Physical and Photocatalytic Properties of CeO₂/ZnO/ZnAl₂O₄ Ternary Nanocomposite Prepared by Co-precipitation Method

Abstract

ZnAl₂O₄ spinel nanoparticles and CeO₂/ZnO/ZnAl₂O₄ 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 ZnAl₂O₄ spinel nanoparticles was 3.220 eV. When 1.0 mmol Ce(NO₃)₃•6H₂O 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 CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites was investigated. The CeO₂/ZnO/ZnAl₂O₄ nanocomposite prepared with 1.0 mmol Ce(NO₃)₃^•6H₂O 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 CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites exhibited better photocatalytic performance than pure ZnAl₂O₄ spinel nanoparticles and 100% degradation of aqueous MB solution was achieved within 60 min when using the CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposite photocatalyst synthesized with 1.0 mmol Ce(NO₃)₃^•6H₂O.

Keywords:
nanocomposite; chemical synthesis; optical properties; photocatalytic properties

1. 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 methods¹. 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.

Table 1
Summary of photocatalysts used in dye degradation processes.

Researcher	Catalyst	Dye	Light source	Irradiation time (min)	%Degradation
Suwanboon et al.2	ZnO	MB	UV black light (18 W)	180	100
		MB	Visible light	180	98
Akika et al.4	Ni0.2Cu0.8Al2O4	Congo red (CR)	Solar light (750 W/m2)	180	90.6
Chaudhary5	ZnAl2O4	CR	Mercury lamp (500 W)	80	97.2
		Methyl orange (MO)	Mercury lamp (500 W)	80	96.9
		MB	Mercury lamp (500 W)	80	90.2
Zhang et al.6	25%ZnO/ZnAl2O4	MO	Xenon lamp (150 W)	60	98.7
Sumathi et al.7	ZnAl1.98Ce0.02O4	MB	UV light (32 W)	25	99.3
			Tungsten lamp (500 W)	240	45.6
Suwanboon et al.8	ZnO	MB	UV black light (15 W)	90	100
Suwanboon et al.9	Zn0.95La0.05O	MB	UV black light (18 W)	150	98
	Zn0.95Mg0.05O	MB	UV black light (18 W)	120	100
Khan et al.10	p-CeO2	MB	Tungsten lamp (400 W)	360	30
Klubnuan et al.11	ZnO	MB	UV black light (18 W)	75	100
Lv et al.12	1%CeO2/ZnO	MB	Mercury lamp (500 W)	110	100
Zhang et al.13	ZnO/NiO/ZnAl2O4	MO	Xenon lamp (150 W)	60	97.3
Balamurugan et al.14	CdO/Al2O3	Metanil yellow (MY)	Visible light	75	82.1
Chen et al.15	ZnO/Zn foil	MO	Mercury lamp (300 W)	240	100
Kirankumar et al.16	ZnAl1.99Bi0.01O4	MB	Xenon lamp (250 W)	240	100
Saleh et al.17	MWCNT/TiO2	MO	UV lamp (40 W)	100	93
Saravanan et al.18	PANI/ZnO	MO	Projection lamp (250 W)	180	98.3
		MB	Projection lamp (250 W)	180	99.2
Saravanan et al.19	95%ZnO/5%CuO	MO	Projection lamp (250 W)	120	97.2
		MB	Projection lamp (250 W)	120	87.7
Saravanan et al.20	Hg:ZnO	MO	Projection lamp (250 W)	120	90
		MB	Projection lamp (250 W)	60	100
Saleh et al.21	MWCNT/WO3	Rhodamine B (RhB)	Solar	150	100
Saravanan et al.22	ZnO/Ag/CdO	MB	Projection lamp (250 W)	90	98.3
Saravanan et al.23	ZnO/Ag/Mn2O3	Textile effluent	Visible light	180	98
Zhao et al.24	Au-Ag@Bi4Ti3O12	RhB	Xenon lamp	120	95.5
		RhB	Mercury lamp	120	99.9
		RhB	Halogen-tungsten lamp	120	49.6
Yan et al.25	Bi@BiOCl	RhB	Xenon lamp	30	91.1

