Design of atomically dispersed catalytic sites for photocatalytic CO2 reduction

Asymmetric Atomic Dual-Sites for Photocatalytic CO2 Reduction

Atomically dispersed active sites in a photocatalyst offer unique advantages such as locally tuned electronic structures, quantum size effects, and maximum utilization of atomic species. Among these, asymmetric atomic dual-sites are of particular interest because their asymmetric charge distribution generates a local built-in electric potential to enhance charge separation and transfer. Moreover, the dual sites provide flexibility for tuning complex multielectron and multireaction pathways, such as CO2 reduction reactions. The coordination of dual sites opens new possibilities for engineering the structure-activity-selectivity relationship. This comprehensive overview discusses efficient and sustainable photocatalysis processes in photocatalytic CO2 reduction, focusing on strategic active-site design and future challenges. It serves as a timely reference for the design and development of photocatalytic conversion processes, specifically exploring the utilization of asymmetric atomic dual-sites for complex photocatalytic conversion pathways, here exemplified by the conversion of CO2 into valuable chemicals.

Synergistic effect of atomically dispersed Cu species and Ti-defects for boosting photocatalytic CO2 reduction over hierarchical TiO2

The photocatalytic water-mediated CO2 reduction reaction, which holds great promise for the conversion of CO2 into valuable chemicals, is often hindered by inefficient separation of photogenerated charges and a lack of suitable catalytic sites. Herein, we have developed a glycerol coordination assembly approach to precisely control the distribution of atomically dispersed Cu species by occupying Ti-defects and adjusting the ratio between Cu species and Ti-defects in a hierarchical TiO2. The optimal sample demonstrates a ∼4-fold improvement in CO2-to-CO conversion compared to normal TiO2 nanoparticles. The high activity could be attributed to the Ti defects, which enhance the photogenerated charge separation and simultaneously facilitate the adsorption of water molecules, thereby promoting the water oxidation reaction. Moreover, by means of in situ EPR and FTIR spectra, we have demonstrated that Cu species can effectively capture photogenerated electrons and facilitate the adsorption of CO2, so as to catalyze the reduction of CO2. This work provides a strategy for the construction of atomic-level synergistic catalytic sites and the utilization of in situ techniques to reveal the underlying mechanism.

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.

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO₂ reduction reaction (CRR) is an effective strategy because it affords the conversion of CO₂ into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst-promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.

Permanently polarized hydroxyapatite, an outstanding catalytic material for carbon and nitrogen fixation

Hydroxyapatite (HAp) is a well-known ceramic material widely used in the biomedical field. This review summarizes the very recent developments on permanently polarized HAp (pp-HAp), a HAp variety with tuned electrical properties that confer remarkable catalytic activity. pp-HAp is obtained by applying a thermal stimulation polarization process (TSP), which consists on a DC electric voltage of 500 V at 1000 °C, to previously sintered HAp. The TSP not only increases the crystallinity, reducing the defects in the crystal lattice, but also creates charges that accumulate at the crystalline boundaries and at the surface of microscopic grains, boosting the electrical conductivity. Finally, the successful utilization of pp-HAp in the catalytic fixation of carbon and nitrogen from CO₂ and N₂ gases, respectively, is reported and the formation of different products of chemical interest (e.g. amino acids, ethanol and ammonium) as a function of the reaction conditions (i.e. feeding gases and presence/absence of UV illumination) and catalyst plasticity is discussed. pp-HAp exhibits important advantages with respect to other consolidated catalysts, which drastically increases the final energetic net balance of the reactions.

Fabrication of 2H/3C-SiC heterophase junction nanocages for enhancing photocatalytic CO2 reduction

The morphology and structure of photocatalyst play an important role in photocatalytic activity. SiC semiconductor is considered as a promising material for the photocatalytic CO₂ reduction due to its negative conduction band position. Herein, SiC nanocages is creatively synthesized by simple low-temperature molten-salt-mediated magnesiothermic reduction method with using SiO₂ as template. The morphology and phase composition of SiC nanocages can be controlled by magnesium dosage and reaction temperature. The 2H and 3C crystal phase in SiC nanocage can form heterophase junctions uniformly to effectively accelerate the photogenerated electron transfer, and plays a key role in improving the photocatalytic activity of 2H/3C-SiC samples. The optimal SiC nanocage sample possesses a CO generation rate of 4.68 μmol g^-1h^-1 for photocatalytic CO₂ reduction, which is 3.25 times higher than that of commercial SiC.

