The multiple roles of rare earth elements in the field of photocatalysis

Abstract

When illuminated by light, different electron transitions occur within the energy levels due to the unique 4f energy level structure of rare earth elements. If rare earth elements are introduced into clean and sustainable photocatalysis technology, the electronic transitions in the energy level structure of the catalyst may change, forming a unique reaction mechanism and producing excellent catalytic performance. In recent years, there have been more and more studies and reports on rare earth photocatalysts due to the large number and different properties of rare earth elements. It is essential to collate and summarize these reports to gain a comprehensive understanding of their functions and applications. This article presents a review of the role and application scope of rare earth photocatalysts and provides a broad outlook on their potential future uses. We hope that this work can provide a new perspective for the development and design of novel and efficient rare earth photocatalysts.

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Jing An

Jing An received her B.S. degree in chemistry from Chifeng University in 2021. Now she is a postgraduate student at Heilongjiang University, under the supervision of Prof. Guofeng Wang and Prof. Ruihong Wang. Her current research interests focus on photocatalytic CO2 reduction.

Yang Qu

Qu Yang is currently a full professor at Heilongjiang University. He received his Ph.D. degree in Physical Chemistry from the Institute of Theoretical Chemistry at Jilin University. Since July 2014, he has been working at the Key Laboratory of Functional Inorganic Materials Chemistry, Ministry of Education, at Heilongjiang University. His research mainly focuses on the development of nano-composite oxide-based photocatalysts for environmental applications and the study of their mechanisms in pollutant transformation and control processes. To date, he has published over 50 research papers in high-impact SCI journals such as Angewandte Chemie International Edition, Advanced Materials, Advanced Science, Applied Catalysis B: Environmental, Chemical Engineering Journal, Nano Research, Journal of Hazardous Materials, etc., with more than 3200 citations to his work. Currently, he serves as the editorial board member for Chinese Chemical Letters, Rare Metals, and Eco-Environment & Health (EEH).

Guofeng Wang

Prof. Guofeng Wang received her M.S. degree in Physics from Northeast Normal University in 2005 and her Ph.D. degree in Electronic Science and Engineering from Jilin University in 2008. She joined the research group of Professor Yadong Li as a postdoctoral associate in 2009. She has been working at Heilongjiang University since 2010. She is the head of the Department of Materials Chemistry of the School of Chemistry and Materials Science at Heilongjiang University. She won the title of excellent postdoctoral student of Tsinghua University. She was awarded the title of “Outstanding Person of the New Century” by the Ministry of Education.

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1. Introduction

As is well known, with the continuous population growth, the demand for non-renewable resources such as coal, oil, and natural gas is rapidly increasing.^1,2 However, since fossil energy reserves are limited, they cannot be exploited indefinitely.^3,4 It is very important to find sustainable and clean energy to replace fossil energy to maintain normal lifestyles. In 1972, with the proposal of the Honda–Fujishima effect for the first time, it was considered as the beginning of a new era of multi-phase photocatalytic reactions.⁵ In the following decades, photocatalytic technology has developed rapidly.^6–11 People have discovered that photocatalytic reactions play significant roles in many research fields, including photocatalytic CO₂ reduction, pollutant degradation, multi-carbon-based material synthesis, water pollution control, and others.^12–22 The mechanism of the photocatalytic reaction has also been confirmed by scientists: when a beam of light is irradiated on the semiconductor photocatalyst, the energy of the incident light (hυ) is higher than the forbidden bandwidth (E_g) of the semiconductor, the electrons (e⁻) in the valence band are excited to transition to the conduction band, and at the same time, the valence band produces holes (h⁺), and under the action of the electric field, the e⁻ and h⁺ diffuse to the surface of the catalyst, and the e⁻ and h⁺ are separated efficiently so that the conduction band and the valence band have the capacity to oxidize and reduce, respectively, which can have a redox reaction with the surface adsorption of the material to produce a product in line with the expectation of the people.^23–26 Therefore, photocatalytic reactions are one of the most promising solutions to the environmental and energy crisis through the efficient and rational use of sunlight for redox reactions. However, despite the relatively mature development of photocatalytic technology currently, photocatalysts still commonly encounter many issues, such as insufficient active sites, high photogenerated electron–hole recombination rates, small light absorption ranges, poor product selectivity, etc., which seriously hinder large-scale practical application of photocatalysts.^27–32

In 1794, the first rare earth element yttrium (Y) was extracted from an ore, and in the following 150 years, a total of 17 rare earth elements were extracted.³³ With the deepening of research, people have discovered that rare earth elements have special electronic structures. The unfilled 4f electron sublayer (4f₀–4f₁₄) is shielded by the outermost 5s and 5p orbitals, allowing 4f level electrons to localize within the atom.³⁴ As a result, rare earth elements are capable of producing very rich electronic energy levels and unique optical properties.³⁵ The introduction of rare earth elements into photocatalysts may form unique electronic transitions between the energy levels of the catalyst, greatly improving the activity of the photocatalyst. Since different rare earth elements have different energy level structures, rare earth elements have different roles and effects in different systems and reaction conditions. Due to the varying electronic structures and response to light of each rare earth element, rare earth elements will exhibit unique mechanisms of action and catalytic effects in different catalytic systems. For example, Joanna Nadolna and his colleagues characterized and analyzed TiO₂ doped with Er³⁺, Ho³⁺, Nd³⁺, and Tm ions. They analyzed the action mechanism of the catalysts by referring to the energy level structures and excitation paths of the different rare-earth elements, and then further verified it by theoretical calculations and summarized the mechanism of action of lanthanide metals in the catalysts.³⁶ Usually, the doping of rare earth ions can enhance the activity of catalysts in many ways, such as narrowing the band gap of the catalyst, enhancing optical adsorption ability, expanding the light response range, improving the efficiency of electron–hole separation, and forming defect vacancies.^37–39 If the energy level structure and mechanism of rare earth elements can be reasonably designed for photocatalytic reactions, the application range of rare earth elements in various fields will be expanded, and more new choices will be provided for the catalytic field. Currently, photocatalytic reactions involving rare earth ions are mainly carried out in four forms: rare earth ion-doped photocatalysts, heterogeneous composite photocatalysts, single atom photocatalysts, and other rare earth based new photocatalysts.^40–43 Here, rare-earth ionic photocatalysts reported in recent years are summarized and analysed, and the role of rare-earth photocatalysts in the reaction process is outlined.^44–46 The challenges and prospects of rare-earth photocatalysts in practical applications are also discussed, and the specific work is categorized as shown in Fig. 1. We hope that this article will provide new ideas and insights for the rational design of rare earth based photocatalysts.

Fig. 1 Role and application areas of rare earth photocatalysts.

2. Role of rare earth ions in photocatalysts

2.1 Role of rare earth ions in photocatalysts

Rare earth ion doping in photocatalysts mainly refers to introducing rare earth ions into the lattice of the catalyst, so that the rare earth ions replace other ions or occupy vacancies to form new active sites.^47,48 It has been shown that the doping of rare earth ions in photocatalysts can significantly improve the activity of the catalysts.^49–51 Among them, rare earth ions in the catalyst mainly play the roles of stabilizing the catalyst structure, adjusting the semiconductor band gap, etc. Here, we summarize several typical action mechanisms as follows.

