Microwave-assisted synthesis of Ni-doped europium hydroxide for photocatalytic degradation of 4-nitrophenol

Abstract

Microwave-assisted synthesis method was used to prepare europium hydroxide (Eu(OH)₃) and different percentages of 1, 5, and 10 % nickel-doped Eu(OH)₃ (Ni–Eu(OH)₃) nanorods (NRs). X-ray diffraction study showed a hexagonal phase with an average crystallite size in the range of 21 – 35 nm for Eu(OH)₃ and Ni–Eu(OH)₃ NRs. FT-IR and Raman studies also confirmed the synthesis of Eu(OH)₃ and Ni–Eu(OH)₃. The synthesized materials showed rod-like morphology with an average length and diameter between 27 – 50 nm and 8 – 13 nm, respectively. The band gap energies of Ni–Eu(OH)₃ NRs were reduced (4.06 – 3.50 eV), which indicates that the doping of Ni²⁺ ions has influenced the band gap energy of Eu(OH)₃. The PL study exhibited PL quenching with Ni doping. The photocatalytic degradation of 4-nitrophenol (4-NP) by the synthesized materials under UV light irradiation was investigated, in which 10 % Ni–Eu(OH)₃ NRs showed the best response. A kinetic study was also conducted which shows pseudo-first-order kinetics. Based on this, Ni–Eu(OH)₃ NRs have shown a potential to be a UV-light active material for photocatalysis.

Highlights

• •Microwave-assisted fabrication of Eu(OH)₃ and Ni-doped Eu(OH)₃.
• •Doping influenced the structural, optical, and morphological properties of Eu(OH)₃.
• •Band gap energy of Eu(OH)₃ was reduced with Ni-doping.
• •Photoluminescence peak intensity was quenched.
• •Improved photocatalytic degradation of 4-nitrophenol using Ni-doped Eu(OH)₃.

1. Introduction

In general, nanomaterials show excellent optical, electronic, magnetic, and mechanical properties owing to their unique small size, surface, and quantum effects. These materials have been widely applied in catalysis [[1], [2], [3]], environment [4], biomedicine [5], energy [6], etc. Their size, crystal form, and morphology determine the physical and chemical properties of nanomaterials, in which materials with the same chemical composition may show different properties if the materials have differences in their characteristics [[7], [8], [9], [10], [11]]. These materials could be 0D, 1D, 2D, or 3D [12].

Europium hydroxide (Eu(OH)₃) exhibit electronic, optical, catalytic, and chemical properties that arise from electron transitions within the 4f shell [7,13,14]. Many studies have been made to synthesize Eu(OH)₃ using various methods, for instance, Zeng et al. synthesized Eu(OH)₃ nanorods via hydrothermal method at 150 °C for 24 h [15]. Short, rod-like morphology have been acquired with a diameter of around 140 nm and lengths between 100 and 500 nm. Similarly, Lie et al. prepared Eu(OH)₃ nanorods using the same synthesis method at 160 °C [13]. However, in this case, cationic CTAB was used. Eu(OH)₃ nanorods of 450 nm in length and 10–30 nm in diameter were obtained. Patra et al. synthesized Eu(OH)₃ using a microwave-assisted method [16]. Rod-like Eu(OH)₃ was obtained which were 200–300 nm in length and 35–50 nm in diameter. Zhang et al., also prepared Eu(OH)₃ nanoclusters through microwave assisted-hydrothermal at 150 °C and 150 W for 10 min [17]. The Eu(OH)₃ nanoclusters were composed of approximately 5 nm Eu(OH)₃ NPs.

p-nitrophenol (4-nitrophenol, 4-NP) is known to be anthropogenic and inhibitory in nature in which at high concentrations, it would be difficult to mineralize or degrade [18]. Untreated water with this contaminant might result in adverse effects to both human beings and animals, which include severe vomiting, headaches, and organ failure such as the liver, kidney, and central nervous systems [2]. Moreover, due to its stability and high solubility, it might also result in ecological stress. Therefore, researchers have developed materials for the degradation of 4-NP [[19], [20], [21]].

As per the authors’ knowledge, the synthesis of Ni-doped Eu(OH)₃ nanorods (NRs) using the microwave-assisted synthesis method has not been reported. Ni²⁺ as a dopant has shown enhanced photocatalytic degradation of pollutants [22,23]. Therefore, in this study, the microwave-assisted synthesis method was employed for the first time to synthesize Ni-doped Eu(OH)₃. In order to study the influence of Ni-doping, the structural, optical, and morphological properties of Eu(OH)₃ and Ni–Eu(OH)₃ NRs were studied using various characterization techniques. Moreover, the activity of Eu(OH)₃ and Ni–Eu(OH)₃ NRs as photocatalysts were also investigated for the photocatalytic degradation of 4-NP, which is a colourless pollutant, under UV light irradiation for 5 h.

