Photocatalytic reduction of CO2 into CH4 over Ru-doped TiO2: Synergy of Ru and oxygen vacancies

Fe-TiO2-x/TiO2 S-scheme homojunction for efficient photocatalytic CO2 reduction

CO2-to-high value-added chemicals via a photocatalytic route is of interest but strangled by the low efficiency. Herein, a novel Fe-TiO2-x/TiO2 S-scheme homojunction was designed and constructed by using a facile surface modification approach whereby oxygen vacancy (OV) and Fe introducing on the TiO2 nanorod surface. The as-synthesized Fe-TiO2-x/TiO2 S-scheme homojunction exhibits positive properties on promoting photocatalytic CO2 reduction: i) the nanorod structure provides numerous active sites and a radical charge transfer path; ii) the doped Fe and OV not only synergistically enhance light utilization but also promote CO2 adsorption; iii) the Fe-TiO2-x/TiO2 S-scheme homojunction benefits photoexcited charge separation and retains stronger redox capacity. Thanks to those good characters, the Fe-TiO2-x/TiO2 homojunction exhibits superior CO2 reduction performances with optimized CO/CH4 generation rates of 122/22 μmol g-1h-1 which exceed those of pure TiO2 by more than 9.4/7.3 folds and most currently reported catalytic systems. This manuscript develops a facile and universal approach to synthesize well-defined homojunction and may inspire the construction of other more high-efficiency photocatalysts toward CO2 reduction and beyond.

Enhancing photocatalytic CO2 reduction with TiO2-based materials: Strategies, mechanisms, challenges, and perspectives

The concentration of atmospheric CO2 has exceeded 400 ppm, surpassing its natural variability and raising concerns about uncontrollable shifts in the carbon cycle, leading to significant climate and environmental impacts. A promising method to balance carbon levels and mitigate atmospheric CO2 rise is through photocatalytic CO2 reduction. Titanium dioxide (TiO2), renowned for its affordability, stability, availability, and eco-friendliness, stands out as an exemplary catalyst in photocatalytic CO2 reduction. Various strategies have been proposed to modify TiO2 for photocatalytic CO2 reduction and improve catalytic activity and product selectivity. However, few studies have systematically summarized these strategies and analyzed their advantages, disadvantages, and current progress. Here, we comprehensively review recent advancements in TiO2 engineering, focusing on crystal engineering, interface design, and reactive site construction to enhance photocatalytic efficiency and product selectivity. We discuss how modifications in TiO2's optical characteristics, carrier migration, and active site design have led to varied and selective CO2 reduction products. These enhancements are thoroughly analyzed through experimental data and theoretical calculations. Additionally, we identify current challenges and suggest future research directions, emphasizing the role of TiO2-based materials in understanding photocatalytic CO2 reduction mechanisms and in designing effective catalysts. This review is expected to contribute to the global pursuit of carbon neutrality by providing foundational insights into the mechanisms of photocatalytic CO2 reduction with TiO2-based materials and guiding the development of efficient photocatalysts.

Advances in Defect Engineering of Metal Oxides for Photocatalytic CO2 Reduction

Photocatalytic CO2 reduction technology, capable of converting low-density solar energy into high-density chemical energy, stands as a promising approach to alleviate the energy crisis and achieve carbon neutrality. Semiconductor metal oxides, characterized by their abundant reserves, good stability, and easily tunable structures, have found extensive applications in the field of photocatalysis. However, the wide bandgap inherent in metal oxides contributes to their poor efficiency in photocatalytic CO2 reduction. Defect engineering presents an effective strategy to address these challenges. This paper reviews the research progress in defect engineering to enhance the photocatalytic CO2 reduction performance of metal oxides, summarizing defect classifications, preparation methods, and characterization techniques. The focus is on defect engineering, represented by vacancies and doping, for improving the performance of metal oxide photocatalysts. This includes advancements in expanding the photoresponse range, enhancing photogenerated charge separation, and promoting CO2 molecule activation. Finally, the paper provides a summary of the current issues and challenges faced by defect engineering, along with a prospective outlook on the future development of photocatalytic CO2 reduction technology.

