Elsevier

Ceramics International

Volume 41, Issue 10, Part A, December 2015, Pages 13692-13701
Ceramics International

In-situ manganese doping of TiO2 nanostructures via single-step electrochemical anodizing of titanium in an electrolyte containing potassium permanganate: A good visible-light photocatalyst

https://doi.org/10.1016/j.ceramint.2015.07.158Get rights and content

Abstract

It is desirable to induce remarkable red-shift in the optical absorption edges of TiO2 so that this class of low-cost materials can be used as optimum photocatalysts beyond the ultraviolet range. This work reports on the room-temperature electrochemical fabrication of manganese-doped TiO2 nanotubes (MnTNT) and the investigation of their optical property and photocatalytic activity. TiO2 nanotubes (TNT) were doped with different concentrations of manganese and prepared nanostructures have been characterized using different techniques for analyzing their structure, morphology, composition, band gap and optical absorption property. Characterization of the as-prepared samples indicated that dopant concentration in anodizing solution significantly influenced the morphology, structure, photoabsorption and photocatalytic activity of fabricated films. The optical absorption range of TiO2 film gradually expands and shifts to the red with increasing dosages and band gap of the TiO2 films were reduced. Moreover, the photocatalytic activity of the TiO2 films for removal of methylene blue was enhanced by doping with an appropriate amount of manganese. The sample MnTNT4 exhibited better photocatalytic activity than the undoped TNT and MnTNT fabricated using other manganese concentrations.

Introduction

A photocatalytic process is a process which is accelerated in presence of light and a photocatalyst. A photocatalyst has no influence on the process under dark conditions. However, in the presence of illumination, it is able to accelerate a reaction. Most photocatalysts are semiconductors containing an electronic structure with specific band gap between a valence band (VB) and a conduction band (CB). The photocatalytic reaction starts with the transport of electrons from VB to CB induced by light absorption. To overcome the band gap in this process, the absorbed light needs to have certain energy, which is in relation to the wavelength of light. For this, the photoreaction is started by the absorption of light with a certain maximal wavelength determined by the band gap of the photocatalyst [1], [2]. If light is used with greater than the optimum wavelength (lower energy than the amount required), the photoreaction cannot start, because the light does not contain the necessary energy to transport electrons from the VB to CB. If light with energy greater than the band gap hits a semiconductor, an electron is promoted from the VB to CB and thus creating a hole (h+) in VB. Hole and electron diffuse or migrate on their respective band to the semiconductor surface and react with a suitable redox species in the environment. In an aqueous environment, electrons and holes can participate in water splitting or direct formation of hydroxyl (OH•) radicals. These radicals are able to oxidize a wide range of compounds. In fact, most organic compounds can be oxidized by hydroxyl radicals fully to CO2 and H2O; in other words, such reactions can be utilized for the oxidative destruction of organic pollutants. Among various semiconductors, TiO2 is frequently used as an optimal photocatalyst since it is nontoxic, low cost and environmental friendly [3]. In order to achieve a maximum rate for the reactions, a high surface area is desired. This is usually created by using nanoparticles that either are suspended in the reaction environment or compacted to a photoelectrode. Over the past decades, various TiO2 morphologies were investigated for their photocatalytic performance and were found in many cases better to nanoparticles [4]. A main drawback of any powder form is that either the process has to be conducted in a suspension (which requires post-reaction separation of the loose material from the solvent), or the photocatalyst has to be immobilized on a carrier by compacting [3]. Several morphologies of TiO2, such as wires, rods or nanotubes, can directly be grown on a titanium substrate and these structures can directly be used as photoanodes in different processes [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Wide band gap of TiO2 (3.2 eV) limits the efficiency of photocatalytic reactions (which means that only UV light is efficiently absorbed and thus in view of solar light driven processes, only≈4–7% of the entire solar spectrum can be exploited). In recent years, considerable researches are directed to reduce its band gap by doping or band-gap engineering to develop new photocatalysts capable of utilizing visible light accounting for ~43% of sunlight [3]. In order to extend the photocatalytic capability into the visible light range, extensive efforts have been made to narrow the band gaps of TiO2 by doping the compounds with metal or nonmetal atoms [16], [17], [18], [19], [20]. Doping TiO2 with transition metal cations is an effective strategy to increase photocatalytic efficiency. Unlike most of the 3d transition metals, manganese doped TiO2 has generated considerable interests as a photocatalyst showing optical response in the visible region [21], [22], [23], [24]. According to some theoretical calculations, among all the 3d transition metals, manganese has the greatest potential in permitting significant optical absorption in the visible or even the infrared solar light, through the combined effects of narrowed band gap and the introduction of intermediate bands within the forbidden gap, so manganese demonstrates the best potential as an alternative dopant in TiO2 [25], [26], [27]. Unlike most of the 3d dopants which tend to induce defect states in the forbidden gap of TiO2, intermediate bands owing to manganese doping are of significant curvature and hence adequate carrier mobility [27].

