Cerium-Doped Oxide-Based Materials for Energy and Environmental Applications

Abstract

Cerium is a rare-earth metal commonly used as a dopant in various metal oxides to enhance their performances or provide optoelectronic properties. Cerium oxide (ceria) is particularly valuable owing to its unique properties and applications in various fields, such as biomedical research, photovoltaics, and industrial catalytic processes. This review focuses on the use of cerium and ceria doping in the synthesis of SiO₂ and ZnO. Studies have shown that Ce-doped SiO₂ thin films exhibit luminescence properties and proton shielding capabilities, and that Ce-doped ZnO has potential applications in gas sensors. In this review, we highlight the potential for controlling the luminescence and optical characteristics of these materials via cerium doping, opening up possibilities for various technological advancements and potential applications of cosmic ray shielding in space photovoltaics.

1. Introduction

Cerium is the most abundant rare-earth metal in the Earth’s crust. Cerium minerals include carbonates, phosphates, silicates, and hydroxides. Cerium is the first element in the periodic table to have orbitals partially filled with f and d ([Xe]4f¹5d¹6s²) [1]. As a lanthanide, it has two stable oxidation states: Ce³⁺ and Ce⁴⁺. The latter state has a noble gas structure with an atomic radius of 0.90 Å [2]. Owing to their superior redox characteristics and oxygen storage capacity in stable Ce³⁺ and Ce⁴⁺ oxide states, cerium ions have long been utilized as well-studied dopants in catalytic systems [3,4]. The Ce⁴⁺/Ce³⁺ redox property effectively improves the separation of electron–hole pairs in the photocatalyst system [5,6]. In addition, the 4f levels of cerium are shielded by Ce³⁺, resulting in well-defined, narrow optical transitions between spin–orbit levels. Cerium strongly absorbs and emits within the visible range of ultraviolet–visible light (UV-Vis) and photoluminescence (PL) spectra.

Cerium oxide (ceria) holds significant technological value owing to its distinctive properties. Ce exists in two oxidation states: Ce³⁺ and Ce⁴⁺. Consequently, ceria exists in two forms in bulk materials: CeO₂ (with Ce⁴⁺) and Ce₂O₃ (with Ce³⁺). The Ce oxide lattice has a nanoscale cubic fluorite structure, and Ce³⁺ and Ce⁴⁺ can coexist on its surface. Oxygen vacancies in the lattice compensate for the charge deficit owing to the presence of Ce³⁺. Therefore, Ce oxide contains intrinsic oxygen defects [7]. These oxygen defects are the most decisive reason ceria is an important dopant. Numerous studies have focused on the wide range of applications of ceria-based structures. They are important in biomedical research [8,9,10,11], photovoltaics [12,13,14], ionic conductivity [15,16,17], solid oxide fuel cells [18,19,20], industrial catalytic processes [21], automotive catalysts [22], and soot oxidation catalysts [23,24,25].

In this review, we focus on cerium-doped metal oxide, particularly SiO₂ and ZnO. We introduce studies on the luminescence properties, improvement in chemical durability, solar cell characteristics of Ce-doped SiO₂, and its proton-shielding properties in aerospace applications, which have not been extensively studied in other reviews. We also discuss studies on the physical properties of Ce-doped ZnO, a group II-V semiconductor, and its applications in UV photodetectors, reusable water treatment materials, and gas sensors. In addition, we provide insights into the potential use of Ce-doped metal oxide as a galactic cosmic ray (GCR) protection layer in space photovoltaic applications.

2. Ce-Doped SiO₂

Ce-containing or -doped glasses have been studied mainly for UV or X-ray shielding, radiation measurements, and the prevention of darkening effects caused by GCR radiation [26,27]. When Ce is incorporated into the SiO₂ system, a new localized impurity band appears between the valence band maximum (VBM) and the conduction band minimum (CBM) of the SiO₂ system, which is primarily induced by the Ce-4f orbitals. The new localized impurity band that forms after Ce incorporation into SiO₂ is significant because it forms a bridge between the valence and conduction bands, making electronic transitions much more accessible. The Ce-doped SiO₂ system has a lower bandgap energy and a higher absorption coefficient than those of the pure SiO₂ system, which makes it a promising material for optoelectronic applications such as photovoltaic devices, light-emitting diodes, and photocatalysts [28].

J.S. Stroud (1961) measured the changes in the light absorption of Ce-containing glasses and electron spin resonance spectra when exposed to near-UV light [29]. The study showed that Ce³⁺ in SiO₂ was photoionized to Ce⁴⁺ under UV irradiation using samples of 75% SiO₂, 25% Na₂O, and 0.06% cerium oxide, which were melted under various reducing and oxidizing conditions. G.A. Haynes (1970) first reported that Ce-doped cover glass can improve radiation resistance performance. Synthetic fused quartz, which has been used as a radiation-resistant material, is prone to breakage when made thin, and sapphire is expensive [30]. This result shows that the cover glass doped with cerium oxide can perform similarly to the synthetic fused quartz. When the cover glass was doped with 1–2 wt% Ce, the radiation resistance performance improved for electron radiation in the range of ~1 MeV and proton radiation in the range of ~22 MeV. Based on the initial results of these UV-responsive and radiation-resistant properties, further research on their application in specific fields was conducted.

