Chemical doping of TiO 2 Nano-tube array for enhancing hydrogen production through photoelectrochemical water splitting
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Chemical doping of TiO2 Nano‑tube array for enhancing hydrogen production through photoelectrochemical water splitting Mehdi Farahmand1 · Ali Allahverdi1 Received: 24 March 2020 / Accepted: 21 August 2020 © Springer Nature Switzerland AG 2020
Abstract Nano-tube array is one of the most important types of titanium dioxide-based nanostructures that attracted researchers’ attention because of its superior photocatalytic properties. In this research, a thin layer of a titanium dioxide nano-tube array was first prepared by a two-step electrochemical anodizing process by using a titanium sheet. The synthesized nano-tube array was then calcined to obtain a good crystalline structure for enhanced photocatalytic properties. The length, diameter, and thickness of the nano-tubes were measured using SEM imaging technique. In order to enhance the photocatalytic properties, the synthesized nano-tube array was then doped individually with three different chemicals including silver nitrate, potassium ferricyanide and copper sulfate using the chemical bath deposition technique. The photocatalytic properties of the synthesized nano-tube array and the doped samples were evaluated and compared by photoelectrochemical water splitting. The results show that the nano-tube array doped with copper sulfate increases the rate of hydrogen production by up to three times producing as much as 21.65 mmol after 5 h. Keywords Titanium dioxide · Nano-tube array · Anodization · Photoelectrochemical water splitting
1 Introduction Hydrogen is an ideal, renewable and clean fuel that burns out and produces energy and water, and is likely to be a good alternative to fossil fuels due to the possibility of eliminating carbon emission [1, 2]. An environmentally friendly method of hydrogen production is photocatalytic water. Photocatalysts are semiconductors that are capable of creating electron–hole pairs upon absorbing light photons of sufficient energy. The energy absorbed from light photons results in the excitement of electrons generally filling the outermost shell of the atom (valence state) into higher energy orbitals (conduction state) that are generally empty [3, 4]. The produced electron–hole pairs are then capable of carrying out redox reactions including the water-splitting reaction to produce hydrogen and oxygen [5]. In 1972, titanium dioxide ( TiO2) was the first catalyst
used by Fujishima and Honda as photoanode in photoelectrochemical cells (PECs) for water splitting and hydrogen production [6]. In 2001, Grimes et al. [7] synthesized titanium dioxide nano-tubes array (TDNA) that attracted scientists’ attention due to its highly ordered structure, very high surface area, and chemical stability. Titanium dioxide is excited only by UV light because of its relatively large energy band gap of 3.2 eV [7]. In order to reduce its energy band gap and increase the hydrogen production rate, attempts have been devoted to dope TDNA with various elements [8]. In addition, different techniques including chemical vapor deposition (CVD), physical vapor
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