Nanoarchitectonics of Metal-Free Porous Polyketone as Photocatalytic Assemblies for Artificial Photosynthesis

The main component of natural gas is methane, whose combustion contributes to global warming. As such, sustainable, energy-efficient, nonfossil-based methane production is needed to satisfy current energy demands and chemical feedstocks. In this article, we have constructed a metal-free porous polyketone (TPA-DPA PPK) with donor-acceptor (D-A) groups with an extensive π-conjugation by facile Friedel-Crafts acylation reaction between triphenylamine (TPA) and pyridine-2,6-dicarbonyl dichloride (DPA). TPA-DPA PPK is a metal-free catalyst for visible-light-driven CO₂ photoreduction to CH₄, which can be used as a solar fuel in the absence of any cocatalyst and sacrificial agent. CH₄ production (152.65 ppm g^-1) is ∼5 times greater than that of g-C₃N₄ under the same test conditions. Charge-density difference plots from excited-state time-dependent density functional theory (TD-DFT) calculations indicate a depletion and accumulation of charge density among the donor/acceptor functional groups upon photoexcitation. Most notably, binding energies from DFT demonstrate that H₂O is more strongly bound with the pyridinic nitrogen group than CO_2, which shed insight into mechanistic pathways for photocatalytic CO₂ reduction.

Significantly Enhanced Photocatalytic CO2 Reduction by Surface Amorphization of Cocatalysts

Rational phase engineering of reduction cocatalyst offers a promising route to modulate the photocatalytic activity and selectivity in the conversion of CO2 to chemical feedstocks. However, it remains a great challenge to choose a suitable phase given that high-crystallinity phase is more conducive to the charge transfer and separation, while amorphous phase is more favorable for the adsorption and activation of CO2 molecules. To resolve this dilemma, herein, with Pd as a well-defined model, a surface amorphization strategy has been developed to fabricate crystalline@amorphous semi-core-shell cocatalysts based on the transformation of outer layer atoms of crystalline cocatalysts to disorder phase. According to the theoretical and experimental analysis, in the heterostructured cocatalysts, crystalline core shuttles the photoexcited electrons from light-harvesting semiconductor to amorphous shell due to its strong electronic coupling with both components. Meanwhile, amorphous shell provides efficient active sites for preferential activation and conversion of CO2 and suppression of undesirable proton reduction. Benefiting from the synergistic effects between crystalline core and amorphous shell, the optimized heterophase cocatalyst with suitable thickness of amorphous shell achieves superior CO (22.2 µmol gcat-1 h-1 ) and CH4 (38.1 µmol gcat-1 h-1 ) formation rates with considerable selectivity and high stability in comparison with crystalline and amorphous counterparts.

Atomically Dispersed Catalytic Sites: A New Frontier for Cocatalyst/Photocatalyst Composites toward Sustainable Fuel and Chemical Production

Photocatalysis delivers a promising pathway toward the clean and sustainable energy supply of the future. However, the inefficiency of photon absorption, rapid recombination of photogenerated electron-hole pairs, and especially the limited active sites for catalytic reactions result in unsatisfactory performances of the photocatalytic materials. Single-atom photocatalysts (SAPCs), in which metal atoms are individually isolated and stably anchored on support materials, allow for maximum atom utilization and possess distinct photocatalytic properties due to the unique geometric and electronic features of the unsaturated catalytic sites. Very recently, constructing SAPCs has emerged as a new avenue for promoting the efficiency of sustainable production of fuels and chemicals via photocatalysis. In this review, we summarize the recent development of SAPCs as a new frontier for cocatalyst/photocatalyst composites in photocatalytic water splitting. This begins with an introduction on the typical structures of SAPCs, followed by a detailed discussion on the synthetic strategies that are applicable to SAPCs. Thereafter, the promising applications of SAPCs to boost photocatalytic water splitting are outlined. Finally, the challenges and prospects for the future development of SAPCs are summarized.