Doping with rare earth elements can usually maintain the structural stability of the catalyst, while the newly formed chemical bonds can retain the original catalytic activity of the catalyst that was unstable during the preparation process and play a role in the overall bonding structure of the catalyst. For example, Gerardo Colón and his colleagues found that BiVO₄ doped with Er³⁺ can maintain a stable tetragonal crystalline phase.⁵² Photocatalytic water decomposition was carried out in AgNO₃ solution to produce oxygen. The experiment showed that the O₂ generation rate of the Er³⁺ doped BiVO₄ sample was 1014 mol h⁻¹ g⁻¹, which is 20 times higher than that of the undoped sample. Researchers analyzed the crystal structure of the catalyst and found that during the synthesis process of BiVO₄, Er³⁺ partially replaced Bi to form ErVO₄. Therefore, a mixture of tetragonal and monoclinic phases was formed in the catalyst, and the two heterogeneous structures significantly increased the activity of the catalyst (Fig. 2a and b).⁵²Fig. 2c shows the catalytic mechanism of the reaction; T-BiVO₄ serves as an internal sensitizer for m-BiVO₄, providing photogenerated photons with sufficient energy. The mixed phase heterostructure formed by this doping is expected to promote the separation of photo-induced electron–hole pairs. Not coincidentally, Katherine Villa and colleagues doped WO₃ with La for photocatalytic methane oxidation to produce methanol. WO₃ is an ordered mesoporous metal oxide with a large surface area and good light capture performance, which usually leads to the excessive oxidation of the products of photocatalytic reactions to CO.⁵³ However, with the introduction of La atoms, the methanol conversion rate of the catalyst is higher than that of other metal ion doping. Compared with pure WO₃, the methanol yield of the catalyst added with the La atom increased by two times, and the CO yield significantly decreased (Fig. 2d).⁵³ This may be due to La avoiding particle sintering during the WO₃ calcination process, maintaining the well-ordered mesoporous structure of WO₃, thereby improving the photocatalytic activity of the catalyst.

Fig. 2 (a) Evolution of the cell volume and the tetragonal phase fraction with the Er³⁺ content. (b) Evolution of cell parameters for monoclinic and tetragonal crystalline phases for the Er-BiVO₄ series. (c) Envisaged electronic scheme involved in the photocatalytic mechanism upon solar-like excitation.⁵² Copyright 2014, Elsevier. (d) Yield of products and methanol selectivity in the photocatalytic oxidation of CH₄ on blank, WO₃ and WO₃/La at ∼55 °C under UVC–visible light irradiation. Catalyst dosage is 1 g L⁻¹. Data corresponding to 2 h of irradiation at a continuous methane flow rate of 4.5 mL min⁻¹.⁵³ Copyright 2016, Elsevier.

In addition to this, the doping of rare earth ions can create vacancy defects on the surface of the catalyst. Under visible light irradiation, newly formed oxygen vacancies can capture photoelectrons, forming transition states and new energy levels between energy level structures, enhancing the light absorption range and effectively limiting the recombination of photo-generated electrons and holes.⁵⁴ In addition, the formation of surface defects on the catalyst can also enhance its adsorption capacity for reactants and generate more free radicals, thereby improving photocatalytic activity.⁵⁵ For example, Kuo's group reported Gd-doped TiO₂ nanorods subsequently complexed with poly-o-phthalimide (PoPD) to form a heterojunction photocatalyst for simultaneous oxidation of benzyl alcohol (BA) and reduction of p-dinitrobenzene (p-DNB).⁵⁶ The results showed that BA was selectively oxidized by photogenerated holes to produce benzaldehyde in 70.5% yield in water and 90.6% yield in acetonitrile solvent. In contrast, p-DNB underwent selective reduction in acetonitrile and water to produce p-nitroaniline (p-NA) in acetonitrile in 89.1% yield and p-phenylenediamine (p-PDA) in water in 85.0% yield (Fig. 3a–d).⁵⁶ This can be attributed to the doping of Gd³⁺ induced by the generation of coordination unsaturated sites with unpaired electrons on the surface of the catalyst, forming oxygen vacancy defects. The presence of oxygen vacancies can cause defective energy levels in the band gap, adjust the structure of the energy bands, promote carrier separation, and improve the activity of the photocatalytic reaction (Fig. 3e).⁵⁶ Zhao's team introduced two doping elements, Ti and Ce, into Nb₂O₅ photocatalysts and compared them by the performance of photocatalytic selective alcohol oxidation (Fig. 4a).⁵⁷ The results showed that the performance of Ce-doped Nb₂O₅ was superior to that of Ti-doped Nb₂O₅. Researchers attributed this to the fact that Ce doping distorts the Nb₂O₅ structure less than Ti doping, resulting in the formation of more oxygen vacancies during the catalytic process, as shown in Fig. 4b and c,⁵⁷ which is beneficial for the interface electron migration process. The presence of oxygen vacancies also increased the adsorption of alcohol molecules on the catalyst and enhanced the photocatalytic performance (Fig. 4d).⁵⁷ Therefore, exploring rare earth ions compatible with photocatalysts and introducing them into the lattice of the catalysts can lead to a significant increase in the activity of the catalysts.

Fig. 3 Selective oxidation of BA (a) in ACN and (b) in water. Selective reduction of p-DNB (c) in ACN and (d) in water. (e) Proposed mechanism for simultaneous oxidation of BA and reduction of p-DNB over GTP-10 under simulated solar light irradiation.⁵⁶ Copyright 2023, Elsevier.

Fig. 4 (a) Proposed mechanism of photocatalytic selective oxidation of alcohols to aldehydes over defective Nb₂O₅ with deposited Pt NPs. (b) Room temperature EPR spectra of Ti-NbO-3, Ce-NbO-3 and B-NbO (inset graph for 77 K EPR spectra of Ce-NbO-3 (above) and Ti-NbO-3 (below)). (c) Room temperature EPR spectra of Ti-NbO-3 and Ce-NbO-3 before and after adsorbing BA. (d) Schematic representation of the mechanism of the LMCT process in the case of adsorbed alcoholate species and doped Nb₂O₅.⁵⁷ Copyright 2021, Elsevier. (e) The possible photocatalytic mechanism and transfer pathway of generated charges in the BiOBr/Gd composite.⁶¹ Copyright 2020, Elsevier. (f) Schematic diagram shows the mechanism of improving the photocatalytic activity by Tm-induced 4f levels promoting electron-to-electron transfer transition between Tm and TiO₂.⁶² Copyright 2021, Elsevier.