Construction synergetic adsorption and activation surface via confined Cu/Cu2O and Ag nanoparticles on TiO2 for effective conversion of CO2 to CH4

High-performance photocatalysts for catalytic reduction of CO2 are largely impeded by inefficient charge separation and surface activity. Reasonable design and efficient collaboration of multiple active sites are important for attaining high reactivity and product selectivity. Herein, Cu-Cu2O and Ag nanoparticles are confined as dual sites for assisting CO2 photoreduction to CH4 on TiO2. The introduction of Cu-Cu2O leads to an all-solid-state Z-scheme heterostructure on the TiO2 surface, which achieves efficient electron transfer to Cu2O and adsorption and activation of CO2. The confined nanometallic Ag further enhances the carrier's separation efficiency, promoting the conversion of activated CO2 molecules to •COOH and further conversion to CH4. Particularly, this strategy is highlighted on the TiO2 system for a photocatalytic reduction reaction of CO2 and H2O with a CH4 generation rate of 62.5 μmol∙g-1∙h-1 and an impressive selectivity of 97.49 %. This work provides new insights into developing robust catalysts through the artful design of synergistic catalytic sites for efficient photocatalytic CO2 conversion.

Ru-Induced Defect Engineering in Co3O4 Lattice for High Performance Electrochemical Reduction of Nitrate to Ammonium

Amidst these growing sustainability concerns, producing NH4 + via electrochemical NO3 - reduction reaction (NO3RR) emerges as a promising alternative to the conventional Haber-Bosch process. In a pioneering approach, this study introduces Ru incorporation into Co3O4 lattices at the nanoscale and further couples it with electroreduction conditioning (ERC) treatment as a strategy to enhance metal oxide reducibility and induce oxygen vacancies, advancing NH4 + production from NO3RR. Here, supported by a suite of ex situ and in situ characterization measurements, the findings reveal that Ru enrichment promotes Co species reduction and oxygen vacancy formation. Further, as evidenced by the theoretical calculations, Ru integration lowers the energy barrier for oxygen vacancy formation, thereby facilitating a more energy-efficient NO3RR-to-NH4 + pathway. Optimal catalytic activity is realized with a Ru loading of 10 at.% (named 10Ru/Co3O4), achieving a high NH4 + production rate (98 nmol s-1 cm-2), selectivity (97.5%) and current density (≈100 mA cm-2) at -1.0 V vs RHE. The findings not only provide insights into defect engineering via the incorporation of secondary sites but also lay the groundwork for innovative catalyst design aimed at improving NH4 + yield from NO3RR. This research contributes to the ongoing efforts to develop sustainable electrochemical processes for nitrogen cycle management.

Constructing Cu defect band within TiO2 and supporting NiOx nanoparticles for efficient CO2 photoreduction

Effectively harnessing solar energy for the conversion of CO2 into valuable chemical energy presents a viable solution to address energy scarcity and climate change concerns. Nonetheless, the limited light absorption and sluggish charge kinetics significantly hinder the photoreduction of CO2. In this study, we employed a facile sol-gel method combined with wetness impregnation to synthesize Cu-doped TiO2 coated with NiOx nanoparticles. Various characterizations verified the successful incorporation of Cu ions into the TiO2 crystal lattice and the formation of NiOx co-catalysts within the composites. The optimal performance attained with CTN-0.5 demonstrates an output of 11.85 μmol h-1 g-1 for CO and 9.51 μmol h-1 g-1 for CH4, which represent a 4.4-fold and 15.6-fold increase, respectively, compared to those achieved with pure TiO2. The induced Cu defect band broadens the light absorption by decreasing the conduction band edge of TiO2, while NiOx upshifts the valence band of TiO2 because of the interaction of valence orbitals. Light irradiation EPR and FTIR tests suggest that the collaboration of CuOx and NiOx promotes the formation of oxygen vacancies/defects and a rapid charge transfer pathway, thereby provides numerous active sites and electrons to enhance CO2 photoreduction performance.

