Photocatalytic Activity and Stability of Porous Polycrystalline ZnO Thin-Films Grown via a Two-Step Thermal Oxidation Process

## James C. Moore, Robert Louder and Cody V. Thompson

Abstract: The photocatalytic activity and stability of thin, polycrystalline ZnO films was studied. The oxidative degradation of organic compounds at the ZnO surface results from the ultraviolet (UV) photo-induced creation of highly oxidizing holes and reducing electrons, which combine with surface water to form hydroxyl radicals and reactive oxygen species. Therefore, the efficiency of the electron-hole pair formation is of critical importance for self-cleaning and antimicrobial applications with these metal-oxide catalyst systems. In this study, ZnO thin films were fabricated on sapphire substrates via direct current sputter deposition of Zn-metal films followed by thermal oxidation at several annealing temperatures (300–1200 ◦C). Due to the ease with which they can be recovered, stabilized films are preferable to nanoparticles or colloidal suspensions for some applications. Characterization of the resulting ZnO thin films through atomic force microscopy and photoluminescence indicated that decreasing annealing temperature leads to smaller crystal grain size and increased UV excitonic emission. The photocatalytic activities were characterized by UV-visible absorption measurements of Rhodamine B dye concentrations. The films oxidized at lower annealing temperatures exhibited higher photocatalytic activity, which is attributed to the increased optical quality. Photocatalytic activity was also found to depend on film thickness, with lower activity observed for thinner films. Decreasing activity with use was found to be the result of decreasing film thickness due to surface etching.

Reprinted from *Coatings*. Cite as: Moore, J.C.; Louder, R.; Thompson, C.V. Photocatalytic Activity and Stability of Porous Polycrystalline ZnO Thin-Films Grown via a Two-Step Thermal Oxidation Process. *Coatings* 2014, *4*, 651–669.

## 1. Introduction

Zinc oxide (ZnO) is a highly useful and practical wide bandgap semiconducting material with a broad range of applications, including self-cleaning and anti-fogging surfaces, sterilization, gas sensing, energy production and environmental purification [1–4]. Specifically, ZnO efficiently absorbs ultraviolet (UV) light and has surface electrical properties sensitive to the environment at the interface, with device applications that include gas sensors, photovoltaic cells, light emitting diodes and photocatalysts [1,5–10]. The photocatalytic effects of ZnO are being exploited for use within self-cleaning paints, in environmental remediation applications and prophylactics with nanoparticle and colloidal suspensions demonstrating high photodegradation efficiency for organic compounds [11]. Thin films have received recent interest due to their reusability and transparency, which is essential for applications, such as self-cleaning glass and antimicrobial coatings on solid surfaces and flexible plastics [12–14]. Transparent ZnO films could also find use as fingerprint-resistant barriers on touch screen devices, such as cell phones and tablet computers.

Many environmental pollutants are organic in nature, and many proposed methods of environmental decontamination involve oxidation of the organic pollutants [15]. However, using semiconductor photocatalysts to oxidize and remove such pollutants from the local environment has many advantages over alternative methods [16]. ZnO materials in particular are nontoxic and present little additional harm to the environment in which they are used, contrary to most other methods of decontamination. However, there is some concern about the dissolution of ZnO particles and resulting Zn toxicity in marine environments [4,17]. Furthermore, ZnO photocatalysts do not need to be re-activated after undergoing photoinduced oxidation and reduction reactions. Conversely, activated carbon, a popular choice for water purification, requires expensive and potentially polluting reactivation [18].

Another traditional means of decontamination involves microorganisms, such as bacteria, which biologically degrade toxic organics [18]. However, these processes occur at a much slower rate compared to photocatalytic oxidation by semiconductors, such as ZnO, and are inefficient at concentrations below parts-per-million (ppm) levels, while ZnO photocatalysts have been shown to oxidize pollutants present in extremely low concentrations. Additionally, many pollutants can also be toxic to the microorganisms themselves, reducing their catalytic activity with time. ZnO photocatalysts degrade most organic pollutants non-selectively, though their stability is a topic of study [19].

Extensive work has gone into investigating the photocatalytic properties of ZnO nanoparticles and colloid suspensions [20,21]. For environmental remediation purposes, nanoparticle powders are particularly effective, since they can be readily mixed with the contaminated solution and have a high surface area. However, separating the catalyst from solution is challenging, which makes their use in these applications potentially cost-prohibitive [22]. As mentioned, Zn toxicity is also a concern for these systems, especially if allowed to remain in the environment. Fujishima *et al.* suggest that the nanofilm form of these semiconductors is preferable to particles for use in fluid decontamination exactly because the nanoparticles need to eventually be collected and removed from the fluid [23,24].

There has been little discussion in the literature concerning the sorts of structural and photophysical properties that can directly affect photocatalytic activity for film-based catalysts. As an example, whether degradation occurs predominantly due to reaction with free-radicals or directly with the holes themselves is controversial, with some groups even proposing a predominant electron-based catalytic pathway on ZnO single-crystal surfaces [23,25]. For surface-based applications, challenges exist with nanoparticle-based films, such as adhesion and optical transparency [26]. Nanoparticles do have a high surface area per volume, which increases the number of available surface states to serve as reaction sites. However, increased crystallinity associated with larger particle sizes typically results in greater optical efficiency and, therefore, higher electron-hole pair production efficiency [23,27,28]. Many applications of decontamination using semiconductor photocatalysts involve the Sun as a practical source of UV illumination, although only 2%–3% of solar radiation will induce semiconductor-catalyzed oxidation [29]. Accordingly, the photodegradation efficiencies of the semiconductor photocatalysts designed for these uses need to be optimized in order to be of practical use. Both high surface area and optical efficiency are required for high photocatalytic activity with metal-oxides. For TiO<sup>2</sup> and ZnO thin, polycrystalline films, a decrease in grain size typically results in increased surface roughness and surface area; however, small grain size typically also corresponds to an increase in deep-level defects that lower the number of photo-induced holes at the surface available for catalysis [28]. These competing mechanisms must be balanced.

In previous work, we have found that thin (<200 nm) ZnO films grown via thermal oxidation of Zn-metal at relatively low temperature (300 ◦C) result in high surface roughness with low deep-level defects [14,28]. Increasing surface-level Zn interstitials could also result in greater catalytic activity and a favorable shift in wavelength into the visible spectrum. We have also found that significant blue emission associated with Zn interstitials near the surface and very little deep-level emission from bulk-related defects can be obtained via tailoring of the films thickness and grain size, resulting in a potential increase in photocatalytic activity due to a favorable balance of features [28,30].

In this study, we measure the photocatalytic activity and stability of thin, polycrystalline ZnO films fabricated on sapphire substrates via direct current (DC) sputter deposition of Zn-metal films, followed by thermal oxidation at several annealing temperatures. In particular, we describe growth parameters that result in highly porous, polycrystalline films demonstrating high surface roughness while simultaneously exhibiting a high excitonic-to-green emission ratio. We also investigate the time-dependent stability of these films.

In Section 2, we describe the process by which films have been fabricated and characterized, and we discuss the method used for determining catalytic activity. In Section 3, we discuss the resulting morphological, structural and optical properties of the fabricated films. In Section 4, we discuss surface catalysis pathways and the reaction kinetic models used to describe the catalytic activity of these films. Finally, in Section 5, we discuss the balance between crystal grain size and the optical efficiency that affects the photocatalytic activity of polycrystalline ZnO films.