While bulk Si does not emit light, Si nanocrystals in silicon-rich silicon oxide films exhibit luminescence in the visible-near-infrared range, and the optical properties of these films can be further tuned via doping with optically active lanthanide atoms. Among them, Ce³⁺ ions have drawn interest because they are expected to luminesce in the ultraviolet blue range, making Ce-based materials useful for various applications. Weimmerskirch-Aubatin and Jennifer et al. (2015) investigated the optical properties of Ce-doped SiO₂ thin films, specifically their luminescence behavior with respect to the Ce content and annealing temperature [31]. Ce-doped thin films were prepared by co-evaporating Ce from a Knudsen cell heated to 1400 °C in an ultra-high vacuum chamber and SiO₂ powder from an electron beam gun at room temperature (RT). The concentration of Ce, defined as [Ce]/([Si] + [O] + [Ce]), was varied from 1 to 6% and the thickness was controlled at 200 nm using a quartz microbalance. The detection of significant blue luminescence at RT from the as-deposited films suggests that these materials have the potential for efficient light emission. At 1100 °C, developing a cerium silicate phase opens up the possibility of synthesizing novel Ce-based materials with customized optical characteristics (Figure 1). These findings imply that the luminescence of the films can be controlled by altering the Ce content and annealing temperature, enabling the creation of blue light-emitting diodes compatible with Si technology.

In addition to enabling wavelength tuning, Ce doping also improves luminescence characteristics. M. L. Su et al. (2022) investigated the enhanced luminescence characteristics achieved via the Ce doping of a CsPbBr₃ precursor and nanocrystal glass [32]. CsPbBr₃ has high quantum efficiency and color-tunable radioluminescence. However, their environmental stabilities remain unclear. Glass ceramics embedded with nanocrystals possess excellent luminescent properties and good mechanical and chemical stability related to the glass matrix [33]. Additionally, in borosilicate glass, the Ce³⁺ ion exhibits efficient blue emission (~420 nm), matching the excitation spectrum of CsPbBr₃ nanocrystals [34]. Incorporating Ce³⁺ ions into the glass matrix effectively transfers the X-rays absorbed by the glass to the CsPbBr₃ nanocrystals, thereby enhancing the luminescence intensity.

To begin, SiO₂, B₂O₃, CaO, Li₂CO₃, Sb₂O₃, PbBr₂, CsBr, and CeO₂ were mixed in a crucible and melted at 1200 °C for 15 min. The glass melt was placed in a warmed copper mold at 350 °C, annealed at 400 °C for six hours, and then cooled down to RT to reduce the thermal tension inside the precursor glass (PG). Subsequently, the samples were heat-treated at 550 °C for different durations, indicated as CsPbBr₃ nanocrystals–glass (CsPbBr₃ NG). Differential thermal analysis (DTA), X-ray diffraction (XRD), UV/VIS/NIR spectrophotometry, PL spectroscopy, and quantum efficiency measurements were performed.

They incorporated Ce³⁺ ions into the borosilicate glass matrix and enhanced green emission under 365 nm UV and X-ray irradiation. The luminous quantum efficiency reached up to 28% after heat treatment at 550 °C for 12 h. The X-ray-stimulated luminescence intensity of Ce³⁺-doped CsPbBr₃ nanocrystals-glass was approximately 190% greater than that of undoped CsPbBr₃ nanocrystals–glass, showing the effective absorption of ray energy by Ce³⁺ ions. Following heat treatment, CsPbBr₃ nanocrystals precipitated from the glass matrix, resulting in high light absorption below 475 nm. The transparency of the CsPbBr₃ nanocrystals–glass decreased marginally with an increasing heat treatment time owing to the progressive development of nanocrystals. The CsPbBr₃ NG 550-12 h sample demonstrated high luminous intensity and reasonably high transmittance (80% at 527 nm) in the emission range. The photoluminescence peaks of the CsPbBr₃ nanocrystals changed from 490 to 527 nm as the heat treatment duration increased, indicating a quantum size impact. The PL and XEL intensities of Ce³⁺-doped CsPbBr₃ NG 550-12 h were enhanced by 1.4 and 1.9 times, respectively, compared to those of the undoped CsPbBr₃ NG (Figure 2).

The CsPbBr₃ nanocrystal–glass composite scintillators demonstrate vigorous luminescent intensity and moderately high transmittance, making them appropriate for scintillator-based indirect-type X-ray detectors; the introduction of Ce³⁺ ions into the borosilicate glass matrix enhanced the green emission and higher X-ray-stimulated luminescence intensity in CsPbBr₃ nanocrystal–glass composite scintillators. The generated CsPbBr₃ nanocrystal scintillators based on Ce³⁺-doped borosilicate glass are considered viable candidates for scintillator-based indirect-type X-ray detectors owing to their low cost, low detection limit, color-tunable emission, and fast decay characteristics.

