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,
2,
3,
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,
6,
7,
8,
9,
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,
13,
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
2 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.
2. Experiment
Zinc films where deposited on c-plane sapphire substrates via direct current sputter deposition. Metallic Zn targets were obtained commercially and had a purity of 99.99%. Before deposition, substrates were cleaned via immersion in acetone, ultrasonically cleaned in methanol and rinsed in deionized water. The chamber base pressure was maintained between 1.0 × 10-5 and 2.5 × 10-5 mbars. A gate valve between the chamber and the pump was utilized as a throttle to maintain an Ar pressure of approximately 2 × 10-2 mbars. Sputtering power was maintained between 10 and 20 W with the substrate located approximately 10 cm from the sputter source. Deposition times ranged from 15 to 40 min, resulting in film thicknesses between 100 nm and 600 nm as measured via the AFM profile and reflectometry. Thermal oxidation of the Zn metal films was carried out in an air-ambient muffle furnace. For all samples, Zn films were initially annealed at 300 °C for 9–24 hours to ensure complete oxidation. Some films were then re-annealed for 1 hour at 600 °C, 900 °C and 1200 °C.
Zinc oxide films were characterized via X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL). Structural properties of the ZnO films were measured using Cu-Kα radiation in the range from 30° to 50°. The morphology of the films was determined via dynamic-mode AFM using an Anfatec Level AFM and approximately 300 kHz resonant aluminum backside silicon tips. Photoluminescence spectra were obtained at room temperature using a HeCd laser as an excitation source and a power of P = 0.3 W/cm2. UV illumination was provided by a deuterium lamp.
Following morphological and optical characterization, the photocatalytic activities of the films were characterized by measuring the degradation of Rhodamine B dye (rhoB) in solution. RhoB was used to simulate an organic environmental contaminant, because its concentration in solution can be accurately measured spectrophotometrically. Oxidized rhoB products do not absorb visible light, so the concentration and the resulting absorbance of the rhoB solutions decrease as the rhoB is oxidized by the ZnO photocatalysts. The overall photocatalytic activities of the materials therefore correlate to the rate of change in the concentration of the rhoB solutions as measured by UV-Vis spectrophotometry.
The UV-Vis spectrophotometer was first calibrated in order to determine the relationship between absorbance measurements and rhoB concentrations. This was accomplished by measuring the absorbance of known concentrations of rhoB and constructing a calibration curve from the resulting measurements (not shown). The concentrations in the rest of the experiment were then calculated from the absorbance measurements. The linear range of the absorbance-concentration relationship was determined to be between 1 and 8 ppm rhoB, so an initial concentration of 8 ppm rhoB in solution was used for the photodegradation experiments.
Each ZnO thin film was incubated in an 8 ppm rhoB solution while irradiated with UV light at a constant power density of 80 μW/cm. The absorbance of the solution was taken after 10, 20, 30, 45, 60, 90, 120, 150 and 180 min of UV irradiation at the peak absorbance wavelength (553 nm) for maximum sensitivity, and the corresponding rhoB concentration was calculated using the equation of the rhoB calibration curve. In order to account for differences in the surface areas of the films, the total change in concentration of rhoB at each time interval was divided by the specific surface area of the film and plotted against the time of UV irradiation.
4. Surface Catalysis and Reaction Kinetics
Metal-oxide semiconductors use light energy to catalyze oxidation-reduction reactions via electron-hole pair production at the material surface [
41,
42]. The photogenerated holes on the semiconductor surface have a high oxidation potential, while the photogenerated electrons have a high reduction potential [
19,
23]. Several types of aqueous reactions catalyzed by electron-hole pairs lead to the formation of the hydroxyl radicals and reactive oxygen species that are directly responsible for the oxidative degradation of organic compounds. One of these reactions involves the oxidation of water (H
2O) by a hole (h
+) into hydrogen ions (H
+) and a hydroxyl radical (O
*):
Another reaction involves the reduction of molecular oxygen (
) into a superoxide radical (
) by a photogenerated electron (
):
The superoxide radical can be further reduced by another electron and then paired with two
ions to form hydrogen peroxide (
):
The hydrogen peroxide can then be reduced by an electron to form hydroxyl radicals:
By these reactions, either the electrons or the holes of the photogenerated electron-hole pairs can produce hydroxyl and superoxide radicals that can subsequently degrade organic compounds [
19,
23]. It has also been proposed that the electrons or holes themselves may be responsible for at least some of the degradation of organics. 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]. Most likely, however, a large combination of chemical reactions involving various intermediates ultimately leads to the degradation of the organic compounds, which complicates the mathematical descriptions of the rates at which the overall degradation occurs [
19].
