A Floatable Piezo-Photocatalytic Platform Based on Semi-Embedded ZnO Nanowire Array for High-Performance Water Decontamination
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ZnO nanowires were securely immobilized onto a floatable photocatalytic platform, which had a uniform diameter (55 ± 5 nm) and length (1.5 ± 0.3 μm).
An additional 20% of the probe pollutant (methylene blue) was degraded by piezocatalysis-assisted photocatalytic degradation.
The crude oil pollutant was decomposed up to 20% within 6 h.
KeywordsSemi-embedded structure Photocatalysis Piezocatalysis ZnO nanowire arrays
In the past few decades, water pollution has been becoming a serious global issue owing to the rapid development of the industry and widespread use of chemical and biological synthetic materials. Therefore, research on the aspect of pollutant removal for a natural water system is highly urgent and essential. Among numerous strategies, photocatalysis is one promising technology for disinfection and detoxification and has been demonstrated to be effective against various types of organic pollutants originating from industrial and agricultural wastes. This process mainly involves two reactions: oxidation and reduction. With the assistance of solar energy, photocatalysis is economical and applicable to versatile environmental conditions so that it has attracted considerable interest in recent years. For heterogeneous photocatalysis, the process is commonly realized by semiconductor photocatalysts, which can be categorized into two kinds in terms of their configurations: (1) slurry photocatalysts [1, 2, 3] and (2) immobilized photocatalysts. Among them, immobilized photocatalysts involve immobilized films/nanocomposites [4, 5, 6] and immobilized vertically grown nanowires (NWs) [7, 8, 9]. Slurry photocatalysts, including particles and 1D nanostructures, are the most commonly studied configurations, which are considered to possess high degradability owing to their large surface area-to-volume ratios and high homogeneity in aqueous solutions. With proper modification, excellent photocatalytic activities of these photocatalysts can be achieved, which are comparable to that of Degussa P25 [10, 11, 12, 13, 14]. However, some challenges such as easy agglomeration, complicated recovery from liquids, and increased absorption along the penetration depth of UV light hindered these photocatalysts from practical use. As an alternative route, immobilized film-like photocatalysts are preferable as practical options. This type of configuration possesses high recyclability, whereas their limited active surface area and mass transfer restraint become additional drawbacks. Recently, a photoreactor, especially microchannel-based photoreactor, has been suggested as one effective way to improve the performance of immobilized photocatalysts by increasing the surface-to-volume ratio and mass transfer and reducing the molecular diffusion distance. Such photoreactors serving as tools for the degradation of organic pollutants have demonstrated excellent reproducibility, stability, and degradability [15, 16, 17, 18]. Moreover, a modification toward this design, such as employing immobilized NW photocatalysts, further improves its photodegradation rate owing to the larger active surface area in comparison with that of a film [19, 20]. However, most of these reactors require external power to drive the polluted solution to flow across the reaction chamber, and a specific platform is usually built to bring it closer to the light source, which inevitably increase the cost for scalable water cleanliness. In this regard, a configuration possessing immobilized NW-like photocatalysts both autonomously floating and freely moving atop the water is desired to improve the utilization of sunlight and overcome the mass transfer issue.
It is well known that for immobilized photocatalysts, a good support possesses features including reliable substrate-to-photocatalyst contact, stable property against generated oxidants, long lifetime in water, and environmental benignity. Furthermore, for a floatable immobilized platform, one more feature that needs to be taken into account in fabrication is a lightweight. Polymers were found suitable to use as floatable substrates [21, 22, 23]. Therefore, in this study, this novel configuration utilized a polymer as a substrate. Owing to the advantages of cost-effectiveness, substrate compatibility, and excellent degradability, ZnO was employed as the photocatalyst [24, 25, 26, 27]. To our knowledge, a floatable photocatalyst is rarely explored. This work proved the concept of a photocatalytic platform that was able to address the aforementioned problems that other platforms encountered.