Optical or thermally stimulated luminescence has been reported in Ce-doped glass rather than the steady-state luminescence mentioned above. G. Okada et al. (2016) reported that Ce-doped SiO₂ glass exhibited optically stimulated luminescence (OSL) and thermally stimulated luminescence (TSL) [35]. Ce-doped SiO₂ glass was prepared using spark plasma sintering, a solid-state reaction technique, instead of conventional methods such as sol–gel synthesis. SiO₂ (99.9% purity) glass nano-sized powder and CeO₂ (99.99% purity) powder were mixed in a weight ratio of 100:1. The mixture was then loaded into a graphite die and sealed with graphite punches. Subsequently, the assembly was sintered in a furnace under vacuum, with a temperature increase from 600 °C to 1300 °C at a rate of 10 °C/min and a pressure of 70 MPa. The prepared samples were characterized using various techniques. The PL spectrum was measured using 265 nm excitation light, and the X-ray-induced luminescence (XL) spectrum was measured using an X-ray tube. The optically stimulated luminescence (OSL) emission spectrum was measured via stimulation using a 630 nm LED, and the thermally stimulated luminescence (TSL) glow curve was measured using a reader (Figure 3). The linear relationship between the OSL and TSL intensities and the incident radiation dose over the range of 1 mGy to 2 Gy indicates the potential for accurate dose measurement. Thus, Ce-doped glass is a good radiophotoluminescent (RPL) material for practical applications.

Cerium oxide has also drawn interest owing to its potential therapeutic applications. Its close electronegativity and ionic radii make it a suitable replacement for calcium in bioactive systems. Ce³⁺ ions possess antimicrobial properties and low toxicity [36,37]. They are recognized for their ability to produce collagen, which increases the mechanical strength and elasticity of skin [38].

P. Kaur et al. (2020) showed that the addition of cerium oxide (CeO₂) to bioceramic samples affected their bioactivity, degradation rate, cell survival response, and antioxidant characteristics [39]. The bioceramic samples were created via a sol–gel technique, employing Si(OC₂H₅)₄ and (C₂H₅)₃PO₄ precursors and alkaline earth metal sources, including Ca(NO₃)₂·4H₂O, Mg(NO₃)₂·6H₂O, and Ce(NO₃)₃·6H₂O. The acid–catalyst sol–gel technique involves six phases and utilizes nitric acid as a hydrolysis catalyst. The samples were kept, aged, calcined, and described using agar and mortar. The Ce-doped CaO–P₂O₅–MgO–SiO₂ bioceramic samples were subsequently crushed for fine powder manufacture.

XRD analysis was performed to obtain XRD patterns and compare them with standard data files. Raman spectroscopy was used to examine the vibrational modes before and after in vitro analysis. Zeta potential experiments were performed to measure the fluctuations in zeta potential before and after in vitro analysis.

Samples with higher cerium concentrations exhibited a decrease in the growth rate of hydroxyapatite (HA) and the synthesis of cerium phosphate instead of HA on their surfaces. Despite the decrease in the HA growth rate, samples with more significant concentrations of cerium exhibited maximum cell survival and were protected against oxidative stress and death caused by hydrogen peroxide. The sol–gel synthesis technique enhanced the growth rate of the hydroxyapatite layer compared to the conventional melt-quenching route. Zeta potential measurements indicate shifting surface charge signs with time when the samples were immersed in simulated bodily fluid, indicating the adsorption of cations and precipitation of phosphate ions.

Overall, the addition of Ce to the bioceramic samples improved their chemical durability and antioxidant capabilities, whereas the development rate of HA was impaired at higher Ce concentrations (Figure 4).

Conventional crystalline silicon solar cells (c-Si) have limitations in terms of energy loss, particularly the ineffective utilization of short-wavelength photons in the UV region [40]. To improve the efficiency of c-Si solar cells, the use of down-conversion (DC) [41,42], down-shifting (DS) [43,44,45], and up-conversion (UP) layers [40,46] in solar cells has received attention. DC layers can convert high-energy UV photons into lower-energy visible or near-infrared photons, thereby improving the performance of the solar cells. Rare-earth-doped phosphors, such as SiO₂ co-doped with Ce and Tb, have been explored as DC layers to enhance the performance of solar cells.

D. A. Kumi et al. (2020) examined the ideal mole percentage ratios of SiO₂–Ce³⁺ Tb³⁺-based phosphors for achieving the highest emission intensity [47]. The nanophosphors were integrated into EVA films and placed as a DC layer over commercial silicon solar cells.

