SILICA SUPPORTED WO3/CU2O HETEROSTRUCTURED NANOPARTICLES FOR PHOTOCATALYTIC DEGRADATION OF HORMONES

In this study, a WO3/Cu2O based heterojunction was synthesized via a facile solvothermal method. The asprepared nanocomposite was characterized by XRD, SEM, and UV-vis spectroscopy. The photocatalytic activity of the as-prepared heterojunction was evaluated by the means of estradiol (E2) and 17αethinylestradiol (EE2) hormone degradation under UV LED illumination wavelength at λmax~ 365 nm. The degradation results showed that the prepared photocatalyst was able to achieve considerable photoactivity owing to the intrinsic heterojunction charge transfer mechanism. Photocatalytic degradation rates of 27 and 35 % were achieved for the E2 and EE2 hormones, respectively.


INTRODUCTION
Over the last few decades, extensive use of chemicals and its production on industrial scale have resulted in serious environmental and ecological issues, mainly due to the contamination of aqueous resources vital for sustaining the life cycle [1]. Among the commonly discharged chemical affluents, pharmaceuticals like antibiotics and hormonally active agents, also classified as endocrine-disrupting chemicals (EDC) are of particular importance due to relatively high bio toxicity, even at minuscule concentrations [2]. The elimination of these chemical toxicants including EDC is imperative since the exposure of such chemicals are associated with several chronic diseases such as cancer, ADHD, learning disability, brain development and retarded sexual development. Therefore, it is imperative to develop sustainable water treatment systems capable of eliminating these toxic chemicals with low cost and energy requirements. So far, various strategies have been implemented to address this issue, including adsorption, biological degradation, electrochemical oxidation (EO), membrane technology, sonolysis, and photocatalysis, each having specific advantages and disadvantages [3,4]. For example, frequent replacement of electrodes and membranes due to fouling increases the operational costs of EO and membrane filtration techniques, respectively [5]. Photocatalysis, in this regard, is currently considered the most sustainable method and has been employed in various applications such as pollutants removal, fuel generation, pathogen elimination, and nitrogen fixation [6][7][8]. A typical photocatalysis process requires only a suitable semiconductor photocatalyst, reaction medium and solar light irradiation as an energy input. The process is initiated by the absorption of photons having energy equal to or greater than the bandgap of the photocatalyst. This results in the excitation of the electrons in the valence band (VB) to the conduction band (CB) and subsequent generation of electron-hole pairs. These excited electrons and holes then participate in the reduction and oxidation of the water molecules and the adsorbed chemical species on the surface of the photocatalyst. However, currently developed photocatalysts suffer from several critical issues which limit their photoactivity, i.e., the wide bandgap of commonly used photocatalysts results in poor visible light harvesting and futile recombination of the generated photoexcited charge carriers [8]. To address these fundamental drawbacks, several modification strategies have been implemented so far to-date such as surface engineering, sensitization, defect engineering, doping, and heterojunction [9,10]. Out of these, the construction of heterojunction has compelling advantages of addressing the previously mentioned issues associated with the single component photocatalysts, namely, the suppression of charge recombination and extension of light absorption to visible range. Numerous heterojunctions systems have been investigated so far with promising photocatalytic activities [11]. Among several dual photocatalytic systems, construction of WO3 and Cu2O based heterojunction presents an attractive approach primarily due to favorable to band position.

Synthesis of WO3/Cu2O nanocrystals
In the first step, WO3 was prepared from starting solution of peroxotungstic acid ([WO2(O2)H2O]·nH2O, PTA) which was obtained by dissolving 1 g of tungsten powder in 20 mL of H2O2. Tungsten powder dissolved quickly under intense stirring and at elevated temperature (55-60°C). Next, 5 g of silica was added into the prepared starting solution, and by a Teflon stick thoroughly mixed to the resulting paste. This paste was dried at 45 °C for 24 hours. The as-prepared powder was crushed in a mortar with a pestle. Subsequently, this powder was annealed for 2 hours at 550 °C in a muffle furnace. The obtained WO3 powder prepared by this synthesis method was used in the next step for Cu2O decoration as follows: 300 mg of previously prepared WO3 powder and 8 mL of ammonia solution with 75 mg of CuSO4 were mixed with a magnetic stirrer. After a short time, 150 mg of glucose was added with continuous stirring, followed by adjusting pH of the mixture with NaOH.

