Selective Photocatalytic CO2 Reduction in Water through Anchoring of a Molecular Ni Catalyst on CdS Nanocrystals

Unveiling the role of proton concentration in dinuclear metal complexes for boosting photocatalytic CO2 reduction

The reaction kinetics of photocatalytic CO2 reduction is highly dependent on the transfer rate of electrons and protons to the CO2 molecules adsorbed on catalytic centers. Studies on uncovering the proton effect in catalysts on photocatalytic activity of CO2 reduction are significant but rarely reported. In this paper, we, from the molecular level, revealed that the photocatalytic activity of CO2 reduction is closely related to the proton availability in catalysts. Specifically, four dinuclear Co(II) complexes based on Robson-type ligands with different number of carboxylic groups (-nCOOH; n = 0, 2, 4, 6) were designed and synthesized. All these complexes show photocatalytic activity for CO2 reduction to CO in a water-containing system upon visible-light illumination. Interestingly, the CO yields increase positively with the increase of the carboxylic-group number in dinuclear Co(II) complexes. The one containing -6COOH shows the best photocatalytic activity for CO2 reduction to CO, with the TON value reaching as high as 10,294. The value is 1.8, 3.4, and 7.8 times higher than those containing -4COOH, -2COOH, and -0COOH, respectively. The high TON value also makes the dinuclear Co(II) complex with -6COOH outstanding among reported homogeneous molecular catalysts for photocatalytic CO2 reduction. Control experiments and density functional theory calculation indicated that more carboxylic groups in the catalyst endow the catalyst with more proton relays, thus accelerating the proton transfer and boosting the photocatalytic CO2 reduction. This study, at a molecular level, elucidates that more carboxylic groups in catalysts are beneficial for boosting the reaction kinetics of photocatalytic CO2 reduction.

Charge Transport Approaches in Photocatalytic Supramolecular Systems Composing of Semiconductor and Molecular Metal Complex for CO2 Reduction

The design of photocatalytic supramolecular systems composing of semiconductors and molecular metal complexes for CO2 reduction has attracted increasing attention. The supramolecular system combines the structural merits of semiconductors and metal complexes, where the semiconductor harvests light and undertakes the oxidative site, while the metal complex provides activity for CO2 reduction. The intermolecular charge transfer plays crucial role in ensuring photocatalytic performance. Here, we review the progress of photocatalytic supramolecular systems in reduction of CO2 and highlight the interfacial charge transfer pathways, as well as their state-of-the-art characterization methods. The remaining challenges and prospects for further design of supramolecular photocatalysts are also presented.

Synthesis of a Triangle-Fused Six-Pointed Star and Its Electrocatalytic CO2 Reduction Activity

As electrocatalysts, molecular catalysts with large aromatic systems (such as terpyridine, porphyrin, or phthalocyanine) have been widely applied in the CO2 reduction reaction (CO2RR). However, these monomeric catalysts tend to aggregate due to strong π-π interactions, resulting in limited accessibility of the active site. In light of these challenges, we present a novel strategy of active site isolation for enhancing the CO2RR. Six Ru(Tpy)2 were integrated into the skeleton of a metallo-organic supramolecule by stepwise self-assembly in order to form a rhombus-fused six-pointed star R1 with active site isolation. The turnover frequency (TOF) of R1 was as high as 10.73 s-1 at -0.6 V versus reversible hydrogen electrode (vs RHE), which is the best reported value so far at the same potential to our knowledge. Furthermore, by increasing the connector density on R1's skeleton, a more stable triangle-fused six-pointed star T1 was successfully synthesized. T1 exhibits exceptional stability up to 126 h at -0.4 V vs RHE and excellent TOF values of CO. The strategy of active site isolation and connector density increment significantly enhanced the catalytic activity by increasing the exposure of the active site. This work provides a starting point for the design of molecular catalysts and facilitates the development of a new generation of catalysts with a high catalytic performance.

Reversible Ligand Detachment from CdSe Quantum Dots Following Photoexcitation

The nanocrystal-ligand boundaries of colloidal quantum dots (QDs) mediate charge and energy transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. We used time-resolved infrared spectroscopy combined with transient electronic spectroscopy to probe vibrational modes of the carboxylate anchoring groups of stearate ligands attached to cadmium selenide (CdSe) QDs that were optically excited in solid nanocrystal films. The vibrational frequencies of surface-bonded carboxylate groups revealed their interactions with surface-localized holes in the excited states of the QDs. We also observed transient and reversible photoinduced ligand detachment from CdSe nanocrystals within their excited state lifetime. By probing both surface charge distributions and ligand dynamics on QDs in their excited states, we open a pathway to explore how the nanocrystal-ligand boundary can be understood and controlled for the design of QD architectures that most effectively drive charge transfer processes in solar energy harvesting and photoredox catalysis applications.

Photocatalytic CO2 Reduction Using CO2-Binding Enzymes

Novel concepts to utilize carbon dioxide are required to reach a circular carbon economy and minimize environmental issues. To achieve these goals, photo-, electro-, thermal-, and biocatalysis are key tools to realize this, preferentially in aqueous solutions. Nevertheless, catalytic systems that operate efficiently in water are scarce. Here, we present a general strategy for the identification of enzymes suitable for CO2 reduction based on structural analysis for potential carbon dioxide binding sites and subsequent mutations. We discovered that the phenolic acid decarboxylase from Bacillus subtilis (BsPAD) promotes the aqueous photocatalytic CO2 reduction selectively to carbon monoxide in the presence of a ruthenium photosensitizer and sodium ascorbate. With engineered variants of BsPAD, TONs of up to 978 and selectivities of up to 93 % (favoring the desired CO over H2 generation) were achieved. Mutating the active site region of BsPAD further improved turnover numbers for CO generation. This also revealed that electron transfer is rate-limiting and occurs via multistep tunneling. The generality of this approach was proven by using eight other enzymes, all showing the desired activity underlining that a range of proteins is capable of photocatalytic CO2 reduction.

