**Yu-Hsiang Wang 1, Kazi Hasibur Rahman 2, Chih-Chao Wu <sup>3</sup> and Kuan-Chung Chen 1,4,\***


Received: 26 April 2020; Accepted: 25 May 2020; Published: 27 May 2020

**Abstract:** Titanium dioxide (TiO2) thin films are used for a broad range of applications such as wastewater treatment, photocatalytic degradation activity, water splitting, antibacterial and also in biomedical applications. There is a wide range of synthesis techniques for the deposition of TiO2 thin films, such as chemical vapor deposition (CVD) and physical vapor deposition (PVD), both of which are well known deposition methods. Layer by layer deposition with good homogeneity, even thickness and good adhesive nature is possible by using the PVD technique, with the products being used for photocatalytic applications. This review studies the effects of magnetron sputtering conditions on TiO2 films. This innovative technique can enhance the photocatalytic activity by increasing the thickness of the film higher than any other methods. The main purpose of this article is to review the effects of DC and RF magnetron sputtering conditions on the preparation of TiO2 thin films for photocatalysis. The characteristics of TiO2 films (i.e., structure, composition, and crystallinity) are affected significantly by the substrate type, the sputtering power, the distance between substrate and target, working pressure, argon/oxygen ratio, deposition time, substrate temperature, dopant types, and finally the annealing treatment. The photocatalytic activity and optical properties, including the degree of crystallinity, band gap (*Eg*), refractive index (*n*), transmittance (*T*), and extinction coefficient (*k*), of TiO2 films are dependent on the above- mentioned film characteristics. Optimal TiO2 films should have a small particle size, a strong degree of crystallinity, a low band gap, a low contact angle, a high refractive index, transmittance, and extinction coefficient. Finally, metallic and nonmetallic dopants can be added to enhance the photocatalytic activity of TiO2 films by narrowing the band gap.

**Keywords:** magnetron sputtering; titanium dioxide (TiO2) film; photocatalytic activity; metal and non-metal doping; optical properties

### **1. Introduction**

Titanium dioxide (TiO2) is a low-cost non-toxic oxide semiconductor that is extensively employed in various industries due to its optical, electronic, and photocatalytic properties. It is mostly used in thin film form, which enhances the quantum efficiency. Because TiO2 has high transparency and a high refractive index, it can be employed for optical coatings such as those for dielectric interference filters [1],

multilayer mirrors, and anti-reflection coatings [2]. Its high dielectric constant and lower resistivity (~10−7Ω·cm−1) give TiO2 the potential for use in the fabrication of capacitors for microelectronic devices [3,4]. In addition, the high chemical stability and modest band gap of TiO2 make it suitable for fabricating dye-sensitized solar cells [5] and photocatalysts. Photocatalysis using TiO2 films has been used to clean up environmental problems [6,7]. TiO2 films have gradually replaced conventional TiO2 powder, which requires stirring during the reaction process and is difficult to separate after the end of reaction [8].

According to electrochemical properties, TiO2 is typically characterized as an n-type semiconductor [9]. TiO2 can exist as an amorphous layer or one of three crystalline phases: anatase (tetragonal, *a* = 0.3785 nm, *c* = 0.9514 nm), rutile (tetragonal, *a* = 0.4594 nm, *c* = 0.2958 nm), and brookite (orthorhombic, a = 0.9184 nm, *b* = 0.5447 nm, *c* = 0.5145 nm) (Figure 1). In all crystalline forms of TiO2, titanium atoms surrounded by six oxygen atoms form TiO6 [10]. In the anatase phase, corner (vertice)-sharing octahedra form (001) planes (Figure 1a) and result in a tetragonal structure. In the rutile phase, the octahedral share edges at (001) planes, therefore, forming a tetragonal structure (Figure 1b). In the brookite phase, both edges and corners are shared, which create an orthorhombic structure (Figure 1c) [11].

**Figure 1.** Building-block representation of TiO2 (**a**) rutile phase, (**b**) anatase phase, and (**c**) brookite phase. Reprinted with permission from [11], Copyright 2009, Elsevier.