2. Experimental

2.1. Chemicals used

Europium acetate hydrate (Eu(CH₃COO)₃·H₂O, 99 %) and nickel (II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 99 %) were obtained from Alfa-Aesar and Acros, respectively. Sodium hydroxide (NaOH, 99.9 %) was obtained from Merck. Throughout the experiments, water was purified using Aquatron (England). For the photocatalysis experiment, 4-nitrophenol (C₆H₅NO₃, 99 %) was used and obtained from Merck. For the trapping experiments, isopropanol and benzoquinone were obtained from Acros.

2.2. Instruments used

X-ray diffraction ((XRD) Shimadzu XRD-7000 X-ray diffractometer) was used to investigate the structural properties of Eu(OH)₃ and Ni–Eu(OH)₃ with Cu Kα radiation (λ = 1.5418 Å). Raman spectra of the samples were analyzed using a Raman spectrophotometer (NRS-5100, JASCO) at a laser wavelength of 785 nm. The presence of vibrational modes of Eu(OH)₃ and Ni–Eu(OH)₃ was studied using Fourier Transform-Infrared Spectroscopy (FT-IR, Shimadzu IRPrestige-21 Fourier Transform-Infrared Spectrophotometer) from 450 to 4000 cm⁻¹. Field emission transmission electron microscopy (FE-TEM) was utilized to analyze the morphology of the materials and the selected area electron diffraction (SAED) was conducted with JEM-F200 (JEOL Ltd., Tokyo, Japan). A photoluminescence (PL) study of Eu(OH)₃ and Ni–Eu(OH)₃ was carried out using F-7000 Fluorescence spectroscopy (Hitachi High Tech) at an excitation wavelength of 370 nm. The band gap energies of Eu(OH)₃ and Ni–Eu(OH)₃ were determined using UV–Vis diffuse reflectance spectroscopy (DRS) (Shimadzu, UV-2600). X-ray photoelectron spectroscopy (XPS) of the synthesized materials was performed on Kratos Analytical, AXIS Nova. The photocatalytic degradation of 4-NP was conducted using a photochemical reactor (Toption, TOPT-V) irradiated by a 300 W UV lamp. The absorbance of 4-NP and the reaction solution were monitored using a UV–visible spectrophotometer (Shimadzu UV-1900, Japan).

2.3. Microwave-assisted synthesis of Eu(OH)₃ NRs

Microwave-assisted synthesis method was used to prepare Eu(OH)₃ NRs according to the literature [24]. In short, Eu(CH₃COO)₃·H₂O solution (0.05 M) was prepared in a microwave vessel prior to the addition of 2.4 mL of 1 M NaOH into the prepared solution. The solution was heated slowly to 180 °C for 10 min in the microwave reactor with stirring and it was maintained at 180 °C for 15 min at 850 W microwave power. The obtained product was centrifuged and washed three times with distilled water. It was dried at 80 °C in the oven before it was ground to produce Eu(OH)₃ powder.

2.4. Microwave-assisted synthesis of Ni–Eu(OH)₃ NRs

Ni–Eu(OH)₃ NRs were synthesized using the same method as mentioned above. In brief, a specific amount of Ni(NO₃)₂·6H₂O (1, 5, and 10 % Ni–Eu(OH)₃) was added to the 0.05 M aqueous Eu(CH₃COO)₃·H₂O solution prepared earlier. Subsequently, 1 M NaOH solution was added dropwise into the solution. The microwave reaction temperature was increased to 180 °C in 10 min and the synthesis reaction was carried out at 180 °C for 15 min at 850 W microwave power. The product was obtained by centrifuging, washed three times with distilled water and then dried at 80 °C. The obtained products were coded as 1 % Ni- Eu(OH)₃, 5 % Ni- Eu(OH)₃, and 10 % Ni- Eu(OH)₃.