Photocatalytic CO2 Reduction Using Homogeneous Carbon Dots with a Molecular Cobalt Catalyst

A simple and precious-metal free photosystem for the reduction of aqueous CO2 to syngas (CO and H2) is reported consisting of carbon dots (CDs) as the sole light harvester together with a molecular cobalt bis(terpyridine) CO2 reduction co-catalyst. This homogeneous photocatalytic system operates in the presence of a sacrificial electron donor (triethanolamine) in DMSO/H2O solution at ambient temperature. The photocatalytic system exhibits an activity of 7.7 ± 0.2 mmolsyngas gCDs -1 (3.6 ± 0.2 mmolCO gCDs -1 and 4.1 ± 0.1 mmolH2 gCDs -1) after 24 hours of full solar spectrum irradiation (AM 1.5G). Spectroscopic and electrochemical characterization supports that this photocatalytic performance is attributed to a favorable association between CDs and the molecular cobalt catalyst, which results in improved interfacial photoelectron transfer and catalytic mechanism. This work provides a scalable and inexpensive platform for the development of CO2 photoreduction systems using CDs.

The role of surface functionalization in quantum dot-based photocatalytic CO2 reduction: balancing efficiency and stability

Photocatalytic CO2 reduction offers a promising strategy to produce hydrocarbons without reliance on fossil fuels. Visible light-absorbing colloidal nanomaterials composed of earth-abundant metals suspended in aqueous media are particularly attractive owing to their low-cost, ease of separation, and highly modifiable surfaces. The current study explores such a system by employing water-soluble ZnSe quantum dots and a Co-based molecular catalyst. Water solubilization of the quantum dots is achieved with either carboxylate (3-mercaptopropionic acid) or ammonium (2-aminoethanethiol) functionalized ligands to produce nanoparticles with either negatively or positively-charged surfaces. Photocatalysis experiments are performed to compare the effectiveness of these two surface functionalization strategies on CO2 reduction and ultrafast spectroscopy is used to reveal the underlying photoexcited charge dynamics. We find that the positively-charged quantum dots can support sub-picosecond electron transfer to the carboxylate-based molecular catalyst and also produce >30% selectivity for CO and >170 mmolCO gZnSe-1. However, aggregation reduces activity in approximately one day. In contrast, the negatively-charged quantum dots exhibit >10 ps electron transfer and substantially lower CO selectivity, but they are colloidally stable for days. These results highlight the importance of the quantum dot-catalyst interaction for CO2 reduction. Furthermore, multi-dentate catalyst molecules create a trade-off between photocatalytic efficiency from strong interactions and deleterious aggregation of quantum dot-catalyst assemblies.

Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst

Inspired by natural enzymes, this study presents a nickel-based molecular catalyst, [Ni‖ (N2 S2 )]Cl2 (NiN2 S2 , N2 S2 =2,11-dithia[3,3](2,6)pyridinophane), for the photochemical catalytic reduction of CO2 under visible light. The catalyst was synthesized and characterized using various techniques, including liquid chromatography-high resolution mass spectrometry (LC-HRMS), UV-Visible spectroscopy, and X-ray crystallography. The crystallographic analysis revealed a slightly distorted octahedral coordination geometry with a mononuclear Ni2+ cation, two nitrogen atoms and two sulfur atoms. Photocatalytic CO2 reduction experiments were performed in homogeneous conditions using the catalyst in combination with [Ru(bpy)3 ]Cl2 (bpy=2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor. The catalyst achieved a high selectivity of 89 % towards CO and a remarkable turnover number (TON) of 7991 during 8 h of visible light irradiation under CO2 in the presence of phenol as a co-substrate. The turnover frequency (TOF) in the initial 6 h was 1079 h-1 , with an apparent quantum yield (AQY) of 1.08 %. Controlled experiments confirmed the dependency on the catalyst, light, and sacrificial electron donor for the CO2 reduction process. These findings demonstrate this bioinspired nickel molecular catalyst could be effective for fast and efficient photochemical catalytic reduction of CO2 to CO.

Generating Long-Lived Charge Carriers in CdS Quantum Dots by Cu-Doping for Photocatalytic CO2 Reduction

Converting CO2 into high-value-added chemicals has been recognized as a promising way to tackle the fossil fuel crisis. Quantum dots (QDs) have been extensively studied for photocatalytic CO2 reduction due to their excellent optoelectronic properties. However, most of the photogenerated charge carriers recombine before they participate in the photocatalytic reaction. It is crucial to regulate the charge carriers to minimize undesired charge recombination, thus, promoting surface photocatalysis. Herein, we report a copper-doped CdS (Cu:CdS) QD photocatalyst for CO2 reduction. Density functional theory simulations and experimental results demonstrate that Cu dopants create intermediate energy levels in CdS QDs that can extend the lifetime of exciton charge carriers. Furthermore, the long-lived charge carriers can be harnessed for the photocatalytic reaction on Cu:CdS QDs. The resultant Cu:CdS QDs exhibited a significantly enhanced photocatalytic activity toward CO2 reduction compared to the pristine CdS QDs. This work highlights the importance of charge regulation in photocatalysts and opens new pathways for the exploration of efficient QD photocatalysts.