SiO₂ codoped with Ce and Tb was synthesized using an ultrasonification approach combined with the modified Strober method. The precursors were dissolved in ethanol, followed by the addition of tetraethyl orthosilicate and threefold distilled water to initiate precursor hydrolysis. The resulting precipitates were washed, dried, and annealed in a H₂/Ar atmosphere. The luminescent and optical properties of the produced phosphors were studied, and the phosphors were integrated into poly EVA films overlaid on a commercial silicon solar cell. The transmittance and UV absorption capability of the phosphor films were measured, and current density–voltage tests were performed on Si solar cells with and without the phosphor–EVA composite layers to evaluate the photocurrent efficiency.

SEM images revealed that the SiO₂-Ce³⁺ Tb³⁺ phosphor powder contained spherical particles with a size distribution ranging from approximately 25 to 275 nm and an average diameter of 148 nm. The phosphor films showed good transmittance (approximately 76%) and UV absorption capability compared with conventional poly EVA films. The photocurrent efficiency of the silicon solar cell improved when the phosphor–EVA composite layers were placed on top, with specific compositions exhibiting an increase in efficiency (2.39%) and others showing negligible decreases relative to that of plain EVA films (Figure 5).

The study provides insights into the energy transmission between Ce and Tb ions in phosphors, which can be employed to adjust the composition of phosphor EVA composite films for enhanced performance in solar cell devices. The results of this study will contribute to the development of more efficient and cost-effective solar cell technologies that adopt DC layers to optimize the usage of short wavelength photons and improve the overall performance of c-Si solar cells.

As mentioned above, the Ce-doped cover glass studied by NASA is commercially available. Borosilicate glass doped with cerium dioxide is being used in space applications. It is used for UV, electron, and proton radiation protection, and its utilization is further enhanced by matching its thermal expansion coefficient to that of Si or GaAs [48]. G. Yang et al. (2016) analyzed the performance degradation of high-specific-power CdTe thin-film solar cells produced via high-energy proton irradiation [49]. The study identified uniformly distributed defects and optical transmittance loss as the main factors contributing to the reduction in cell performance, specifically in terms of current-level diminution, such as short-circuit current density (J_SC) (Figure 6). This is mainly owing to the transmittance loss caused by the darkening effect, as mentioned in the introduction.

To avoid performance degradation caused by the darkening effect, D. A. Lamb et al. (2017) introduced Ce-doped cover glasses as substrates for CdTe solar cells [50]. CdTe solar cells were directly fabricated on the Ce-doped cover glasses, and the CdTe devices were irradiated under various proton irradiation conditions. Consequently, at the lowest fluence of 10¹² cm⁻², the relative efficiency of the solar cells was reduced by just 5%. This dosage of protons may plausibly be anticipated to represent that encountered by a 20-year geostationary Earth orbit (GEO) mission. Once the proton dosage was raised to 10¹³ cm⁻² and 10¹⁴ cm⁻², the solar cell relative efficiency declined to 82% and 4%, respectively (Figure 7). This reaction to proton radiation was better than that observed in prior research employing CdTe and can be ascribed to the utilization of the Ce-doped cover glass. The degradation of CdTe solar cells owing to high-intensity proton doses was explained using SCAPS modeling. The 10¹³ cm⁻² dose sample showed an excellent fit with a decrease in the acceptor concentration (N_a) without any change in the trap density, which was unexpected but supported by the C-V measurements. This study provides insights into the effects of proton irradiation on CdTe solar cells, including changes in carrier concentration, acceptor doping, and the formation of interstitial H⁺ created by proton irradiation, resulting in shallow donor levels.

Perovskite solar cells have drawn significant attention as potential alternatives to traditional silicon solar cells, owing to their high power conversion efficiency and low cost. Owing to their low cost and high specific power (Watt/gram) [51], they can replace conventional CdTe and Si photovoltaics in outer space applications. However, harsh conditions, such as the presence of high-energy particles, thermal cycling, and a vacuum, can lead to solar cell degradation in outer space. This degradation can cause ionization, atomic movement, and defects in the molecular structure, resulting in a decreased output and potential damage to semiconductor devices. Cyclic radiation, notably protons with energies close to 1 MeV, can cause severe damage to solar cells owing to their high-stop cross-section. To prevent damage to perovskite thin films, it is necessary to develop measures against proton collisions for space solar applications.

In 2015, proton radiation studies on PSCs were reported [52]. In 2016, F. Lang et al. reported that the irradiation of perovskite solar cells with protons with an energy of 68 MeV did not affect the open-circuit voltage (V_oc) and fill factor (FF). However, proton irradiation led to J_sc degradation, thus reducing the power conversion efficiency of the device (Figure 8a–c) [53]. In 2019, the same group evaluated the performance of superstrate-configuration (inverted structure) perovskite solar cells by irradiating protons with energies varying from 10 to 1000 MeV, instead of a single energy. They found that all PV parameters slightly decreased after proton irradiation (Figure 8d–f) [54]. Space solar cells use Ce-doped glass as a substrate; however, they are heavy and expensive. Ce-doped metal oxide is expected to protect space solar cells from GCR. As the specific power of space solar cells is essential to minimize their weight, a Ce-doped metal oxide deposited on a lightweight substrate can replace the current heavy space glasses. In addition, space solar cells can be made flexible if Ce-doped metal oxides and perovskites are deposited onto flexible substrates.