2.5. Photocatalytic degradation of 4-nitrophenol

The photocatalytic degradation of 4-NP using Eu(OH)₃ and 1, 5, and 10 % Ni–Eu(OH)₃ was investigated under UV light irradiation. Eu(OH)₃ and 1, 5, and 10 % Ni–Eu(OH)₃ NRs (10 mg) were put in 50 mL of 10 ppm 4-NP aqueous solution. It was first sonicated for 3 min and proceeded with stirring in the dark for 3 min. Then, the photocatalytic reaction was carried out with a 300 W UV light irradiation for 5 h. The absorbance of the 4-NP solution was measured at every hour. The percentage of photocatalytic degradation of 4-NP was obtained using the following equation (Eq. 1):

$$\%photocatalytic4-NPdegradation=\frac{\left(A_{blank}-A_{sample}\right)}{A_{blank}}\times 100$$ (1)

in which A_blank is the absorbance of 4-NP only and A_sample is the absorbance of 4-NP after photocatalytic degradation reaction with the respective catalyst.

2.6. Active species trapping experiment

The radical trapping experiment was conducted following the same photocatalytic degradation of the 4-NP procedure. In brief, 10 mg of 10 % Ni–Eu(OH)₃ was put in 50 mL of 10 ppm 4-NP aqueous solution. Exactly 500 μL of isopropanol (10 mmol/L) and benzoquinone (1 mmol/L) was added separately. Then, it was first sonicated for 3 min and proceeded with stirring in the dark for 3 min. Then, the trapping experiment was carried out with a 300 W UV light irradiation for 5 h. The absorbance of the 4-NP solution was measured at every hour and the percentage of the photocatalytic degradation of 4-NP in the presence of trapping agents was obtained using Eq. (1). The experiment was conducted in triplicates.

3. Results and discussion

3.1. X-ray diffraction

The structural properties of the synthesized Eu(OH)₃ and Ni–Eu(OH)₃ NRs were determined using powder X-ray diffraction (XRD) analysis (Fig. 1). Peaks were visible at approximately 2θ = 16.10, 28.08, 29.58, 32.58, and 41.04° corresponding to the (010), (110), (011), (020), and (021) planes, respectively, which are in good agreement with the literature [7,25,26]. All peaks were indexed to a pure hexagonal phase (space group = P6₃/m) of Eu(OH)₃ (JCPDS no. 98-020-0488) [27]. No impurities were observed in Eu(OH)₃, 1 % Ni–Eu(OH)₃, 5 % Ni–Eu(OH)₃, and 10 % Ni–Eu(OH)₃ NRs suggesting that the materials were successfully synthesized and Ni²⁺ ions were incorporated into the lattice. Moreover, it was observed that the peak intensity was higher in the case of 1 % Ni–Eu(OH)₃ NRs in comparison to Eu(OH)₃. However, further increase in Ni-doping concentration i.e., 5 % Ni–Eu(OH)₃ and 10 % Ni–Eu(OH)₃ has resulted in the lowering of the peak intensity. The lowering of peak intensity may be due to the difference in electron densities of Ni and Eu, which mainly depends on factors such as scattering and structure factors [28].

Fig. 1.

XRD patterns of Eu(OH)₃ and Ni–Eu(OH)₃ NRs.

The average crystallite size was calculated to study the effect of doping on the structural properties of the Eu(OH)₃. Hence, their average crystallite size was calculated using the Debye-Scherrer's equation (Eq. 2):

$$\mathrm{D}=\mathrm{k}\lambda /\mathrm{\beta }\cos \mathrm{\theta }$$ (2)

where λ represents the wavelength of the X-ray, θ indicates the Bragg angle, and β is the FWHM of the characteristic peaks. The average crystallite size of Eu(OH)₃ was found to be 35.84 nm. However, when 1 % and 5 % Ni were doped into the Eu(OH)₃ lattice, the average crystallite size was reduced greatly to 22.09 and 21.88 nm, respectively (Table 1). In the case of 10 % Ni–Eu(OH)₃, the crystallite size increased slightly to 24.14 nm. This might be due to the difference in the ionic radii of Eu³⁺ and Ni²⁺. As the amount of Ni²⁺ replacing the Eu³⁺ is more than 10 %, this might lead to an increase in internal strain [29]. Moreover, the effect of Ni doping was also studied based on the average lattice strain. It was also calculated using Eq. (3) [30,31]:

$$\epsilon =\frac{{\beta }_{hkl}}{4\mathit{tan}\theta }$$ (3)

where θ is the diffraction angle and β is the full-width half maxima (FWHM) in radians. It was found that the average lattice strain for Eu(OH)₃ and 1, and 5 % Ni–Eu(OH)₃ NRs showed no significant change (Table 1). However, 10 % Ni–Eu(OH)₃ showed a smaller average lattice strain value which could lead to an increase in crystallite size [29].

Table 1.

Average crystallite size (nm), lattice parameters (Å), cell volume (ų), and average lattice strain (ε) of Eu(OH)₃, and Ni–Eu(OH)₃ NRs.