Anchoring of Metal Complexes on Au25 Nanocluster for Enhanced Photocoupled Electrocatalytic CO2 Reduction

Atomically precise Au nanoclusters (NCs) with discrete energy levels can be used as photosensitizers for CO2 reduction. However, tight ligand capping of Au NCs hinders CO2 adsorption on its active sites. Here, a new hybrid material is obtained by anchoring of thiol functionalized terpyridine metal complexes (metal=Ru, Ni, Fe, Co) on Au NCs by ligand exchange reactions (LERs). The anchoring of Ru and Ni complexes on Au25 NC (Au25 -Ru and Au25 -Ni) leads to adequate CO2 to CO conversion for photocoupled electrocatalytic CO2 reduction (PECR) in terms of high selectivity, with Faradaic efficiency of CO (FECO ) exceeding 90 % in a wide potential range, remarkable activity (CO production rate up to two times higher than that for pristine Au25 PET18 ) and extremely large turnover frequencies (TOFs, 63012 h-1 at -0.97 V for Au25 -Ru and 69989 h-1 at -1.07 V vs. RHE for Au25 -Ni). Moreover, PECR stability test indicates the excellent long-term stability of the modified NCs in contrast with pristine Au NCs. The present approach offers a novel strategy to enhance PECR activity and selectivity, as well as to improve the stability of Au NCs under light illumination, which paves the way for highly active and stable Au NCs catalysts.

2D Bi2MoO6/Zn3V2O8 heterojunction photocatalyst for efficient photocatalytic reduction of CO2 to CO and CH4

Two-dimensional (2D) "face-to-face" heterojunctions promote interfacial charge transfer and separation in composite photocatalysts. Here, we report an efficient 2D/2D step-scheme (S-scheme) photocatalyst composed of Bi2MoO6/Zn3V2O8 (BMO/ZVO), which has been designed and prepared via the self-assembly of BMO and ZVO nanoflakes. The heterojunction with an optimized composition of 30% BMO/ZVO showed extended light absorption capacity and enhanced separation efficiency of photogenerated carriers. Density functional theory (DFT) calculation on work function and charge density revealed the presence of a built-in electric field at the interface region, which should facilitate the separation of photogenerated electron-hole pairs. This work showed that it is essential to select two photocatalysts with interlaced band arrangement and to fine-tune the heterojunction interface for the preparation of S-scheme heterojunctions to achieve high photocatalytic efficiency.

Selective Metal-Free CO2 Photoreduction in Water Using Porous Nanostructures with Internal Molecular Free Volume

The conversion of CO2 to a sole carbonaceous product using photocatalysis is a sustainable solution for alleviating the increasing levels of CO2 emissions and reducing our dependence on nonrenewable resources such as fossil fuels. However, developing a photoactive, metal-free catalyst that is highly selective and efficient in the CO2 reduction reaction (CO2RR) without the need for sacrificial agents, cocatalysts, and photosensitizers is challenging. Furthermore, due to the poor solubility of CO2 in water and the kinetically and thermodynamically favored hydrogen evolution reaction (HER), designing a highly selective photocatalyst is challenging. Here, we propose a molecular engineering approach to design a photoactive polymer with high CO2 permeability and low water diffusivity, promoting the mass transfer of CO2 while suppressing HER. We have incorporated a contorted triptycene scaffold with "internal molecular free volume (IMFV)" to enhance gas permeability to the active site by creating molecular channels through the inefficient packing of polymer chains. Additionally, we introduced a pyrene moiety to promote visible-light harvesting capability and charge separation. By leveraging these qualities, the polymer exhibited a high CO generation rate of 77.8 μmol g-1 h-1, with a high selectivity of ∼98% and good recyclability. The importance of IMFV was highlighted by replacing the contorted triptycene unit with a planar scaffold, which led to a selectivity reversal favoring HER over CO2RR in water. In situ electron paramagnetic resonance (EPR), time-resolved photoluminescence spectroscopy (TRPL), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) techniques, further supported by theoretical calculations, were employed to enlighten the mechanistic insight for metal-free CO2 reduction to exclusively CO in water.

Efficient and Direct Functionalization of Allylic sp3 C-H Bonds with Concomitant CO2 Reduction

Solar-driven CO2 reduction integrated with C-C/C-X bond-forming organic synthesis represents a substantially untapped opportunity to simultaneously tackle carbon neutrality and create an atom-/redox-economical chemical synthesis. Herein, we demonstrate the first cooperative photoredox catalysis of efficient and tunable CO2 reduction to syngas, paired with direct alkylation/arylation of unactivated allylic sp3 C-H bonds for accessing allylic C-C products, over SiO2 -supported single Ni atoms-decorated CdS quantum dots (QDs). Our protocol not only bypasses additional oxidant/reductant and pre-functionalization of organic substrates, affording a broad of allylic C-C products with moderate to excellent yields, but also produces syngas with tunable CO/H2 ratios (1 : 2-5 : 1). Such win-win coupling catalysis highlights the high atom-, step- and redox-economy, and good durability, illuminating the tantalizing possibility of a renewable sunlight-driven chemical feedstocks manufacturing industry.