In detail, a two-step synthesis route (hydrothermal growth on a rigid substrate and a second hydrothermal growth on a flexible substrate) enabled a ZnO NW array to be securely and uniformly fixed onto the polymer substrate (ZP film). As a result, the light and flexible polymer allowed the ZP film to freely float and move on top of water, while the semi-embedded configuration tackled some problems such as re-suspension and light utilization. Besides, the investigation of the mechanical property and thermal stability confirmed the high durability and reusability of the ZP film to harsh environments. In the photodegradation experiment, the surface area and mass transfer, as two primary aspects, were found affecting the photodegradation of the ZP film. The enhanced photocatalytic activity was obtained at a large NW length and high mass flow rate. Moreover, the piezo-photocatalytic effect, as an additional functionality, was demonstrated to further improve the degradability of the ZP film by around 20%. Other than methylene blue (MB) dye, the pollutant removal capability of the ZP film toward crude oil was confirmed, which revealed the potential of the ZP film in use for various industrial wastes in open water systems.
2.1 Initial Growth of ZnO Nanowire Array
All the chemicals are of analytical grade and used without further purification. A ZnO NW array was synthesized via the hydrothermal method as previously described . In the first step, 10 mM Zn(CH3COO)2·2H2O (99.9% Sigma-Aldrich) was dissolved in methanol (Fisher Scientific). After vigorous stirring at 60 °C for 2 h, the mixed solution was spin-coated onto a glass substrate at 1000 rpm for 30 s. Prior to the seed deposition, the glass substrate was cleaned by acetone, isopropanol alcohol, and deionized (DI) water, consecutively. The seeded glass substrate was heated to 150 °C for 1 h for dehydration. The growth solution of 25 mM Zn(NO3)2·6H2O (Sigma-Aldrich) and 25 mM hexamethylenetetramine (HMTA, Sigma-Aldrich) was prepared and preheated in a convection oven at 90 °C for 1 h to achieve thermal equilibrium. The annealed substrate was then immersed into the growth solution at 90 °C for 6 h. Consequently, a vertically grown ZnO NW array was obtained on the glass substrate after thoroughly rinsing by DI water.
2.2 Secondary Growth of ZnO Nanowire Array on PDMS
In the second step, a certain amount of polydimethylsiloxane (PDMS) solution (10 parts of SYLGARD 184 pre-polymer and 1 part of the curing agent, Dow Corning Co.) was fully covered and spin-coated on the NW-grown glass substrate. The spin coating speed was varied in terms of the desired thickness of the PDMS layer. For example, spin coating at 1000 rpm for 90 s produced a PDMS film with around 300 µm thickness. To avoid potential defects, the spin coating speed for the PDMS layer should not exceed 3000 rpm. The PDMS layer was thermally cured at 60 °C for 12 h and cooled to the room temperature overnight. The PDMS layer was then detached from the glass substrate. To facilitate the peeling of PDMS (extremely thin to be handled on its own), a polyethylene terephthalate (PET) film was utilized as a backing layer. The PET film was brought into intimate contact with the PDMS layer prior to its detachment. The as-fabricated PET/PDMS stack was placed atop the ZnO growth solution with the embedded NW array facing down at 70 °C for 12 h. The sample was thoroughly rinsed with DI water and completely dried. Finally, the PET film was separated from the PDMS layer to obtain a vertically aligned ZnO NW array on the PDMS membrane (denoted as the ZP film).
2.3 Material Characterization
The morphology, crystallinity, and composition of the ZnO NW array both on the glass and PDMS substrates were characterized by scanning electron microscopy (SEM, JEOL 6610LV, Hitachi S-4700II), transmission electron microscopy (TEM, TALOS F200X), X-ray diffraction (XRD, Bruker-AXS), energy-dispersive X-ray spectrometer (EDS, JEOL 6610LV), and photoluminance spectrometer (PL, Ocean Optics, Inc. USB spectrometer and optical fiber) with the excitation laser at 337 nm.