Recently, AB₂O₄ spinel oxides, in which A is a divalent metal ion and B is a trivalent metal ion, have gained attention from many research groups^4,26-31. This interest has led to their applications in water splitting, gas sensing, transparent conducting materials and photocatalysis. Zinc aluminate (ZnAl₂O₄), a spinel oxide with a wide band gap of about 3.8 eV, is an important member of the AB₂O₄ spinel oxides. Applications of ZnAl₂O₄ include dosimetry³², opto-electronic devices³³, gas sensing³⁴, ceramic support³⁵ and photocatalysis⁵. The unique properties of ZnAl₂O₄ spinel nanoparticles depend on various parameters and researchers have improved these properties by doping with divalent^36-38 or trivalent^39-41 metal ions, and loading with secondary metal oxide powders^6,41. ZnAl₂O₄ spinel nanoparticles have been synthesized by vibrational ball milling⁴², hydrothermal synthesis⁴³, sol-gel synthesis³⁹, combustion⁴¹ and co-precipitation⁴⁴. 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 CeO₂/ZnO/ZnAl₂O₄ 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 CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites was characterized to determine its dependence on structural, morphological and optical properties of the photocatalyst.

2. Experimental

2.1 Material

Zinc nitrate tetrahydrate (Zn(NO₃)₂^•4H₂O, Emsure®, Germany), aluminium nitrate nonahydrate (Al(NO₃)₃^•9H₂O, Sigma-Aldrich, Germany), cerium (III) nitrate hexahydrate (Ce(NO₃)₃^•6H₂O, Aldrich, China), potassium hydroxide (KOH, Emsure®, Germany), and methylene blue (C₁₆H₁₈ClN₃S, Emsure®, Germany) were purchased and used without further purification.

2.2 Synthesis of ZnAl₂O₄ spinel nanoparticles

ZnAl₂O₄ spinel nanoparticles were synthesized by a co-precipitation method. Following stoichiometric calculations, 0.005 mol Zn(NO₃)₂^•4H₂O and 0.01 mol Al(NO₃)₃^•9H₂O were weighed and dissolved in 100 mL distilled water for 15 min under moderate stirring by a magnetic bar. Then, 0.04 mol KOH dissolved in 100 mL distilled water was added dropwise into the prepared mixture solution of Zn²⁺ and Al³⁺ ions. The white precipitates obtained were continuously stirred and heated at 70^°C for 1 h. After the reaction was terminated and cooled to room temperature, the precipitates were washed three times with 200 mL distilled water, filtered and dried at 80^°C for 2 h. Finally, the as-synthesized powders were calcined in air at 800^°C for 1 h and the calcined powders were later characterized by various techniques.

2.3 Synthesis of CeO₂/ZnO/ZnAl₂O₄ nanocomposites

To enable investigation of the effect of CeO₂ and ZnO loadings on the ZnAl₂O₄ nanocomposites, 0.2, 0.4, 0.6, 0.8 and 1.0 mmol Ce(NO₃)₃^•6H₂O were introduced separately into 100 mL mixture solutions prepared, as described in section 2.2, from 0.005 mol Zn(NO₃)₂^•4H₂O and 0.01 mol Al(NO₃)₃^•9H₂O. The precursor solutions were then precipitated with 0.04 mol KOH dissolved in 100 mL distilled water. The synthesis then proceeded in the same way as the synthesis of pure ZnAl₂O₄ spinel nanoparticles.

2.4 Characterization

Thermal gravimetric analysis (TGA) was used to investigate the thermal behavior of as-synthesized ZnAl₂O₄ 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 ZnAl₂O₄ spinel nanoparticles and secondary phases (CeO₂ 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).

2.5 Photocatalytic test

The photocatalytic activity of ZnAl₂O₄ spinel nanoparticles and CeO₂/ZnO/ZnAl₂O₄ 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.

3. Results and Discussion

3.1 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 ZnAl₂O₄ 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 water⁴⁵. The second weight loss of about 27%, between 180 and 500^°C, derived from the elimination of structural water⁸. The third weight loss of about 3%, between 500 and 750^°C, was attributed to the removal of nitrates⁴⁶. No weight loss occurred above 750^°C. Therefore, before characterization, the powders were calcined at 800^°C in air for 1 h.

Figure 1
Thermal behavior of representative sample of as-synthesized ZnAl₂O₄ powder.