Nickel chloride complexes with substituted 4'-phenyl-2',2':6',2″-terpyridine ligands: synthesis, characterization, anti-proliferation activity and biomolecule interactions

A series of Ni(II) sandwich-like coordinated compounds were synthesized by the reaction of nickel dichloride and ten 4'-(4-substituent phenyl)-2',2':6',2″-terpyridine ligands, and their structures were confirmed by elemental analysis, FT-IR, ESI-MS, solid state ultraviolet spectroscopy and X-ray single crystal diffraction analysis. Three human cancer cell lines and a normal human cell line were used for anti-proliferation potential study: human lung cancer cell line (A549), human esophageal cancer cell line (Eca-109), human liver cancer cells (Bel-7402) and normal human liver cells (HL-7702). The results show that these nickel complexes possess good inhibitory effects on the cancer cells, outperforming the commonly used clinical chemotherapy drug cisplatin. Especially, complexes 3 (-methoxyl) and 7 (-fluoro) have strong inhibitory ability against Eca-109 cell line with IC50 values of 0.223 μM and 0.335 μM, complexes 4 and 6 showed certain cell selectivity, and complex 6 can inhibit cancer cells and slightly poison normal cells when the concentration was controlled. The ability of these complexes binding to CT-DNA was studied by UV titration and CD spectroscopy, and CD spectroscopy was also used to study the secondary structural change of BSA under the action of the complexes. The binding of these complexes with DNA, DNA-Topo I and bovine serum protein has been simulated by molecular docking software, and the docking results and optimal binding conformation data showed that they interacted with DNA in the mode of embedded binding, which is consistent with the experimental results. These complexes are more inclined to move to the cleavage site when docking with DNA-Topo I, so as to play a role of enzyme cleavage, while BSA promotes the action of the complexes by binding to effective binding sites.

Colloidal semiconductor nanocrystals: from bottom-up nanoarchitectonics to energy harvesting applications

Colloidal semiconductor nanocrystals (NCs) have been extensively investigated owing to their unique properties induced by the quantum confinement effect. The advent of colloidal synthesis routes led to the design of stable colloidal NCs with uniform size, shape, and composition. Metal oxides, phosphides, and chalcogenides (ZnE, CdE, PbE, where E = S, Se, or Te) are few of the most important monocomponent semiconductor NCs, which show excellent optoelectronic properties. The ability to build quantum confined heterostructures comprising two or more semiconductor NCs offer greater customization and tunability of properties compared to their monocomponent counterparts. More recently, the halide perovskite NCs showed exceptional optoelectronic properties for energy generation and harvesting applications. Numerous applications including photovoltaic, photodetectors, light emitting devices, catalysis, photochemical devices, and solar driven fuel cells have demonstrated using these NCs in the recent past. Overall, semiconductor NCs prepared via the colloidal synthesis route offer immense potential to become an alternative to the presently available device applications. This feature article will explore the progress of NCs syntheses with outstanding potential to control the shape and spatial dimensionality required for photovoltaic, light emitting diode, and photocatalytic applications. We also attempt to address the challenges associated with achieving high efficiency devices with the NCs and possible solutions including interface engineering, packing control, encapsulation chemistry, and device architecture engineering.

Ultrathin Carbon Nitride Nanosheets Exfoliated and In Situ Modified with a Nickel Bis(Chelate) Complex for Boosting Photocatalytic Performances

Exfoliation and interfacial modification of two-dimensional (2D) polymeric carbon nitride (CN) are considerably vital for applications in photo/electrocatalysis fields. Here, a grinding-ultrasonic route was designed to construct nickel bis(chelate) complex (Ni(abt)₂, abt = 2-aminobenzenethiolate)-modified CN ultrathin nanosheets. Under the assistance of the shear force derived from the grinding process, Ni(abt)₂ was implanted into the interlamination of bulk CN, resulting in the formation of ultrathin CN (UCN) nanosheets. Simultaneously, Ni(abt)₂ molecules were anchored on the surfaces of as-formed UCN nanosheets due to the π-π stacking interaction. Interestingly, compared with single Ni(abt)₂ and UCN, the as-obtained Ni(abt)₂/UCN nanosheets exhibited excellent photocatalytic hydrogen evolution capability. A molecule-semiconductor internal electron transmission mechanism was suggested for explaining the separation and transfer of electron-hole pairs. Density functional theory (DFT) calculations demonstrated that the interface-induced electron redistribution tuned the electron density and hydrogen adsorption of the active centers, thus enhancing the photocatalytic performance of the hybrid catalyst. In addition, the as-obtained Ni(abt)₂/UCN nanosheets could also catalyze the reduction of nitroaromatics in the presence of NaBH₄. It was found that under the simulated sunlight irradiation, the conversion efficiency of nitroaromatic compounds to amino aromatic ones was up to 97.3%, far higher than that under the condition without light irradiation (51.7%), suggesting that the photocatalytic-produced hydrogen took part in the reduction of nitroaromatic compounds.

Methyldisulfide groups enable the direct connection of air-stable metal bis(terpyridine) complexes to gold surfaces

We show that a new terpyridine ligand comprising a directly-connected methyldisulfide group (tpySSMe) can be used to prepare a modular series of metal bis(terpyidine) complexes, [M(tpySSMe)2](PF6)2 (M = Fe, Co, Zn), suitable for the functionalization of metal surfaces. Critically, we find these complexes are air-stable in solution for >7 d, in stark contrast to their thiol-substituted analogues, [M(tpySH)2](PF6)2 (M = Fe, Co), which decompose in <1 d. While CoSH has previously been utilized in several important studies, we explicitly detail its synthesis and characterization here for the first time. We subsequently probe the electrochemical properties of [M(tpySSMe)2](PF6)2 in solution, showing that the (electro)chemical reactions associated with disulfide reduction significantly increase the complexity of the voltammetric response. In preliminary surface voltammetry studies, we confirm that CoSS and FeSS form solution-stable self-assembled monolayers (SAMs) on gold with comparable electrochemical properties to those formed from CoSH. Taken together, this work provides a robust foundation for future studies of this prominent class of complexes as redox-active components of SAMs or single-molecule junctions.

