Spray Coated of ZnO Particles on ePA Nylon-Carbon Fiber Substrate and Its Photocatalytic Activity Evaluation

This work studies a simple deposition of ZnO particles on the ePA Nylon–Carbon Fiber substrate by sol-gel-spray-coating technique. The ZnO films were deposited on the substrate with deposition parameters such as suspension concentration, nozzle-to-substrate distance, nozzle size, coats, pump pressure, and deposition temperature. The ZnO gel suspension concentrations were varied (ZnO-0.06g/ml; ZnO-0.03g/ml), and the post-heated treatment was carried out at 120°C in vacuum condition. The temperature at 45°C during spray coating deposition was also applied to ZnO with the concentration of 0.03g/ml, which was noted as ZnO-0.03 TS. The films were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS), and Contact Angle (CA) to determine the structure, morphology, and hydrophilicity. The XRD pattern revealed that all samples have a hexagonal wurtzite structure. The SEM results showed that ZnO-0.06 relatively shows a dense surface, while ZnO-0.03 and ZnO-0.03 TS have a porous surface. The atomic ratio (Zn:O) revealed from the EDS measurement of the films was stoichiometric. The CA measurement showed that the film surfaces are hydrophilic. In addition, the photocatalytic activity was evaluated by the degradation of 2 mg/L methylene blue irradiated by sunlight for 180 mins—the photodegradation efficiencies were 88.37 %, 90.22 %, and 89,47 %, respectively.


Introduction
Generally, ZnO as a photocatalyst is developed as powder suspended in photocatalyst reactors, such as nanoparticles, nanowires, nanorods, nanosheets, and nanotubes.ZnO powder certainly possesses a large specific surface area and excellent photocatalytic performance.However, there is still a limitation to use, a requirement of stirring process during the reaction, and the powder has to be separated after the photocatalytic reaction.ZnO film deposited on a substrate could solve such limitations.The ZnO film could be easily dissolved in water and efficiently recycled.On the other hand, ZnO film as a photocatalyst in water treatment also has several limitations, such as a relatively small surface area for absorbing visible light or UV irradiation and a low contact area.Depositing the ZnO nanoparticles onto a modified substrate is one of the methods to overcome the film limitations via elevating the contact area between the pollutants; thus, the photocatalytic activity could also increase [1][2][3].
A modified substrate could be proposed by designing a specific morphology for the deposition.3D printer technology allows us to design and produce an object with shape and morphology as required, which has a large surface area [4,5].Carbon fiber-reinforced nylon-based material is available as a 3D printer filament.ePA Nylon-Carbon Fiber (polyamide of 80%, carbon of 20 %) is one of the filament materials with high strength, rigidity, abrasive resistance, and high dimensional stability.The heat distortion temperature is 155℃; thus, it is suitable for use as a substrate in the ZnO low deposition process [6].Before modifying the shape of the substrate, it is necessary to know whether ZnO particles can be appropriately deposited on the surface of the ePA Nylon-Carbon Fiber.Considering our previous result, ZnO nanoparticles have been successfully synthesized at 150℃ [7].Therefore, this work is a preliminary study of depositing ZnO particles on the ePA Nylon-Carbon Fiber by a sol-gelspray-coating technique.The ZnO films were spray-coated on the substrate by considering the deposition parameters, such as ZnO gel suspension concentration, nozzle-to-substrate distance, nozzle size, coats, pump pressure, and deposition temperature.

3D-Printed Substrates Production
Flat rectangular-shaped (7.5 cm × 2.5 cm × 0.5 mm) substrates were prepared by the Fused Deposition Modelling technique using ePA Nylon-Carbon Fiber as filament with a printing temperature of 80-90°C.The substrates building process involves heating, pushing, stacking, and cooling the filaments layer-by-layer (0.1 mm nozzle diameter) according to the required shape.