Samples	Average crystallite size (nm)	Lattice parameter (Å)		Cell volume (Å3)	Average lattice strain (ε)
		a	c
Eu(OH)3	35.84	6.36	4.19	146.8	0.0018
1 % Ni–Eu(OH)3	22.09	6.34	4.22	146.9	0.0021
5 % Ni–Eu(OH)3	21.88	6.36	4.22	147.8	0.0018
10 % Ni–Eu(OH)3	24.14	6.34	4.20	146.2	0.0015

The lattice parameters (a and c) were calculated for Eu(OH)₃ and Ni–Eu(OH)₃ NRs and it was found that there is no significant change in the lattice parameter for the synthesized materials. Similarly, no significant difference in the cell volume was observed. This shows that there is not much distortion in the lattice of Eu(OH)₃ after doping with Ni²⁺ except minor changes in the average crystallite size.

3.2. Fourier transform infrared and Raman spectroscopy

The different vibrational modes of Eu(OH)₃, 1 % Ni–Eu(OH)₃, 5 % Ni–Eu(OH)₃, and 10 % Ni–Eu(OH)₃ NRs were studied using FT-IR as shown in Fig. 2(a). A vibration band of Eu-O-H was present at approximately 645 cm⁻¹, in all the samples [32]. The band intensity was seen to decrease as the concentration of Ni²⁺ doping increased. This might correspond to the successful doping of Ni²⁺ into the Eu(OH)₃ lattice. Symmetric and asymmetric stretching of O–C–O were observed in all synthesized materials at ∼1394 cm⁻¹ [25]. Moreover, O–H vibration at 1600 cm⁻¹ was observed from the absorbed moisture on the surface of the Eu(OH)₃, 1 % Ni–Eu(OH)₃, 5 % Ni–Eu(OH)₃, and 10 % Ni–Eu(OH)₃ NRs [27]. The broad shoulder at about 3400 cm⁻¹ may be ascribed to absorption of water molecules. The sharp band at ∼3550 cm⁻¹ was observed in Eu(OH)₃, 1, 5, and 10 % Ni–Eu(OH)₃ NRs is assigned to the coordinated O–H stretch [27].

Fig. 2.

(a) FT-IR of Eu(OH)₃, 1, 5, and 10 % Ni–Eu(OH)₃ NRs, and (b) Raman spectra of Eu(OH)₃, 1, and 10 % Ni–Eu(OH)₃ NRs.

Raman study of Eu(OH)₃, 1, and 10 % Ni–Eu(OH)₃ NRs was carried out and the respective Raman spectra are shown in Fig. 2(b). Eu(OH)₃ showed three obvious Raman peaks at 303.22, 377.37, and 485.83 cm⁻¹. These peaks were assigned to A_g translatory, E_2g translatory, and E_1g libration modes of Eu(OH)₃ NRs, respectively [32]. Enhancement of the Raman peaks intensity was observed for the 1 % Ni–Eu(OH)₃ NRs with no significant change in the peak position. However, in the case of 10 % Ni–Eu(OH)₃, lower Raman intensity was observed. Moreover, Eu(OH)₃ translatory and libration modes of 10 % Ni–Eu(OH)₃ NRs have become broader which might be due to the breakdown of the lattice periodicity and long-range translational crystal symmetry caused by the doping of Ni²⁺ into the crystal lattice [33].

3.3. Transmission electron microscopy

TEM analysis of Eu(OH)₃, 1 % Ni–Eu(OH)₃, and 5 % Ni–Eu(OH)₃ was carried out to determine the morphology and size. Fig. 3 shows the TEM images and SAED patterns of Eu(OH)₃, 1 % Ni–Eu(OH)₃, and 5 % Ni–Eu(OH)₃. In general, Eu(OH)₃ shows rod-like morphology [7,13,15,25,34,35]. It was observed that the synthesized materials also possess rod-like morphology as can be observed in Fig. 3. Fig. 3(a) shows the TEM image of Eu(OH)₃ NRs with an average length of 27 nm and 8 nm in diameter. In general, the growth of particles is influenced by initial pH, reaction time, and temperature [36]. In this case, the initial pH, temperature, and reaction time were all consistent. Therefore, the difference in the particle size mainly depends on the addition of Ni²⁺ ions. When 1 % Ni was incorporated into Eu(OH)₃, the TEM image of 1 % Ni–Eu(OH)₃ shows nanorods with an average length of 34 nm and diameter of 10 nm as shown in Fig. 3(c). It was observed that the particle size has slightly increased with Ni²⁺ doping. The particle size was observed to be similar to 1 % Ni–Eu(OH)₃ when 5 % Ni was incorporated. The 5 % Ni–Eu(OH)₃ NRs showed an average length of 33 nm and diameter of 9 nm (Fig. 3(e)). This suggests that 5 % Ni doping has no significant influence on the particle size of Eu(OH)₃. However, when 10 % Ni was incorporated, the particle length was increased to 50 nm and the diameter was increased to 13 nm (Fig. 3(g)). This shows that a further increase in the percentage doping of Ni²⁺ significantly influenced the particle size of Eu(OH)₃. The increase of the particle size might be due to the kinetics of the formation reaction of Eu(OH)₃ and Ni–Eu(OH)₃, the different particle growth rate or the presence of structural disorder and strain in the lattice due to different ionic radii [37].