Simplified mathematical descriptions of the degradation reactions have been used to experimentally quantify and compare photocatalytic activities. For most purposes, especially when the concentration of the contaminant is less than 10 ppm, the reaction can be considered first order, meaning that the time
t rate of change in concentration
c of the contaminant,
, follows the general relationship:
where
k is the rate constant [
19]. This equation has been modified for solid photocatalysts to account for the surface area of the photocatalyst and the intensity of the incident light, as follows:
where
I is the intensity of light used and
is the total surface area of the photocatalyst [
23]. The solution to this differential equation is as follows:
where
is the initial concentration of the contaminant. In order to make the concentration-time relationship linear, the equation can be rewritten as the integrated rate law, as follows:
When the natural log of the normalized concentration
is plotted as a function of time, the rate constant
k can be easily determined from the slope of the linear best-fit. The rate constant is the figure of merit referred to in the literature for quantitative comparisons of photocatalytic activity [
19].
Zero- and half-order rate laws have also been used to describe the photocatalyst degradation reactions. In particular, the half-order rate law has been shown to more accurately model photocatalytic reactions on metal-oxide surfaces, most likely due to the combination of zero- and first-order chemical reactions involved in photodegradation [
19]. The differential and integrated forms of the half-order rate law are as follows:
and:
respectively. In this study, we calculate
and plot this as a function of the UV irradiation time, with the half-order rate constant determined from the slope of the linear best-fit. While the light intensity is a factor in calculating the rate constant, the same light intensity was used for each experiment in this study, and its effect on the rate of each reaction can be negated.
6. Discussion
Extensive work has gone into investigating the photocatalytic properties of ZnO nanoparticles and colloid suspensions [
20]. 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. For the porous polycrystalline films discussed in this study, both optical and morphological properties have been shown to significantly affect the photodegradation of organics at the interface.
Polycrystalline films demonstrating high surface roughness and with small crystal grain size do typically have a high effective surface area, which increases the number of available surface states to serve as reaction sites. The concern with respect to the engineering of effective metal-oxide-based catalysts, though, is that the decreased crystallinity associated with these surfaces typically results in reduced optical efficiency and, therefore, lower electron-hole pair production efficiency [
23,
27,
28]. Both high surface area and optical efficiency are required for high photocatalytic activity with metal-oxides. For
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 could lower the number of photo-induced holes at the surface available for catalysis [
28]. These competing mechanisms must be balanced. Thermal oxidation of Zn-metal films at low annealing temperatures applied over many hours results in a balance between high effective surface area and optical quality. Furthermore, the porous nature of these films results in increased effective surface area with increased film thickness, without a corresponding decrease in optical quality, at least with respect to electron-hole pair production efficiency.
However, a significant problem with metal-oxide systems, such as ZnO, is long-term stability. The most photoactive films discussed in this study decrease in effectiveness by 50% in approximately 24 hours. There is also some concern about the dissolution of ZnO particles and resulting Zn toxicity in marine environments [
4,
17]. Although most of the work on ZnO dissolution has been with respect to nanoparticles and colloid suspensions, there is increasing evidence that Zn toxicity should be a concern for film-based systems and even for the interface of the single-crystal bulk [
4,
25]. Continued work needs to be done to find metal-oxide systems that exhibit high catalytic activity and that are passivated from preferential etching at termination sites, resulting in increased stability.