CdO decorated CdS nanorod for enhanced photocatalytic reduction of CO2 to CO

Solar-driven CO₂ reduction into fuels and sustainable energy has attracted increasing attention around the world. However, the photoreduction efficiency remains low due to the low efficiency of separation of electron-hole pairs and high thermal stability of CO₂. In this work, we prepared a CdO decorated CdS nanorod for visible light driven CO₂ reduction. The introduction of CdO facilitates the photoinduced charge carrier separation and transfer and acts as an active site for adsorption and activation of CO₂ molecules. Compared with pristine CdS, CdO/CdS exhibits a nearly 5-fold higher CO generation rate (1.26 mmol g^-1 h^-1). In situ FT-IR experiments indicated that CO₂ reduction on CdO/CdS may follow a COOH* pathway. This study reports the pivotal effect of CdO on photogenerated carrier transfer in photocatalysis and on CO₂ adsorption, which provides a facile way to enhance photocatalytic efficiency.

Engineering Single Cu Sites into Covalent Organic Framework for Selective Photocatalytic CO2 Reduction

Photocatalytic CO₂ conversion into value-added chemicals is a promising route but remains challenging due to poor product selectivity. Covalent organic frameworks (COFs) as an emerging class of porous materials are considered as promising candidates for photocatalysis. Incorporating metallic sites into COF is a successful strategy to realize high photocatalytic activities. Herein, 2,2'-bipyridine-based COF bearing non-noble single Cu sites is fabricated by chelating coordination of dipyridyl units for photocatalytic CO₂ reduction. The coordinated single Cu sites not only significantly enhance light harvesting and accelerate electron-hole separation but also provide adsorption and activation sites for CO₂ molecules. As a proof of concept, the Cu-Bpy-COF as a representative catalyst exhibits superior photocatalytic activity for reducing CO₂ to CO and CH₄ without photosensitizer, and impressively, the product selectivity of CO and CH₄ can be readily modulated only by changing reaction media. Experimental and theoretical results reveal the crucial role of single Cu sites in promoting photoinduced charge separation and solvent effect in regulating product selectivity, which provides an important sight onto the design of COF photocatalysts for selective CO₂ photoreduction.

Effect of a redox-mediating ligand shell on photocatalysis by CdS quantum dots

Semiconductor quantum dots (QDs) are efficient organic photoredox catalysts due to their high extinction coefficients and easily tunable band edge potentials. Despite the majority of the surface being covered by ligands, our understanding of the effect of the ligand shell on organic photocatalysis is limited to steric effects. We hypothesize that we can increase the activity of QD photocatalysts by designing a ligand shell with targeted electronic properties, namely, redox-mediating ligands. Herein, we functionalize our QDs with hole-mediating ferrocene (Fc) derivative ligands and perform a reaction where the slow step is hole transfer from QD to substrate. Surprisingly, we find that a hole-shuttling Fc inhibits catalysis, but confers much greater stability to the catalyst by preventing a build-up of destructive holes. We also find that dynamically bound Fc ligands can promote catalysis by surface exchange and creation of a more permeable ligand shell. Finally, we find that trapping the electron on a ligand dramatically increases the rate of reaction. These results have major implications for understanding the rate-limiting processes for charge transfer from QDs and the role of the ligand shell in modulating it.

Spin State in Homoleptic Iron(II) Terpyridine Complexes Influences Mixed Valency and Electrocatalytic CO2 Reduction

Two homoleptic Fe(II) complexes in different spin states bearing superbasic terpyridine derivatives as ligands are investigated to determine the relationship between spin state and electrochemical/spectroscopic behavior. Antiferromagnetic coupling between a ligand-centered radical and the high-spin metal center leads to an anodic shift of the first reduction potential and results in a species that shows mixed valency with a moderately intense intervalence-charge-transfer band. The differences afforded by the different spin states extend to the electrochemical reactivity of the complexes: while the low-spin species is a precatalyst for electrocatalytic CO₂ reduction and leads to the preferential formation of CO with a Faradaic efficiency of 37%, the high-spin species only catalyzes proton reduction at a modest Faradaic efficiency of approximately 20%.

Single-site bipyridine cobalt complexes covalently embedded into graphitic carbon nitride with excellent photocatalytic activity and selectivity towards CO2 reduction

A combination of a semiconductor-based photosensitizer with molecular catalysts via covalent bonds is an effective way to utilize solar energy to reduce CO₂ into value-added chemicals with high efficiency and selectivity. In this study, 2,2'-bpy-5,5'-dialdehyde functioned as organic ligands and were embedded into the skeleton of g-CN through imine bonds via thermal copolymerization. The introduction of 2,2'-bpy can not only chelate with earth-abundant Co as single-site catalytic centers but also can optimize the properties of original g-CN such as the enlarged specific surface area and extended visible light absorption range. The CO evolution rate of g-CN-bpy-Co can reach up to 106.3 μmol g^-1 h^-1 with a selectivity of 97% over proton reduction, which was 82-fold than that of g-CN-Co. The different coordination environments and valence states of cobalt were also studied simultaneously and the results showed that Co(II) exhibited superior catalytic activity towards Co(III). Control experiments demonstrated that the covalent linkage between g-CN and Co-2,2'-bpy plays a vital role in photocatalytic activity and selectivity. Besides, the CO generation rate demonstrated linear growth upon visible light irradiation up to 72 h and preferable recyclability. This research provides a new facile way to fabricate low-priced photocatalysts with high activity and selectivity and bridge homogeneous and heterogeneous catalysis.