Fig. 3.

TEM images and SAED patterns of (a)–(b) Eu(OH)₃, (c)–(d) 1 % Ni–Eu(OH)₃, (e)–(f) 5 % Ni–Eu(OH)₃, and (g)–(h) 10 % Ni–Eu(OH)₃, respectively.

Moreover, to evaluate the phase and crystallinity of Eu(OH)₃, 1 % Ni–Eu(OH)₃, and 5 % Ni–Eu(OH)₃ NRs, SAED was used and shown in Fig. 3(b), (d), and (f). Five broad rings were observed for Eu(OH)₃, 1 % Ni–Eu(OH)₃, and 5 % Ni–Eu(OH)₃ NRs which are attributed to the (110), (011), (020), (121), and (012) reflections of the hexagonal Eu(OH)₃ (P6₃/m) structure. Fig. 3(h) shows three rings of 10 % Ni–Eu(OH)₃ which are attributed to (110), (011), and (020). Original SAED images can be observed in Fig. S1. This is in agreement with the XRD results as discussed in Section 3.1. This further confirms the successful synthesis of Eu(OH)₃ and doping of Ni²⁺ in the Eu(OH)₃.

3.4. UV–Vis diffuse reflectance spectroscopy and photoluminescence spectroscopy

UV–Vis DRS analysis was used to estimate the band gap energies of Eu(OH)₃, 1, 5, and 10 % Ni–Eu(OH)₃ NRs. Fig. 4(a) depicts the Tauc plot obtained from the Kubelka-Munk function using the following equation (Eq. 4):

$$F\left(R\right)={\left(\frac{{\left(1-R\right)}^2}{2R}\times h\nu \right)}^{\frac{1}{2}}$$ (4)

where R is the measured absolute reflectance of the samples. The band gap energy of Eu(OH)₃ and Ni–Eu(OH)₃ NRs was obtained from the plots of [F(R)hv]^1/2 versus hv and shown in Table 2.

Fig. 4.

(a) Tauc plot obtained from Kubelka-Munk function for band gap energy estimation and (b) PL spectra of Eu(OH)₃ and 1, 5, and 10 % Ni–Eu(OH)₃ NPs (inset: zoomed PL of Ni–Eu(OH)₃).

Table 2.

Calculated band gap energies and PL wavelength of Eu(OH)₃ and Ni–Eu(OH)₃ NRs.

Sample	UV–Vis DRS	PL wavelength (nm)
	Band gap energy (eV)	5D0 → 7F0	5D0 → 7F1	5D0 → 7F2	5D0 → 7F3	5D0 → 7F4
Eu(OH)3	3.80	578	592	617	651	691 and 697
1 % Ni–Eu(OH)3	4.06	–	592	616	–	691 and 697
5 % Ni–Eu(OH)3	3.69	–	592	616	–	690 and 697
10 % Ni–Eu(OH)3	3.50	–	593	616	651	690 and 697

The effect of doping on the band gap energy was observed. Eu(OH)₃ showed a band gap energy of 3.80 eV which is in good agreement with the literature [13]. The band gap energy was increased slightly to 4.06 eV after doping with 1 % Ni. This might be due to the high crystallinity of the 1 % Ni–Eu(OH)₃ as can be observed from the XRD analysis. High crystallinity can be associated with fewer defects and less disorder which can lead to a wide band gap energy [38]. However, the band gap energy was decreased to 3.69 eV as expected when 5 % Ni was doped into Eu(OH)₃. Similarly, a further increase in doping concentration would result in the decrease of the band gap energy as observed in the case of 10 % Ni–Eu(OH)₃ which showed about 3.50 eV. The band gap energies of Eu(OH)₃ and Ni–Eu(OH)₃ NRs are tabulated in Table 2.

