Physical and Photocatalytic Properties of CeO 2 /ZnO/ZnAl 2 O 4 Ternary Nanocomposite Prepared by Co-precipitation Method

2020 ZnAl 2 O 4 spinel nanoparticles and CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites were synthesized by a co-precipitation method. The structural, morphological, optical properties and chemical compositions of the products were analyzed respectively by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS) and X-ray fluorescence (XRF) spectroscopy. The optical band gap of ZnAl 2 O 4 spinel nanoparticles was 3.220 eV. When 1.0 mmol Ce(NO 3 ) 3 •6H 2 O was added to the synthesis reaction, the optical band gap of the obtained ternary nanocomposite was 3.170 eV. The influence of phase composition, optical band gap, oxygen vacancy and specific surface area on photocatalytic activity over CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites was investigated. The CeO 2 /ZnO/ZnAl 2 O 4 nanocomposite prepared with 1.0 mmol Ce(NO 3 ) 3 •6H 2 O showed the lowest recombination rate of photoexcited electron-hole pairs, the narrowest optical band gap (3.170 eV) and the highest oxygen vacancy concentration or highest Urbatch energy (0.299 eV). These parameters produced the best photocatalytic activity toward methylene blue (MB) under UV irradiation. The CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites exhibited better photocatalytic performance than pure ZnAl 2 O 4 spinel nanoparticles and 100% degradation of aqueous MB solution was achieved within 60 min when using the CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposite photocatalyst synthesized with 1.0 mmol Ce(NO 3 ) 3 •6H 2


Introduction
In recent years, the paper, textile, leather and cosmetic industries have developed rapidly worldwide. These industries attract customers by coloring their products with a range of synthetic organic dyes. When the use of synthetic organic dyes increases, the amount of wastewater produced also increases. To reduce pollution, factories must remove synthetic organic dyes in wastewater before they are discharged into natural waterways. The elimination of synthetic organic dyes from wastewater is accomplished by biological, coalescence and adsorption methods 1 . However, since these methods cannot completely get rid of the synthetic organic dyes in a single step, further treatment is necessary. The photocatalytic process is another popular method of eliminating dyes. Removing the remaining dye in wastewater by photocatalytic degradation has several advantages; for example, photocatalytic conditions are mild, the use of chemical reagents is reduced, and synthetic organic dyes can be degraded to small non-toxic molecules 2,3 . In photocatalytic degradation, the photocatalyst used is very important to the process. Therefore, the choice of photocatalyst is the primary consideration and a summary of the many different photocatalysts that have been used is presented in Table 1.
Recently, AB 2 O 4 spinel oxides, in which A is a divalent metal ion and B is a trivalent metal ion, have gained attention from many research groups 4,[26][27][28][29][30][31] . This interest has led to their applications in water splitting, gas sensing, transparent conducting materials and photocatalysis. Zinc aluminate (ZnAl 2 O 4 ), a spinel oxide with a wide band gap of about 3.8 eV, is an important member of the AB 2 O 4 spinel oxides. Applications of ZnAl 2 O 4 include dosimetry 32 , opto-electronic devices 33 , gas sensing 34 , ceramic support 35 and photocatalysis 5 . The unique properties of ZnAl 2 O 4 spinel nanoparticles depend on various parameters and researchers have improved these properties by doping with divalent [36][37][38] or trivalent [39][40][41] metal ions, and loading with secondary metal oxide powders 6,41 . ZnAl 2 O 4 spinel nanoparticles have been synthesized by vibrational ball milling 42 , hydrothermal synthesis 43 , sol-gel synthesis 39 , combustion 41 and co-precipitation 44 . The advantages of the co-precipitation method include low temperature preparation, high purity products, simple procedure and easy scalability 7,8 .
The present work proposes a co-precipitation synthesis of CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites using KOH solution as the precipitating agent. This process has not, to our knowledge, been reported previously. The synthesized nanocomposites were used in the photocatalytic degradation of MB. The chosen dye model enabled the assessment of the suitability of the photocatalyst for applications in several industries. Photocatalytic activity over the CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites was characterized to determine its dependence on structural, morphological and optical properties of the photocatalyst.

