Surfactant-Free Microwave-Assisted Synthesis of Fe-Doped ZnO Nanostars as Photocatalyst for Degradation of Tropaeolin O in Water under Visible Light

Iron-doped zinc oxide nanostar was synthesized by the microwave-assisted surfactant-free hydrolysis method. The as-synthesized Fe-doped ZnO nanostars catalyst was fully characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (XRD), and diffuse reflectance UV-vis spectroscopy (UV-DRA). The photocatalytic activity of the photocatalyst was investigated for the photocatalytic degradation of Tropaeolin O under visible light irradiation. It is observed that the doping of Fe ions enhances the absorption of the visible light and thus the photocatalytic degradation rate of Tropaeolin O would increase. Despite the Taguchi orthogonal experimental designmethod,thephotocatalyticconversioncouldbeachievedat99.8%intheFe-dopedZnOcatalyzedphotodegradationreactionundertheoptimalreactionconditionsofcatalystloading(30mg),temperature(60 ∘ C), light distance (0 cm), initial pH (pH = 9), and irradiation time (3h). The Fe-doped ZnO photocatalyst can also be easily recovered and directly reused for eight cycles with over 70% conversion.


Introduction
In the well-developed centuries, textile, paper, and other industries continuously discharge wastewater contaminated with organic dye, definitely harmful to human life and aquatic system. In the recent years, nanostructured metal oxide semiconductor materials played an important role in degradation of organic dyes to maintain an environmental sustainability [1][2][3][4][5]. Zinc oxide (ZnO) is considered as a promising semiconductor compared to titanium dioxide (TiO 2 ) because of the similar band gap at 3.37 eV, lower cost, and larger exciton binding energy of 60 mV at room temperature [6][7][8]. However, there are some limitations due to the lower solar utilization and higher charge carrier recombination rate. Some metal nanoparticles such as gold (Au) [9], platinum (Pt) [10], and silver (Ag) [11] were supported on ZnO to enhance the photocatalytic activity on the degradation of organic dye. However, these supported ZnO photocatalysts were only excited by UV light with a limited wavelength shorter than 350 nm. In order to increase the solar utilization and expand the light absorption wavelength to visible light region, some literatures investigated the modification of semiconductors by an incorporation of transition metal (Mn, Cu, Ni, or Co) as dopant into ZnO [12][13][14][15][16]. It is understandable that the introduction of the dopant would narrow the band gap since the delocalized d-energy level insert in between the conduction band and the valence band in which less energy (i.e., longer wavelength) is required to generate a hole [17][18][19][20]. Furthermore, semiconductors would greatly inhibit the recombination of photoinduced charge carrier (electron-hole pair) and thus enhance the photocatalytic activity [18,21].
Various synthetic protocols for synthesis of different nanostructures of ZnO and Fe-doped ZnO have been widely reported by literature including direct calcination of metal salt [21], hydrothermal synthesis [22], sol-gel method [23], and simply hydrolysis followed by calcination [24]. Among all synthetic methods, hydrothermal synthesis is relatively simple and operates under a relatively low temperature.
However, it takes a relatively long reaction time. Microwave irradiation is widely used as heating source which provides a fast and simple synthetic route to heat up the reaction system homogeneously.
In this present study, we reported the synthesis of iron-(Fe-) doped ZnO nanostars by a surfactant-free microwaveassisted hydrolysis towards the photocatalytic degradation of Tropaeolin O under visible light. The as-synthesized Fedoped ZnO nanostars were fully characterized by SEM, TEM, EDX, XRD, and UV-DRA. Meanwhile, the optimal photocatalytic conditions for degradation of Tropaeolin O under visible light catalyzed by Fe-doped ZnO were determined by Taguchi analysis which has not been reported so far.
Stepwise approach has been extensively used; however, it is relatively time consuming and difficult to estimate the optimized conditions as some of the reaction conditions simultaneously affect the photocatalytic degradation. Taguchi analysis is a powerful, cost effective, and relatively time saving optimization approach which consists of orthogonal array experimental design and range analysis [25][26][27][28]. The relationship between photocatalytic activity and the reaction conditions including catalyst loading, temperature, light distance, initial pH, and irradiation time was being evaluated in this present study. The photocatalytic durability of the Fedoped ZnO nanostars catalyst was also examined.

Catalyst Preparation.
Fe-doped ZnO nanostars catalyst was synthesized by a surfactant-free microwave-assisted hydrolysis without any thermal calcination treatment. Similar synthetic protocol has been reported by Han and coworkers [22] with some modifications. Zinc acetate dihydrate (330 mg) and iron(III) nitrate nonahydrate (3.6 mg) were completely dissolved in milli-Q water (150 mL) to form a transparent mixed metal solution followed by adding aqueous ammonia water (0.16 M). The solution was then heated using a microwave synthesis system (Micro SYNTH) at a fixed temperature of 80 ∘ C with a constant microwave power of 400 W for 30 minutes. The precipitate was then washed by milli-Q water and ethanol three times and finally dried at 80 ∘ C.