Ruthenium doping: An effective strategy for boosting nitrite electroreduction to ammonia over titanium dioxide nanoribbon array

Electrochemical reduction of nitrite (NO₂^-) not only removes NO₂^- contaminant but also produces high-added value ammonia (NH₃). This process, however, needs efficient and selective catalysts for NO₂^--to-NH₃ conversion. In this study, Ruthenium doped titanium dioxide nanoribbon array supported on Ti plate (Ru-TiO₂/TP) is proposed as an efficient electrocatalyst for the reduction of NO₂^- to NH₃. When operated in 0.1 M NaOH containing NO₂^-, such Ru-TiO₂/TP achieves an ultra-large NH₃ yield of 1.56 mmol h^-1 cm^-2 and a super-high Faradaic efficiency of 98.9%, superior to its TiO₂/TP counterpart (0.46 mmol h^-1 cm^-2, 74.1%). Furthermore, the reaction mechanism is studied by theoretical calculation.

Oxygen Vacancy Mediated Band-Gap Engineering via B-Doping for Enhancing Z-Scheme A-TiO2/R-TiO2 Heterojunction Photocatalytic Performance

Fabrication of Z-scheme heterojunction photocatalysts is an ideal strategy for solving environmental problems by providing inexhaustible solar energy. A direct Z-scheme anatase TiO₂/rutile TiO₂ heterojunction photocatalyst was prepared using a facile B-doping strategy. The band structure and oxygen-vacancy content can be successfully tailored by controlling the amount of B-dopant. The photocatalytic performance was enhanced via the Z-scheme transfer path formed between the B doped anatase-TiO₂ and rutile-TiO₂, optimized band structure with markedly positively shifted band potentials, and the synergistically-mediated oxygen vacancy contents. Moreover, the optimization study indicated that 10% B-doping with the R-TiO₂ to A-TiO₂ weight ratio of 0.04 could achieve the highest photocatalytic performance. This work may provide an effective approach to synthesize nonmetal-doped semiconductor photocatalysts with tunable-energy structures and promote the efficiency of charge separation.

First-principles screening of transition metal doped anatase TiO2(101) surfaces for the electrocatalytic nitrogen reduction

The electrocatalytic nitrogen reduction reaction (eNRR) has been attracting intensive scientific attention as a potential alternative to the industrial Haber-Bosch process for ammonia production. Although many materials have been investigated, optimal catalysts for the reaction remain to be found. In this work, we performed the theoretical screening of 3d-5d transition metal doped anatase TiO₂ for the eNRR. The most favorable doping site of each transition metal on the (101) surface was located. We found that the doping of transition metals promotes the formation of oxygen vacancies which are beneficial for the reaction. The scaling relations between the energies of the key intermediates were investigated. Using a machine learning algorithm (SVM), we identified two adsorption modes for the end-on adsorbed *HNN, which exhibited different scaling relations with *NH₂. From a two-step process, we screened out several candidates, among which Au and Ta were proposed to be the most efficient dopants. Electronic structure analysis reveals that they can efficiently lower the energy of the intermediates. These results should be helpful for the design of more efficient TiO₂-based catalysts for the eNRR.

Photocatalytic and Antibacterial Properties of Doped TiO2 Nanopowders Synthesized by Sol−Gel Method

For environmental applications, nanosized TiO2-based materials are known as the most important photocatalyst and are intensively studied for their advantages such as their higher activity, lower price, and chemical and photoresist properties. Zn or Cu doped TiO2 nanoparticles with anatase crystalline structure were synthesized by sol−gel process. Titanium (IV) butoxide was used as a TiO2 precursor, with parental alcohol as a solvent, and a hydrolysing agent (ammonia-containing water) was added to obtain a solution with pH 10. The gels were characterized by TG/DTA analysis, SEM, and XPS. Based on TG/DTA results, the temperature of 500 °C was chosen for processing the powders in air. The structure of the samples thermally treated at 500 °C was analysed by XRD and the patterns show crystallization in a single phase of TiO2 (anatase). The surface of the samples and the oxidation states was investigated by XPS, confirming the presence of Ti, O, Zn and Cu. The antibacterial activity of the nanoparticle powder samples was verified using the gram−positive bacterium Staphylococcus aureus. The photocatalytic efficiency of the doped TiO2 nanopowders for degradation of methyl orange (MO) is here examined in order to evaluate the potential applications of these materials for environmental remediation.

Ni3B as p-Block Element-Modulated Cocatalyst for Efficient Photocatalytic CO2 Reduction

Due to the multiple electron and proton transfer processes involved, the photogenerated charges are easily recombined during the photocatalytic reduction of CO₂, making the generation of the eight-electron product CH₄ kinetically more difficult. Herein, Ni₃B nanoparticles modulated by p-block element were combined with TiO₂ nanosheets to construct a novel Schottky junction photocatalyst (Ni₃B/TiO₂) for the selective photocatalytic conversion of CO₂ to CH₄. The formed Ni₃B/TiO₂ photocatalyst with Schottky junction ensures a transfer pathway of photogenerated electrons from TiO₂ to Ni₃B, which facilitates the accumulation of electrons on the surface of Ni₃B and subsequently improves the activity of photocatalytic CO₂ reduction to CH₄. The optimized Ni₃B/TiO₂ Schottky junction shows an improved CH₄ yield of 30.03 μmol g^-1 h^-1, which was much higher than those of TiO₂ (1.62 μmol g^-1 h^-1), NiO/TiO₂ (2.44 μmol g^-1 h^-1), and Ni/TiO₂ (4.3 μmol g^-1 h^-1). This work demonstrated that the introduction of p-block elements can alleviate the scaling relationship effect of pure metal cocatalysts to a certain extent, and the modified Ni₃B can be used as a promising new cocatalyst to effectively improve the selective photocatalytic of CO₂ to CH₄.

Heterostructured Nanoscale Photocatalysts via Colloidal Chemistry for Pollutant Degradation

With the further acceleration in the industrialization process, organic pollutants and gas pollution in the environment have posed severe threats to human health. It has been a global challenge regarding achieving an efficient solution to pollutant degradation. In such a context, photocatalysts have attracted researchers’ attention for their simplicity, efficiency, cleanliness and low cost. However, the single photocatalyst is facing a research bottleneck owing to its narrow light absorption spectrum and high photocarrier recombination rate. Given that heterojunctions can achieve efficient separation of photogenerated carriers in space, constructing heterostructured photocatalysts has become the most perspective method to improve the performance of photocatalysts. Furthermore, nanoparticles prepared through colloidal chemistry have the characteristics of high dispersion, stability and adsorption, further enhancing the degradation efficiency of heterostructured photocatalysts. This article reviews the primary methods for preparing heterostructured photocatalysts through colloidal chemistry, classifies the heterojunction types by transport routes of photogenerated carriers and summarizes the recent progress of heterostructured photocatalysts in pollutant degradation. To implement environmental remediation, it is crucial to explore economical and efficient photocatalysts for practical applications. It is hoped that this review will stimulate further exploration of colloidal heterostructured photocatalysts for pollutant degradation.

Boosted CO2 photoreduction performance on Ru-Ti3CN MXene-TiO2 photocatalyst synthesized by non-HF Lewis acidic etching method

Photocatalytic CO₂ reduction to produce value-added products is considered a promising solution to solve the global energy crisis and the greenhouse effect. In this study, Ti₃CN MXene was synthesized using a Lewis acidic etching method without the usage of toxic hydrofluoric acid (HF). Ti₃CN MXene was then used as a support for the in situ hydrothermal growth of TiO₂ and Ru nanoparticles. In the presence of 0.5 wt% Ru, Ru-Ti₃CN-TiO₂ shows CO and CH₄ production rates of 99.58 and 8.97 μmol/g, respectively, in 5 h under Xenon lamp irradiation, more than 20.5 and 9.3 times that of commercial P25. The enhancement in photocatalytic activity was attributed to the synergy between the in-situ growth of TiO₂ on Ti₃CN MXene and Ru nanoparticles. It was proven experimentally that Ti₃CN MXene can provide abundant pathways for electron transfer. The separation and transfer of the photo-induced charge were further increased with the help of Ru and Ti₃CN MXene, leaving more electrons to participate in the subsequent CO₂ reduction reaction. We believe that this work will encourage more attention to designing environment-friendly MXene-based photocatalysts for CO₂ photoreduction using the non-HF method.

Interface modification by defect engineering for g-C3N4/LaPO4−x nanorods towards efficient CO2 photoreduction

In this work, a series of specific surface defects are introduced at the interface of LaPO4/g-C3N4 composites.