In addition to the UV–Vis DRS, PL analysis of Eu(OH)₃ and Ni–Eu(OH)₃ NRs was carried out to understand the optical behavior of the samples. Fig. 4(b) exhibits the PL emission spectra of Eu(OH)₃, 1 % Ni–Eu(OH)₃, 5 % Ni–Eu(OH)₃, and, 10 % Ni–Eu(OH)₃ NRs at an excitation wavelength of 321 nm in which the emission peaks were observed in the range of 500 – 750 nm. The PL peaks were associated with the ⁵D₀ → ⁷F_J (J = 0–4) transitions. Eu(OH)₃ NRs showed PL peaks at approximately ∼ 578, 592, 617, 651, and 690 nm which correspond to ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃, and ⁵D₀ → ⁷F₄ transitions, respectively [15]. However, the PL peaks were quenched greatly when 1 % Ni was doped into Eu(OH)₃. The PL intensity was further decreased for 5 % Ni–Eu(OH)₃ NRs. On the other hand, 10 % Ni–Eu(OH)₃ showed an enhanced PL intensity which was higher than both 1 % Ni–Eu(OH)₃ NRs and 5 % Ni–Eu(OH)₃ NRs. It is stated that high recombination of charge carriers would result in high PL intensity [39]. Therefore, in this case, pure Eu(OH)₃ showed the highest PL intensity while Ni–Eu(OH)₃ showed lower intensity suggesting low recombination of charge carriers. Amongst Ni–Eu(OH)₃ NRs, it was observed that 10 % Ni–Eu(OH)₃ might show more recombination of photogenerated electron-hole pair than 1 and 5 % Ni–Eu(OH)₃ NRs. Table 2 shows the wavelength of each PL peak of Eu(OH)₃ and Ni–Eu(OH)₃ NRs. It was observed that the PL peaks corresponding to ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₃ were flattened. This might be due to the forbidden transition for ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₃ both in magnetic and electric dipole schemes [40].

3.5. X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was performed at room temperature to investigate the chemical state and the electronic structure of the elements in Eu(OH)₃, 1 % Ni–Eu(OH)₃, 5 % Ni–Eu(OH)₃ and 10 % Ni–Eu(OH)₃ (Fig. 5). Fig. 5(a) shows the complete survey scan spectra of the samples, which confirmed the presence of Ni 2p, O 1s, and Eu 4d. The Eu 4d core level peak is shown in Fig. 5(b). Eu³⁺ 4d_3/2 and Eu³⁺ 4d_5/2 were found at 141.9 and 136.2 eV, respectively [41]. A slight shift was observed in the binding energy of Eu³⁺ 4d_3/2 and Eu³⁺ 4d_5/2 when doped with Ni²⁺. The binding energy was shifted to higher energy when doped with more Ni²⁺ ions.

Fig. 5.

XPS: (a) Survey scan, (b) Eu 4d, (c) O 1s, (d) Ni 2p, and (e) C 1s of Eu(OH)₃ and Ni–Eu(OH)₃ NRs.

Fig. 5(c) shows the XPS spectrum of O 1s where all samples showed one major peak at approximately 528 eV. This indicates the presence of nucleophilic oxygen species which is OH⁻ anion, in Eu(OH)₃ and Ni–Eu(OH)₃ NRs [42]. The XPS peak intensity decreased slightly from Eu(OH)₃–10 % Ni–Eu(OH)₃ NRs. However, no significant change in the peak position was observed for all the samples. Ni 2p spectra of Ni–Eu(OH)₃ NRs can be observed in Fig. 5(d). Two prominent peaks can only be observed for 10 % Ni–Eu(OH)₃ at 854.26 eV and 871.50 eV, which corresponds to Ni 2p_3/2 and Ni 2p_1/2, respectively [43]. The XPS spectra of 1 % and 5 % Ni–Eu(OH)₃ NRs showed weak signals which might be due to the small content of Ni in the samples [43]. Satellite peaks at 860.22 eV and 877.83 eV suggest the presence of Ni²⁺ ions in the Eu(OH)₃ lattice [44,45]. The typical C 1s were observed at about 282 eV in the spectra (Fig. 5(e)). The atomic concentrations of C 1s, O 1s, Eu 4d, and Ni 2p are listed in Table 3.

Table 3.

The atomic concentration of C 1s, O 1s, Eu 4d, and Ni 2p of Eu(OH)₃, 1 % Ni–Eu(OH)₃. 5 % Ni–Eu(OH)₃, and 10 % Ni–Eu(OH)₃ NRs.