ZnO Nanoparticles Synthesis
The ZnO nanoparticle synthesis followed our previous work without modification [7].Firstly, ZnO nanoparticles were prepared by sol-gel method in reflux condition at 65 o C by adding 1 g Zn (CH3COO)2 2H2O (≥ 98%, Sigma Aldrich) as a precursor into 42 ml methanol as a solvent.The solution was then stirred until a transparent solution was formed.The transparent solution turned into a milky white solution after titration with 0.3 M NaOH due to the change in the pH level of the solution.The solution was continuously aged for 48 hours to produce gel precipitation.The ZnO gel precursor was then washed using methanol and n-Hexane (1:1) and subsequently centrifuged to collect the cleaned gel precipitation, and this route was repeated three times.The following reactions in equation (1-3) explain the ZnO gel precursor formation by sol-gel method [8].

ZnO Deposition
Following the sol-gel process, ZnO gel precursor was dissolved with methanol and became a gel precursor suspension.The gel precursor suspension was stirred using a magnetic stirrer until a homogenous and stable suspension was achieved.Before deposition, the substrates were cleaned with acetone and then methanol using an ultrasonic bath for 15 minutes consecutively.The next step is to place the gel precursor suspension into a bottle of airbrush sprayer.Next, the suspension was sprayed onto the substrates from left to right with a nozzle distance to the substrate of 5 cm, pump pressure of 30 psi, and nozzle size of 5 cm.After spraying the suspension towards the substrate, the precipitate was pre-heated at 100 o C for about 1 minute.In order to produce a thick, homogenous layer, the spraying and pre-heating steps were repeated for 22 times.Finally, a post-heating step was carried out at 120C in vacuum condition for 12 hours to crystallize ZnO nanoparticles.Some ZnO films were prepared using a similar steps with several deposition parameters as shown in Table 1. (

Characterizations and Photocatalytic Activity Evaluation
X-ray diffraction (XRD) measurements were performed to determine the samples crystal structure and the other crystalline-related parameters.The film morphology and elemental composition were examined using scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS).Then, the wetting properties of ZnO films were measured using contact angle (CA).Furthermore, the photocatalytic activity was evaluated by degrading 2 mg/L methylene blue under sun irradiation.The prepared ZnO films were immersed and soaked into the solution within a beaker glass in a dark condition for 48 hours, and each sample consisted of three pieces of ZnO film.The Irradiation process was done using sunlight for 180 minutes with 30-minute intervals to observe methylene blue degradation.The workflow of this experiment is shown in Figure 1.

XRD Analysis
Figure 2 shows the diffraction patterns for all ZnO films and ePA Nylon-Carbon Fiber.The XRD patterns show that the peaks correspond to a standard COD 9008877, meaning the ZnO films possess a hexagonal wurtzite structure [9].The ZnO-0.06 intensities of (100), (101), and (110) are higher, whereby the ZnO gel suspension concentration is higher than the other samples.2. The interplanar spacing (dhkl) was calculated according to Bragg's law [10].Since the films have a hexagonal structure (=≠c), the lattice parameters were calculated using equation ( 5) that is the formula for the hexagonal crystal systems (Equation 5) [11].

    
The Degree of Crystallinity (DOC) was calculated using Equation ( 6) by determining the area of the crystalline phase (Ac) and amorphous phase (Aa) from the under the XRD spectrum curve [12].The lattice constants and DOC of the ZnO films are shown in Table 3.The crystallite size of each film is about 80-90 nm.Similar to the obtained FWHM, the crystallite size has no significant difference.It means that deposition parameters such as ZnO gel suspension concentration and deposition temperature do not affect the crystallite size.Furthermore, the lattice parameters "" and "c" are nearest to those of pure hexagonal wurtzite of ZnO.This indicates that the obtained ZnO films have a great definite structure.Regarding the Degree of Crystallinity or DOC, the DOC of ZnO-0.06,ZnO-0.03, and ZnO-0.03TS are 76.80%, 46.51%, and 46.87%, respectively.ZnO-0.06 has the highest DOC related to the denser film morphology, so the entire surface of the substrate was covered.The other two, which have a lower DOC, were associated with the amorphous structure that was identified as originating from the ePA substrate.