2.4 Photocatalytic/Piezocatalytic Assessment and Activities
2.4.1 Immobilized, Floating Photocatalytic Platform
Photocatalytic degradation was carried out in an aluminum foil-sealed case with a magnetic stirrer stage inside. Two 9 W florescent tubes (Philips λ = 375 nm) were placed 30 cm above the dye solution container. A 10 mM methylene blue (MB, 1.5%, Sigma-Aldrich) aqueous solution was prepared as a model polluted water. Four pieces of the ZP film (1 × 1 in.2 each piece) were utilized afloat on top of 40 mL of the dye solution (with the ZnO NW side facing down). Prior to irradiation, the dye solution was kept in dark for 30 min to establish the absorption/desorption equilibrium. The UV light irradiation was established perpendicular and with a distance of 30 cm to the floating ZP film. One milliliter of solution was extracted from the container every 30 min as the photodegradation progressed. The extracted solution was centrifuged at 10,000 rpm to remove any impurities and examined for absorbance in a UV–Vis spectrometer (Lambda). The absorption peak at 667 nm corresponded to MB, and the peak height can be correlated with the MB concentration. The photodegradation experiment was repeated three more times under the same working condition to test reusability/recyclability. The same degradation experiment (but only one 1 × 1 in.2 ZP film used for comparison) was carried out in 20 mL MB solution to evaluate the role of the secondary growth of the ZnO NW array on its photocatalytic performance. The effect of the mass transfer on the floating immobilized ZP film was investigated by turning on or off the magnetic stirrer during the degradation experiments.
2.4.2 Mechanical Stability of ZnO NW Array on PDMS: Piezocatalytic Degradation
The mechanical stability of the two-step grown ZnO NW array on the PDMS film was tested using the standard tape testing and compared to a ZnO NW array directly grown on a PDMS film. The bendability and stretchability of the PDMS film allowed the ZP film to be placed on a curved surface with a small radius of curvature (i.e., an 8-mm-diameter glass rod). The rod mounted with the ZP film was connected to the overhead mixer (Dragon Lab, OS20-S) and tested for photodegradation. The piezocatalytic effect of the ZP film was tested in an ultrasonic bath (Fisher Scientific, 160 W, 40 kHz), as a floating catalytic configuration and/or an overhead mixer configuration. One piece of the ZP film (1 × 1 in.2) was utilized for piezocatalytic degradation experiments to treat 40 mL of the dye solution. Photocatalysis-assisted piezocatalytic degradation was also performed in the presence of UV irradiation (the same condition as stated above).
2.4.3 Crude Oil Degradation
The floating ZP film was tested for crude oil degradation. Each sample had 50 mg of crude oil (Saudi Arabian Berri crude oil) dispersed on 2 mL DI water. One piece of the ZP film (circular, 16 mm diameter) was placed on top of the oil solution. After degradation, 5 mL of petroleum ether (Sigma-Aldrich) was used to dilute the remaining crude oil prior to the UV–Vis spectrometry measurement.
3 Results and Discussion
3.1 Two-Step Synthesis of ZnO NW Array on PDMS
In a typical hydrothermal synthesis, the substrate is deposited with a seed layer by various means prior to the growth of vertically aligned ZnO NW array. The structural stability of the NW array on the substrate is, thus, highly dependent on the adhesion between the seed layer and substrate. When a rigid inorganic substrate (e.g., glass, silicon) is used, the seed layer can be thermally treated at relatively high temperatures (often over 200 °C), which significantly improves the adhesion. However, this high-temperature treatment cannot be applied for polymeric substrates. Moreover, polymeric substrates are flexible and bendable, making it easy for the seed layer to be delaminated. Therefore, the NW array directly grown on a polymer substrate typically suffers from weak adhesion and is easy to be detached from the substrate. Our approach to improving the NWs’ adhesion to the polymer substrate is to utilize a two-step hydrothermal growth and secure the NW array by partially embedding the NWs in the polymer matrix.