3.2 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 (CeO₂) appeared at 2θ diffraction angles of 28.58^°, 33.18^° and 79.37^° (JCPDS 34-0394). The diffraction peaks of zinc oxide (ZnO) appeared at 2θ diffraction angles of 31.80^°, 34.47^°, 36.30^°, 47.58^°, 56.59^°, 62.95^°, 67.95^° and 69.36^° (JCPDS 36-1451) and the diffraction peaks of zinc aluminate (ZnAl₂O₄) appeared at 2θ diffraction angles of 31.60^°, 36.97^°, 44.78^°, 55.56^°, 59.72^° and 65.58^° (JCPDS 05-0669). A pure ZnAl₂O₄ spinel phase formed without a ZnO secondary phase when Ce(NO₃)₃^•6H₂O was not present in the precursor solution. Therefore, the chemical reactions that occurred in this process can be expressed as reactions (1-3)⁴⁷:

Figure 2
XRD patterns of CeO₂/ZnO/ZnAl₂O₄ nanocomposites prepared at different Ce(NO₃)₃^•6H₂O concentrations.

$$Zn{\left(N{O}_3\right)}_{2(aq)}+2Al{\left(N{O}_3\right)}_{3(aq)}+8KO{H}_{(aq)}\to Zn{\left(OH\right)}_{2(s)}+2Al{\left(OH\right)}_{3(s)}+8KN{O}_{3(aq)}$$ (1)

$$Zn{\left(OH\right)}_{2(s)}+2Al{\left(OH\right)}_{3(s)}+8KN{O}_{3(aq)}\stackrel{\Delta }{\to }{(ZnO\cdot A{l}_2{O}_3)}_{amorphous}+8KN{O}_{3(aq)}+4H_2{O}_{(l)}$$ (2)

$${(ZnO\cdot A{l}_2{O}_3)}_{amorphous}\stackrel{\Delta }{\to }ZnA{l}_2{O}_{4(s)}$$ (3)

On the other hand, when Ce(NO₃)₃^•6H₂O was added to the precursor solution, Ce³⁺ ions could not form a substitutional solid solution as ZnAl_2-xCe_xO₄. They were unable to do so because the ionic radius of the Ce³⁺ ion (101 pm) is significantly larger than that of the Al³⁺ ion (53 pm). Consequently, the Ce³⁺ ions could not replace the Al³⁺ ions at Al sites in the ZnAl₂O₄ spinel structure. According to the Hume-Rothery rule⁹, an extensive substitutional solid solution occurs only if the relative difference between the ionic radius of Al³⁺ and Ce³⁺ 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 Al³⁺ ions with Ce³⁺ ions could not occur. However, the Ce³⁺ ions could react with hydroxide ions to form CeO₂ according to reactions (4-6)^48,49:

$$Ce{\left(N{O}_3\right)}_3\cdot 6H_2{O}_{(s)}+H_2{O}_{(l)}\to C{e}^{3+}{}_{(aq)}+3N{O}_3{{}^{-}}_{(aq)}$$ (4)

$$C{e}^{3+}{}_{(aq)}+3O{H}^{-}{}_{(aq)}\to Ce{\left(OH\right)}_3{}_{\left(s\right)}$$ (5)

$$Ce{\left(OH\right)}_3{}_{\left(s\right)}\stackrel{\Delta }{\to }Ce{O}_{2(s)}+H_2{O}_{(g)}+0.5H_{2(g)}$$ (6)

Simultaneously, ZnO could be generated according to reactions (7-10)⁵⁰:

$$Zn{\left(N{O}_3\right)}_2\cdot 4H_2{O}_{(s)}+H_2{O}_{(l)}\to Z{n}^{2+}{}_{(aq)}+2N{O}_3{{}^{-}}_{(aq)}$$ (7)

$$Z{n}^{2+}{}_{(aq)}+4O{H}^{-}{}_{(aq)}\leftrightarrow {\left[Zn{\left(OH\right)}_4\right]}^{2-}{}_{(aq)}$$ (8)

$${\left[Zn{\left(OH\right)}_4\right]}^{2-}{}_{(aq)}\leftrightarrow Zn{\left(OH\right)}_{2(s)}+2O{H}^{-}{}_{\left(aq\right)}$$ (9)

$$Zn{\left(OH\right)}_{2(s)}\stackrel{\Delta }{\to }Zn{O}_{(s)}+H_2{O}_{(g)}$$ (10)

Therefore, the products formed as CeO₂/ZnO/ZnAl₂O₄ 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 CeO₂ and ZnO increased as a function of Ce(NO₃)₃^•6H₂O concentration. Therefore, it could be summarized that amounts of CeO₂ and ZnO formed increasingly.