Atomically Structural Regulations of Carbon-Based Single-Atom Catalysts for Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO₂ RR) converting CO₂ into value-added chemicals and fuels to realize carbon recycling is a solution to the problem of renewable energy shortage and environmental pollution. Among all the catalysts, the carbon-based single-atom catalysts (SACs) with isolated metal atoms immobilized on conductive carbon substrates have shown significant potential toward CO₂ RR, which intrigues researchers to explore high-performance SACs for fuel and chemical production by CO₂ RR. Especially, regulating the coordination structures of the metal centers and the microenvironments of the substrates in carbon-based SACs has emerged as an effective strategy for the tailoring of their CO₂ RR catalytic performance. In this review, the current in situ/operando study techniques and the fundamental parameters for CO₂ RR performance are first briefly presented. Furthermore, the recent advances in synthetic strategies which regulate the atomic structures of the carbon-based SACs, including heteroatom coordination, coordination numbers, diatomic metal centers, and the microenvironments of substrates are summarized. In particular, the structure-performance relationship of the SACs toward CO₂ RR is highlighted. Finally, the inevitable challenges for SACs are outlined and further research directions toward CO₂ RR are presented from the perspectives.

Integrated nano-architectured photocatalysts for photochemical CO2 reduction

Recent advances in nanotechnology, especially the development of integrated nanostructured materials, have offered unprecedented opportunities for photocatalytic CO2 reduction. Compared to bulk semiconductor photocatalysts, most of these nanostructured photocatalysts offer at least one advantage in areas such as photogenerated carrier kinetics, light absorption, and active surface area, supporting improved photochemical reaction efficiencies. In this review, we briefly cover the cutting-edge research activities in the area of integrated nanostructured catalysts for photochemical CO2 reduction, including aqueous and gas-phase reactions. Primarily explored are the basic principles of tailor-made nanostructured composite photocatalysts and how nanostructuring influences photochemical performance. Specifically, we summarize the recent developments related to integrated nanostructured materials for photocatalytic CO2 reduction, mainly in the following five categories: carbon-based nano-architectures, metal-organic frameworks, covalent-organic frameworks, conjugated porous polymers, and layered double hydroxide-based inorganic hybrids. Besides the technical aspects of nanostructure-enhanced catalytic performance in photochemical CO2 reduction, some future research trends and promising strategies are addressed.

Recent Advances on Metalloporphyrin-Based Materials for Visible-Light-Driven CO2 Reduction

Photocatalytic CO₂ reduction is a promising technology to mitigate environmental issue and the energy crisis. The four nitrogen atoms in the porphyrin ring can incorporate transition metals to form stable active sites for CO₂ activation and photoreduction. Nevertheless, the photocatalytic efficiency of metalloporphyrins is still low due to the insufficient photoelectron injection to drive CO₂ photoreduction upon visible light irradiation. To address this issue, considerable efforts have been made to introduce photosensitizers for constructing homogeneous or heterogeneous metalloporphyrin-based photocatalytic systems. In this Review, recent advances of metalloporphyrin-based materials for visible-light-driven CO₂ reduction were summarized. The methods for the modulation of photosensitizing process at molecular level were presented for the promotion of photocatalytic performance. The mechanism of CO₂ activation and photocatalytic conversion was illustrated. Better insight into the structure-activity relationship provides guidance to the design of metalloporphyrin-related photocatalytic systems.

The synthetic strategies for single atomic site catalysts based on metal-organic frameworks

Metal-organic frameworks (MOFs) are a good platform for the fabrication of single atomic site catalysts (SACs) due to their large specific surface area, rich pore structure, large number of unsaturated coordination metal sites and their intriguing and controllable structures. The influencing factors of each strategy used to synthesize SACs based on MOFs, such as the finetuning ligand strategy, heteroatom doping (N, P, S) strategy, space restriction strategy, bimetallic strategy, metal cluster defect strategy, substrate to capture strategy, and various post-treatment strategies have not been discussed. Here, we will discuss the influencing factors of each strategy and the relationship between the different methods, which are used to synthesize SACs based on MOFs.