Characterization
Thermal gravimetric analysis (TGA) was used to investigate the thermal behavior of as-synthesized ZnAl 2 O 4 spinel nanoparticles. The TGA thermogram was recorded by thermogravimetric analyzer (TGA 7, Perkin Elmer) under nitrogen gas. X-ray diffraction (XRD) was used to analyze ZnAl 2 O 4 spinel nanoparticles and secondary phases (CeO 2 and ZnO). XRD patterns were recorded by powder X-ray diffractometer (XRD, X′Pert MPD, Philips). X-ray fluorescence spectrometry (XRF, Zetium, PANalytical) was used to analyze the chemical composition of synthesized products. Scanning electron microscope (SEM) was used to observe the morphology of samples. The secondary electron images (SEI) were obtained by scanning electron microscope (SEM, Quanta 400, FEI). Brunauer-Emmett-Teller (BET) surface area analysis was used to determine the specific surface area (SA) of powders. The adsorption isotherm was measured by BET surface area analyzer (Autosorb 1MP, Quantachrome). Diffuse reflection spectroscopy (DRS) was used to study the optical behavior and evaluate the optical band gap of powders. Absorbance spectra were measured by UV-Vis spectrophotometer (UV-Vis 2450, Shimadzu). To evaluate remaining MB concentration, the absorbance of MB solutions was measured by UV-Vis spectroscopy and temporal changes were recorded by UV-Vis spectrophotometer (UV-Vis Lambda 25, Perkin Elmer).

Photocatalytic test
The photocatalytic activity of ZnAl 2 O 4 spinel nanoparticles and CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites was evaluated by the degradation of aqueous MB solution under UV irradiation (3 parallel blacklight tubes, 15 W). In the typical procedure, 150 mg of photocatalyst were introduced into a 250 mL beaker containing 150 mL of 1×10 -5 M MB solution. Before irradiation, the suspension was continuously stirred with a magnetic bar for 30 min in darkness to attain adsorption-desorption equilibrium between the MB and photocatalyst. The suspension was then irradiated and 3 mL of the suspension were withdrawn every 30 min and centrifuged at 4000 rpm for 5 min to separate the photocatalyst. The absorbance of the supernatant was recorded between 400 and 800 nm to determine the remaining MB and calculate the percentage of MB degradation.

Thermal analysis
In the experimental procedure, reagents were mixed in distilled water and reacted with each other to form a new compound. The thermal decomposition of representative as-synthesized powders was analyzed to determine an appropriate calcination temperature to obtain a pure ZnAl 2 O 4 phase. The thermal analysis proceeded from room temperature to 1,000°C at a heating rate of 10°C/min under nitrogen gas.
Thermal decomposition comprised three steps ( Figure 1). The first weight loss of about 9%, between room temperature and 180°C, was due to the removal of physically adsorbed molecular water 45 . The second weight loss of about 27%, between 180 and 500°C, derived from the elimination of structural water 8 . The third weight loss of about 3%, between 500 and 750°C, was attributed to the removal of nitrates 46 . No weight loss occurred above 750°C. Therefore, before characterization, the powders were calcined at 800°C in air for 1 h.

X-ray diffraction study
The phase formation of nanocomposites was identified from X-ray diffraction patterns of calcined samples ( Figure 2). The diffraction peaks of cerium dioxide (CeO 2 ) On the other hand, when Ce(NO 3 ) 3 •6H 2 O was added to the precursor solution, Ce 3+ ions could not form a substitutional solid solution as ZnAl 2-x Ce x O 4 . They were unable to do so because the ionic radius of the Ce 3+ ion (101 pm) is significantly larger than that of the Al 3+ ion (53 pm). Consequently, the Ce 3+ ions could not replace the Al 3+ ions at Al sites in the ZnAl 2 O 4 spinel structure. According to the Hume-Rothery rule 9 , an extensive substitutional solid solution occurs only if the relative difference between the ionic radius of Al 3+ and Ce 3+ is less than 15%. If the difference in ionic radius is more than 15%, a limited substitutional solid solution occurs. In this study, the difference was about 90%. Therefore, the replacement of Al 3+ ions with Ce 3+ ions could not occur. However, the Ce 3+ ions could react with hydroxide ions to form CeO 2 according to reactions (4−6) 48 Simultaneously, ZnO could be generated according to reactions (7-10) 50 Therefore, the products formed as CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites when they were calcined at 800°C in air for 1 h. In this study, it was observed that the intensity of the principal peaks of CeO 2 and ZnO increased as a function of Ce(NO 3 ) 3 •6H 2 O concentration. Therefore, it could be summarized that amounts of CeO 2 and ZnO formed increasingly.
In this study, chemical composition was determined by XRF technique. When the content of Ce(NO 3 ) 3

