Investigation of DC magnetron-sputtered TiO₂ coatings: Effect of coating thickness, structure, and morphology on photocatalytic activity

Abstract

The photocatalytic performance of magnetron-sputtered titanium dioxide (TiO₂) coatings of different thickness in anatase crystalline structure deposited on aluminium 1050 alloy substrates was investigated using a combination of photo-electrochemistry, methylene blue decomposition, and microscopic and spectroscopic methods, such as high resolution scanning and transmission electron microscopy, atomic force microscopy and ellipsometry. The reaction resistance was measured by AC impedance, while photocurrent measurements were carried out using the zero resistance ammetry (ZRA) method. The results showed that the TiO₂ grains grow in dipyramidal columns having a linear increase in surface area with increased coating thickness. The refractive index values indicate also an evolutionary growth. The refractive index values obtained for the thin coatings on aluminium substrate were well below the values reported for monocrystalline anatase. The photocatalytic performance increased with increased coating thickness, though more rapidly over a range of 100–500 nm thickness. The dielectric constant also increased linearly with coating thickness.

Introduction

There is growing interest in titanium dioxide (TiO₂) in the anatase crystalline form due to the usefulness of its photocatalytic activity for number applications. These include solar energy conversion [1], [2] electro-chromic devices [3], photocatalytic applications in air and water purification [4], self-cleaning and antimicrobial surfaces [5]. Therefore, significant research is being undertaken all over the world to understand the relationship between photocatalysis and morphology of TiO₂ as a means to enhance its performance [6], [7], [8], [9].

The high photocatalytic activity of anatase TiO₂ is primarily due to the large band gap between the valence and conduction bands, resulting in high redox power [6]. Electromagnetic radiation with energy equal or higher than the band gap of TiO₂ (3.2 eV) can excite electrons to the conduction band (CB), similar to other semiconducting materials. However, because of the large band gap, excitation of TiO₂ produces high energy electrons and holes. The oxidative power of the holes is sufficient to decompose water molecules into hydroxyl radicals, while the high reductive power of the exited electrons will generate superoxide from oxygen. Both the hydroxyl radicals and the superoxide are useful in imparting photodecomposition of the dissolved organic material either by a reductive or oxidative process. The independent consumption of the generated electron–hole pair makes the material and its photocatalytic activity sustainable (Fig. 1).

The photocatalytic activity of TiO₂ is influenced by a number of parameters, including the morphology of the TiO₂ particles or coatings, processing methods, crystallographic orientation, and the presence of dopants. It has been reported that the 0 0 4 face of TiO₂, due to constrained alignment of the surface atoms [10], is more active than the 1 0 1 face [10], [11], [12]. A number of investigations have demonstrated that larger surface area increases the photocatalytic activity, and the photocatalytic activity was improved by addition of various dopants such as Cr, Mo, V, Mn, Fe, Co and Ni [13], [14].

Most investigations in the literature related to photocatalysis of TiO₂ focused on nano-particles and coatings on glass substrates. A review by Debold [15] is a good summary of research done on TiO₂ and shows the increased interest in this area as reflected by the increased number of publications, especially on single-crystalline TiO₂. Moreover, this paper gives a brief introduction on metal/titanium dioxide interfaces and highlights its relevancy for further studies. It has been reported that the behaviour of TiO₂ coatings on a metallic substrate is different from that of a non-metallic substrate due to the interplay between the conduction band of the substrate metal, band structure of the interfacing oxide, and band structure of TiO₂ [16]. Previous studies of TiO₂ coatings on metallic substrates have demonstrated that the substrate can assist the charge separation and electron transfer, and increased coating thickness increases the activation depth and UV-absorption until a saturation depth of the coating is reached [17].

The change in coating thickness can influence the morphology and surface area of the top layer of the coating, which will have significant impact on the photocatalytic performance. The surface morphology of TiO₂ also depends on the synthesis method. There are various techniques for synthesising TiO₂ coatings [3]. Among, and the most common methods are chemical solution deposition (CSD, Sol gel) [18], [19] and physical vapour deposition (PVD) [20], [21]. PVD is known to produce compact coatings with good adhesion to the substrate and the processing parameters, such as temperature and pressure, are controlled precisely and hence provide good reproducibility.

When a coating is deposited by vacuum techniques such as PVD, the size of the crystallites can increases with increased coating thickness due to the evolutionary nature of the growth [22]. In larger crystallites, the lifetime of the electron/hole is also lengthened as the pair migrates a greater distance in large crystallites than in smaller crystallites [23]. Greater electron–hole recombination distance increases the photocurrent, allowing more photocatalytic decompositions to take place.

The investigation reported in this paper focuses on the magnetron-sputtered TiO₂ coatings of different thicknesses on an aluminium alloy (AA1050) substrate. Microstructural and surface morphological investigations were performed by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and glow discharge optical emission spectroscopy (GDOES). The photocatalytic activity was determined by measuring: (i) the reaction resistance under UV illumination based on impedance measurements, (ii) photocurrent upon UV illumination using zero resistance ammetry (ZRA), and (iii) the photocatalytic decomposition of methylene blue on TiO₂ surfaces using UV light.