In this study, chemical composition was determined by XRF technique. When the content of Ce(NO₃)₃^•6H₂O in the reactions was 0.2, 0.4, 0.6, 0.8 and 1.0 mmol, the CeO₂ content of the products was about 0.92, 2.15, 3.52, 4.30 and 6.06%, respectively, and the ZnO contents were 11.68, 14.41, 16.81, 17.97 and 18.90%. The amount of CeO₂ and ZnO in the product increased as a function of Ce(NO₃)₃^•6H₂O concentration. This was in good agreement with XRD results.

3.3 Morphological study

As presented in Figure 3, the morphology of pure ZnAl₂O₄ 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 = 11⁵⁰. In this study, ZnO particles formed as rod structures along the c-axis, which had the growth velocity in the following order: $v_{\left(0001\right)}>v_{\left(\overline{1}010\right)}>v_{\left(\overline{1}011\right)}>v_{\left(000\overline{1}\right)}.$ At the same time, fluffy particles of CeO₂ formed on the surfaces of ZnAl₂O₄ 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 decreased⁵² and a more stable system resulted.

Figure 3
SEM images of CeO₂/ZnO/ZnAl₂O₄ nanocomposites prepared at different Ce(NO₃)₃^•6H₂O concentrations.

3.4 Optical properties

Figure 4 shows the UV-vis diffuse reflectance spectra of ZnAl₂O₄ spinel nanoparticles and CeO₂/ZnO/ZnAl₂O₄ nanocomposites. The absorption edge of CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites shifted towards longer wavelengths or lower energies compared to the absorption edge of pure ZnAl₂O₄ spinel nanoparticles. This shift towards longer wavelengths occurred as a function of the Ce³⁺ 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 energy².

Figure 4
Diffuse reflectance spectra of CeO₂/ZnO/ZnAl₂O₄ nanocomposites prepared at different Ce(NO₃)₃^•6H₂O concentrations.

The optical band gap of samples was evaluated from Tauc plots via Equation 11 ²:

$${(\alpha h\upsilon )}^2=A(h\upsilon -E_g)$$ (11)

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.

Figure 5
Plots of (αhυ)² versus hυ of CeO₂/ZnO/ZnAl₂O₄ nanocomposites prepared at different Ce(NO₃)₃^•6H₂O concentrations.

Table 2
Principal characterization data of ZnAl₂O₄ spinel nanoparticles and CeO₂/ZnO/ZnAl₂O₄ nanocomposites.

Ce(NO3)3•6H2O (mmol)	Phase	Shape	Eg (eV)	Eu (eV)	SA (m2/g)
0	ZnAl2O4	Irregularly agglomerated sphere	3.220	0.120	37.78
0.2	CeO2/ZnO/ZnAl2O4	Rod + Irregularly agglomerated sphere	3.192	0.282	27.98
0.4	CeO2/ZnO/ZnAl2O4	Rod + Irregularly agglomerated sphere	3.184	0.284	40.64
0.6	CeO2/ZnO/ZnAl2O4	Rod + Irregularly agglomerated sphere	3.180	0.287	41.45
0.8	CeO2/ZnO/ZnAl2O4	Rod + Irregularly agglomerated sphere	3.178	0.294	39.45
1.0	CeO2/ZnO/ZnAl2O4	Rod + Irregularly agglomerated sphere	3.170	0.299	37.06

The optical band gap of ZnAl₂O₄ spinel nanoparticles obtained from this experiment was narrower than the optical band gap of bulk ZnAl₂O₄ 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 resulted in the reduction in optical band gap of ZnAl₂O₄ spinel nanoparticles in a previous work⁵⁰.

As Ce³⁺ ions were introduced into the precursor solution, secondary phases of CeO₂ and ZnO formed. The value of the optical band gap slightly decreased as a function of the concentration of Ce³⁺ 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 CeO₂ and they found that reductions in the optical band gap were due to the presence of Ce³⁺ 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 CeO₂/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 CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites and the augmentation in CeO₂ and ZnO secondary phases was in good agreement with the XRD results (Figure 2). The increases in CeO₂ and ZnO contents generated more oxygen vacancies in the ternary nanocomposite systems and as a result the optical band gap reduced. To confirm the presence of oxygen vacancies in the samples, the Urbatch energy or band tail energy (E_u) was taken into account. The Urbatch energy was expressed as Equation 12 ²:

$\text{α}={\text{α}}_0\left(\frac{\text{E}}{{\text{E}}_{\text{u}}}\right)$ (12)

where α is the absorption constant, α₀ 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(NO₃)₃^•6H₂O in the solution was greater. This behavior was attributed to increments of oxygen vacancy due to increased Ce(NO₃)₃^•6H₂O concentration and the resultant reductions in optical band gap value.

