Ti(IV)-MOF with Specific Facet-Ag Nanoparticle Composites for Enhancing the Photocatalytic Activity and Selectivity of CO2 Reduction

Modulation of interface structure on titanium-based metal-organic frameworks heterojunctions for boosting photocatalytic carbon dioxide reduction

Rational regulation of interface structure in photocatalysts is a promising strategy to improve the photocatalytic performance of carbon dioxide (CO2) reduction. However, it remains a challenge to modulate the interface structure of multi-component heterojunctions. Herein, a strategy integrating heterojunction with facet engineering is developed to modulate the interface structure of metal-organic frameworks (MOF)-based heterojunctions. A series of core-shell UiO-66 (Zr-MOF)-loaded MIL-125 (Ti-MOF) heterojunctions with exposed specific facets were prepared to enhance the separation efficiency of photogenerated electrons-holes in CO2 photoreduction. Impressively, MIL-125to@UiO-66 with exposed {1 1 1} facet exhibits an excellent CO production rate (56.4 μmol g-1 h-1) and selectivity (99 %) under visible light irradiation without any photosensitizers/sacrificial agents, being 1.4 and 11.3 times higher than individual MIL-125to and UiO-66, respectively. The type-II heterojunction significantly enhances the separation of photogenerated electrons-holes in physical space. The photogenerated electrons migrate from Zr in UiO-66 to Ti in MIL-125to, promoting a spatial synergy between CO2 reduction on MIL-125to and H2O oxidation on UiO-66. Compared with MIL-125rd@UiO-66 with exposed {1 1 0} facet and MIL-125ds@UiO-66 with exposed {0 0 1} facet, MIL-125to@UiO-66 with exposed {1 1 1} facet improves the exposure of surface-active Ti sites, thereby enhancing the adsorption/activation of CO2 to generate the *COOH intermediate. This work provides an effective strategy for designing MOF-based heterojunction photocatalysts to improve photocatalytic performance.

NH2-MIL-125(Ti) and its functional nanomaterials - a versatile platform in the photocatalytic arena

Titanium (Ti)-based MOFs are promising materials known for their porosity, stability, diverse valence states, and a lower conduction band (CB) than Zr-MOFs. These features support stable ligand-to-metal charge transfer (LMCT) transitions under photoirradiation, enhancing photocatalytic performance. However, Ti-MOF structures remain a challenge owing to the highly volatile and hydrophilic nature of ionic Ti precursors. The discovery of MIL-125 marked a breakthrough in Ti-cluster coordination chemistry. Combining it with NH2 chromophores to form NH2-MIL-125 enhanced its structural design and extended its activity into the visible light region. This review delves into the high-performance photocatalytic properties of NH2-MIL-125, focusing on its applications in H2O2 and H2 production, CO2 and N2 reduction, drug and dye degradation, photocatalytic sensors, and organic transformation reactions. The discussion considers the influence of the Ti precursor, coordination environment, synthesis process, and charge transfer mechanisms. Numerous strategic methods have been discussed to improve the performance of NH2-MIL-125 by incorporating linker modification, metal node modification, encapsulation of active species, and post-modification for enhancing light absorption ability, promoting charge separation, and improving photocatalytic efficiency. Moreover, future perspectives include methods to investigate how the efficiency of NH2-MIL-125-based materials can be planned in promoting research by highlighting their versatility and potential impacts in the area of photocatalysis.

One-Pot Synthesis of Hydrophobic Porphyrin Zirconium-Based MOFs for the Photoreduction of CO2 to Formate