Characterization methods
Phase structure of all samples was investigated by XRD diffractometer MiniFlex600 (Japan, RIGAKU) with a Co-Kα X-ray source (λ = 1.7903 Å) in the diffraction angle range of 5-90° 2θ. Crystallite sizes were estimated according to the Scherrer's equation as follows: where d is the diameter and the shape factor K is 0.89, since no preferential orientation was observed, λ is the wavelength (CoKα 1,2) = 0.179 nm, the angle θ is the full width half maximum FWHM of the corresponding highest diffraction peak, i.e., (002) and (111) line for WO3 and Cu2O, respectively, and β is the line broadening at half the maximum intensity corrected for the instrumental response. Sample analysis was performed by NovaNanoSEM 450 microscope (The Netherland, FEI company). Microscopic images were taken using an ETD (topographic contrast) and CBS (material contrast) detector accelerated at 5 kV and 15 kV voltages.

Photocatalytic experiment
The photocatalytic adsorption and degradation experiment was performed using WO3/Cu2O as a photocatalyst in an aqueous mixture of hormone solution containing E2 and EE2 hormones. Each photocatalytic test was conducted by transferring 5 mg of the powdered catalyst into a beaker containing 10 mL of hormone solution with each hormone at a concentration of 0.2 mg/L and a total solution concentration of 0.8 mg/L. Separate beakers were used to evaluate the hormones adsorption rate under constant magnetic stirring at 450 rpm. Additionally, a separate beaker containing solution only was kept as a reference to calculate the removal percentage from the initial concentration. Results were obtained from high-performance liquid chromatography (HPLC) using calibration after running each sample twice.

HPLC Method
HPLC analysis was performed on a Dionex UltiMate 3000 Series equipped with a diode array detector (Thermo Fisher Scientific, Germany) according to the previously reported technique [12]. The concentration of hormones was calculated from the results of the 200 nm performed test (concentration of calibration standards 0.20 -0.02 mg/L; Data were recorded and processed in Chromeleon 7.2 software (Thermo Fisher Scientific) [13].

RESULTS AND DISCUSSION
The crystalline phase structures of the WO3/Cu2O heterojunction are shown in Figure 1. For reference, peroxotungstic acid (PTA) and silica-supported WO3 diffractograms are also given. The XRD pattern of the PTA exhibits intense peaks in the range of small Braggs angles, typically associated with layered compounds. The as-prepared silica supported WO3 exhibits several characteristic peaks, indexed to (002)  The morphology of the as-prepared samples was investigated using SEM as shown in Figure 2. The SEM images of silica-supported PTA precursor exhibits course morphology with small grain size, which is consistent with poor crystallinity as evidenced by the XRD results (Figure 2A-B). For the annealed samples, WO3 and WO3/Cu2O, an increase in the temperature led to smaller grain size and a large number of Cu2O particles were observed on the surface of the WO3 embedded with silica. No agglomeration of Cu2O particles was noticed. The electronic structure of semiconductor which plays a crucial role in the photoactivity is closely related to its bandgap and its alignment. The as-prepared WO3/Cu2O sample was also characterized by UV-vis analysis to evaluate the optical bandgap value via Kubelka-Munk equation, as previously reported [14].
[ ℎ ] = (ℎ − ) where α is the optical absorption coefficient, hν stands for quantized photon energy, A is the constant of proportionality, and the value p indicates the electronic transition type. The value of the exponent, p, was determined by plotting a [F(R)·hν] 1/2 vs hν graph and calculating the best fit, which turned out to be ½, implying an indirect allowed transition. Finally, by plotting a [F(R)·hν] 1/2 vs hν graph and extrapolation of the graph slope to F(R) → 0, the optical bandgap values were obtained as shown in Figure 4.

Figure 4
Reflectance spectrum and the calculated bandgap energies (inset) of the silica-supported WO3/Cu2O composite.
The photocatalytic activity of the as-prepared nanocomposite was evaluated via E2 and EE2 hormones degradation having an initial concentration of 0.2 mg/L under UV light irradiation (Roithner LaserTechnik, UVLUX 340-HL-3) with a maxima wavelength of 343 nm (corresponding to Eg ~ 3.6 eV). From Figure 5, it is obvious that the photocatalytic activity gradually increased with increasing time, reaching 26 and 35 % after 1 h duration, for E2 and EE2, respectively. However, it should be noted that the reference sample without light irradiation also recorded some hormonal removal due to adsorption, with contribution reaching as high as 5.4 (E2) and 7.8 % (EE2), within 1 h duration. This high degree of hormonal adsorption could be ascribed due to the presence of a large number of silica particles, possessing highly porous structure and large surface area.

CONCLUSION
A silica supported WO3/Cu2O heterojunction was prepared via a facile solvothermal method employing peroxotungstic acid and copper sulfate as precursors. XRD and SEM analysis showed the formation of a suitable heterojunction was successful. The as-prepared WO3/Cu2O showed significant photocatalytic activity towards estrogenic hormonal degradation under UV light irradiation. The enhanced photoactivity was attributed due to the advantageous charge transfer route of heterojunction. This work may provide insights into developing new highly efficient materials for environmental pollutants degradation.