*Catalysts* **2020**, *10*, 598

The thermal stability of crystallineTiO2 is affected by the film structure and particle size [10]. Rutile is a thermodynamically stable phase, whereas anatase and brookite are metastable. It has been reported that for pure TiO2 films, the metastable anatase phase can easily transform into the more stable rutile phase at high temperature [12,13]. Each crystalline form has practical applications. Rutile is desirable for optical applications, whereas anatase has suitable photocatalytic properties [14,15]. The bandgaps of anatase, rutile, and brookite are 3.2, 3.0, and ∼3.2 eV, respectively [16]. In photocatalysis, TiO2 is excited by photons with energy equal to or higher than their band gap energy level and electrons on the TiO2 surface are excited to the conduction band (eCB−, CB), generating positive holes in the valence band (hVB<sup>+</sup>, VB) (Equation (1)). Moreover, the CB and VB can further react with water and molecular oxygen, respectively, resulting in the formation of the hydroxyl radical (·OH) (Equation (2)) and the superoxide radical anion (O2 ·−) (Equation (4)). O2 ·− subsequently reacts with H<sup>+</sup> to generate the hydroperoxyl radical (·OOH) (Equation (6)), and then it may further react with itself to produce H2O2 (Equation (8)). The radicals produced by photocatalysis are powerful oxidants that can efficiently oxidize organic species, with mineralization producing mineral salts, CO2, and H2O (Equations (3), (5) and (7)). For TiO2, the photocatalysis reactions (1) to (8) are listed below [16]:

$$\text{TiO}\_2 \xrightarrow{hv} e^-\_{\text{CB}} + h^+\_{\text{VB}} \tag{1}$$

$$\cdot \text{H}\_2\text{O} + h\_{\text{VB}}^+ \rightarrow \cdot \text{OH} + \text{H}^+ \tag{2}$$

$$\text{-OH} + \text{pollutant} \rightarrow \text{H}\_2\text{O} + \text{CO}\_2\tag{3}$$

$$\text{O}\_2 + \text{e}\_{\text{CB}}^- \rightarrow \text{O}\_2^{\cdot -} \tag{4}$$

$$\rm{CO\_2^- + pollutant} \rightarrow \rm{H\_2O + CO\_2} \tag{5}$$

$$\rm O\_2^{\cdot -} + H^+ \to \cdot \rm OOH \tag{6}$$

$$\text{-OOH} + \text{pollutant} \rightarrow \text{H}\_2\text{O}\_2 + \text{CO}\_2\tag{7}$$

$$\cdot\text{-OOH} + \cdot\text{OOH} \rightarrow \text{H}\_2\text{O}\_2 + \text{O}\_2\tag{8}$$

TiO2 films have been modified to enhance their photoelectric properties and make them excitable by visible light (narrowing the band gap) [17]. Many studies have demonstrated that the addition of dopants (metal and non-metal) onto TiO2 films can enhance the effect of photocatalytic activity [16].

Metal dopants can act as electron traps at the semiconductor interface that prevent recombination (see Figure 2) [18,19], and non-metal dopants substitute the lattice O atom in the structure of TiO2 to increase the absorption of light in the visible spectrum (see Figure 3) [20,21].

**Figure 2.** Schematic of metal-TiO2 photocatalytic mechanism.

**Figure 3.** Schematic models in a 2 × 2 × 1 supercell for (**a**) oxygen vacancy and (**b**) substitutional N-doping. Reprinted with permission from [21], copyright 2009, Elsevier.