In this work, we reported the effectiveness of manganese doped TiO2 nanotube in photocatalytic degradation of methylene blue (MB) under visible light. We demonstrate for the first time, a simple and facile electrochemical method to introduce manganese in to the TiO2 nanotube film during an anodizing process. We report an easy approach to fabricating Mn-doped TiO2 nanotubes (MnTNT) films by a single-step anodizing of titanium substrate in an organic bath consisted of DMSO-HF electrolyte containing potassium permanganate. The anodizing method has been chosen, since it offers considerable advantages in terms of low cost owing to the simple process; fabrication of Mn-doped TiO2 nanotubes in one-step; control of morphology, length, diameter, thickness and surface area of the nanotubes by controlling various parameters of anodizing such as anodizing voltage, time, electrolyte temperature, electrolyte type and concentration. Also in these layers, desired dopant with different concentrations was doped during synthesis of TiO2 nanotubes and this reduces the cost of prepared material. For the first time, we used KMnO4 as the manganese source. TiO2 nanotubes (TNT) with different amounts of manganese were obtained by controlling the concentration of potassium permanganate in anodizing electrolyte. The morphology and structure were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). Optical properties were investigated by UV–vis diffuse reflectance spectra. The effect of the manganese doping on the photocatalytic activity of MnTNT samples was evaluated through the degradation of methylene blue (MB). Till now, very less research has been done to introduce manganese in to the TiO2 nanotube array by anodizing process. Also, to our knowledge, the quantity effect of manganese doped in TNT films on photocatalytic activities of MnTNTs are lacking.

Section snippets

Experimental

Dimethyl sulfoxide (DMSO), Potassium permanganate (KMnO4), HF, H2SO4, and HNO3 were of analytical grade. Other chemicals were obtained as analytical reagent grade and used without further purification. The solutions were prepared with distilled water. Titanium (Ti) foils (purity>99.99%, 1 mm thickness) were used. Methylene blue (purity, 99%) was used as received. Its molecular formula is C16H18N3SCl (molecular weight, 319.85 g/mol). Methylene blue (MB) is a cationic dye, used extensively for

Results and discussion

The morphology of samples prepared by anodizing in dimethyl sulfoxide (DMSO) electrolyte containing different concentrations of potassium permanganate were observed by SEM. Fig. 2 shows FE-SEM images of different samples, which clearly shows formation of films on the surface of titanium. In this figure, different samples displayed nanotube arrays with open ends at the surface. In Fig. 2(a) and (b), diameter of the tubes is around 50–80 nm and some nanotubes were covered with debris. In Fig. 2

Conclusion

In summary, we report on the fabrication, optical and photocatalytic properties of Mn-doped TiO2 nanotubes synthesized via a one-step electrochemical anodizing technique. Films of Mn-doped TiO2 were synthesized via anodizing of titanium foil in organic electrolytes containing hydrogen fluoride and various concentrations of KMnO4. The morphology and structure were characterized by FE-SEM, XRD and EDX. The morphology and quality of the fabricated materials were found to be significantly affected

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