Substituents make a difference: 6,6″-modified terpyridine complexes with helix configuration and enhanced emission

A series of complexes L22-M (L2: 6,6″-bis(4-methoxyphenyl)-4'-phenyl-2,2':6',2″-terpyridine, M: Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) were synthesized by coordinating p-methoxyphenyl 6,6″-substituted terpyridine ligand with first-row transition metal ions and characterized by NMR, ESI-MS, and X-ray single crystal diffraction techniques. Single-crystal structures demonstrated that the steric hindrance of p-methoxyphenyl substituents endowed complexes L22-M with obvious longer coordination bond lengths and larger bond angles and dihedral angles compared with unmodified L12-M (L1: 4'-phenyl-2,2':6',2″-terpyridine). The chiral helix geometry was observed for L22-M, in which 2,2':6',2″-terpyridine moiety dramatically twisted to a spiral form in comparison to the nearly coplanar structure of the parent L12-M, resulting in plentiful intramolecular and intermolecular π-π interactions. Also, the appealing racemic (P and M) double helix packed structure for 6,6″-modified bisterpyridine complex L22-Cu was formed in the crystal. The consequent appealing charge transfer (CT) emission for L22-Zn in the solution and solid were investigated via UV-vis and fluorescence spectroscopy techniques and time-dependent density functional theory (TD-DFT) calculations. This work afforded a new method to achieve intriguing chiral geometry and CT optical properties via the subtle design and modification of terpyridine ligands.

Synthesis and characterisation of Ga- and In-doped CdS by solventless thermolysis of single source precursors

We report a facile and low temperature synthesis of Ga- and In-doped CdS nanoparticles from molecular precursors. Diethyldithiocarbamate complexes of Cd(II), Ga(III), and In(III), were synthesised and decomposed in tandem through solventless thermolysis, producing Ga- or In-doped CdS. The resultant M_xCd_1-xS_1+0.5x (where M = Ga/In at x values of 0, 0.02, 0.04, 0.06, 0.08 and 0.1) particulate powder was analysed by powder X-ray diffraction, which showed that both Ga (through all doping levels) and In (at doping levels <8 mol%) were successfully incorporated into the hexagonal CdS lattice without any impurities. Raman spectroscopy also showed no significant change from CdS. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to investigate the morphology and elemental dispersion through the doped CdS materials, showing homogenous incorporation of dopant. The optical and luminescent properties of the doped M_xCd_1-xS_1+0.5x materials were examined by UV-Vis absorption and photoluminescence spectroscopies respectively. All materials were found to exhibit excitonic emission, corresponding to band gap energies between 2.7 and 2.9 eV and surface defect induced emission which is more prominent for Ga than for In doping. Additionally, moderate doping slows down charge carrier recombination by increasing the lifetimes of excitonic and surface state emissions, but particularly for the latter process.

A novel concept of photosynthetic soft membranes: a numerical study

We focus on a novel concept of photosynthetic soft membranes, possibly able to allow the conversion of solar energy and carbon dioxide (CO[Formula: see text]) into green fuels. The considered membranes rely on self-assembled functional molecules in the form of soap films. We elaborate a multi-scale and multi-physics model to describe the relevant phenomena, investigating the expected performance of a single soft photosynthetic membrane. First, we present a macroscale continuum model, which accounts for the transport of gaseous and ionic species within the soap film, the chemical equilibria and the two involved photocatalytic half reactions of the CO[Formula: see text] reduction and water oxidation at the two gas-surfactant-water interfaces of the soap film. Second, we introduce a mesoscale discrete Monte Carlo model, to deepen the investigation of the structure of the functional monolayers. Finally, the morphological information obtained at the mesoscale is integrated into the continuum model in a multi-scale framework. The developed tools are then used to perform sensitivity studies in a wide range of possible experimental conditions, to provide scenarios on fuel production by such a novel approach.

Rich Landscape of Colloidal Semiconductor-Metal Hybrid Nanostructures: Synthesis, Synergetic Characteristics, and Emerging Applications

Nanochemistry provides powerful synthetic tools allowing one to combine different materials on a single nanostructure, thus unfolding numerous possibilities to tailor their properties toward diverse functionalities. Herein, we review the progress in the field of semiconductor-metal hybrid nanoparticles (HNPs) focusing on metal-chalcogenides-metal combined systems. The fundamental principles of their synthesis are discussed, leading to a myriad of possible hybrid architectures including Janus zero-dimensional quantum dot-based systems and anisotropic quasi 1D nanorods and quasi-2D platelets. The properties of HNPs are described with particular focus on emergent synergetic characteristics. Of these, the light-induced charge-separation effect across the semiconductor-metal nanojunction is of particular interest as a basis for the utilization of HNPs in photocatalytic applications. The extensive studies on the charge-separation behavior and its dependence on the HNPs structural characteristics, environmental and chemical conditions, and light excitation regime are surveyed. Combining the advanced synthetic control with the charge-separation effect has led to demonstration of various applications of HNPs in different fields. A particular promise lies in their functionality as photocatalysts for a variety of uses, including solar-to-fuel conversion, as a new type of photoinitiator for photopolymerization and 3D printing, and in novel chemical and biomedical uses.

Repurposing Photosensitizer Proteins Through Genetic Code Expansion to Facilitate Photo-Biocatalysis

Artificial photoenzymes with noncanonical photo-redox cofactors have paved the way for enzyme rational design and the creation of new-to-nature biocatalysts. Genetically encoded photo-redox cofactors endow photoenzymes with enhanced or novel activities that catalyze numerous transformations with high efficiency. Herein, we describe a protocol of repurposing photosensitizer proteins (PSP) through genetic code expansion to facilitate multiple photocatalytic conversions including photo-activated dehalogenation of aryl halides, CO₂ to CO and CO₂ to formic acid reduction. The methods for expression, purification, and characterization of the PSP are detailed. The installation of the catalytic modules and the utilization of PSP-based artificial photoenzymes for photoenzymatic CO₂ reduction and dehalogenation are also described.