Moreover, non-metal dopants can also increase the thermal stability of anatase [22]. There are many important parameters which control and enhance the photocatalytic performance of the TiO2 thin films. The parameters are listed as follows: different pathways during reaction process, i.e., nucleation growth, formation of different phases like anatase or rutile, surface area, at the interface of the reactants. TiO2 thin films can be deposited by some advances techniques like physical vapor deposition (PVD), chemical vapor deposition (CVD), and wet chemical methods including dip coating, spin coating, spray pyrolysis, etc. Moreover, thermal evaporation, reactive sputtering, ion or electron beam evaporation methods are widely used. These mentioned applications are low cost, and easy to access which is preferable for outdoor applications. The disadvantages of these techniques are that they must be followed by secondary process which include drying and annealing of the thin films to increase the stability and crystallinity of the films, which are some of the essential criteria for the desired enhanced photocatalytic activities. Generally, vapor phase methods present various advantages such as well controlled homogeneity and thickness over a large area and good adhesive nature [23]. The CVD method requires higher temperatures (400–900 ◦C) compared to PVD processes [24]. This disadvantage of requirement of excessive heat can be incompatible with some substrate materials. Some titanium precursors and their byproducts are highly corrosive in nature, which leads to various material handling and storage problems [25]. However, the PVD processes have garnered great interest since they are not limited to deposition only at thermodynamically equilibrium and they run at much lower costs in comparison to CVD processes [26]. There are many PVD methods that are being applied for the deposition of thin films such as oxidation, deposition, DC magnetron sputtering, RF magnetron sputtering, thermal evaporation, pulsed laser deposition (PLD), etc.

Highly homogenous and high-density films are obtained in these processes due to bombardment of energetic particles, thus these energetic hot particles take part in the photocatalysis process followed by surface plasmon resonance mechanism [27]. This method yields highly porous and dense films at relative low temperatures. TiO2 thin films prepared in this technique can exhibit diverse photocatalytic activities, including high surface area, morphology, defect density, and crystallization pathways. In comparison with TiO2 bulk nanoparticles, TiO2 thin films exhibit a limited surface area, which hinders the efficiency of the photocatalysis for the decomposition of the organic compounds [28]. Thus, various strategies are reported to enhance the surface area of thin films for higher photocatalytic dye degradation studies, where novel sputtering methods are established to produce highly porous thin films for better adsorption of the dye molecules on the surface of the photocatalyst TiO2 thin films. Suzuki et al. [28] have reported a sculptured TiO2 thin film providing a higher photocatalytic efficiency. To achieve a high surface area, nowadays another effective process is established, namely to control nano-crack network formation within sputter deposited TiO2 thin films [29,30].

This review is focused on the effect of magnetron sputtering conditions on TiO2 films. Magnetron sputtering allows the structure and composition of TiO2 films to be easily controlled, yields uniform films, and is suitable for large-scale industrial production. Thus, the optimal operating conditions for magnetron sputtering are of interest. The considered operating conditions, which influence the quality of photocatalytic thin films (PTFs) during magnetron sputtering, are the substrate type, sputtering power, distance between substrate and target, working pressure, argon/oxygen ratio, deposition time, substrate temperature, dopant type, and annealing treatment.

#### **2. Working Principle of Magnetron Sputtering Technique**

DC magnetron sputtering techniques are used for depositing thin films over large areas. The continuous current glow discharge that originates the energetic particles that support sputtering process can be obtained by applying a potential difference between two electrodes in the presence of a gas (usually argon) kept at low pressure inside a vacuum chamber. The potential difference can be delivered by a continuous current power supply that can provide tens of thousands of volts, depending on the equipment configuration. An electric field is formed between the electrodes separated by a distance d and having a potential difference V due to the presence of electrons formed by processes such as ionization, thermionic emission and collisions between particles. Here ionization is caused by cosmic rays, but it cannot be maintained. A schematic diagram is shown in Figure 4a.

**Figure 4.** Schematic diagram of the working principle of (**a**) DC and, (**b**) RF magnetron.

The gas pressure inside the chamber should not be very low in order to allow a collision between electrons and some atoms before the electrons hit the anode. If the gas pressure is very high, the electrons will neither reach enough velocity nor achieve enough energy in order to form new ions or excited species in the collisions. Under the electrical field, positive ions are accelerated towards the cathode while electrons are accelerated towards the anode. Once electrons travel a long enough distance before the collision, they will acquire enough kinetic energy to promote a new ionization. The main difference between the RF and DC magnetron sputtering is that they work in AC source and DC source mode, respectively. The main advantage of using RF sputtering techniques is that they work well with insulating targets. Moreover, the electric field inside the plasma chamber changes with RF frequency, which avoids any charge up effects. A schematic diagram of RF magnetron sputtering is shown in Figure 4b.

#### **3. E**ff**ect of Deposition Conditions on Photocatalytic Thin Films**