Figure 6
Plots of ln(α) versus E of CeO₂/ZnO/ZnAl₂O₄ nanocomposites prepared at different Ce(NO₃)₃^•6H₂O concentrations.

3.5 Photocatalytic activity

In this study, an aqueous MB solution was used as a dye model. The degradation of MB molecules over ZnAl₂O₄ spinel nanoparticles and CeO₂/ZnO/ZnAl₂O₄ 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 CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites prepared with 0.2 mmol Ce(NO₃)₃^•6H₂O within 180 min, whereas they degraded by about 70% over pure ZnAl₂O₄ spinel nanoparticles at the same irradiation time.

Figure 7
Temporal change in absorbance of (a) ZnAl₂O₄ spinel nanoparticles and (b) CeO₂/ZnO/ZnAl₂O₄ nanocomposites prepared by 0.2 mmol Ce(NO₃)₃^•6H₂O.

The degradation of aqueous MB solution over all photocatalysts was determined using Equation 13 ²:

$\text{\%degradation}=\frac{{\text{C}}_0-{\text{C}}_{\text{t}}}{{\text{C}}_0}\times 100=\frac{{\text{A}}_0-{\text{A}}_{\text{t}}}{{\text{A}}_0}\times 100$ (13)

where A₀ is the initial absorbance of aqueous MB solution, A_t is the absorbance of aqueous MB solution at time interval t, C₀ 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 CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites than over ZnAl₂O₄ spinel nanoparticles (Figure 8). After exposure to UV radiation for 1 h, MB molecules were completely degraded over the CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites prepared with 1.0 mmol Ce(NO₃)₃^•6H₂O, whereas only about 54% of MB molecules were degraded over pure ZnAl₂O₄ spinel nanoparticles. When irradiation time was increased to 2 h, the MB molecules were completely degraded over the CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites prepared with 0.6, 0.8 and 1.0 mmol Ce(NO₃)₃^•6H₂O. At the same irradiation time, the degree of degradation over pure ZnAl₂O₄ spinel nanoparticles increased from 54% after 1 h to 64%. After irradiation for 3 h, the MB molecules were completely degraded over all the CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites, whereas only 73% of the MB molecules were degraded over pure ZnAl₂O₄ spinel nanoparticles. Besides irradiation time, photocatalytic activity is influenced by various other parameters, including the optical band gap, defects concentration and specific surface area.

Figure 8
Percentage MB degradation over CeO₂/ZnO/ZnAl₂O₄ nanocomposites prepared at different Ce(NO₃)₃^•6H₂O concentrations.

The photocatalysts with narrower optical band gaps exhibited higher photocatalytic activity. Since electrons in valence bands could be easily excited to conduction bands, holes (h⁺) were left in the valence bands. Photoexcited electrons then reacted with adsorbed O₂ and holes reacted with H₂O at the surface of the photocatalysts to generate superoxide (^•${\text{O}}_2^{-}$) radicals and hydroxyl (^•OH) radicals, respectively. These reactive species (^•${\text{O}}_2^{-}$, ^•OH, h⁺) were mostly responsible for the degradation of the MB in solution. The CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites with higher Ce³⁺ concentrations could also adsorb more UV radiation (Figure 4). This result was in good agreement with the higher degradation of aqueous MB solution.

Although the CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites exhibited narrower band gaps than ZnAl₂O₄ spinel nanoparticles, the recombination rate of photoexcited electron-hole pairs was retarded because the heterostructure of the CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposite could promote the transfer of photoexcited electrons via the interfaces of the heterostructure, as occurred in the heterostructure of ZnO/NiO/ZnAl₂O₄¹³. This transfer of photoexcited electrons contributed to the superior photocatalytic activity of CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites compared to pure ZnAl₂O₄ spinel nanoparticles.

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 reduced¹⁴. Moreover, oxygen vacancies can interact with adsorbed O₂ at the surface of photocatalysts, trapping photoexcited electrons to generate ^•${\text{O}}_2^{-}$ radicals. As a result, the degradation of aqueous MB solution improved as a function of the concentration of oxygen vacancies².