Catalyst Characterization.
The size and morphology of the photocatalyst were characterized using a Hitachi S-4800 field emission scanning electron microscope (SEM) which operated at 5 eV and was equipped with energy dispersive spectrometry (EDX) with Horiba EMAX EDS detectors. A JEOL Model JEM-2100F field emission transmission electron microscope (STEM) operated at 200 eV was also used to characterize the photocatalyst. Powder X-ray diffraction (XRD) pattern was collected using a Rigaku SmartLab Xray Diffractometer equipped with CuK ( = 1.54056, 45 kV, 200 mA) radiation with 2 ranging from 10 ∘ to 90 ∘ with a step size of 0.02 ∘ . The optical absorption spectrum was obtained by Varian Cary 4000 UV-vis spectrophotometer in diffuse reflectance mode.

Photocatalytic Activity Study.
Tropaeolin O was employed as a model of organic dye for the evaluation of the photocatalytic activity of the as-synthesized Fe-doped ZnO nanostars catalyst. For each experimental run, the Fe-doped ZnO nanostars catalyst was suspended in aqueous Tropaeolin O solution (20 mL, 10 mg L −1 ) with constant vigorous stirring. The photocatalytic reaction was conducted at predesigned temperature under visible light using the compact fluorescent lamp. The photocatalytic decomposition of Tropaeolin O was monitored by measuring the absorbance of the filtered solution at a wavelength of 430 nm using UVvis spectrophotometer in liquid cuvette configuration with milli-Q water as reference. The photocatalytic conversion ( ) of Tropaeolin O can be calculated according to where 0 is the initial concentration of Tropaeolin O solution and is the concentration of Tropaeolin O solution at time .

Taguchi Analysis for the Fe-Doped ZnO Catalyzed Photocatalytic Degradation of Tropaeolin O.
In this photocatalytic study, the optimal reaction conditions for the photocatalytic degradation of Tropaeolin O were designed according to the orthogonal array experimental design and the results were evaluated through the range analysis. Five main processing factors including the catalyst loading (factor ), temperature (factor ), light distance (factor ), initial pH (factor ), and irradiation time (factor ) were designed in four different levels individually as displayed in Table 1.
According to the OA 16 experimental design, OA 16 matrix model contains five factors with four levels as distributed in Table 2. In principle, each column of the matrix represents a factor affecting the photocatalytic degradation while each row indicates one experimental run in a combination of specific factors at different level.
The mean ( ) values for the factor ( = , , , , and ) at level ( = 1, 2, 3, and 4) were simply calculated using the photocatalytic conversions ( ) of Tropaeolin O. The level of each factor that gave the highest demonstrates that particular level is classified as the optimal level which has a greater influence on the photocatalytic degradation of  Tropaeolin O. The value at different levels for each factor can be calculated as follows: Furthermore, the largest range value of the factor indicates that the particular factor is the most significant towards the photocatalytic degradation. It is simply calculated by the difference between maximum and minimum value according to where is the mean value of level ( = 1, 2, 3, and 4) of factor .

Characterization of Fe-Doped ZnO Nanostars Photocatalyst.
The SEM micrograph as displayed in Figure 1 shows that the as-synthesized Fe-doped ZnO photocatalyst exhibits a star-like morphology with the average particular size of 432.5 ± 5.6 nm. The elemental analysis conducted by EDX shows that the atomic Zn-to-Fe ratio is found to be 99.56 : 0.44. TEM analysis (Figure 2(a)) confirms that the Fe-doped ZnO photocatalyst is exhibiting a star-like morphology with the average size of 300.5 nm. As depicted in HR-TEM (Figure 2  The Fe-doped ZnO nanostars photocatalyst was characterized by XRD as shown in Figure 3. The XRD pattern for Fe-doped ZnO photocatalyst exhibits a high degree of crystallinity and matches well with the standard pattern of hexagonal phased zinc oxide with space group P63mc (ICDD card number 00-036-1451). The valence oxidation state of the Fe ions present is proposed to be +3 since iron(III) nitrate was used as precursor and Fe(III) is the most stable oxidation state. As suggested by EDX and XRD analysis, it is confirmed that Fe(III) ions are successfully doped into ZnO lattice without altering the original crystal structure of ZnO. The average crystalline size is found to be 12.7 nm. The lattice constants for the Fe-doped ZnO photocatalyst are found to be = 3.25Å, = 3.25Å, and = 5.21Å. Figure 4 shows the UV-vis diffuse reflectance spectra of the Fe-doped ZnO and undoped ZnO. The introduction of Fe ions into ZnO crystal lattice exhibits a decrease of the reflectance in the visible region which indicates that the Fe ions doping may enhance the absorption of visible light and thus the Fe-doped ZnO photocatalyst is suitable for an effective photocatalytic degradation of dye under visible light irradiation.