Samples	Atomic concentrations (%)
	C 1s	O 1s	Eu 4d	Ni 2p
Eu(OH)3	11.5	58.9	29.6	–
1 % Ni–Eu(OH)3	5.4	58.5	30.7	5.4
5 % Ni–Eu(OH)3	4.7	57.6	28.7	9.0
10 % Ni–Eu(OH)3	5.3	55.7	25.4	13.6

4. Applications

4.1. Photocatalytic degradation of 4-nitrophenol

The colourless organic pollutant i.e., 4-NP was photocatalytically degraded under UV light irradiation using Eu(OH)₃ and Ni–Eu(OH)₃ NRs. The experiment was carried out for 5 h and the absorbance of the reaction solution was measured at every hour to observe the progress of the photocatalytic activities of Eu(OH)₃ and Ni–Eu(OH)₃ NRs. The experiment was conducted in triplicates to ensure the repeatability of the results and avoid any errors. Fig. 6(a) shows the average percentage and ln C₀/C of the photocatalytic degradation of 4-NP using Eu(OH)₃ and Ni–Eu(OH)₃ NRs, respectively.

Fig. 6.

The average of (a) percentage, (b) ln C₀/C plot against time for the photocatalytic degradation of 4-NP using Eu(OH)₃, 1 % Ni–Eu(OH)₃, 5 % Ni–Eu(OH)₃, and 10 % Ni–Eu(OH)₃, and (c) average percentage photocatalytic degradation of 4-NP for trapping experiment of 10 % Ni–Eu(OH)₃ under UV light irradiation.

Fig. 6(a) at 0 h, shows the adsorption-desorption phase (in the dark), in which Eu(OH)₃ and 5 % Ni–Eu(OH)₃ NRs have low adsorption affinity towards 4-NP which are about 28.45 ± 3.88 % and 28.49 ± 4.07 % respectively, while 10 % Ni–Eu(OH)₃ has the highest adsorption affinity (33.69 ± 3.98 %). When illuminated with UV light, the percentage degradation of 4-NP gradually increased with time. After 5 h of UV irradiation, 10 % Ni–Eu(OH)₃ NRs showed the highest photocatalytic activity with a total of 74.29 ± 7.53 % as shown in Fig. 6(a) and Table 4. The effectiveness of the photocatalytic degradation of 4-NP was observed as follows: 10 % Ni–Eu(OH)₃ > 5 % Ni–Eu(OH)₃ > 1 % Ni–Eu(OH)₃ > Eu(OH)₃. The photocatalytic degradation activity was observed to be enhanced with increasing the percentage of Ni doping. The efficiency of 10 % Ni–Eu(OH)₃ NRs might be due to its band gap energy which is about 3.50 eV. Moreover, 5 % Ni–Eu(OH)₃ also showed slightly higher band gap energy which is about 3.69 eV suggesting a similar response in the photocatalytic activity. One should note that even though the PL intensity of 5 % Ni–Eu(OH)₃ (Fig. 4(b): inset) exhibits the lowest intensity, which in general indicates the maximum separation of charge carriers that is very useful for efficient photocatalytic performance, the 10 % Ni–Eu(OH)₃ showed the narrowest band gap energy which facilitated the UV light adsorption. The doping might help in creating defect states in the band gap which enables UV light absorption and thus inhibits rapid charge carrier recombination [46]. Moreover, at adsorption and desorption, it was found that 10 % Ni–Eu(OH)₃ showed the highest response which might facilitate the overall photocatalytic response [47]. Therefore, 10 % Ni–Eu(OH)₃ showed the best photocatalytic activity. On the other hand, both Eu(OH)₃ and 1 % Ni–Eu(OH)₃ also showed similar photocatalytic responses due to their band gap energies i.e., 3.80 and 4.06 eV, respectively.

Table 4.

The average percentage photocatalytic activities of Eu(OH)₃ and 1, 5, and 10 % Ni–Eu(OH)₃ NRs for 4-NP degradation under UV light irradiation.