High-performance light-driven heterogeneous CO2 catalysis with near-unity selectivity on metal phosphides

Akin to single-site homogeneous catalysis, a long sought-after goal is to achieve reaction site precision in heterogeneous catalysis for chemical control over patterns of activity, selectivity and stability. Herein, we report on metal phosphides as a class of material capable of realizing these attributes and unlock their potential in solar-driven CO₂ hydrogenation. Selected as an archetype, Ni₁₂P₅ affords a structure based upon highly dispersed nickel nanoclusters integrated into a phosphorus lattice that harvest light intensely across the entire solar spectral range. Motivated by its panchromatic absorption and unique linearly bonded nickel-carbonyl-dominated reaction route, Ni₁₂P₅ is found to be a photothermal catalyst for the reverse water gas shift reaction, offering a CO production rate of 960 ± 12 mmol g_cat⁻¹ h⁻¹, near 100% selectivity and long-term stability. Successful extension of this idea to Co₂P analogs implies that metal phosphide materials are poised as a universal platform for high-rate and highly selective photothermal CO₂ catalysis.

There exists an urgent need to develop new materials to convert CO₂ to useful products. Here, authors demonstrate metal phosphide nanoparticles to enable light-driven CO₂ hydrogenation with high activities and near-unity selectivity.

Halide Perovskite Nanocrystal Photocatalysts for CO2 Reduction: Successes and Challenges

In current research, halide perovskite nanocrystals have emerged as one of the potential materials for light-harvesting and photovoltaic applications. However, because of phase sensitivity, their exploration as photocatalysts in polar mediums is limited. It has been recently reported that these nanocrystals are capable of driving solar-to-chemical production through CO₂ reduction. Using bare nanocrystals and also coupling in different supports, several reports on CO₂ reduction in low polar mediums were reported, and the mechanism of involved redox processes was also proposed. Considering the importance of this upcoming catalytic activity of perovskites, in this Perspective, details of the developments in the field established to date and supported by several established facts are reported. In addition, some unestablished stories or unsolved pathways surrounding the redox process and the importance of using a polar solvent which confused the understanding of the exclusive roles of perovskite nanocrystals in catalysis are also discussed. Further, the future prospects of these materials that face challenges in dispersing in polar solvents, a key process in redox catalysis for CO₂ reduction, are also discussed.

Exquisite design of porous carbon microtubule-scaffolding hierarchical In2O3-ZnIn2S4 heterostructures toward efficient photocatalytic conversion of CO2 into CO

Porous carbon microtubule (PCMT)-scaffolding semiconductor heterostructures were exquisitely designed through the in situ growth of ZnIn₂S₄ (ZIS) ultrathin nanosheets onto In₂O₃ nanoparticle layers generated on the surface PCMT (abbreviated as PCMT@In₂O₃/ZIS) toward the efficient photocatalytic conversion of CO₂ into CO. The pronounced photocatalytic performance for CO₂ photoreduction into CO is attributed to a synergistic effect of the following factors: (1) the multistage hopping of the charge carriers among In₂O₃, ZIS, and PCMT greatly reduces the charge recombination in In₂O₃ and ZIS. (2) The mesoporous feature of the PCMT renders the large surface area and abundant active sites to accumulate the local concentration of CO₂ in the heterostructures. (3) The existence of a large amount of carbon defects in PCMT promotes the activity of the absorbed CO₂ molecules. (4) The tubular structures with two open ends of PCMT may favor the fast diffusion of the reactants and products, and the optical absorption can also be increased by multi-light scattering/reflection in the interior void. (5) The unique fabrication route leads to an intimate and tight contact among PCMT, In₂O₃, and ZIS, which is also favorable for the charge migration. This work makes a contribution to the development of a complex hollow photocatalysis system for artificial photosynthesis.