SEM/EDS CA Studies
In our previous result, the ZnO particles formed possessed spherical shape and size were less than 50 nm [7]. Figure 3 shows the morphology of the ZnO films.ZnO-0.06 has a dense surface.Meanwhile, the ZnO-0.03 and ZnO-0.03TS have a porous surface.ZnO-0.03 has an average pore size of 2.038μm, the largest pore is 8.898μm, and the smallest pore is 0.628μm.ZnO-0.03TS has an average pore size of 1.672μm; the largest pore reaches 3.616μm, and the smallest pore is 0.314μm.The smaller pore size of ZnO-0.03TS may be due to the deposition temperature.On the other hand, the formation of a dense surface on ZnO-0.06 is strongly influenced by the ZnO gel suspension concentration parameter, whereby ZnO-0.06 has a more significant ZnO gel suspension concentration than the other samples.
The contact angle for ZnO-0.06,ZnO-0.03, and ZnO-0.03TS are 32.79ᵒ;54.37ᵒ; 51.12ᵒ, respectively.Thus, all film surfaces are hydrophilic since ZnO is a naturally hydrophilic compound due to the presence of hydroxyl groups on its surface [13].The topography of ZnO film morphology influences the wetting properties of its surface.ZnO-0.06, which has a denser surface, shows a more hydrophilic surface than the other films.
The chemical analysis from EDS measurement (Table 4.) reveals that the films are only composed of stoichiometry of Zn and O, whereby close to 1:1.An extra of ZnO elements probably come from Zn interstitial (Zni) or oxygen vacancies on the surface, both of which are natural defects in ZnO.Further characterization is needed to observe the existence of such native defects.It also confirms that the spray-coating techniques and annealing treatment during deposition produce a ZnO hexagonal wurtzite structure.

Photocatalytic Activity Evaluation
The work efficiency of the catalyst films in degrading methylene blue is calculated using the photodegradation efficiency as shown in equation ( 7) [14].whereby ZnO-0.03TS, which shows a porous surface, has a higher efficiency of 81.20% at the first 90 minutes of irradiation.All samples have almost the same efficiency for 180 minutes of irradiation, 88.37%, 90.22%, and 89.47% for ZnO-0.06,ZnO-0.03, and ZnO 0.03 TS, respectively.

Figure 1 .
Figure 1.Workflow of deposition process of ZnO on ePA NCF substrate.

14th 4 Figure 2 .
Figure 2. XRD patterns of the ZnO films compared to ePA Nylon-Carbon Fiber

In Figure 4 ,
the photodegradation efficiency from the sun at minute 180 reached 26.36%.The UV radiation from the sun plays a role in degrading methylene blue because it has high enough energy to break the methylene blue bond.All films generally show almost similar photodegradation for 180 minutes of irradiation.In the first 30 minutes, each film (ZnO-0.06,ZnO-0.03, and ZnO-0.03TS) can degrade with the efficiency of 51.94%, 57.14%, and 59.39%, respectively.ZnO-0.03TS slightly outperforms ZnO-0.03 and ZnO-0.06.In the 60 minutes, ZnO-0.03TS is more efficient in degrading methylene blue than the others, with an efficiency of 70.68%.This situation is potentially due to the porous surface of the catalyst film, whereby ZnO-0.03TS has a porous surface and smaller average diameter.However, at 180 minutes of irradiation, all samples show the same efficiency, around 88 -90 %.

Figure 4 .
Figure 4. Photodegradation of methylene blue using ZnO film on ePA NCF substrate

Table 1 .
ZnO Films prepared on ePA NCF substrate

Table 2 .
The crystallite size of the ZnO films ePA NCF substrate 14th International Symposium on Modern Optics and Its Applications

Table 3 .
Lattice constants and Degree of Crystallinity of the ZnO films ePA NCF substrate.

Table 4 .
Chemical analysis of ZnO on ePA NCF substrate