Using zirconium tetrachloride as a metal source and tetra(4-carboxyphenyl)porphyrin as a ligand and by in situ introducing octadecylphosphonic acid (OPA), three hydrophobic porphyrin zirconium metal-organic frameworks (MOFs) with different structural topologies were constructed, where these MOFs are labeled as OPA@PCN-222, OPA@PCN-223, and OPA@PCN-224. Compared with the original Zr-MOFs without the modification of OPA, the modified porphyrin Zr-MOFs show excellent hydrophobic properties and can maintain excellent stability in a long-term humidity environment. Meanwhile, OPA@PCN-222, OPA@PCN-223, and OPA@PCN-224 exhibit wide absorption of visible light and steadfast and expeditious photocurrent response by leveraging the properties of porphyrin ligands. When illuminated by visible light, the hydrophobic Zr-MOFs demonstrate an efficient reduction of CO2 to HCOO-, achieving average reaction rates of 330, 260, and 258 μmol·h-1·g-1 for OPA@PCN-222, OPA@PCN-223, and OPA@PCN-224. These rates are 1.13-1.41 times higher than that of the original porphyrin Zr-MOFs. The mechanism study shows that both the porphyrin ligands and the Zr-O clusters serve as catalytically active sites, enabling the conversion of CO2 to HCOO-. This research shows that the introduction of hydrophobic alkyl chains can effectively enhance the stability of MOFs under a humid environment while maintaining their catalytic activity, which provides a reference for improving the comprehensive performance of MOF catalysts.

Cr-MOF composited with facet-engineered bimetallic alloys for inducing photocatalytic conversion of CO2 to C2H4

The design of efficient photocatalysts is crucial for photocatalytic CO2 reduction. This study developed photocatalysts based on MIL-101(Cr) composited with a facet-engineered Pt/Pd nanoalloy (PPNA). Photocatalytic performance evaluations show that MIL-101(Cr) loaded with PPNA exposing {111} facets, namely M-A(111), exhibits a CO2 to C2H4 conversion rate of 9.5 μmol g-1 h-1 in addition to the CO and CH4, whereas M-A(100) with PPNA exposing {100} facets gives CO2 conversion rates of 33.2 for CO and 9.3 μmol g-1 h-1 for CH4 without C2H4. In situ FT-IR revealed that M-A(111) can readily form C2 intermediates during the reaction. This work offers a strategy for the design of photocatalysts for CO2 reduction to C2H4.

Regulating the Electronic Band Structure of the Ti-Based Metal-Organic Framework toward Boosting Light-Driven Hydrogen Evolution

The photocatalytic H2 evolution rate on the isomorphic nanosheet-based Ti metal organic-frameworks (MOFs) is regulated through changing the length of aromatic carboxylate ligands. For the series of Ti-MOFs, when increasing the length of organic linkers, the band gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) narrow based on density functional theory (DFT) calculation, accompanied by a degree of increase of organic ligand involvement in the LUMO. When increasing the linker length, both the intensities of photoluminescence (PL) and electron paramagnetic resonance (EPR) signals related to Ti3+ gradually decrease, which are opposite to their photocatalytic performance, where the longer the linkers, the higher the hydrogen evolution rate. It is suggested that the bound photoelectrons by Ti3+ compete with the transfer of photoelectrons for H2 evolution. When increasing the length of the organic linker, more photoelectrons could be generated, in addition to electron transfer overwhelming electrons bound by Ti3+. Both of them engender the super photocatalytic hydrogen evolution. This work highlights a specific way of regulating the electronic structure of Ti-based photocatalysts toward promoting the utilization efficiency of photoelectrons, which will shed light on the design of efficient photocatalysts for the generation of solar fuels.

Synergistic Coordination and Surface Plasmon Resonance of Quantum Dots in Enhancing Photocatalytic Hydrogen Evolution

Recent studies have revealed that the integration of metal nanoparticles (NPs) with photocatalysts allows the metal NPs to serve as cocatalysts, significantly enhancing reactant efficiency near nanostructures through the surface plasmon resonance (SPR) effect. On this basis, we synthesized highly reactive FePt quantum dots (FePt QDs) with tailored configurations, manipulating the Pt coordination environment and combining FePt QDs with ultrathin two-dimensional nickel metal-organic layer (Ni-MOL) nanosheets. X-ray absorption fine spectroscopy (XAFS) confirmed the distinctive Pt-Fe configuration after photoactivation. The optimized loading amount of FePt QDs led to a hydrogen evolution reaction (HER) yield of 113 mmol·g-1·h-1 after activation, with the catalyst remaining stable over five cycles. The improved reaction efficiency stemmed from Pt coordination adjustments and a localized SPR effect, supported by ultraviolet-visible (UV-vis), infrared (IR), Raman, and XAFS characterizations. A comparative analysis was conducted with Ni-MOL deposited with platinum NPs, further underscoring the distinct advantages of FePt QDs and corroborating by density functional theory (DFT) calculations, which revealed favorable d-band center properties. These findings offer a promising avenue for the development of highly efficient and stable nanoalloy photocatalysts.

Metal-organic frameworks (MOFs) for photoelectrocatalytic (PEC) reducing carbon dioxide (CO2) to hydrocarbon fuels

MOF-based photoelectrocatalysis (PEC) using CO2 as an electron donor offers a green, clean, and extensible way to make hydrocarbon fuels under more tolerant conditions. Herein, basic principles of PEC reduction of CO2 and the preparation methods and characterization techniques of MOF-based materials are summarized. Furthermore, three applications of MOFs for improving the photoelectrocatalytic performance of CO2 reduction are described: (i) as photoelectrode alone; (ii) as a co-catalyst of semiconductor photoelectrode or as a substrate for loading dyes, quantum dots, and other co-catalysts; (iii) as one of the components of heterojunction structure. Challenges and future wave surrounding the development of robust PEC CO2 systems based on MOF materials are also discussed briefly.

Decoration of viscose fibers with silver nanoparticle-based titanium-organic framework for use in environmental applications

To effectively remove pharmaceuticals, nitroaromatic compounds, and dyes from wastewater, an efficient multifunctional material was created based on silver nanoparticles (Ag) and MIL-125-NH2 (MOF) immobilized on viscose fibers (VF) as a support substrate. Firstly, silver nanoparticles (Ag) were immobilized on the surface of viscose fibers (VF) via in situ synthesis using trisodium citrate (TSC) as a reducing agent to create (VF-Ag). Then, VF and VF-Ag were decorated with the titanium metal-organic framework MIL-125-NH2 (MOF) to create VF-MOF and VF-Ag-MOF. The influence of VF-Ag, VF-MOF, and VF-Ag-MOF on the sonocatalytic or sonophotocatalytic degradation of sulfa drugs was investigated. The results show that VF-Ag-MOF showed excellent sonocatalytic and sonophotocatalytic activity towards the degradation of sulfa drugs compared to VF-Ag and VF-MOF. Furthermore, sonophotodegradation showed a dramatic enhancement in the efficiency of degradation of sulfa drugs compared to sonodegradation. The sonophotodegradation degradation percentage of sulfanilamide, sulfadiazine, and sulfamethazine drugs in the presence of VF-Ag-MOF was 65, 90, and 95 after 45 min of ultrasonic and visible light irradiation. The catalytic activity of VF-Ag, VF-MOF, and VF-Ag-MOF was evaluated through the conversion of p-nitrophenol (4-NP) to p-aminophenol (4-AP). The results demonstrate that VF-Ag-MOF had the highest catalytic activity, followed by VF-Ag and VF-MOF. The conversion percentage of 4-NP to 4-AP was 69%. The catalytic or photocatalytic effects of VF-Ag, VF-MOF, and VF-Ag-MOF on the elimination of methylene blue (MB) dye were investigated. The results demonstrate that VF-Ag-MOF showed high efficiency in removing the MB dye through the reduction (65%) or photodegradation (71%) after 60 min. VF-Ag-MOF composites structure-activity relationships represent that doping within silver NPs enhanced the photocatalytic activity of MIL-125-NH2, which could be explained as follows: (i) Due to the formation of a Schottky barrier at the junction between MIL-125-NH2 and Ag NPs, the photogenerated electrons in the conduction band of MIL-125-NH2 were supposed to be quickly transferred to the valence band of the Ag NPs, and subsequently, the electrons were transferred to the conduction band of Ag NPs. This considerable electron transferring process, which is reported as Z scheme heterojunction, can efficiently suppress the recombination of electron/hole pairs in VF-Ag-MIL-125-NH2 composites. (ii) Sufficient separation between the photogenerated charge carriers (holes and electrons) and avoiding their recombination enhanced the photocatalytic activity of composites.