Rare earth ions can not only adjust the structure of a single material but also regulate the light response range of the catalyst. As is well known, the most critical aspect of photocatalysis is that after the catalyst absorbs light energy, the oxidation–reduction ability of h⁺ and e⁻ generated at the energy level causes the substances adsorbed on the surface to react. The redox ability of photocatalysts depends on the position of their conduction and valence bands; the more negative the conduction band, the stronger the reduction ability, and the more positive the valence band, the stronger the oxidation ability.^58,59 In addition, whether a photocatalyst can be stimulated by light to produce h⁺ and e⁻ pairs is also related to its energy gap (E_g); the narrower the E_g, the wider the light absorption range and the easier it is to be stimulated by light.⁶⁰ Therefore, modulating the energy band structure of semiconductors is the most common method to broaden the spectral response range. For example, Hou and his colleagues doped Gd³⁺ onto BiOBr microspheres by in situ synthesis.⁶¹ The product catalysts were tested for photocatalytic CO₂ reduction, with the most active BiOBr/Gd-0.05 methanol yielding up to 123.711 μmol g⁻¹, which is a 5-fold increase in photocatalytic activity compared to pure BiOBr. It was shown that the doping of Gd changed the valence band of BiOBr from 3.17 to 1.66 eV and the conduction band from 0.19 eV to −1.24 eV, making the photocatalytic reduction of CO₂ to methanol (−0.60 eV) possible. At the same time, the energy gap of the catalyst was narrowed, which broadened the visible light response range and improved the activity of the photocatalyst. The free radicals ˙CO₂⁻ can capture a photo-generated electron and convert it into HCOOH, which is then reduced to CH₃OH, as shown in Fig. 4e.⁶¹ Sun's team synthesized Tm³⁺-modified TiO₂ catalysts for organic pollutant degradation using a sol–gel method.⁶² The results showed that the catalyst significantly improved the gas-phase degradation of acetaldehyde, o-xylene, and their mixtures compared to pure TiO₂. The photocatalytic activity was gradually enhanced with the increase of the Tm concentration. Among them, the photodegradation efficiency of the 0.5 mol% Tm-TiO₂ sample for acetaldehyde and o-xylene could reach up to 93.1%. The researchers concluded that the improved photocatalytic performance was mainly attributed to the doping of the 4f energy level of Tm between the band gap of TiO₂, which generated new redox sites for the effective separation of photogenerated electrons and holes, as well as increased adsorption capacity for acetaldehyde and o-xylene (Fig. 4f).⁶² In addition to changing the energy level structure of the catalyst, rare earth elements can also act as electron transfer bridges, enabling the directional transfer of photogenerated electrons to the active center of the catalyst, enhancing reactant activation, effectively inhibiting electron and hole recombination, and significantly improving the activity of the catalyst. Liu and his colleagues doped C and Y into the lattice of TiO₂ for photocatalytic methyl blue degradation.⁶³ It was shown that the degradation rate of methyl blue on (C, Y)-co-doped TiO₂ samples was much higher than on other undoped samples. This can be attributed to the synergistic effect of C and Y doping, which narrows the energy gap of TiO₂ from 3.2 eV to about 2.75 eV, and broadens the absorption band edge into the visible light region. When semiconductor nanomaterials are modified with different dielectric materials, a phenomenon occurs: their optical properties are strongly influenced by the surface environment, resulting in dielectric confinement effects. In addition, the doping of Y formed a doping energy level on TiO₂, which made the electron transitions and transfers easier and could effectively inhibit the recombination of photogenerated electron/hole pairs. Meanwhile, the doping of Y can inhibit the growth of TiO₂ particles, increase the specific surface area, and enhance the photocatalytic activity. Not coincidentally, Xu and his colleagues anchored elemental Sm and Ag quantum dots on CeO₂ for photocatalytic water decomposition for hydrogen production.⁶⁴ Compared with pure CeO₂, Sm doping significantly increases the number of CeO₂ oxygen vacancies, which can stimulate photogenerated electrons to form transition states closer to the CB under visible light irradiation, thereby reducing the bandgap of related products and effectively suppressing photogenerated electrons and holes. Meanwhile, the bandgap of CeO₂ narrowed after rare earth ion doping, and the estimated bandgap values of CeO₂, Sm@CeO₂, Ag@CeO₂, and Ag/Sm@CeO₂ were 2.96, 2.91, 2.86, and 2.82 eV, respectively, which indicated that the doping of Sm was able to broaden the response range of light. The results showed that the H₂ yield of the catalyst reached 342.12 μmol g⁻¹ h⁻¹, which was 6.83 times higher than the value of CeO₂, and a CeO₂ photocatalyst with high visible light catalytic performance was realized. In addition, the catalyst has better reproducibility, indicating that the doping of Sm³⁺ makes the structure of the catalyst more stable. In subsequent experiments, the researchers found that the catalyst liquid medium also exhibited high photocatalytic properties, such as the oxidation of bisphenol A (BPA) and the reduction of 4-nitrophenol (4-NP). Wang's group doped Er³⁺ and Eu³⁺ into CaTiO₃ catalysts for comparison of their photocatalytic hydrogen production performances, respectively.⁶⁵ The results showed that CaTiO₃:Er³⁺ nanocrystals had the highest photocatalytic activity of up to 461.25 μmol h⁻¹ under UV irradiation, which was higher than that of CaTiO₃:Eu³⁺ nanocrystals and pure CaTiO₃. The results of UV diffuse reflectance spectroscopy indicate that all sample absorption band edges are in the ultraviolet region (λ < 400 nm), which means that the doping of rare earth ions reduces the bandgap width of CaTiO₃ from 3.5 eV to 3.3 eV, expanding the range of optical response. In addition, the doping of Er³⁺ resulted in enhanced light absorption at 400–700 nm in CaTiO₃ compared to Eu³⁺-doped CaTiO₃, which may be attributed to the special molecular orbital structure of Er³⁺ that gives rise to a unique mode of electron migration in the catalysts.

Wu's team tested the antimicrobial effects of TiO₂ nanoparticles co-doped with Ce and Er against Staphylococcus aureus and Escherichia coli using the sol–gel method.⁶⁶ The results showed that the photocatalytic activity of the catalysts first increased and then decreased with the increase of Er and Ce doping contents. When the Er and Ce doping doses were 0.5 mol% and 0.2 mol%, the 0.5Ce0.2Ti-O calcined at 800 °C had the best antimicrobial performance, with the antimicrobial efficiencies against Staphylococcus aureus and Escherichia coli of 91.23% and 92.8%, respectively. The UV-visible absorption spectra show that the absorption edge of the co-doped samples shows a significant redshift compared to the untreated TiO₂, indicating that the doping of Er and Ce significantly reduces the bandgap of the catalyst, which is related to the incomplete occupation of the 4f and 5d orbitals of the rare earth ions. Special electronic orbitals not only reduce the band energy gap but also suppress the electron–hole recombination rate by providing new energy levels, which can effectively improve their photocatalytic performance. Therefore, the doping of Er³⁺ successfully converted the near-infrared radiation into the visible region, and the incorporation of Ce³⁺ also effectively expanded the spectral response range and improved the photocatalytic activity of co-doped TiO₂. In summary, rare earth ion doping can change the valence band and conduction band of the catalyst, reduce the band gap width, and expand the light response range. In addition, due to the unique energy level structure of rare earth ions, they can also adjust the response of light of different wavelengths. Therefore, exploring suitable rare earth ion photocatalysts to improve the light energy utilization of sunlight will be a breakthrough in the field of photocatalysis.

2.2 Rare earth element based composite photocatalysts

Rare earth elements can also act as electronic additives to the catalyst. The combination of two semiconductor materials with different band structures will form a potential difference, and the electrons generated by light excitation will be transferred from one semiconductor to another semiconductor, effectively separating electrons and holes.^67–72 For example, Zhang's team co-doped Yb³⁺ and Er³⁺ on BiOBr and complexed it with Bi₄O₅I₂ to form a Z-type heterojunction catalyst for two-dimensional materials.⁷³ The catalyst exhibited excellent performance in degrading BPA under full-spectrum and NIR light irradiation. Within 30 min, the degradation rate of BPA reached 100%, which was much higher than the performance of the catalyst before combining. This can be attributed to the fact that the doping of Yb³⁺ and Er³⁺ can capture near-infrared (NIR) light, which can then be converted to visible light by the upconversion function of rare earth ions, expanding the optical response range of the photocatalytic system. Meanwhile, the heterogeneous structure formed made it possible to generate active radicals ˙O₂⁻ and ˙OH by a redox reaction on the catalyst, which effectively inhibited the recombination of photogenerated electrons and holes, and significantly improved the photocatalytic activity. Wang and his colleagues doped Eu³⁺ on BiVO₄ nanoplates, which were then coupled with CeO_x nanoparticles to form -type-II heterojunctions for photocatalytic conversion of CO₂ to CH₄ and degradation of 2,4-dichlorophenol (2,4-DCP).⁷⁴ The results showed that CeO_x/BiVO₄:Eu³⁺ nanoparticles possessed the most visible-light photocatalytic performance, while the photocatalytic degradation of 2,4-DCP by 2C/BVO-2 was 62.5%, which was higher than the degradation rate of 21.3% on BVO and 33.3% on BVO-2. This can be attributed to the fact that the doping of Eu³⁺ changes the electron-transition structure of the catalyst and promotes charge separation. When the catalyst is excited by a visible light source, the photoelectrons produced by BVO-2 can transition to the conduction band of CeO_x. Meanwhile, the doping of Eu³⁺ increases the number of electrons excited by visible light (498 < λ ≤ 530 nm), so that high-energy electrons are transferred from BVO-2 to the conduction band of CeO_x, giving the nanomaterials enough energy to promote oxidation and reduction reactions. In general, no matter which kind of heterojunction is formed, it will lead to effective separation of charges and holes. Therefore, the doping of rare earth ions into the catalyst and the formation of a new energy level structure as a channel connecting the electron transfer of the two semiconductor materials is an important breakthrough in the preparation of high-performance composite catalysts.

2.3 Rare earth single-atom photocatalysts

As is well known, the smaller the particle size of a photocatalyst, the shorter the time it takes for the electron–hole pair to reach the surface. The better the redox ability of semiconductors, the higher the catalytic activity or selectivity of catalysts. Therefore, people have always been committed to the synthesis of catalysts with small particle sizes and highly dispersed.⁷⁵ In 2011, academician Tao Zhang's group at the Dalian Institute of Chemical Technology first proposed the concept of single-atom catalysis, bringing catalytic science to a smaller research scale, and the emergence of single-atom catalysts can maximize the catalytic efficiency of precious metals and reduce the cost of manufacturing for highly loaded metal catalysts.^76–82 Therefore, the ideal state of loaded catalysts is to uniformly distribute the metal in the form of single atoms on the carrier. With the deepening of the research, it has been gradually discovered that single-atom catalysts can have excellent catalytic properties triggered by the reaction, such as sharply increasing the surface free energy, quantum size effect, unsaturated coordination environment, metal–carrier interactions, etc. These unique properties enable single-atom catalysts to have excellent catalytic performance.⁸³ If rare earth elements are introduced into the structure of photocatalysts in the form of single atoms, and the unique 4f energy level of rare earth single atoms is reasonably utilized, the catalysts can take advantage of the dual advantages of the energy level of rare earth elements and the efficiency of single atoms, which will promote the entirely new discovery in the field of photocatalysis and further broaden the scope of application of rare earth photocatalysts.⁸⁴

For example, Zhang and his colleagues doped La single atoms on MoO_3−x for photocatalytic nitrogen fixation using the solvothermal method.⁸⁵ The results showed that La single atoms underwent a strong bonding interaction with MoO_3−x to form O_2c–La–O_2c bonds, and the La single atoms as active centers could enhance the adsorption of N₂ on the surface and make the overall structure of the catalyst more stable. At the same time, the 5d orbital of La can spontaneously provide electrons to the orbitals of molecules with adsorbed N₂, allowing the activation of the inert N≡N bond to produce a continuous hydrogenation to synthesize NH₃. In the absence of a sacrificial agent, the catalyst showed an NH₃ production rate of 209.0 μmol g⁻¹ h⁻¹ under visible light. The researchers attributed it to the doping of La single atoms that optimized the electronic properties of the MoO_3−x carrier and enhanced the surface adsorption activation of the catalysts, which facilitated the photocatalytic nitrogen fixation for NH₃ production (Fig. 5a and b).⁸⁵ Not coincidentally, Wang's team synthesized catalysts with controllable single-atom density dispersion properties (Er₁/CN-NT) by loading rare-earth single-atom Er onto carbon nanotubes using a novel atomic confinement and coordination strategy (Fig. 5c).⁸⁶ Meanwhile, the catalytic activity of the catalyst was excellent. The yields of CO and CH₄ were 47.1 and 2.5 μmol g⁻¹ h⁻¹, respectively, under the conditions of pure water (Fig. 5d–f).⁸⁶ The results of synchrotron radiation photoelectron spectroscopy and theoretical calculations showed that the presence of Er single atoms could improve the adsorption capacity on the catalyst surface. Meanwhile, the electronic structure of the Er single atom as the active center of the catalyst is closely related to the processes of adsorption, activation, and photocatalytic CO₂ reduction, suggesting that rare earth ion-doped photocatalysts can significantly improve the activity of the catalytic reaction. Zhou's team used a single-atom engineered oxygen coordination strategy to successfully dope Pr single atoms with oxygen coordination on CN (Pr₁-N₄O₂/CN), which was used to achieve the efficient photocatalytic reduction of CO₂ to CH₃OH (Fig. 6a).⁸⁷ The experimental results show that the CH₃OH yield of this catalyst is 511.1 μmol g⁻¹ h⁻¹, which is 28.9 and 1.9 times higher than that of CN and Pr₁–N₆/CN, respectively, and this performance outperforms most of the other metal-ion doped ones reported (Fig. 6b–e).⁸⁷ The researchers concluded that Pr₁-N₄O₂ induced the accumulation of photogenerated electrons as an active center and increased the photogenerated electron density in the active site. In addition, the Pr-loaded active center could promote the adsorption and activation of CO₂ and provide high-density active sites for CO₂ photoreduction (Fig. 6f).⁸⁷

Fig. 5 (a) Top view of the MoO_3−x structure and the optimized configuration of the La atom at the terminal oxygen vacancy site. (b) Schematic illustration of a single La atom anchored on the oxygen vacancy site.⁸⁵ Copyright 2022, Elsevier. (c) Schematic process of the synthesis of single-atom Er₁/CN-NT catalysts. The evolution rates of CH₄ (d) and CO (e) during the photocatalytic CO₂ reduction with H₂O over LD-Er1/CN-NT, HD-Er₁/CN-NT, and CN-NT catalysts. (f) Enhancement of product evolution over the LD-Er₁/CN-NT, HD-Er₁/CN-NT, and CN-NT catalysts.⁸⁶ Copyright 2020, Wiley-VCH.

Fig. 6 (a) Schematic of the synthesis process of Pr₁–N₆/CN and Pr₁-N₄O₂/CN. (b) CH₃OH production with time for CN, O/CN, Pr₁–N₆/CN, Pr₁-N₄O₂/CN. (c) The selectivity of CH₃OH and other products. (d) H₂, CO, CH₄, and C₂H₄ production in 4 h. (e) CH₃OH production corresponding to cycle experiments (∼20 h) for the Pr₁-N₄O₂/CN. CH₃OH production of the reference photocatalysts synthesized in this work, such as (f) CN supported different kinds of metal elements, (ii) CN supported Pr single-atom with different densities, and (iii) CN supported Pr single-atom with different coordination configurations.⁸⁷ Copyright 2023, Elsevier.

In addition to the above-mentioned rare-earth single-atom photocatalysts that can be used as active sites of the reaction to directly contact the reactants, rare-earth single atoms can also improve the activity of the reaction by indirectly coming into contact with the reactants, such as the formation of vacancy defects, free radicals, and so on. For example, Wang's group used a hydrothermal method to introduce La single-atoms on P/InVO₄ for photocatalytic CO₂ reduction.⁸⁸ In the absence of any sacrificial agent, the CO yield was 11.96 μmol g⁻¹ h⁻¹ with a selectivity of about 100%. The test results showed that it was the doping of La single-atoms that led to a significant increase in the activity and selectivity of the catalysts, and then the results of theoretical calculations demonstrated that La single-atoms increased O defects in the catalysts, modulated the active sites on the surface of the catalysts, and promoted the adsorption of CO₂ on the surface of the catalysts. As shown in Fig. 7a, two single component samples recombine to form a Z-type heterojunction, and CO₂ is reduced to CO and CH₄ under visible light irradiation.⁸⁸ Wang and his colleagues loaded La single-atoms onto g-C₃N₄ by solvothermal and high-temperature reduction methods and tested the degradation performance of the SALa/CN photocatalytic system using the total organic carbon (TOC) characterization method.⁸⁹ The results showed that SALa/CN exhibited a good removal rate (73.27%) of TC (tetracycline hydrochloride) within 30 min. The higher mineralization activity indicated that SALa/CN was capable of converting TC into non-polluting small molecules such as CO₂ and H₂O. The catalyst was then subjected to photocatalytic degradation experiments of oxytetracycline hydrochloride (OTC) and doxycycline hydrochloride (DOX). The photocatalytic degradation efficiencies of the SALa/CN photocatalyst for OTC and DOX reached 84.49% and 91.71%, respectively, within 30 min. It is shown that SALa/CN has good photocatalytic activity for the removal of antibiotics. The researchers attributed such high photodegradation activity to the effective separation of electrons and holes due to the doping of La single atoms, which improves the utilization of visible light. Meanwhile, the doping of La single-atoms provides a new active site and a special coordination structure, which enables the catalyst to reduce O₂ to the active species O₂⁻ more often and remove pollutants effectively.

Fig. 7 (a) Mechanism of photocatalytic CO₂ reduction under visible light irradiation.⁸⁸ Copyright 2022, Wiley-VCH. (b) The proposed mechanism diagram of photocatalysis.⁹² Copyright 2021, Wiley-VCH. (c) The proposed mechanism diagram of photocatalysis. Production of CO and CH₄ from the CO₂ photoreduction systems without a sacrificial agent for the photocatalytic reaction: (d) CdS:0.015%Ln³⁺, (e) CdS:Tm³⁺. CdS:Er³⁺, and CdS:Dy³⁺ with different doping concentrations, (f) g-CN and CdS:Dy³⁺@g-CN (CSDy/CN-SS) prepared by in situ synthesis methods (the inset shows the photocatalytic production rates).⁹³ Copyright 2021, Wiley-VCH.

In addition, rare earth single atoms can form unique charge transfer channels within single and composite catalysts, and the formation of such electron transfer bridges largely improves the charge separation efficiency and enhances the reactivity of photocatalysts. For example, Dong and her colleagues immobilized La single atoms onto a g-C₃N₄ layer for photocatalytic CO reduction.^90,91 The results showed that the electronic structure of the La–N bridge resulted in a CO yield of 92 μmol g⁻¹ h⁻¹ and a CO selectivity of 80.3%, which is superior to most CN photocatalysts. This can be attributed to the unique 4f and 5d orbital La single atoms that enable the electronic state of the catalyst to be changed. Meanwhile, the hybridization of the p–d orbitals of the La–N structure induces the formation of La–N electron-directed transfer bridges on the catalyst, which improves the charge separation efficiency. At the same time, the presence of La single atoms changed the energy level structure of the catalyst, which led to the expansion of the photoresponsive range of the catalyst. Subsequently, the researchers demonstrated that rare earth ions, as the key active centers of the whole catalyst, can significantly activate CO₂ and promote the desorption of CO, thus improving photocatalytic efficiency. Wang's group composited Er single atom doped g-C₃N₄ with ZnGeO₄ by in situ synthesis to obtain a photocatalytic heterojunction of type-II, which combines the dual advantages of rare-earth single atoms and heterostructures to make ZnGeO₄ responsive in the visible light range (Fig. 7b).⁹² At the same time, the activity of the catalyst was made to be significantly enhanced relative to the semiconductors alone, and a series of experimental results showed that the 4f energy level of Er³⁺ could act as an electron transport bridge between the two semiconductors to enable the effective separation of photogenerated electrons. In subsequent work, Wang's team introduced several different rare earth ions into CdS for photocatalytic CO₂ reduction (Fig. 7c).⁹³ The results showed that Dy³⁺ doping could lead to optimal photocatalytic performance of the catalyst. Subsequently, the researchers carried out a composite trip heterojunction of Dy-doped CdS with CN. In the absence of a sacrificial agent, the CO yield was 23.44 μmol g⁻¹ h⁻¹ and the CH₄ yield was 8.06 μmol g⁻¹ h⁻¹, which were superior to other metal ion doped photocatalysts (Fig. 7d–f).⁹³ Through characterization and theoretical calculations, it is shown that the doping of Dy changes the density of the surrounding electron cloud, prompting Dy³⁺ to act as an electron-transport bridge in two bandgap-matched semiconductors, effectively suppressing the recombination of photogenerated electrons and holes. It can be seen that the chemical bond formed by rare earth doping can increase the density of photogenerated electrons in the active site, so that the photogenerated electrons can be transferred and enriched directionally to the active site of the catalyst, and the effective transfer of energy can be improved, so that the photogenerated electrons and holes can be effectively separated.

2.4 Other rare earth based new photocatalysts

Rare-earth oxides usually have the advantages of strong adsorption selectivity, good thermal stability, electron-type conductivity, and abundance of oxygen vacancies, and thus have received much attention. However, with continuous research, it has been found that a single rare earth oxide photocatalyst has many drawbacks, such as a wide band gap, a small light absorption range, and a high recombination rate of electrons and holes.^94–96 Therefore, pure rare earth oxides are rarely used in practical applications. For example, Karunakaran and his coworkers compared the photocatalytic degradation of 1-naphthol by TiO₂, ZnO, CeO₂, CdO, WO₃, Co₃O₄, Sb₂O₃, ZrO₂, La₂O₃, Y₂O₃, Pr₆O₁₁, Sm₂O₃ and Al₂O₃.⁹⁷ The results showed that the degradation of these oxides follows the Langmuir–Hinshelwood kinetic model (Fig. 8a).⁹⁷ However, the performance of the catalysts was significantly different among them, TiO₂ and ZnO were the most effective in degrading 1-naphthol, and the effects of the other oxide catalysts were not significant compared with those of TiO₂ and ZnO. Although many efforts have been devoted to tuning the intrinsic surface properties of the catalysts, controlling the specific morphology with different surface exposures is essential for improving the catalytic performance of the catalysts. However, pure rare earth oxide photocatalysts still show limited catalytic activity and selectivity. As a result, rare earth oxide photocatalysts have been modified to overcome their drawbacks using methods such as metal ion doping and semiconductor combining. Janardhan Reddy Koduru's team composited graphene oxide (GO) with Gd₂O₃ for photocatalytic water pollution treatment for arsenic (As) removal.⁹⁸ Characterization results showed that the composite of Gd₂O₃ significantly increased the catalyst surface area and increased the stability of the catalyst. Meanwhile, the adsorption capacity of the catalyst for As could reach up to 216.70 mg g⁻¹. In addition, the GO-Gd₂O₃ nanocomposites showed a high bacterial photocatalytic deactivation rate. It shows that the composite of rare earth oxides can significantly improve the catalytic effect of the catalysts. Liu's team synthesized composites of ScO and CN for photocatalytic oxidation of organic pollutants by in situ calcination.⁹⁹ It was shown that under visible light irradiation, the photogenerated electrons of CN migrated onto ScO through the Sc–C channel, which improved the charge separation and photogenerated electron transfer of the catalyst. Meanwhile, the surface-active sites provided by Sc atoms enhanced the adsorption and activation of PMS, which can further improve the rapid oxidation of organic pollutants and activation of PMS, and increase the catalytic activity of the catalysts.

Fig. 8 (a) Decay of 1-naphthol with the illumination time. 0.10 g of oxide loading, 7.8 mL s⁻¹ airflow, 19.2 mg L⁻¹ dissolved O₂, 365 nm, 19.2 μEinstein L⁻¹ s⁻¹, 20 mL of naphthol solution, and CO₂ photoreduction properties of samples.⁹⁷ Copyright 2021, Elsevier. (b) CO and C₂H₆ formation yields as a function of the Au content of ACEO-x.¹⁰² Copyright 2023, Elsevier. (c) Schematic diagram of the photoinduced electron–hole separation process for the P-CeO₂/g-C₃N₄ Z-scheme heterojunction.¹⁰³ Copyright 2021, Elsevier. (d) The internal electric field with band-edge bending near the interface of CeO₂/PCN. (e and f) The S-scheme transfer mechanism of photogenerated electrons under illumination.¹⁰⁴ Copyright 2020, Wiley-VCH.

Tian and his colleagues synthesized a novel S-doped In₂O₃–CeO₂ heterojunction for photocatalytic CO₂ reduction.¹⁰⁰ The results showed a CH₄ yield of 60.6 μmol g⁻¹ h⁻¹ in the absence of a sacrificial agent with a selectivity of 92.4%, which is much higher than the activity of other common catalysts. The researchers concluded that the synergistic effect of heterojunction formation and catalyst surface defects resulted in effective transfer and separation of photogenerated carriers, enhanced visible light utilization and CO₂ adsorption, and expanded catalytic active sites. Currently, there are relatively few areas of research on rare earth oxide photocatalysts. Among them, CeO₂ has become the most widely researched and applied rare earth oxide photocatalyst due to its reversible Ce³⁺/Ce⁴⁺ redox pair, abundant oxygen vacancies, etc.¹⁰¹ Shen's team doped Au into CeO₂ to form nanocomposites with Au–O–Ce sites for the highly selective conversion of photocatalytic CO₂ to CH₃CH₃.¹⁰² The results showed that Au doping significantly improved the conversion of CO₂ with an ethane production rate of 11.07 μmol g⁻¹ h⁻¹ and a product selectivity of 65.3%, which is 42.85 times higher than that of pristine CeO₂ (Fig. 8b).¹⁰² This can be attributed to the better adsorption of CO₂ at the Au–O–Ce active sites on the composites than at the oxygen vacancies on CeO₂. Meanwhile, the doping of Au reduces the bandgap width of the catalyst, which enlarges the photoresponse range of the catalyst and improves the catalytic activity. Dong and his team utilized phosphate-modified octahedral CeO₂{111} and complexed it with g-C₃N₄ for a photocatalytic CO₂ reduction reaction.¹⁰³ The results showed that the CO production rate of the catalyzed reaction was 4.184 μmol g⁻¹. Subsequently, the researchers demonstrated that interfacial electrons were transferred from g-C₃N₄ to CeO₂via a phosphate bridge, forming a Z-type heterojunction instead of a type-II heterojunction (Fig. 8c).¹⁰³ In addition, the phosphate modification increased the active sites for CO₂ adsorption. As a result, it led to a significant improvement in the performance of P-CeO₂/g-C₃N₄. Zhang's team used in situ wet chemistry and subsequent heat treatment to composite polymer CN with CeO₂ to design a 0D/2D S-type heterojunction for visible light photocatalytic inactivation of bacteria.¹⁰⁴ The catalyst showed a photocatalytic bactericidal rate of 88.1% against Staphylococcus aureus under visible light irradiation, which was 2.7 and 8.2 times higher than that of CeO₂ and PCN, respectively. Researchers believe that in the dark, CeO₂ obtains electrons and the band edge bends downwards, resulting in negative charges in the nearby interface area, as shown in Fig. 8e. Therefore, an internal electric field is established at the interface of the heterojunction, which hinders the continuous flow of electrons. Under illumination, excited electrons transfer from CeO₂ to PCN under an internal electric field and Coulomb interactions, forming an S-type heterojunction, as shown in Fig. 8f. The formation of this structure promotes the separation and transfer of photo-generated charge carriers, greatly improving the photocatalytic and antibacterial properties of CeO₂/PCN heterojunction materials. Li and her colleagues constructed a Cu^δ+/CeO₂–TiO₂ photocatalyst for photocatalytic CO₂ conversion to C₂H₄.¹⁰⁵ The results showed that the catalyst produced C₂H₄ at a rate of 4.51 μmol g_cat⁻¹ h⁻¹ under simulated sunlight with a selectivity of 47.5% and an electron utilization selectivity of 73.9% for C₂H₄. The researchers concluded that the heterogeneous structure enhances charge separation and transfer, the Cu^δ+ generated as a reflective active site stabilizes the intermediate, and the synergistic effect of multiple active sites improves the conversion and selectivity of the C₂ product. In summary, rare-earth oxides can improve the catalyst activity by doping metal ions and composite semiconductors to form heterojunctions. However, so far, the real structure of rare earth oxide photocatalysts and their mechanisms in catalytic reactions is not clear. Therefore, future studies should pay more attention to the application mechanism of pure rare earth oxide photocatalysts.¹⁰⁶

With the increasing research on rare earth oxides, many novel photocatalysts have been gradually discovered, such as Dy₂Ce₂O₇, Nd₂Sn₂O₇, La₂Ti₂O₇, and so on.^107–114 These new rare-earth oxide photocatalysts can usually be applied to the photocatalytic reaction alone. For example, Sahar Zinatloo-Ajabshir used the Dy₂Ce₂O₇ photocatalyst for visible light decontamination of organic dyes (methyl orange, rhodamine B, and 2-naphthol pollutants) in water.¹¹⁵ The results showed that Dy₂Ce₂O₇, calcined at 400 °C using grape juice as a catalyst, had good photocatalytic performance for the degradation of methyl orange, rhodamine B, and 2-naphthol pollutants under visible illumination. The degradation efficiency was 94.3% for rhodamine B and 92.4% for methyl orange, and the degradation efficiency of the 2-naphthol pollutant was 96.8% for sample 5 after 20 min of light exposure. This can be attributed to the fact that grape juice acts as a green surface covering agent and reducing agent in the catalytic process, and this green and non-polluting material brings new ideas for photocatalytic degradation. Subsequently, the group successively applied the synthesized Nd₂Sn₂O₇ nanostructures to degrade eosin Y, infrared black T, and methyl violet pollutants in visible light and the erythrosine contaminant in ultraviolet light. Nd₂Sn₂O₇ showed significant degradation properties.^116,117 By coincidence, Wang and his colleagues successfully synthesized a series of cerium zirconium oxide (Ce_xZr_yO₂) nanocomposites using a hydrothermal method for photocatalytic sulfonamide performance testing.¹¹⁸ Among them, element Ce mainly exists in the form of Ce³⁺ and Ce⁴⁺, and element Zr exists in the form of Zr⁴⁺. The researchers found the optimal content ratio for the catalytic reaction by adjusting the content of Ce and Zr. The results showed that the Ce_0.9Zr_0.1O₂ composite had the best photocatalytic performance under simulated sunlight, with an efficiency of 91.33% for the degradation of formamide within 30 min, which can be attributed to the large production of ˙O₂⁻ and ˙OH radicals. Despite the initial applications of novel rare earth oxides, most of the novel rare earth oxide photocatalysts reported so far have very wide band gaps (>3.5) and have no catalytic activity under visible light. Therefore, to efficiently develop and utilize rare earth oxide photocatalysts, they also need to be rationally photocatalytically modified.^119–125 Generally speaking, the modification of rare earth oxide photocatalysts is similar to that of other catalysts, which mainly includes two aspects: broadening the light absorption range of photocatalysts and improving the separation efficiency of photogenerated carriers. Among them, broadening the light absorption range of photocatalysts is mainly realized by metal ion doping. For example, Zhang and her colleagues combined Au nanorods with La₂Ti₂O₇ (Au-LTO NSP) to form a surface heterojunction-based plasma photocatalyst for photocatalytic hydrogen production in visible and near-infrared regions.¹²⁶ The composites showed excellent hydrogen production performance when the light illumination wavelength was greater than 420 nm and 70 nm, respectively. The high apparent quantum efficiency (AEQ) of the catalyst was 1.4% under 920 nm light illumination, indicating that the catalyst could absorb infrared light efficiently. The researchers believe that this is due to the formation of a surface heterojunction between Au nanorods and La₂Ti₂O₇, where a large number of photogenerated electrons are transferred from Au to the (010) surface, and then directionally migrate to the (012) surface of La₂Ti₂O₇. The unique step structure of La₂Ti₂O₇ retards the recombination of photogenerated electrons and holes in the Au nanorods, which proves that the semiconductor surface heterojunction facilitates interfacial electron transfer for efficient photocatalytic reactions (Fig. 9a).¹²⁶ Tsunehiro Tanaka and his coworkers doped Ag in La₂Ti₂O₇ with a layered chalcogenide structure for photocatalytic CO₂ reduction in pure water.¹²⁷ CO, H₂, and O₂ were produced at rates of 5.2, 4.9, and 5.3 mol h⁻¹, respectively, with CO being the major product. With the increase of the calcination temperature and time, the grain size of La₂Ti₂O₇ increases and the surface area decreases, and water acts as an electron donor for the photocatalytic conversion of CO₂. The researchers maximized the photocatalytic activity of La₂Ti₂O₇ by controlling the size and surface area of the grains. Yusuke Asakura and his colleagues obtained nitrogen-doped La₂Ti₂O₇ for photocatalytic NO_x oxidative decomposition by treating La₂Ti₂O₇ nanocrystals in by treating them under an ammonia flow at 600–750 °C.¹²⁸ The structures, optical properties, and photocatalytic activities of the catalysts with different nitrogen doping amounts were also compared (Fig. 9c).¹²⁸ It is shown that the doping of N extends the light absorption range of the wide bandgap UV material La₂Ti₂O₇ into the visible region, and the increase in the specific surface area leads to the enhancement of photocatalytic oxidation activity of NO gas (Fig. 9d).¹²⁸ Not coincidentally, Zhang's team utilized Pt/La₂Ti₂O₇ nanoplates for the photocatalytic reduction and defluorination of perfluorooctanoic acid (PFOA).¹²⁹ The results showed that methanol acted as a sacrificial reagent and electron donor for the catalytic reaction, and the photocatalytic degradation rate of PFOA was significantly improved by reducing PFOA by 40% in the range of 180 min under UV₂₅₄ irradiation (1 mW cm⁻²). This can be attributed to the enhanced separation of photogenerated electrons from holes by the doping of Pt, which broadens the photoresponsive range of the catalyst and improves photocatalytic activity. Huang's team used a sol–gel method to synthesize La₂Zr₂O₇/rGO catalysts for the photocatalytic degradation of tetracycline under visible light.¹³⁰ It was shown that the removal rate and reaction rate of LZO/rGO-3 were 82.1% and 0.3097 min⁻¹, respectively, after 40 min, which were significantly higher than that of La₂Zr₂O₇. The researchers attributed this to the doping of graphene which added more adsorption and reaction sites on the catalyst surface. Meanwhile, the doping of graphene oxide reduced the band gap of the catalyst and expanded the light absorption range (Fig. 10a and b).¹³⁰ In addition, the doping of graphene formed vacancy defects, which enabled the effective separation of electrons and holes and further promoted the generation of active substances (Fig. 10c and d).¹³⁰ Overall, doping metal ions in novel rare earth oxides can reduce the band gap width of the catalysts and expand the photoresponsive range while increasing the active sites of adsorbed reactants and improving the catalytic activity.