Taguchi Analysis for the Fe-Doped ZnO Catalyzed Photocatalytic Degradation of Tropaeolin O.
Sixteen different experimental runs were carried out according to OA 16 matrix. The result as tabulated in Table 3 shows the photocatalytic degradation of Tropaeolin O for each experimental run. The conversion ranged from 24.2% to 99.1%. These data are taken as the original data and used in calculation of values. The highest value for each factor is assigned as the optimal level of reaction condition for the photocatalytic degradation of Tropaeolin O. The highest value for each factor as shown in Table 4 is obvious defined in a combination of 4 4 1 3 4 as the catalyst loading is 30.0 mg (62.1), temperature is 60 ∘ C (56.6), light distance is 0 cm (78.0), initial pH Journal of Nanomaterials 5   light distance (45.6). The light distance (factor ) gives the largest range value which implies that the photocatalytic degradation changes significantly with slight changing of the distance between visible light source and the reaction flask. A significant change of the initial pH of the aqueous Tropaeolin O solution induces slight changing of the photocatalytic conversion due to the smallest range value obtained. It is clear from Figure 6 that there is a gradual increase of photocatalytic conversion with the increase of catalyst loading from 7.5 mg to 22.5 mg. It may due to the availably of more active sites on the photocatalyst surface to generate hydroxyl radicals and enhance the photodegradation of the Tropaeolin O. Further increase of catalyst loading to 30 mg did not enhance the photocatalytic conversion significantly as amounts of active sites were already saturated with the limited amount of Tropaeolin O present. The photocatalytic conversion is found to be enhanced with the temperature increased from 30 ∘ C to 60 ∘ C as observed in Figure 7. The increase in temperature at 60 ∘ C would increase the kinetic energy of the Tropaeolin O which leads to a higher diffusion rate and increases the collision between the Tropaeolin O and Fe-doped ZnO nanostars for faster degradation. In contrast, the increase in distance between the visible light source and the reaction flask would decrease the photocatalytic conversion. The highest photocatalytic conversion was observed with zero light distance (Figure 8), since the shortening of the distance provides a higher light intensity for more efficient electron transfer from valence band to conduction band of the photocatalyst. For pH, the highest photocatalytic conversion is obtained at pH 9 ( Figure 9). Owing to the amphoteric property of the photocatalyst, the initial pH is an important factor governing the kinetics of photodegradation on the catalyst surface. It affects the surface charge property of the photocatalyst. At higher pH, the present of excess hydroxide (OH − ) facilitates the generation of hydroxyl radicals. However, when the pH was further increased to 11, a significant decrease in activity was observed. At such high pH, dissolution of ZnO is observed which is not favorable for the overall activity. For the reaction time factor, it can be observed that the time required for complete degradation of Tropaeolin O requires at least three hours ( Figure 10

Confirmative Experiment.
The UV-vis absorption spectra as depicted in Figure 11 record the photocatalytic degradation of Tropaeolin O by Fe-doped ZnO nanostars in a function of time. Tropaeolin O exhibits a maximum adsorption band at wavelength max = 430 nm. The absorption peak is gradually decreased and finally disappeared, indicating the Tropaeolin O is degraded with time. As shown in Figure 12,   one indication to evaluate the catalyst. In this study, a simple approach of direct reuse of photocatalyst without any further washing steps is employed. The reusability study on the Fedoped ZnO nanostars catalyst for the photocatalytic degradation of Tropaeolin O under optimal conditions was examined. Referring to Figure 13, the Fe-doped ZnO photocatalyst gave 99.8% conversion in the first cycle; however, the conversion gradually decreased to 72.5% at the eighth cycle.

Conclusions
Fe-doped ZnO nanostars photocatalyst was synthesized successfully by the microwave-assisted surfactant-free hydrolysis. The as-synthesized Fe-doped ZnO catalyst was fully characterized by SEM, TEM, EDX, XRD, and UV-DRA. The introduction of Fe ions successfully increases the absorption of visible light and facilitates the photocatalytic degradation of Tropaeolin O. Through the Taguchi analysis and range analysis, the photocatalytic conversion was achieved with 99.8% under optimal reaction conditions of catalyst loading (30 mg), temperature (60 ∘ C), light distance (0 cm), initial pH (pH = 9), and irradiation time (3 h). The light distance gave the highest range value which implies that the photocatalytic degradation changes rapidly with changing of the distance between the visible light source and the reaction flask. Furthermore, the Fe-doped ZnO photocatalyst can also be directly reused for six cycles with over 80% conversion.