	% Photocatalytic degradation of 4-NP
	Ads-des	1 h	2 h	3 h	4 h	5 h
Eu(OH)3	28.45 ± 3.88	35.27 ± 4.18	43.47 ± 1.68	50.20 ± 1.94	58.19 ± 4.46	65.75 ± 5.97
1 % Ni–Eu(OH)3	29.98 ± 3.96	37.58 ± 2.46	43.98 ± 3.47	50.69 ± 1.74	58.03 ± 3.50	65.63 ± 4.89
5 % Ni–Eu(OH)3	28.49 ± 4.07	39.17 ± 4.95	46.64 ± 2.13	54.58 ± 3.24	63.19 ± 5.19	71.43 ± 6.96
10 % Ni–Eu(OH)3	33.69 ± 3.98	42.72 ± 2.31	50.10 ± 2.37	55.45 ± 1.37	66.42 ± 5.58	74.29 ± 7.53

Moreover, the photocatalytic activity of Eu(OH)₃ and Ni–Eu(OH)₃ NRs against 4-NP was studied by applying the pseudo-first-order kinetics as shown below (Eq. (5)):

$$\mathit{ln}\frac{C_0}{C}=Kt$$ (5)

C₀ and C denote the original and the treated concentration of 4-NP while t is the photocatalytic reaction time in h. The rate constant of the pseudo-first-order (K) is expressed in h⁻¹. Fig. 6(b) presents the pseudo-first-order kinetics of the photocatalytic degradation of 4-NP activity. The rate constants of Eu(OH)₃, 1 % Ni–Eu(OH)₃, 5 % Ni–Eu(OH)₃, and 10 % Ni–Eu(OH)₃ NRs were estimated to be 0.1813, 0.1826, 0.2075, and 0.2247 h⁻¹, respectively. It was observed that the rate constant was increased with more Ni-doping. This suggests that the doping of Ni²⁺ into Eu(OH)₃ has improved the photocatalytic degradation of 4-NP. Moreover, radicals trapping experiments were conducted for 10 % Ni–Eu(OH)₃ with the addition of isopropanol and benzoquinone separately. As can be seen in Fig. 6(c), the photocatalytic degradation of 4-NP with the addition of benzoquinone showed a reduction in the photocatalytic response from the beginning of the experiment as compared to the photocatalytic reaction without scavengers. Benzoquinone is responsible for scavenging O₂^·- radicals. Isopropanol which is responsible for the scavenger of •OH radicals also showed a reduction in the photocatalytic degradation activity although not as much as benzoquinone. Despite that, at 5 h, photocatalytic degradation of 4-NP with isopropanol showed a similar response as benzoquinone.

In the photocatalytic degradation of 4-NP, •OH radicals are reported to play an important role [48,49]. Moreover, O₂^·- is also involved in the photocatalytic degradation reaction as suggested by Kang et al. [50]. Based on the radical trapping experiment (Fig. 6(c)), both •OH and O₂^·- radicals were trapped which showed a reduction in the performance of photocatalytic degradation of 4-NP. Therefore, the involvement of •OH and O₂^·- radicals improved the overall photocatalytic activity. Fig. 7 illustrates the general mechanism of the photocatalytic degradation of 4-NP.

Fig. 7.

The proposed mechanism for the photocatalytic degradation of 4-NP using Eu(OH)₃ and Ni–Eu(OH)₃ NRs under UV light irradiation.

5. Conclusions

The synthesis of Eu(OH)₃ and Ni–Eu(OH)₃ NRs was carried out using a microwave-assisted synthesis method. The properties of the synthesized materials such as their structural, optical, morphological properties, and photocatalytic activities were investigated. All the synthesized materials i.e., Eu(OH)₃ and 1, 5, and 10 % Ni–Eu(OH)₃ have acquired hexagonal phases based on the XRD studies. FT-IR and Raman studies confirmed the synthesis of Eu(OH)₃ and Ni–Eu(OH)₃. All the materials showed rod-like morphology with average length and diameter between 27 – 50 nm and 8–13 nm, respectively. The band gap energies of Eu(OH)₃ and Ni–Eu(OH)₃ were decreased with an increase in Ni-doping. Owing to that, the PL study showed a decrease in PL intensity for Ni–Eu(OH)₃ NRs. The photocatalytic degradation of 4-NP activity was conducted and it showed that 10 % Ni–Eu(OH)₃ NRs exhibited enhanced photocatalytic activity under UV light irradiation. The kinetic study was also carried out using the pseudo-first-order kinetics in which 10 % Ni–Eu(OH)₃ showed a rate constant of 0.2247 h⁻¹. Therefore, Ni–Eu(OH)₃ has potential to be a UV light-responsive material for the photocatalytic degradation of colourless pollutants.

Data availability

Data will be supplied upon request.

CRediT authorship contribution statement

Shaidatul Najihah Matussin: Writing – original draft, Methodology, Investigation, Data curation. Fazlurrahman Khan: Formal analysis, Data curation. Mohammad Hilni Harunsani: Writing – original draft, Supervision. Young-Mog Kim: Resources, Formal analysis. Mohammad Mansoob Khan: Writing – review & editing, Supervision, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix ASupplementary data

The following is the Supplementary data to this article: