Influence of Ni Doping on Oxygen Vacancy-Induced Changes in Structural and Chemical Properties of CeO₂ Nanorods

Abstract

In recent years, cerium dioxide (CeO₂) has attracted considerable attention owing to its remarkable performance in various applications, including photocatalysis, fuel cells, and catalysis. This study explores the effect of nickel (Ni) doping on the structural, thermal, and chemical properties of CeO₂ nanorods, particularly focusing on oxygen vacancy-related phenomena. Utilizing X-ray powder diffraction (XRD), alterations in crystal structure and peak shifts were observed, indicating successful Ni doping and the formation of Ni₂O₃ at higher doping levels, likely due to non-equilibrium reactions. Thermal gravimetric analysis (TGA) revealed changes in oxygen release mechanisms, with increasing Ni doping resulting in the release of lattice oxygen at lower temperatures. Raman spectroscopy corroborated these findings by identifying characteristic peaks associated with oxygen vacancies, facilitating the assessment of Ni doping levels. Ni-doped CeO₂ can catalyze the ultrasonic degradation of methylene blue, which has good application prospects for catalytic ultrasonic degradation of organic pollutants. Overall, this study underscores the substantial impact of Ni doping on CeO₂ nanorods, shedding light on tailored catalytic applications through the modulation of oxygen vacancies while preserving the nanorod morphology.

1. Introduction

In recent years, cerium dioxide (CeO₂), as a typical rare earth metal oxide, has received widespread attention due to its excellent performance in photocatalysis [1,2,3], fuel cells [4,5,6,7], sensors [8,9,10,11], CO oxidation [12,13,14,15,16], water–gas shift reaction [17,18,19,20], and other fields [21,22,23]. This is mainly attributed to its two important characteristics. Firstly, in the CeO₂ lattice, rapid conversion of two valence states (Ce⁴⁺/Ce³⁺) is achieved through the formation/loss of oxygen vacancies, thus CeO₂ has excellent redox ability [24]. Secondly, CeO₂ has a cubic fluorite structure containing many oxygen vacancies, which are beneficial for improving oxygen mobility [25,26].

CeO₂ is an n-type semiconductor material with oxygen vacancy sites. The Kroger–Vink formula is expressed as 2CeO₂ = 2Ce’_Ce + Vö + 3O_O^x + 1/2O₂↑, where Ce’_Ce represents the presence of one-unit negative charge at the Ce⁴⁺ position, Vö represents an oxygen vacancy with a two-unit positive charge, and O_O^x represents the oxygen atom on the CeO₂ lattice site. The presence of oxygen vacancies generates Ce³⁺, therefore CeO₂ has a high lattice ion mobility and excellent oxygen storage and release ability and is used as a catalyst in various fields [27].

CeO₂ nanocrystals typically expose low-index crystal planes (111), (110), and (100) [28]. Theoretical calculations indicate that the (110) plane has the lowest vacancy formation energy of 1.99 eV, the (100) plane has a vacancy formation energy of 2.27 eV, and the most stable (111) plane has a maximum vacancy formation energy of 2.60 eV. Consequently, the order of the formation energy of oxygen vacancies on different crystal planes of CeO₂ is: (110) < (100) < (111) [29]. Therefore, the formation of oxygen vacancies on the CeO₂ (110) crystal plane is easier. CeO₂ has different exposed crystal planes based on its morphology. Polyhedral CeO₂ mainly exposes (111) crystal planes, while cubic CeO₂ mainly exposes (100) crystal planes and rod-shaped CeO₂ mainly simultaneously exposes (110) and (100) crystal planes [28]. It is possible to regulate the morphology of CeO₂ to alter oxygen vacancies. Yuan investigated the effect of CeO₂ morphology on the catalytic activity of nitrobenzene hydrogen transfer reduction reaction. Oxygen vacancies and basic sites can selectively activate ethanol molecules to reduce nitro groups. The catalytic activity is sorted in the order: CeO₂ nanorods, CeO₂ nanopolyhedrons, and CeO₂ nanocubes [30].

Doping different metals, such as precious metals, transition metals, alkali metals, and rare earth metals, into the CeO₂ lattice can improve its catalytic activity and stability [31,32]. Through metal doping, lattice distortion can be induced, resulting in abundant oxygen vacancies and Ce³⁺, which are widely used in catalysis [33]. Transition metal catalysts have received considerable attention due to their low cost and excellent activity. Researchers have found that the substitution of Ce⁴⁺ with transition metals can significantly alter the geometric and electronic structures of CeO₂ systems, leading to the reappearance of enriched electronic regions in CeO₂ and the weakening of Ce-O or M-O bonds [34,35]. The results indicate that the atomic radius has a significant impact on the structure of doped CeO₂, and radii larger or smaller than those of Ce⁴⁺ ions usually produce significant geometric distortions. La-doped CeO₂ nanorod shows significantly higher H₂ production compared to CeO₂ in photocatalytic reaction [36]. We selected the fourth-period transition metal element Ni, which has the advantage of low cost, as the doping element to investigate its effect on oxygen vacancies in CeO₂.

Methylene blue is a common dye, and untreated methylene blue can cause serious pollution to the water environment [37]. Organic dye wastewater has a deep chromaticity, and ultrasonic degradation of organic pollutants has been proven to be a useful method [38]. Ultrasound has been found to be an attractive and advanced method for degrading harmful organic compounds in water [39,40]. These local hotspots have a temperature of 5000 K, a pressure of 500 atm, and a lifespan of a few microseconds [41]. The water vapor inside the collapsed bubble undergoes pyrolysis to form hydroxyl and hydrogen radicals [42,43]. Ultrasonic degradation of organic compounds such as dyes is considered a highly effective method. Ni-doped CeO₂ catalysts can generate a large number of oxygen vacancies, and combined with ultrasound methods, may contribute to the degradation of dyes.

In this article, we control the concentration of oxygen vacancies in CeO₂ by combining morphology control and metal ion doping, thereby affecting its catalytic activity. This study prepared a series of Ni-doped CeO₂ nanorods using the hydrothermal method. In addition, the structural, thermal stability, and changes in oxygen vacancy concentration between CeO₂ and the doped CeO₂ nanorods were thoroughly analyzed using a combination of X-ray power diffusion (XRD), transmission electron microscopy (TEM), thermogravimetry analysis (TGA), and Raman spectroscopy. Finally, the degradation reaction of methylene blue was used as a model, the effect of Ni-doped CeO₂ catalytic ultrasonic degradation of methylene blue was studied, and the degradation mechanism was explored.

2. Materials and Methods

2.1. Reagents

Cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.9%, AR) was purchased from Shanghai Civi Chemical Technology Co., Ltd. (Shanghai, China). Nickel nitrate hexahydrate (Ni(NO₃)₃·6H₂O, 99%, AR) was obtained from J&K Scientific (Beijing, China), and sodium hydroxide (NaOH, 99%, AR) was sourced from Shanghai Titan Scientific Co., Ltd. (Shanghai, China). Ultrapure water was produced using a laboratory water purification system (Hitech Instruments Co., Ltd., Hetai, China). All chemicals were used as received without further purification.

2.2. Hydrothermal Synthesis of Ni-Doped CeO₂ Nanorods

Ni-doped CeO₂ nanorods with various Ni²⁺ contents (0, 1, 5, 15 at%) were synthesized using the modified hydrothermal method, as described in Mai’s work [28]. Ce(NO₃)₃·6H₂O and Ni(NO₃)₃·6H₂O with different molar ratios were dissolved in 5 mL of deionized water. Meanwhile, 12.6 g of NaOH was dissolved in 30 mL of deionized water. Afterward, the two solutions were uniformly mixed and stirred at room temperature for 30 min. The resulting mixture was transferred to a 50 mL autoclave, which was then sealed and maintained at 373 K for 24 h. Subsequently, the autoclave was allowed to cool down to room temperature, and the obtained product was centrifuged and washed with deionized water and ethanol three times. The product was then dried overnight at 333 K.

2.3. Catalytic Degradation of Methylene Blue

The degradation of methylene blue solution was conducted under the condition of 293 ± 0.5 K. In a 10 mg/L methylene blue solution, 0.10 g of Ni-doped CeO₂ catalyst was added, and ultrasound (25 kHz, 500 W) was used. Catalysts were taken at different times and filtered using a 0.45 μm microporous filter membrane. The absorbance of the methylene blue (MB) solution was measured at its maximum absorbance wavelength of 664 nm using a 721N Visible Spectrophotometer (INESA Analytical Instrument Co., Ltd., Shanghai, China) [37]. The removal efficiency of methylene blue was calculated according to the following formula:

Removal efficiency = (A₀ − A_t)/A₀ × 100%

A₀ is the absorbance of the original methylene blue solution before degradation. A_t is the absorbance of the degraded methylene blue solution at time t.

Methylene blue (MB) standard solutions of 3 mg/L, 4 mg/L, 5 mg/L, 7 mg/L, and 9 mg/L were prepared using ultrapure water as the reference solution. The absorbance of the methylene blue solutions at a wavelength of 664 nm was measured, and the calibration curve was established. The solution exhibits a good linear relationship within the range of 3–9 mg/L, and its standard curve fits well. The calculated correlation coefficient is R² = 0.99864, and the relationship between absorbance, A and MB concentration, C is A = 0.1762C + 0.02492.

2.4. Characterization

The X-ray powder Diffraction (XRD) patterns were recorded using a Rigaku diffractometer with Cu Kα radiation (λ = 1.5418 nm). The X-rays were operated at 40 kV and 40 mA. Patterns were collected in the 2θ range from 20° to 80°, with a scanning step of 0.02 and a scanning speed of 2.5°/min. The morphology and microstructure of the samples were characterized using a HT7800 transmission electron microscope (TEM) operating at 200 kV. Thermal gravimetric analysis (TGA-50, Shimadzu Corp., Kyoto, Japan) was employed to measure oxygen release with temperature increase. The temperature was raised from room temperature to 800 °C at a rate of 10 °C/min under a nitrogen atmosphere (gas flow rate: 24 mL/min). Raman spectroscopy was utilized to measure the presence of surface Ce³⁺ or oxygen vacancies. This analysis was performed at the Shanghai branch of the Beijing High Voltage Science Research Center (Gaoke, Shanghai, China) using the Renishaw inVia micro- Raman spectrometer from the UK. The experiment employed a 532 nm laser light source with a power of 50 mW, 1800 gr/mm grating, and a 50 times Leica long focal length lens. The required Raman spectrum for this experiment covers a scanning range of 50–1200 cm⁻¹, with a scanning time of 10 s, 10 scanning times, and a resolution of 3 cm⁻¹.

3. Results and Discussion

The phase structure of the products was characterized by X-ray powder diffraction (XRD), as shown in Figure 1, and the related data are summarized in

Table 1. Lattice constant and crystallite size of the samples.

	CeO2	1% Ni-CeO2	5% Ni-CeO2	15% Ni-CeO2
Lattice constant (Å)	5.4110	5.4064	5.3982	5.4230
Crystallite size (nm)	128	65	60	92

. The relationship between grain size and half peak width can be obtained through the Scherrer equation, D = (Kλ)/(βcos θ), where D is the grain size, K is a constant, and β is the half peak width. The half peak width is inversely proportional to the crystallite size.

The characteristic diffraction peaks of pure CeO₂ samples appear at 28.5°, 33.1°, 47.5°, 56.3°, 59.1°, 69.4°, 76.7°, and 79.1°, corresponding to the cubic fluorite crystal planes of CeO₂ for 111, 200, 220, 311, 222, 400, 331, and 420, respectively. As the Ni doping amount increases, the width of the diffraction peaks also increases, indicating a decrease in crystallite size with increasing Ni doping amount (Table 1).

Additionally, upon comparison with the (111) peak of pure CeO₂, the peaks of the doped samples are observed to shift to higher angles. This phenomenon occurs due to the replacement of cerium sites in the cerium oxide lattice by Ni²⁺, with a radius of 0.63 Å, resulting in the observed shift of the cerium oxide peaks [44,45]. This shift indicates the successful doping of metallic nickel into the cerium oxide lattice. According to the Bragg equation, 2dsinθ = nλ, after doping with nickel metal, the increase in θ value leads to a slight decrease in the interplanar spacing of the crystal cell, causing slight lattice shrinking [46].

When the doping amount reaches 15%, a new diffraction peak appears near 38°. The peak in question may correspond to Ni₂O₃, indicating that some of the excess Ni species did not successfully dope into the CeO₂ structure and instead underwent a secondary reaction. Therefore, it can be confirmed that there was hardly any peak shift. Furthermore, it has been observed that as the nickel doping level increases, there is a tendency for the crystallite size to decrease. However, at 15% doping, it has been confirmed that this trend deviates significantly.

Therefore, at doping levels of 1% and 5%, only diffraction peaks corresponding to CeO₂ are observed, confirming a trend of high-angle shifts and decreasing crystallite size. This suggests that nickel can be fully doped into CeO₂ at these levels.

Figure 2a–d presents TEM images of CeO₂ and Ni-doped CeO₂. From Figure 2a, it is evident that rod-shaped CeO₂ nanomaterials have been successfully synthesized. As depicted in Figure 2b, even with a 1% doping amount, the rod-shaped morphology of CeO₂ nanomaterials remains unchanged. With a doping amount of 5% (Figure 2c), the rod-shaped structure becomes wider. In Figure 2d, with a doping amount of 15%, although the structure remains rod-shaped, the length of the rods significantly decreases while the width increases.

Figure 3 shows the thermogravimetric analysis of undoped CeO₂ and Ni-doped CeO₂ nanorods. All the samples were measured without calcination. Up to about 150 °C, the weight loss due to the loss of surface moisture can be ignored. The weight loss rate of undoped CeO₂ is about 3.08%, the weight loss rate of 1% Ni-CeO₂ is about 7.95%, the weight loss rate of 5% Ni-CeO₂ is about 5.88%, and the weight loss rate of 15% Ni-CeO₂ is about 3.33%; therefore, the weight loss above 150 °C can be attributed to the loss of surface or lattice oxygen. It can be observed that the pattern of weight loss changes around 320 °C, indicating different energy release mechanisms between surface and lattice oxygen [47,48]. By analyzing the weight loss in each temperature range (150–320 °C), the weight loss due to surface oxygen release was approximately 2.35 wt% for pure CeO₂, 3.79 wt% for 1% Ni-CeO₂, 5.04 wt% for 5% Ni-CeO₂, and 4.34 wt% for 15% Ni-CeO₂. On the other hand (320–800 °C), the weight loss due to lattice oxygen release was approximately 1.23 wt% for pure CeO₂, 2.62 wt% for 1% Ni-CeO₂, 2.63 wt% for 5% Ni-CeO₂, and 2.08 wt% for 15% Ni-CeO₂. The weight loss rate after Ni doping was higher than that without doping, and generally decreased with increasing doping level (

Table 2. The ratio of surface oxygen/lattice oxygen of samples.

	CeO2	1% Ni-CeO2	5% Ni-CeO2	15% Ni-CeO2
Surface oxygen	2.35 wt%	3.79 wt%	5.04 wt%	4.34 wt%
Lattice oxygen	1.23 wt%	2.62 wt%	2.63 wt%	2.08 wt%
Total oxygen	3.58 wt%	6.41 wt%	7.67 wt%	6.42 wt%

). This may be attributed to the non-equilibrium reactions leading to the formation of NiO_x when Ni is added at high concentrations [49]. Furthermore, as the doping effect increases, the activation of surface oxygen also increases, leading to a greater impact on weight loss at lower temperatures. In other words, in the temperature range below 800 °C, the influence of surface oxygen release becomes more significant with increased doping. Calculating the ratio of surface oxygen to lattice oxygen, it increased to 1.45 for 1% Ni-CeO₂, 1.92 for 5% Ni-CeO₂, and 2.23 for 15% Ni-CeO₂.

As shown in Figure 4, the Raman spectra provide the following insights: both undoped and Ni-doped CeO₂ samples exhibit characteristic peaks around 462 cm^–1, originating from the F₂g vibration of CeO₂ due to the symmetric arrangement of oxygen atoms in the CeO₂ lattice [50]. Additionally, a weak absorption peak is observed near 595 cm^–1, corresponding to Frenkel-type oxygen vacancies [51]. Notably, the characteristic peak of undoped CeO₂ at 462 cm^–1 appears strongest among the four images. As the doping level of Ni increases, the intensity of absorption peaks at this location gradually diminishes due to the substitutional reaction ${\mathrm{C}\mathrm{e}}^{4+}+{\mathrm{N}\mathrm{i}}^{2+}\leftrightarrow {\mathrm{N}\mathrm{i}}^{3+}+{\mathrm{C}\mathrm{e}}^{3+}$, leading to the generation of oxygen vacancies and thus symmetry degradation [52,53]. Furthermore, an increase in Ni doping may lead to a decrease in particle size, which in turn affects the linewidth of the main Raman peak at 462 cm⁻¹ [54].

To gauge the relative concentration of surface oxygen vacancies in the samples, the intensity of the absorption peak at 595 cm⁻¹ (A₅₉₅) was compared to that at 462 cm⁻¹ (A₄₆₂) [55]. A higher ratio indicates a greater presence of oxygen vacancies on the sample surface [56,57]. Specific ratios are provided in

Table 3. The ratio of A₅₉₅/A₄₆₂ in Raman spectra.

	CeO2	1% Ni-CeO2	5% Ni-CeO2	15% Ni-CeO2
A595/A462	9.79%	23.9%	30.5%	26.7%

. As evident from these findings, increasing Ni doping correlates with a rise in the concentration of oxygen vacancies on the surface of cerium dioxide, peaking at a 5% doping level. However, at a 15% doping level, the peak intensity slightly decreases, attributed to incomplete Ni doping and the formation of Ni₂O₃, consistent with previous discussions.

To evaluate the chemical properties of the synthesized nanoparticles, an acoustic cavitation test was conducted using methylene blue, a surrogate for organic pollutants, as shown in Figure 5 (Refer to the Section 2 for the calibration curve in Figure 5a). Acoustic cavitation occurs when ultrasound of sufficient intensity passes through a liquid, causing small bubbles (cavitation nuclei) present in the liquid to rapidly increase under negative pressure. During the subsequent positive pressure cycle, the bubbles undergo adiabatic compression and collapse, creating extremely short and intense pressure pulses at the moment of collapse. High temperatures and high pressures are generated inside the bubbles, making this method very effective for the degradation of organic materials.

As shown in Figure 5b, the results after 20 min of reaction indicate that 1% and 5% Ni-CeO₂ show higher degradation rates than undoped CeO₂. Initially, both samples exhibit similar degradation rates, but as the reaction time increases, the 5% Ni-CeO₂ shows a higher degradation rate, confirming that the higher the surface oxygen concentration, the higher the degradation rate (Table 2 and Table 3). In contrast, 15% Ni-CeO₂ initially shows a degradation rate similar to that of pure CeO₂, as the nickel doping is minimal at the start. However, as the reaction progresses, the degradation rate gradually increases, and after 40 min, it increases even more, reaching a degradation rate of 45.9% at 80 min. This is thought to be due to the additional catalytic action of nickel oxide in forming ·OH radicals, rather than the effect of doping itself [58].

4. Conclusions

Summarizing the research findings, it can be concluded that doping Ni into CeO₂ nanorods significantly impacts their structural, thermal, and chemical properties. XRD analysis revealed changes in crystal structure and peak shifts, indicating successful Ni doping and the formation of Ni₂O₃ at high doping levels due to non-equilibrium reactions. TGA results demonstrated variations in oxygen release mechanisms, with increasing Ni doping leading to the emission of some lattice oxygen at lower temperatures, as confirmed by changes in weight ratio across temperature ranges. Additionally, the presence of characteristic Raman peaks associated with oxygen vacancies allowed for assessing the extent of Ni doping based on peak intensity changes. Catalysts with rich oxygen vacancies under ultrasound help generate more ·OH radicals, promoting the degradation of methylene blue. Therefore, through this study, successful adjustment of properties for catalytic applications was achieved by controlling the Ni doping level while maintaining the CeO₂ nanorod type.