Nanomaterials as Photocatalysts-Synthesis and Their Potential Applications

Increasing demand for energy and environmental degradation are the most serious problems facing the man. An interesting issue that can contribute to solving these problems is the use of photocatalysis. According to literature, solar energy in the presence of a photocatalyst can effectively (i) be converted into electricity/fuel, (ii) break down chemical and microbial pollutants, and (iii) help water purification. Therefore, the search for new, efficient, and stable photocatalysts with high application potential is a point of great interest. The photocatalysts must be characterized by the ability to absorb radiation from a wide spectral range of light, the appropriate position of the semiconductor energy bands in relation to the redox reaction potentials, and the long diffusion path of charge carriers, besides the thermodynamic, electrochemical, and photoelectrochemical stabilities. Meeting these requirements by semiconductors is very difficult. Therefore, efforts are being made to increase the efficiency of photo processes by changing the electron structure, surface morphology, and crystal structure of semiconductors. This paper reviews the recent literature covering the synthesis and application of nanomaterials in photocatalysis.

Encapsulated CdSe/CdS nanorods in double-shelled porous nanocomposites for efficient photocatalytic CO2 reduction

Colloidal quantum dots have been emerging as promising photocatalysts to convert CO₂ into fuels by using solar energy. However, the above photocatalysts usually suffer from low CO₂ adsorption capacity because of their nonporous structures, which principally reduces their catalytic efficiency. Here, we show that synchronizing imine polycondensation reaction to self-assembly of colloidal CdSe/CdS nanorods can produce micro-meso hierarchically porous nanocomposites with double-shelled nanocomposites. Owing to their hierarchical pores and the ability to separate photoexcited electrons, the self-assembled porous nanocomposites exhibit remarkably higher activity (≈ 64.6 μmol g⁻¹ h⁻¹) toward CO₂ to CO in solid-gas regime than that of nonporous solids from self-assembled CdSe/CdS nanorods under identical conditions. Importantly, the length of the nanorods is demonstrated to be crucial to correlate their ability to long-distance separation of photogenerated electrons and holes along their axial direction. Overall, this approach provides a rational strategy to optimize the CO₂ adsorption and conversion by integrating the inorganic and organic semiconductors.

The authors design double shelled hollow superstructures from self-assembled CdSe/CdS nanorods in covalent organic frameworks for CO2 photo-reduction at a gas/solid interface.

Molecular oxidation-reduction junctions for artificial photosynthetic overall reaction

Constructing redox semiconductor heterojunction photocatalysts is the most effective and important means to complete the artificial photosynthetic overall reaction (i.e., coupling CO₂ photoreduction and water photo-oxidation reactions). However, multiphase hybridization essence and inhomogeneous junction distribution in these catalysts extremely limit the diverse design and regulation of the modes of photogenerated charge separation and transfer pathways, which are crucial factors to improve photocatalytic performance. Here, we develop molecular oxidation-reduction (OR) junctions assembled with oxidative cluster (PMo₁₂, for water oxidation) and reductive cluster (Ni₅, for CO₂ reduction) in a direct (d-OR), alternant (a-OR), or symmetric (s-OR) manner, respectively, for artificial photosynthesis. Significantly, the transfer direction and path of photogenerated charges between traditional junctions are obviously reformed and enriched in these well-defined crystalline catalysts with monophase periodic distribution and thus improve the separation efficiency of the electrons and holes. In particular, the charge migration in s-OR shows a periodically and continuously opposite mode. It can inhibit the photogenerated charge recombination more effectively and enhance the photocatalytic performance largely when compared with the traditional heterojunction models. Structural analysis and density functional theory calculations disclose that, through adjusting the spatial arrangement of oxidation and reduction clusters, the energy level and population of the orbitals of these OR junctions can be regulated synchronously to further optimize photocatalytic performance. The establishment of molecular OR junctions is a pioneering important discovery for extremely improving the utilization efficiency of photogenerated charges in the artificial photosynthesis overall reaction.

Construction of Ultrathin S-Scheme Heterojunctions of Single Ni Atom Immobilized Ti-MOF and BiVO4 for CO2 Photoconversion of nearly 100% to CO by Pure Water

To rationally design single-atom metal-organic framework (MOF)-involving photocatalysts remains an ongoing challenge for efficient CO₂ conversion. Here, cuppy microstructures, consisting of a Ti(IV)-oxo node and three linked carboxylic moieties, in the single-coordination-layer Ti₂ (H₂ dobdc)₃ MOF (NTU-9) are exploited to immobilize abundant single Ni(II) sites (Ni@MOF). The coupling of Ni@MOF with BiVO₄ (BVO) nanosheets by H-bonding-induced assembly process obtains wide-spectrum 2D heterojunctions. The optimal heterojunction exhibits competitive performance and enables around 66-fold CO₂ conversion of that for BVO nanoparticles by pure water, with nearly 100% CO selectivity. The exceptional photoactivity is attributed to favorable S-scheme charge transfer from BVO to MOF then to single Ni(II) sites. Noteworthily, single Ni(II) sites anchored by the Ti(IV)-oxo node and vicinal carboxylic moieties serving as a unique local microenvironment (LME) are found to synergistically catalyze CO₂ conversion. Specifically, the hydroxyl groups of carboxylic moieties can form H-bonds with CO₂ to promote its adsorption on single Ni(II) sites, and also can provide accessible protons to facilitate H-assisted CO₂ reduction. Moreover, the CO desorption and subsequent CO₂ adsorption on single Ni(II) sites with LME is proved to be thermodynamically favored, and hence dominates the high CO selectivity. This work highlights the significance of modulating the LME of single atoms to rationally design photocatalysts for realizing carbon neutralization.

