morphological trends on porous anodic TiO2 substrates obtained at different annealing temperatures

    Int. J. Hydrogen Energy

    (2020)
  • L. Aïnouche et al.

    Interfacial barrier layer properties of three generations of TiO2 nanotube arrays

    Electrochim. Acta

    (2014)
  • C. Venkata Reddy et al.

    Synthesis and photoelectrochemical water oxidation of (Y, Cu) codoped α-Fe2O3 nanostructure photoanode

    J. Alloys Compd.

    (2020)
  • L. Zaraska et al.

    Nanoporous tin oxides synthesized via electrochemical anodization in oxalic acid and their photoelectrochemical activity

    Electrochim. Acta

    (2016)
  • T. Umebayashi et al.

    Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculations

    J. Phys. Chem. Solids

    (2002)
  • K. Arivalagan et al.

    Nanomaterials and its potential applications

    Int. J. Chemtech Res.

    (2011)
  • P. Schmuki

    Self-organized oxide nanotube layers on titanium and other transition metals

  • J. Dong et al.

    Self-organized ZnO nanorods prepared by anodization of zinc in NaOH electrolyte

    RSC Adv.

    (2016)
  • Cited by (42)

    View all citing articles on Scopus
    View full text