3. Ce-Doped ZnO

Zinc oxide (ZnO) is a white, odorless, tasteless, solid type II–VI semiconductor. It is relatively abundant and inexpensive to produce. ZnO has a wide bandgap (3.37 eV at room temperature [55]; therefore, it is transparent in the visible region of its spectrum. ZnO exhibits piezoelectric properties, indicating that it can generate a mechanically deformed electric field. Owing to its desirable characteristics, ZnO has been employed in various applications, such as optoelectronics [56,57,58], electronics [59,60,61], and healthcare [62,63]. Optoelectronic devices are widely used in LEDs, solar cells, and photodetectors. In addition, they are used in transistors and sensors. Ce can be doped into ZnO to provide better optoelectronic performance, and many studies have reported improved Ce-doped ZnO performance.

Khuili et al. (2022) discovered that doping ZnO with rare-earth elements has a substantial impact on its optoelectronic and magnetic properties, resulting in an enlarged bandgap and a shift in the Fermi level to the conduction band, indicating n-type features [64]. The electrical conductivity of doped ZnO increases, making it suitable for use in integrated optoelectronic devices. Furthermore, rare-earth dopants create significant magnetic moments in the doped ZnO, broadening their potential uses in spintronics. The band structure, partial density of states (PDOS), and total density of states (DOS) were calculated to comprehend the electronic properties of the doped ZnO compounds (Figure 9). The DFT and reflectivity spectra provided insights into the potential applications of Tm-, Yb-, and Ce-doped ZnO compounds in integrated optoelectronic devices, solar cells, and spintronics.

Kulandaisamy et al. (2016) investigated the effect of Ce dopants on the acetone and ethanolamine sensing characteristics of ZnO, providing insights into the microstructural and sensing mechanisms of Ce-doped ZnO nanostructured films prepared using spray pyrolysis [65]. Ce-doped ZnO thin films were deposited on a glass substrate via spray pyrolysis and characterized using XRD and SEM to determine their structural characteristics, lattice parameters, average crystallite size, and surface morphologies. They controlled the doping concentration with the precursor amount, and the Ce-doped ZnO thin films exhibited a decreased grain size and increased bandgap with an increase in the Ce dopant concentration. The sensors exhibited better RT sensing responses to acetone and ethanolamine at specific Ce dopant concentrations (Figure 10). The study demonstrates the potential of using Ce-doped ZnO sensors as a noninvasive and cost-effective method for disease detection with high selectivity to acetone and ethanolamine and swift response and recovery times.

Zhang et al. (2021) used a simple and controlled hydrothermal process to fabricate porous Ce-doped ZnO (CZO) nanosheets [66]. To prepare the CZO composites, different concentrations of Ce (0.5, 1, 3, and 5 at%) were doped into ZnO. This synthesis involves the substitution of Zn ions with Ce ions, which causes lattice deformation and the production of oxygen vacancy (OV) defects. The physical properties of the samples were evaluated using XRD, SEM, TEM, and AFM microscopy. The sensing capabilities, including their responsiveness and the selectivity of the CZO composites, were examined at RT. They found that the sensing capabilities of the sensors increased owing to the increased number of oxygen vacancies formed by Ce doping. Based on density functional theory (DFT) calculations, they discovered that Ce doping effectively reduced the band gap and increased the absorption energy between ZnO and aniline (Figure 11).

The OV in CZO has been shown to facilitate CO₂ adsorption and activation, suggesting that OV defect engineering can be used to improve CO₂ electrocatalytic reduction activity. Ce doping was utilized to introduce OV into ZnO, and the doping concentration of the Ce³⁺ dopant may have adjusted the concentration of OV in Ce_xZn_1-xO. Ren et al. (2021) proposed a method for generating high-performance CO₂ electrocatalytic reduction (ER) catalysts by varying the number of oxygen vacancies [67]. A hydrothermal technique was employed to fabricate the Ce-doped ZnO catalysts with different OV concentrations. The concentration of the Ce³⁺ dopant was varied to regulate the concentration of oxygen vacancies. The increased OV in Ce-doped ZnO improves CO₂ adsorption, lowers the thermodynamic energy barrier, and accelerates electron transfer kinetics during CO₂ ER (Figure 12). The use of Ce-doped ZnO catalysts with regulated OV concentrations can help produce efficient and selective CO₂ reduction electrocatalysts, most likely enabling CO₂ conversion into valuable fuels to reduce climate change.