Fig. 9 (a) Schematic illustration of the three different electron decay processes in the Au-LTO NSP system.¹²⁶ Copyright 2018, ACS Catal. (b) The reaction mechanism for photoreduction of CO₂ by H₂O on MgO/LaFeO₃:Er³⁺ and LaFeO₃ under visible-light irradiation.¹⁴⁰ Copyright 2020, Wiley-VCH. (c) Valence band spectra vs. Fermi level and (d) schematic illustration of the band structure diagram of LTO-NC before and after heat treatment in NH₃ gas flow at different temperatures (600–750 °C) (the valence band energy and conduction band energy is vs. Fermi level).¹²⁸ Copyright 2021, Elsevier.

Fig. 10 (a) Tauc plot (ahv)² − hv curves of LZO and LZO/rGO-3; the inset shows UV–vis absorption spectra. (b) Kubelka–Munk plot (F(R∞)hυ)² − hv curves of LZO and LZO/rGO-3; the inset shows UV–vis diffuse reflectance spectra. (c) Band gap, valence band and conductive band of LZO, GO and LZO/rGO-3; (d) photocatalytic mechanism of LZO/rGO.¹³⁰ Copyright 2020, Elsevier. (e) The work functions of α-Fe₂O₃ and LTON before contact. (f) The internal electric field and band edge bending at the interface of α-Fe₂O₃/LTON after contact. (g) The Z-scheme charge transfer mechanism between α-Fe₂O₃ and LTON under light irradiation.¹⁴¹ Copyright 2021, Elsevier.

Since these new rare-earth oxides are usually compounded by reacting rare-earth oxides with other compounds, these catalysts usually have the multiple advantages of rare-earth elements and other compounds.^131–134 Among them, the most widely studied is the combination of rare-earth oxides with chalcogenides, which gives the catalysts a variety of advantages, such as special electron-transitioning properties, high stability, and non-toxicity.^135–139 However, owing to its disadvantage of high charge sufficiency, if the new rare earth oxide photocatalyst is compounded with other semiconductors, both efficient charge separations can be realized. For example, Wang's group doped Er into LaFeO₃ and composited it with MgO to test its photocatalytic CO₂ reduction ability under visible light.¹⁴⁰ The experimental results showed that the Er-doped composites have better CO₂ adsorption capacity and higher carrier separation efficiency, and the researchers attribute the enhancement of the catalyst photoactivity to the inhibition of the recombination of photogenerated electrons and holes by the presence of Er³⁺, which leads to a significant increase in the electron–hole separation ratio (Fig. 9b).¹⁴⁰

Su's team complexed LaTiO₂N with α-Fe₂O₃ for photocatalytic CO₂ reduction.¹⁴¹ In the absence of a sacrificial agent, the CO and methane yields were 29.0 and 38.0 μmol g⁻¹, respectively, with a total utilized photoelectron number (UPN) of 362.0 μmol g⁻¹ after 3 h of light exposure, which was significantly better than that of the original LaTiO₂N photocatalyst. The researchers believe that this is due to the formation of Z-type heterojunctions that enable efficient separation and transfer of photogenerated electrons and holes, resulting in an increase in the overall redox capacity of the catalyst and the realization of efficient overall gas-phase CO₂ conversion (Fig. 10e–g).¹⁴¹ Yang's group combined La₂Ti₂O₇ with a narrow bandgap semiconductor to construct a 2D/2D N-doped La₂Ti₂O₇/ZnIn₂S₄ heterojunction for photocatalytic water decomposition for hydrogen production.¹⁴² The experimental results showed that the optimized catalyst has the best photo-driven H₂ generation activity with a hydrogen production rate of 44.62 μmol h⁻¹, which is 5.2 times higher than that of the pristine ZnIn₂S₄ sample. This is due to its unique 2D/2D heterostructure that generates high-speed charge transfer channels, which significantly improves the separation and migration efficiency of photocarriers. Meanwhile, N doping narrows the band gap of La₂Ti₂O₇, which greatly inhibits the recombination of electron–hole pairs. Xu's team synthesized an ultrathin 2D type-II p–n heterojunction, La₂Ti₂O₇/In₂S₃, for visible-light photocatalytic hydrogen production.¹⁴³ It was shown that the activity of the catalyst was significantly better than that of La₂Ti₂O₇ and In₂S₃, with an 18-fold increase in hydrogen production. This is attributed to the effective separation of photogenerated charges and holes due to the formation of the heterojunction, which prolongs the charge lifetime and enhances the photocatalytic activity. In summary, rare-earth oxide photocatalysts and new rare-earth oxide photocatalysts can be widely used in various fields of photocatalysis and show good catalytic activity, and if they can be reasonably modified and designed and applied with rare-earth element-containing oxides, they will show great potential in the future photocatalytic field.

3. Conclusions and future perspective

In summary, the role of rare earth ions in photocatalysts mainly includes the following aspects: (i) rare earth ion doping can inhibit the phase change of the catalyst and maintain structural stability. (ii) Rare earth ions can separate the lattice oxygen from the surface of the photocatalyst, creating oxygen vacancies. (iii) The introduction of rare earth elements will cause lattice distortion of the catalyst, making redox reactions more likely to occur. (iv) Rare earth ions can act as a bridge for electron transfer between two semiconductor materials to effectively separate photogenerated electrons. (v) The doping energy level can play a role in trapping photogenerated electrons or holes, effectively inhibiting the recombination of electrons and holes. (vi) The introduction of rare earth elements can change the energy level structure of the catalyst, allowing catalysts with a narrow band gap to obtain a larger photoresponse range. (vii) Rare earth elements can enhance the adsorption ability of the catalyst to reactants, etc. Based on these advantages of rare earth elements, the rational design of rare earth based photocatalysts is expected to occupy an important position in the field of catalysis in the future. However, it is worth noting that not all rare earth elements act positively on catalysts. Many studies have shown that the impact on catalyst performance depends on the type and doping concentration of rare earth ions. Therefore, selecting appropriate rare earth elements and doping concentrations to design ideal photocatalysts remains a huge challenge. In addition, it is necessary to overcome many difficulties to develop rare earth photocatalysts on a large scale, such as the high cost of raw materials and low yield of the reaction, which means that more energy is needed to promote the reaction. The current reaction conditions are relatively harsh, usually requiring conditions such as high temperature and high pressure. To some extent, it limits practical applications.¹⁴⁴ Therefore, it is necessary to optimize the process preparation of catalysts.

In addition to the above applications, scientists also need to explore some little-known fields, for example, using rare earth elements in other industries such as construction, automotive, and textile to develop new materials, devices, and products with photocatalytic functions. It can also be combined with other technologies, such as optoelectronics and biotechnology, to develop more efficient, environmentally friendly, and promising composite technologies. In summary, rare earths, as a group of similar and different elements, have special electronic structures and spectral characteristics, which can play various important roles in catalysts and show great potential for application. Deeply studying the deep interaction mechanisms and specific advantages of different rare earth ions, and utilizing the energy level structure of each rare earth ion for catalyst design are expected to provide new ideas for the design and development of efficient or novel photocatalyst systems.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22271080).

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