Morphological study
As presented in Figure 3, the morphology of pure ZnAl 2 O 4 spinel nanoparticles was an irregular sponge-like structure made up of agglomerated spherical nanoparticles 7,39,51 but ZnO particles formed a facet structure in a strongly alkaline solution at pH = 11 50 . In this study, ZnO particles formed as rod structures along the c-axis, which had the growth velocity in the following order: ( ) ( ) ( ) ( ) At the same time, fluffy particles of CeO 2 formed on the surfaces of ZnAl 2 O 4 spinel nanoparticles. In agglomerations, many small particles are attracted to one another through chemical bonds and physical forces at interfaces. However, some crystals form a faceted structure due to the different surface energies present at different crystal facets. If particles agglomerated to form a large cluster or a faceted structure, overall surface energy decreased 52 and a more stable system resulted. Figure 4 shows the UV-vis diffuse reflectance spectra of ZnAl 2 O 4 spinel nanoparticles and CeO 2 /ZnO/ZnAl 2 O 4 nanocomposites. The absorption edge of CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites shifted towards longer wavelengths or lower energies compared to the absorption edge of pure ZnAl 2 O 4 spinel nanoparticles. This shift towards longer wavelengths occurred as a function of the Ce 3+ ion concentration in the precursor solution and absorption edges shifted to longer wavelengths as the optical band gaps of samples narrowed. Therefore, electrons in valence bands were excited to conduction bands by consuming less photon energy 2 .

Optical properties
The optical band gap of samples was evaluated from Tauc plots via Equation 11 2 : where α is an absorption coefficient, hυ is the photon energy (h is the Planck's constant and υ is the photon frequency) and E g is the optical band gap. The plots of (αhυ) 2 versus hυ for all samples are presented in Figure 5. To obtain the optical band gap, the linear region was extrapolated to (αhυ) 2 = 0. The values of the obtained optical band gaps are given in Table 2.
The optical band gap of ZnAl 2 O 4 spinel nanoparticles obtained from this experiment was narrower than the optical band gap of bulk ZnAl 2 O 4 spinel (3.8 eV) 53 . This may have been due to the formation of a subband between valence and conduction bands caused by the formation of localized energy states of defects such as oxygen vacancies, which the concentration of Ce 3+ ions in the precursor solution. The reduction in the optical band gap could be attributed to increments in the secondary phases. Khan et al. 10 studied the optical properties of CeO 2 and they found that reductions in the optical band gap were due to the presence of Ce 3+ ions at grain boundaries, which generated localized energy states from oxygen vacancies within the forbidden band. Consequently, electrons in valence bands could be excited to localized energy states with lower photon energy. In addition, Suwanboon et al. 11 found that the optical band gap of ZnO nanoparticles decreased due to the presence of defects in the ZnO nanoparticles. Band tail energy was created within the forbidden band of ZnO and this event resulted in a reduction in the optical band gap of ZnO nanoparticles. Reports by other research groups 12,54,55 indicated that the optical band gap of CeO 2 /ZnO nanocomposites decreased when the mole ratio of Ce to Zn was increased. The reductions were attributed to an increase in the concentration of oxygen vacancies.
In this study, the products formed as CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites and the augmentation in CeO 2 and ZnO secondary phases was in good agreement with the XRD results ( Figure 2). The increases in CeO 2 and ZnO contents generated more oxygen vacancies in the ternary nanocomposite systems where α is the absorption constant, α 0 is the constant, E is the photon energy and E u is the Urbatch energy. Urbatch energy was determined from the reciprocal of the slope in the linear region of the plot of ln(α) versus E ( Figure 6). The values of obtained Urbatch energy were presented in Table 2. Urbatch energy was greater when the amount of Ce(NO 3 ) 3 •6H 2 O in the solution was greater. This behavior was attributed to increments of oxygen vacancy due to increased Ce(NO 3 ) 3 •6H 2 O concentration and the resultant reductions in optical band gap value.