UV photodetectors (PDs) are essential for various applications, including sensors, military defense, and optical communications [68,69,70]. ZnO is a promising material for UV PDs owing to its outstanding features, such as large carrier concentration, significant absorption coefficient, and acceptable bandgap [71]. Doping ZnO with Ce can boost its performance by decreasing the carrier recombination [72]. However, the effect of Ce on the UV photoresponse of ZnO is yet to be established.

M. Shkir et al. (2022) explored Ce-doped ZnO thin-film devices for UV photodetection [73]. Spray pyrolysis can be used to produce ZnO photodetectors with varying wt.% values of Ce dopants. The Zn and Ce precursors were dissolved in water, mixed, and coated onto pre-cleaned glass via spray pyrolysis. During film deposition, the temperature of the glass substrate was uniformly controlled at 450 °C via a temperature controller. The 2D piston motion in both the x- and y-dimensions and the spraying conditions were accurately controlled by software.

An evaluation of the UV detection capabilities of the constructed devices showed that the ZnO:Ce(2.0%) sample displayed good UV photodetection properties, with a maximum responsivity of 0.27 A/W, detectivity of 63%, and external quantum efficiency of 2.18 × 10¹⁰ Jones. This highlights their potential use as UV photodetectors. The photoresponse results showed that the current increased with increasing Ce concentrations and was saturated at 2 wt%, indicating the higher conductivity of ZnO:Ce(2.0). The optical characteristics of ZnO were altered by Ce doping, as the dopant altered the electronic structure. The absorption spectra indicate the influence of Ce on the optical properties of ZnO. The topographies and morphologies of the ZnO and ZnO:Ce films were examined using AFM. Ce doping altered the surface topography, with the ZnO:Ce(2.0) film showing uniformly sized grains without any fissures.

The fast transfer of photogenerated charge carriers was enabled by the ohmic contact between the ZnO device and the Ag electrode, contributing to the short response and recovery times of the devices. The photodetection mechanism involved interactions between the excitons and O₂ ions in the depletion layer, contributing to the improved performance of the ZnO:Ce devices (Figure 13). These promising device properties, including responsivity, detectivity, and EQE, render the ZnO:Ce(2.0%) sample a potential candidate for UV photodetection applications.

Ce-doped ZnO nanowires have the potential to be used in water treatment. Semiconductor metal oxides, such as TiO₂ and ZnO, have been extensively used for photocatalytic treatment because of their excellent photodegradation activity, high abundance, low cost, non-toxicity, and high stability [74,75,76]. However, ZnO semiconductors have limitations owing to their broad bandgap energy, which limits their photocatalytic capabilities [77,78]. Doping ZnO nanostructures with rare-earth metals has been shown to be useful because it boosts the light absorption capabilities and the separation of charge carriers.

S. Choudhary et al. (2023) investigated a simple and rapid process for fabricating Ce-doped ZnO nanowires using a thermal decomposition approach [79]. Moreover, they discussed the impact of the Ce doping concentration on the lattice characteristics, crystallite size, and light absorption capacity. Ce-doped ZnO nanowire manufacturing involves thermal decomposition using zinc acetate and cerium nitrate as precursor ingredients. A mixture of the precursor salt and cerium dopant is blended to achieve a final weight of 2 g. The mixture is then transferred to alumina crucibles for thermal treatment at 350 °C for 30 min in a muffle furnace. The off-white powder elements are finely pulverized using a mortar and pestle. Pure ZnO samples are classified as ZPW, while samples containing a variable Ce dopant are denoted as ZCW1 (Ce 0.5 wt%), ZCW2 (Ce 1 wt%), and ZCW3(Ce 1.5 wt%). The impact of cerium doping on the nanowire structure, optical characteristics, and photodegradation behavior was studied using XRD, FE-SEM, Raman spectroscopy, XPS, PL, and UV-vis absorption spectroscopy. The photocatalytic response of the nanowires was tested for the degradation of cationic and anionic dyes, and the photodecomposition efficiency was calculated.

Ce-doped ZnO nanowires displayed remarkable photodegradation efficacy in the removal of organic contaminants from water. ZnO nanowires doped with 0.5% Ce demonstrated the best photodecolorization performance, with photodecomposition efficiencies of 96.6% for MB, 92.2% for MG, 67.1% for MO, and 77.7% for RhB dye removal in 16 min of sunlight exposure (Figure 14a). The strain of the produced samples was estimated using the Williamson–Hall equation, which provides information on the lattice spacing and crystallite size.

The remarkable reusability and stability of the nanowires further enhance their practical use in wastewater treatment systems (Figure 14b,c). Utilizing the thermal decomposition approach for the synthesis of Ce-doped ZnO nanowires provides a facile and rapid method for their manufacture, which can be extended to large-scale manufacturing.