Recent Advances in Solar-Driven Carbon Dioxide Conversion: Expectations versus Reality

Solar-driven carbon dioxide (CO₂) conversion to fuels and high-value chemicals can contribute to the better utilization of renewable energy sources. Photosynthetic (PS), photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic plus electrochemical (PV+EC) approaches are intensively studied strategies. We aimed to compare the performance of these approaches using unified metrics and to highlight representative studies with outstanding performance in a given aspect. Most importantly, a statistical analysis was carried out to compare the differences in activity, selectivity, and durability of the various approaches, and the underlying causes are discussed in detail. Several interesting trends were found: (i) Only the minority of the studies present comprehensive metrics. (ii) The CO₂ reduction products and their relative amount vary across the different approaches. (iii) Only the PV+EC approach is likely to lead to industrial technologies in the midterm future. Last, a brief perspective on new directions is given to stimulate discussion and future research activity.

Current Trends in MXene-Based Nanomaterials for Energy Storage and Conversion System: A Mini Review

MXene is deemed to be one of the best attentive materials in an extensive range of applications due to its stupendous optical, electronic, thermal, and mechanical properties. Several MXene-based nanomaterials with extraordinary characteristics have been proposed, prepared, and practiced as a catalyst due to its two-dimensional (2D) structure, large specific surface area, facile decoration, and high adsorption capacity. This review summarizes the synthesis and characterization studies, and the appropriate applications in the catalysis field, exclusively in the energy storage systems. Ultimately, we also discussed the encounters and prospects for the future growth of MXene-based nanomaterials as an efficient candidate in developing efficient energy storage systems. This review delivers crucial knowledge within the scientific community intending to design efficient energy storage systems.

Photocatalytic CO2 reduction of C/ZnO nanofibers enhanced by an Ni-NiS cocatalyst

The photocatalytic reduction of CO₂ into valuable hydrocarbon fuels via solar energy is a promising strategy for carbon utilization. In the present paper, a hierarchical Ni-NiS/C/ZnO photocatalyst was prepared via the in situ photodeposition of compact Ni-NiS nanosheets onto C/ZnO electrospun nanofibers. The existence of metallic Ni and NiS was confirmed by X-ray photoelectron spectroscopy. Photoluminescence (PL) and time-resolved PL spectra revealed that the cocatalyst Ni-NiS enhanced the charge separation efficiency of the C/ZnO nanofibers. The as-prepared Ni-NiS/C/ZnO showed enhanced CO₂ reduction activity, with CO and CH₄ production rates 10 and 15 times greater than those of pristine C/ZnO under 350 W visible light illumination. The intermediates of CH₃O^-, HCHO, and HCOO^- were detected by in situ Fourier transform infrared spectroscopy, confirming that CO₂ reduction is a complex reaction with multiple steps. The ¹³C isotopic tracer method proved that CH₄ and CO were obtained from the reduction of CO₂ rather than from other carbon species in the environment. The amorphous carbon in C/ZnO could promote optical absorption, improve conductivity and reduce the interfacial charge transport resistance. Ni-NiS improved the electron-hole-pair separation of the C/ZnO nanofibers. The observed enhancement in photocatalytic activity was largely attributed to higher light utilization and effective electron-hole separation. This work proves that Ni-NiS is a promising cocatalyst to ZnO for photocatalytic CO₂ reduction.

A ruthenium/polyoxometalate for efficient CO2 photoreduction under visible light in diluted CO2

A system containing polyoxometalate ([Co-POM]^2-) and [Ru(bpy)₃]²⁺ as constructed for visible-light-induced CO₂ conversion to syngas. In diluted CO₂, high efficiency of 56.8 mmol g^-1 h^-1 in syngas production was gained, exceeding that of reported systems with [Ru(bpy)₃]²⁺ participation in similar conditions. Mechanism studies revealed efficient photo-induced charge separation is achieved in the system and CO₂ reduction tends to occur on [Ru(bpy)₃]²⁺.