Guest-Induced Multilevel Charge Transport Strategy for Developing Metal-Organic Frameworks to Boost Photocatalytic CO2 Reduction

Encapsulating photogenerated charge-hopping nodes and space transport bridges within metal-organic frameworks (MOFs) is a promising method of boosting the photocatalytic performance. Herein, this work embeds electron transfer media (9,10-bis(4-pyridyl)anthracene (BPAN)) in MOF cavities to build multi-level electron transfer paths. The MOF cavities are accurately regulated to investigate the significance of the multi-level electron transfer paths in the process of CO2 photoreduction by evaluating the difference in the number of guest media. The prepared MOFs, {[Co(BPAN)(1,4-dicarboxybenzene)(H2 O)2 ]·BPAN·2H2 O} and {[Co(BPAN)2 (4,4'-biphenyldicarboxylic acid)2 (H2 O)2 ]·2BPAN·2H2 O} (denoted as BPAN-Co-1 and BPAN-Co-2), exhibit efficient visible-light-driven CO2 conversion properties. The CO photoreduction efficacy of BPAN-Co-2 (5598 µmol g-1  h-1 ) is superior to that of most reported MOF-based catalysts. In addition, the enhanced CO2 photoreduction ability is supported by density functional theory (DFT). This work illustrates the feasibility of realizing charge separation characteristics in MOF catalysts at the molecular level, and provides new insight for designing high-performance MOFs for artificial photosynthesis.

Photocatalytic Reduction of Carbon Dioxide to Methanol: Carbonaceous Materials, Kinetics, Industrial Feasibility, and Future Directions

Photocatalytic carbon dioxide reduction (PCCR) for methanol synthesis (CH₃OH) targeting renewable energy resources is an attractive way to create a sustainable environment and also balance the carbon-neutral series. The application of PCCR to methanol enables the generation of solar energy while reducing CO₂, killing two birds with one stone in terms of energy and the environment. In recent years, research on CO₂ utilization has focused on hydrogenation of CO₂ to methanol due to global warming. This article mainly focuses on selective carbonaceous materials such as graphene, mesoporous carbon, and carbon nanotubes (CNTs) as catalysts for heterogeneous photocatalytic CO₂ reduction to methanol. In addition, special emphasis will be placed on the state of the art of PCCR catalysts as this type of research will be of great benefit for further development in this field. The main features of the reaction kinetics, techno-economic study, and current technological developments in PCCR are covered in detail.

Facile Synthesis of Ag NP Films via Evaporation-Induced Self-Assembly and the BA-Sensing Properties

Herein, we design and prepare large-area silver nanoparticle (Ag NP) films based on evaporation-induced self-assembly, which offers the visual and real-time detection of chilled broiler meat freshness. The color change is based on the fact that an increase in the biogenic amine (BA) concentration causes a change in the absorption wavelength of Ag NPs caused by aggregation and etch of the Ag NPs, resulting in a yellow to brown color change, thus enabling a naked-eye readout of the BA exposure. The Ag NP films exhibit a rapid, sensitive, and linear response to BAs in a wide detection range of 2 µM to 100 µM. The Ag NP films are successfully applied as a quick-response, online, high-contrasting colorimetric sensor for visual detection of the freshness of chilled broiler meat.

Rational Construction of Metal Organic Framework Hybrid Assemblies for Visible Light-Driven CO2 Conversion

Photocatalytic reduction of CO₂ to value-added chemicals is known to be a promising approach for CO₂ conversion. The design and preparation of ideal photocatalysts for CO₂ conversion are of pivotal significance for the sustainable development of the whole society. In this work, we integrated two functional organic linkers to prepare a novel metal organic framework (MOF) photocatalyst {[Co(9,10-bis(4-pyridyl)anthracene)_0.5(bpda)]·4DMF} (Co-MOF). The existence of anthryl and amino groups leads to a wide range of visible light absorption and efficient separation of photogenerated electrons. To extend the lifetime of photogenerated electrons in the photocatalytic system, we modified Co-MOF particles onto g-C₃N₄. As expected, Co-MOF/g-C₃N₄ composites exhibited an ultrahigh selectivity (more than 97%) in the photocatalytic process, and the highest CO production rate (1824 μmol/g/h) was 7.1 and 27.2 times of Co-MOFs and g-C₃N₄, respectively. What's more, we also discussed the reaction mechanism of the Co-MOF/g-C₃N₄ photocatalytic CO₂ reduction, and this work paves the pathway for designing photocatalysts with ideal CO₂ reduction performance.