3.2 Material Characterization
3.3 Structural Integrity of ZnO NW Array on PDMS
This unique configuration of the ZnO NW array on PDMS allows the immobilized film to be bent and stretched without undermining the structural integrity. Figure 5c, d shows a ZP film wrapped on a finger and stretched to more than 50% of the original length, respectively. If the NW array is deposited or directly grown onto the PDMS surface, such bending or stretching of the substrate may cause the delamination of the NW array. In the ZP film, the individual ZnO NWs are vertically oriented and strongly anchored onto PDMS; therefore, the NWs appear to be intact in spite of such harsh handling (see Fig. 5e). Finally, no apparent morphological change is observed (see Fig. 5f) in the ZP films that are immersed in the common solvents (e.g., water and alcohols) and subject to vigorous mixing conditions or various temperature fluctuations (60 °C for 12 h). We believe that this superior structural robustness of the ZnO NW array on PDMS will present a reliable immobilization platform and well accommodate the harsh environments for practical applications.
3.4 Photodegradation: Floatable Immobilized Photocatalytic Platform
Secondarily, another important factor limiting the degradation performance of immobilized photocatalytic systems is a slow mass transfer rate, i.e., a sluggish transport of the reactants and products to and from the catalyst surface. The photocatalytically generated oxidants, such as hydroxyl radicals and super oxide ions, are short-lived species and available only in the vicinity of the immobilizing substrate. Thus, the overall chemical reactions are governed by the transport rate of the pollutants to (or byproducts from) the substrate. Without an external field, this transport is dependent merely on diffusion, which is sluggish. Here, we investigated the effect of mass transfer on the degradability of the ZP film with secondary ZnO NW growth by implementing magnetic stirring (at 700 rpm). As shown in Fig. 7b, the degradation rate is significantly improved when magnetic stirring is employed. Apparent reaction rate constant k increases from 0.0018 min−1 (without stirring) to 0.0074 min−1 (with stirring). It can be concluded from these two sets of experiments that the enhancement of the mass transfer rate has a more significant impact on the degradability of the ZP film. The combination of the ZP film with the secondary growth and active stirring is favored in the photodegradation experiments.
3.5 Immobilized Platform for Piezocatalysis
3.6 Photocatalysis of Crude Oil by Immobilized Platform
It is worth mentioning that a quantitative comparison of the ZP film and other photodegradation techniques (e.g., slurry system, immobilized photoreactors, and floatable photocatalysts) would be highly desirable. However, it is often not feasible to quantitatively compare these methods because the outcomes are highly dependent on the light source and intensity, ratio of the photocatalyst to the pollutant, and water volume and some other experimental conditions. Therefore, more future works will be done to make the comparison by employing the similar operation conditions as other techniques.
In this study, we fabricated a new type of immobilized photocatalytic platform for photodegradation. This platform was proposed possessing a number of advantages: (1) avoided the contamination caused by photocatalyst re-suspension, (2) increased the utilization of the incident light, and (3) facilitated the mass transfer. According to the results, the floating ZP film exhibited a considerable degradability and reusability against MB dye. From the degradation mechanism, because the redox reaction only occurred at the interface of the solution and NWs, the longer exposed NW part resulted in a better photocatalytic performance. In addition, the ZP film, which moved energetically, also had a high degradability. In the experiment, we also investigated the piezocatalytic and photocatalytic properties of the ZP film. Meanwhile, a proper platform was proposed to effectively improve the piezocatalytic capability of the ZP film. Considering the practical challenge in treating floating pollutants, crude oil as a model pollutant was tested by the ZP film. The result confirmed that a slow but valid photocatalytic degradation process took place in the presence of the ZP film. For a better performance, a high-power light source needs to be introduced to accelerate the process. Sunlight irradiation and other types of pollutants need to be introduced to further evaluate the ZP film in future works.
We would like to express our gratitude to Dr. Rebecca Anthony in the Department of Mechanical Engineering of our college for the equipment and helpful discussion about the material characterization. We also thank Dr. Baokang Bi in the Department of Physics and Astronomy of our college for the support of instruments.
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