Immobilization of catalytic sites on quantum dots by ligand bridging for photocatalytic CO2 reduction

Harvesting solar energy to convert carbon dioxide (CO₂) into fossil fuels shows great promise to solve the current global problems of energy crisis and climate change. To achieve this goal, it is desirable to develop efficient catalysts with visible light response to cater for the solar spectrum. CdTe QDs are ideal candidates for absorbing visible light, but it is difficult to directly perform CO₂ reduction due to the lack of effective catalytic sites. Herein, we report a strategy for the activation of mercaptopropionic acid (MPA)-capped CdTe QDs for visible-light-driven CO₂ reduction, in which iron ions (Fe²⁺) are immobilized onto CdTe QDs using l-cysteine as a bridging ligand (CdTe-b-Fe). This ligand bridging strategy can immobilize Fe²⁺ ions on the surface of CdTe QDs as catalytic sites, and these catalytic sites can be conveniently adjusted by directly adding different types or numbers of metal ions. In addition to effectively immobilizing catalytic sites, the bridging ligands can also provide a pathway for electron transport between CdTe QDs and the catalytic sites. The CdTe-b-Fe QD system based on the ligand bridging strategy exhibits excellent catalytic properties: the yield of CH₄/CO (two products together) is 126 μmol g^-1 h^-1, and the selectivity for carbon-based products approaches 98%. This work presents a facile strategy for immobilizing catalytic sites on QDs and provides a platform for designing efficient visible-light driven catalysts for CO₂ reduction.

Intentional construction of high-performance SnO2 catalysts with a 3D porous structure for electrochemical reduction of CO2

Herein, SnO₂-NC (SnO₂-nanocube) and SnO₂-NF (SnO₂-nanoflake) electro-catalysts featuring a large specific surface area and 3D porous structure were successfully constructed via acid etching and sulfurization-desulphurization methods, respectively. As catalysts for the electrochemical reduction of CO₂, the faradaic efficiency (F_HCOO^-+CO = 82.4%, 91.5%, respectively) and partial current density (j_HCOO^-+CO = 10.7 and 11.5 mA cm^-2, respectively) of SnO₂-NCs and SnO₂-NFs were enhanced in comparison with SnO₂-NPs (SnO₂-nanoparticles, F_HCOO^-+CO = 63.4%, j_HCOO^-+CO = 5.7 mA cm^-2) at -1.0 V vs. RHE. The enhanced catalytic activity is attributed to their uniform 3D porous structure, high specific surface area and excellent wettability. Additionally, the morphology of SnO₂-NCs and SnO₂-NFs was largely preserved after electrolyzing for 12 h (after 12 h of electrolysis), indicating the effective buffering effect of the 3D structure in electrolysis. Naturally, the current density and faradaic efficiency of the SnO₂-NC and SnO₂-NF catalysts remained nearly unchanged after long-term stability measurements, revealing great stability.

Phase-Selective Disordered Anatase/Ordered Rutile Interface System for Visible-Light-Driven, Metal-Free CO2 Reduction

Visible-light-driven photocatalytic CO₂ reduction using TiO₂ that can absorb light of all wavelengths has been sought for over half a century. Herein, we report a phase-selective disordered anatase/ordered rutile interface system for visible-light-driven, metal-free CO₂ reduction using a narrow band structure, whose conduction band position matches well with the reduction potential of CO₂ to CH₄ and CO. A mixed disordered anatase/ordered rutile (A_d/R_o) TiO₂ was prepared from anatase and rutile phase-mixed P25 TiO₂ at room temperature and under an ambient atmosphere in sodium alkyl amine solutions. The A_d/R_o TiO₂ showed a narrow band structure due to multi-internal energy band gaps of Ti³⁺ defect sites in the disordered anatase phase, leading to high visible light absorption and simultaneously providing fast charge separation through the crystalline rutile phase, which was faster than that of pristine P25 TiO₂. The band gap of A_d/R_o TiO₂ is 2.62 eV with a conduction band of -0.27 eV, which matches well with the reduction potential of -0.24 V_NHE of CO₂/CH₄, leading to effective electron transfer to CO₂. As a result, the A_d/R_o TiO₂ provided the highest CH₄ production (3.983 μmol/(g h)), which is higher than that of even metal (W, Ru, Ag, and Pt)-doped P25, for CO₂ reduction under visible light.