4. Conclusions

Ce-doped materials, specifically Ce-doped SiO₂ and ZnO, have shown promising results in various optoelectronic applications. Ce-doped SiO₂ has been studied for its UV and X-ray shielding properties, light emission, and proton protection in space applications. Ce-doped SiO₂ thin films have been found to exhibit luminescence in the visible-near-infrared range, and their luminescence behavior can be controlled by altering the Ce content and annealing temperature. Incorporating Ce³⁺ ions into a borosilicate glass matrix improved the green emission of the nanocrystal–glass composite and increased the X-ray-stimulated luminescence intensity. Ce doping has also been used to improve the chemical stability and antioxidant capabilities of bioceramics, the short-circuit current of c-Si solar cells, and the OSL and TSL of RPL materials. A Ce-doped cover glass can be utilized as a substrate for CdTe solar cells to alleviate the performance degradation caused by high-energy proton irradiation. The relative efficiency of the solar cells was reduced by only 5% at the lowest fluence of protons, and even at higher doses, the degradation was better than that in previous research on CdTe without Ce-doped glass.

Ce-doped ZnO has been investigated owing to its improved optoelectronic and magnetic properties. Doping ZnO with rare-earth elements, including Ce, has been found to increase the bandgap and electrical conductivity of ZnO, making it suitable for integrated optoelectronic devices and spintronics. Ce-doped ZnO has also shown potential for gas sensing applications with improved sensing characteristics for sensing acetone and ethanolamine. The sensing capabilities of Ce-doped ZnO sensors can be further enhanced by increasing the Ce concentration, which creates increased oxygen vacancies and improves the responsiveness and selectivity. Doping ZnO with 2 wt% Ce resulted in a 63% improvement in UV detection performance over the undoped sample, and doping the nanowire-shaped sample demonstrated its performance as a reusable water treatment material. Table 1 summarizes the research on various applications of Ce-doped SiO₂ and ZnO introduced in this paper.

Overall, Ce-doped materials, such as Ce-doped SiO₂ and Ce-doped ZnO, have great potential in various fields, including radiation shielding, light emission, and gas sensing. Further research and development in these areas could lead to advanced materials with customized optical and electrical properties for various applications. Overall, the commercialization of Ce-doped SiO₂ and ZnO is still in its early stages. However, growing interest in the industry and investment in technology development suggest that these materials have the potential to be widely used in the future.

Author Contributions

Investigation and resources, M.C.; writing—original draft preparation and visualization, S.K., writing—review and editing, supervision, J.P. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2022R1I1A3069502), and this work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (RS-2023-00257494).

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Table

Figure 1. (a) Room-temperature steady-state PL spectrum (blue solid line) from an as-deposited 3% Ce-doped SiO2 film excited at 325 nm and the corresponding PL excitation (PLE) spectrum (blue dotted line) measured at 450 nm. (b) Evolution of the PL spectra of the 3% Ce-doped SiO2 film as a function of the annealing temperature (adapted with permission from Ref. [31]. Copyright 2014 Elsevier).

Figure 2. PL spectra under 365 nm excitation. (a) CsPbBr3 NG in different heat treatment times; (b) undoped Ce3+ CsPbBr3 NG and 0.3 mol% Ce3+-doped CsPbBr3 NG. (c) Fluorescence decay curves of three different samples under the excitation of 365 nm; (d) XEL spectra of 0.3 mol Ce3+-doped PG, undoped, and Ce3+-doped CsPbBr3 NG 550-12 h samples (adapted with permission from Ref. [32]. Copyright 2021 Elsevier).

Figure 3. (a) Normalized OSL emission and stimulation spectra of SiO2:Ce3+ glass and (b) TSL spectrum recorded at 250 °C (adapted with permission from Ref. [35]. Copyright 2016 Elsevier).

Figure 4. Apoptosis induction in MG-63 cells after 24 h treatment with test samples as analyzed using flow cytometry ((live cells (LL quadrant), early apoptotic (LR quadrant), late apoptotic (UR quadrant) and necrotic (UL quadrant)). Results indicate the maximum live cells for the Ce 5 sample (adapted with permission from Ref. [39]. Copyright 2019 Elsevier).

Figure 5. (a) SEM image of SiO2 doped with Ce3+, Tb3+, and (b) current–voltage (I–V) curve for the sample which showed the highest increase in conversion efficiency with the poly EVA film layer (adapted with permission from Ref. [47]. Copyright 2019 Elsevier).

Figure 6. (a) Magnet for proton beam adjustment and (b) proton exit. (c) Schematic diagram of proton irradiation on CdS/CdTe solar cell grown on the ultra-thin glass substrate. (d) Transmittance graph and (e) photograph of 100 mm thick glass substrates before and after proton irradiation (15 MeV, 1 × 1014 cm−2) (adapted with permission from Ref. [49]. Copyright 2016 Wiley-VCH).

Figure 7. (a) Structure and (b) photograph in Perspex sample holder of CdTe solar cell on ultra-thin cerium-doped cover glass. (c) SRIM simulation of 0.5 MeV proton longitudinal penetration in the CdTe solar cell. Mean J-V parameters of each sample after proton irradiation, expressed as a ratio to unirradiated values versus proton fluence (cm−2). (d) Mean efficiency; (e) mean Jsc; (f) mean Voc; (g) mean FF.