Photocatalytic activity
In this study, an aqueous MB solution was used as a dye model. The degradation of MB molecules over ZnAl 2 O 4 spinel nanoparticles and CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites was observed under UV irradiation.
The strongest intensity of the absorbance peak of the aqueous MB solution centered at a wavelength of 664 nm decreased as a function of irradiation time (Figure 7) as MB contents in the solution were reduced by the photocatalytic reaction. In this study, the MB molecules completely degraded over CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites prepared with 0.2 mmol Ce(NO 3 ) 3 •6H 2 O within 180 min, whereas they degraded by about 70% over pure ZnAl 2 O 4 spinel nanoparticles at the same irradiation time.
The degradation of aqueous MB solution over all photocatalysts was determined using Equation 13 2 : where A 0 is the initial absorbance of aqueous MB solution, A t is the absorbance of aqueous MB solution at time interval t, C 0 is the initial concentration of aqueous MB solution and C t is the concentration of aqueous MB solution at time interval t. The degradation of aqueous MB solution was higher over CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites than over ZnAl 2 O 4 spinel nanoparticles ( Figure 8). After exposure to UV radiation for 1 h, MB molecules were completely degraded over the CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites prepared with 1.0 mmol Ce(NO 3 ) 3 •6H 2 O, whereas only about 54% of MB molecules were degraded over pure ZnAl 2 O 4 spinel nanoparticles. When irradiation time was increased to 2 h, the MB molecules were completely degraded over the CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites prepared with 0.6, 0.8 and 1.0 mmol Ce(NO 3 ) 3 •6H 2 O. At the same irradiation time, the degree of degradation over pure ZnAl 2 O 4 spinel nanoparticles increased from 54% after 1 h to 64%. After irradiation for 3 h, the MB molecules were completely degraded over all the CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites, whereas only 73% of the MB molecules were degraded over pure ZnAl 2 O 4 spinel nanoparticles. Besides irradiation time, photocatalytic activity is influenced by various other parameters, including the optical band gap, defects concentration and specific surface area.
The photocatalysts with narrower optical band gaps exhibited higher photocatalytic activity. Since electrons in    Oxygen vacancy is another important parameter that can enhance photocatalytic activity for dye degradation. During the photocatalytic process, oxygen vacancies accept electrons and recombination rates of photoexcited electron-hole pairs are reduced 14 . Moreover, oxygen vacancies can interact with adsorbed O 2 at the surface of photocatalysts, trapping photoexcited electrons to generate • 2 O − radicals. As a result, the degradation of aqueous MB solution improved as a function of the concentration of oxygen vacancies 2 .
Photocatalytic activity was also affected by particle shape 2 . ZnO particles with a rod-like structure could enhance photocatalytic activity. The degradation of the aqueous MB solution was improved as a function of {0001} surfaces in which the {0001} facets are strongly reactive in the degradation of MB molecules 15 . ZnO rod structures consist of a positively charged Zn-(0001) terminate and a negatively charged O-(000 1) terminate that created an internal electric field between the positive and negative planes by spontaneous polarization 15 . Therefore, under the influence of this internal electric field, photoexcited electrons transferred to the positive (0001) plane and holes transferred to the negative ( 000 1 ) plane. This phenomenon can improve the reduction reaction at the positive (0001) plane and the oxidation reaction at the negative (000 1) plane, so the formation of ZnO in a rod structure can promote the degradation of aqueous MB solution.
The specific surface area of a photocatalyst plays a crucial role in the photocatalytic process. A higher specific surface area provided more active sites; therefore, the photocatalytic reactions involved were accelerated 16 . Considering the specific surface areas listed in Table 2 Table 3 and a schematic diagram of the electron-hole separation and transport mechanism is presented in Figure 9. To conclude, the possible mechanism of photocatalytic MB degradation over this system irradiated with UV radiation (λ = 315-400 nm) can be proposed as Equations 16-24 57 .

Conclusion
CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites were successfully synthesized by a facile co-precipitation method in which the addition of Ce 3+ ions to the precursor solution disturbed the reaction equilibrium of spinel formation.  58 3.20 0.620 -2.600 SEM revealed the different particle shapes of CeO 2 (fluffy particles), ZnO (rod-like) and ZnAl 2 O 4 (irregular sponge-like). The optical band gap of CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites slightly shifted to a longer wavelength compared with ZnAl 2 O 4 spinel nanoparticles. The defect concentration of oxygen vacancies increased as a function of Ce 3+ ion concentration. The photocatalytic activity of the CeO 2 /ZnO/ZnAl 2 O 4 ternary nanocomposites depended significantly on the particle shape of the loading, the optical band gap and the defect concentration.

Acknowledgement
This work was supported by the budget revenue of Prince of Songkla University under the contract number SCI6202036S. The authors would like to thank the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Ministry of Higher Education, Science, Research and Innovation, Thailand. The authors would like to acknowledge Mr. Thomas Duncan Coyne for assistance with the English.