Photocatalytic activity was also affected by particle shape². 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 molecules¹⁵. ZnO rod structures consist of a positively charged Zn-(0001) terminate and a negatively charged O-($000\overline{1}$) terminate that created an internal electric field between the positive and negative planes by spontaneous polarization¹⁵. Therefore, under the influence of this internal electric field, photoexcited electrons transferred to the positive (0001) plane and holes transferred to the negative ($000\overline{1}$) plane. This phenomenon can improve the reduction reaction at the positive (0001) plane and the oxidation reaction at the negative ($000\overline{1}$) 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 accelerated¹⁶. Considering the specific surface areas listed in Table 2, the specific surface area of the CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposite prepared at 0.2 mmol Ce(NO₃)₃^•6H₂O was smaller than that of the pure ZnAl₂O₄ spinel nanoparticle and the specific surface area of the ternary nanocomposite prepared at 1.0 mmol Ce(NO₃)₃^•6H₂O was equal to the specific surface area of the pure ZnAl₂O₄ spinel nanoparticle. However, the photocatalytic activity of both those CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites was greater than the photocatalytic activity of pure ZnAl₂O₄ spinel nanoparticles. Therefore, the specific surface areas of the CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites did not significantly influence their photocatalytic activity in this study.

The positions of the valence band potential (E_VB) and conduction band potential (E_CB) were calculated from Equations 14 and 15 ⁵⁶:

$$E_{VB}=X-E^e+0.5E_g$$ (14)

$$E_{CB}=E_{VB}-E_g$$ (15)

where X is the geometric mean of the Mulliken’s electronegativity of CeO₂, ZnO, ZnAl₂O_4, E^e is the energy of free electrons on the hydrogen scale (4.50 eV for a normal hydrogen electrode (NHE)) and E_g is the optical band gap energy of CeO₂, ZnO, ZnAl₂O₄. The X, E^e, E_g, E_VB and E_CB values of CeO₂, ZnO, ZnAl₂O₄ are listed in 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.

Table 3
X, E_g, E_VB and E_CB of CeO₂, ZnO and ZnAl₂O₄.

Constituents	X	Eg (eV)	EVB (eV)	ECB (eV)
CeO2	5.5756	2.88	0.370	-2.510
ZnO	5.7956	3.22	0.310	-2.890
ZnAl2O4	5.4958	3.20	0.620	-2.600

Figure 9
Schematic diagram of electron-hole separation and transport mechanism under UV irradiation of CeO₂/ZnO/ZnAl₂O₄ nanocomposite photocatalyst.

$$\begin{array}{l}Ce{O}_2/ZnO/ZnA{l}_2{O}_4+h\upsilon \to \\ Ce{O}_2/ZnO/ZnA{l}_2{O}_4(e^{-})+Ce{O}_2/ZnO/ZnA{l}_2{O}_4(h^{+})\end{array}$$ (16)

$$O_2+e^{-}\to {}^{·}O{}_2^{-}$$ (17)

${}^{·}O{}_2^{-}+H_{2}O\to H{O}_2+O{H}^{-}$ (18)

$${}^{·}H{O}_2+H_{2}O\to H_2{O}_2+{}^{·}OH$$ (19)

$H_2{O}_2+e^{-}\to O{H}^{-}+{}^{·}OH$ (20)

$h^{+}+O{H}^{-}\to {}^{·}OH$ (21)

$h^{+}+H2O\to {}^{·}OH+H^{+}$ (22)

$MB+{}^{·}OH/{}^{·}O{}_2^{-}\to degradationproduct$ (23)

$MB+h^{+}\to degradationproduct$ (24)

4. Conclusion

CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites were successfully synthesized by a facile co-precipitation method in which the addition of Ce³⁺ ions to the precursor solution disturbed the reaction equilibrium of spinel formation. SEM revealed the different particle shapes of CeO₂ (fluffy particles), ZnO (rod-like) and ZnAl₂O₄ (irregular sponge-like). The optical band gap of CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites slightly shifted to a longer wavelength compared with ZnAl₂O₄ spinel nanoparticles. The defect concentration of oxygen vacancies increased as a function of Ce³⁺ ion concentration. The photocatalytic activity of the CeO₂/ZnO/ZnAl₂O₄ ternary nanocomposites depended significantly on the particle shape of the loading, the optical band gap and the defect concentration.

5. 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.

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Publication Dates

• Publication in this collection
  20 Mar 2020
• Date of issue
  2020

History

• Received
  11 Nov 2019
• Reviewed
  20 Jan 2020
• Accepted
  24 Jan 2020