Figure 8. (a) Structural diagram of perovskite solar cell with glass/ITO/PEDOT:PSS/MALHs/PCBM/BCP/Ag structure, (b) SEM cross-sectional image, (c) PV parameter comparison with crystalline silicon device after proton irradiation (adapted with permission from Ref. [53]. Copyright 2016 Wiley-VCH), (d) simulation results of proton irradiation on SiO2 thin film, (e) structure of glass/ITO/PTAA/CsMAFAPb/C60/BCP/Cu structure of a perovskite solar cell, and (f) SEM cross-sectional image (adapted with permission from Ref. [54]. Copyright 2019 RSC Publishing).

Figure 9. Band structure of (a) pure ZnO and (b) Ce-doped ZnO. (c) The partial density (PDOS) and total density of the states (TDOS) of pure and rare-earth-doped ZnO (adapted with permission from Ref. [64]. Copyright 2020 Elsevier).

Figure 10. SEM images of undoped (a) and Ce-doped ZnO (b–f) samples, and (g) sensing response of undoped samples. Ce-doped ZnO samples to 100 ppm of ethanolamine, acetone, ammonia, and acetaldehyde vapors (adapted with permission from Ref. [65]. Copyright 2016 Elsevier).

Figure 11. (a) Response of pristine ZnO and CZO-1 to various gases at RT. (b) Long-term pure ZnO and CZO-1 aniline stability at room temperature. The favorable configurations of (c) ZnO@aniline and (d) OV-CZO@aniline. (e) Band structures for pristine ZnO (green) and ZnO@aniline (blue). (f) Band structures for OV-CZO (green) and OV-CZO@aniline (blue) (adapted with permission from Ref. [66]. Copyright 2021 Elsevier).

Figure 12. (a) Ce 3d XPS spectra of CexZn1−xO, (b) the calculated ratio of integral areas for the OV, (c) CO2 adsorption isotherms at 25 °C and (d) CO2-TPD spectra (adapted with permission from Ref. [67]. Copyright 2021 Elsevier).

Figure 13. (a) Photoluminescence spectra of ZnO and ZnO:Ce thin films and (b) responsivity, EQE, and the detectivity of the fabricated ZnO and Ce:ZnO devices (adapted with permission from Ref. [73]. Copyright 2022 Elsevier).

Figure 14. (a) Decomposition efficiencies of the ZCW1 sample in the removal of various organic dyes, (b) recyclability of ZnO nanowires doped with Ce in sample ZCW1 for four successive cycles to the photodecomposition of MB under sunlight irradiation, and (c) XRD patterns of the ZCW1 photocatalyst after undergoing repetitive photocatalytic treatments under identical experimental conditions (adapted with permission from Ref. [79]. Copyright 2022 Elsevier).

Table 1

Summary of Ce-doped SiO₂ and ZnO research results for various applications.

Materials	Applications	Purpose	Results	Fabrication Method	Ref.
Ce doped SiO2	Optoelectronic	Tuning of optical properties	strong blue luminescence at room temperature	Co-evaporation	[31]
		luminescence characteristics	vigorous luminous intensity and reasonably high transmittance (80% at 527 nm)	Meltquenching	[32]
			a strong band emission in the UV-blue range due to Mie scattering	spark plasma sintering	[35]
	Biomedical	chemical durability and antioxidant capabilities	a decrease in growth rate of HA and protection against the H2O2 stress	Sol-gel	[39]
	Photovoltaic	Improvement of solar cell efficiency	2.39% efficiency increase in the c-Si solar cell applied to the EVA	Strobermethod	[47]
		proton radiation protection	5% relative efficiency decrease in the solar cells at a proton fluence of 1012cm−2	Commercial product	[50]
Ce doped ZnO	Principle	Analysis of optoelectronic and magnetic properties	Identification of changes in the optoelectronic and magnetic properties of ZnO due to the presence of 4f electrons originating from rare earth element doping	Simulation	[64]
	Gas sensor	acetone and ethanolamine sensing	acetone response(S = 252 for 500 ppm) ethanolamine response(S = 1850 for 500 ppm)	spray pyrolysis	[65]
		anilinesensing	aniline response (S = 15.1 for 100 ppm)	hydrothermal	[66]
		electrocatalytic CO2 reduction to CO	current density of 24 mA cm−2 and Faradaic efficiency of 88% for CO at −1.0 V versus RHE	hydrothermal	[67]
	Optoelectronic	UV detection	UV responsivity: 0.27 A/W; detectivity: 63%; EQE: 2.18 × 1010 Jones.	Spraypyrolysis	[73]
	Water treatment	removal of various organic dyes	Improvement in decomposition efficiencies in removal of various organic dyes	thermal decomposition	[79]

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