Photocatalytic Reduction of CO2 to CO in Aqueous Solution under Red-Light Irradiation by a Zn-Porphyrin-Sensitized Mn(I) Catalyst

This work demonstrates photocatalytic CO₂ reduction by a noble-metal-free photosensitizer-catalyst system in aqueous solution under red-light irradiation. A water-soluble Mn(I) tricarbonyl diimine complex, [MnBr(4,4′-{Et₂O₃PCH₂}₂-2,2′-bipyridyl)(CO)₃] (1), has been fully characterized, including single-crystal X-ray crystallography, and shown to reduce CO₂ to CO following photosensitization by tetra(N-methyl-4-pyridyl)porphyrin Zn(II) tetrachloride [Zn(TMPyP)]Cl₄ (2) under 625 nm irradiation. This is the first example of 2 employed as a photosensitizer for CO₂ reduction. The incorporation of −P(O)(OEt)₂ groups, decoupled from the core of the catalyst by a −CH₂– spacer, afforded water solubility without compromising the electronic properties of the catalyst. The photostability of the active Mn(I) catalyst over prolonged periods of irradiation with red light was confirmed by ¹H and ¹³C{¹H} NMR spectroscopy. This first report on Mn(I) species as a homogeneous photocatalyst, working in water and under red light, illustrates further future prospects of intrinsically photounstable Mn(I) complexes as solar-driven catalysts in an aqueous environment.

A Mn(I) bipyridyl tricarbonyl complex, where the diimine ligand is functionalized with water-solubilizing phosphonate ester groups, has been prepared and is shown to catalytically convert CO₂ to CO in aqueous solution following photosensitization from a water-soluble Zn(II) porphyrin under red-light irradiation.

Reductive Carbon-Carbon Coupling on Metal Sites Regulates Photocatalytic CO2 Reduction in Water Using ZnSe Quantum Dots

Colloidal quantum dots (QDs) consisting of precious-metal-free elements show attractive potentials towards solar-driven CO₂ reduction. However, the inhibition of hydrogen (H₂ ) production in aqueous solution remains a challenge. Here, we describe the first example of a carbon-carbon (C-C) coupling reaction to block the competing H₂ evolution in photocatalytic CO₂ reduction in water. In a specific system taking ZnSe QDs as photocatalysts, the introduction of furfural can significantly suppress H₂ evolution leading to CO evolution with a rate of ≈5.3 mmol g^-1  h^-1 and a turnover number (TON) of >7500 under 24 h visible light. Meanwhile, furfural is upgraded to the self-coupling product with a yield of 99.8 % based on the consumption of furfural. Mechanistic insights show that the reductive furfural coupling reaction occurs on surface Zn-sites to consume electrons and protons originally used for H₂ production, while the CO formation pathway at surface anion vacancies from CO₂ remains.

Exploiting consecutive photoinduced electron transfer (ConPET) in CO2 photoreduction

The consecutive photoinduced electron transfer (ConPET) process of 1,2,3,5-Tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) in CO₂ photoreduction to achieve powerful reducing species has been disclosed by activating a bis(terpyridine)ruthenium(II) complex bearing a high overpotential for selective light-driven reduction of CO₂ to CO in homogeneous solution.

Aqueous CO2 Reduction on Si Photocathodes Functionalized by Cobalt Molecular Catalysts/Carbon Nanotubes

Photoelectrochemical reduction of CO₂ is a promising approach for renewable fuel production. We herein report a novel strategy for preparation of hybrid photocathodes by immobilizing molecular cobalt catalysts on TiO₂ -protected n⁺ -p Si electrodes (Si|TiO₂ ) coated with multiwalled carbon nanotubes (CNTs) by π-π stacking. Upon loading a composite of Co^II (BrqPy) (BrqPy=4',4''-bis(4-bromophenyl)-2,2' : 6',2'' : 6'',2'''-quaterpyridine) catalyst and CNT on Si|TiO₂ , a stable 1-Sun photocurrent density of -1.5 mA cm^-2 was sustained over 2 h in a neutral aqueous solution with unity Faradaic efficiency and selectivity for CO production at a bias of zero overpotential (-0.11 V vs. RHE), associated with a turnover frequency (TOF_CO ) of 2.7 s^-1 . Extending the photoelectrocatalysis to 10 h, a remarkable turnover number (TON_CO ) of 57000 was obtained. The high performance shown here is substantially improved from the previously reported photocathodes relying on covalently anchored catalysts.

Insight on Reaction Pathways of Photocatalytic CO2 Conversion

Photocatalytic CO₂ conversion to value-added chemicals is a promising solution to mitigate the current energy and environmental issues but is a challenging process. The main obstacles include the inertness of CO₂ molecule, the sluggish multi-electron process, the unfavorable thermodynamics, and the selectivity control to preferable products. Furthermore, the lack of fundamental understanding of the reaction pathways accounts for the very moderate performance in the field. Therefore, in this Perspective, we attempt to discuss the possible reaction mechanisms toward all C₁ and C₂ value-added products, taking into account the experimental evidence and theoretical calculation on the surface adsorption, proton and electron transfer, and products desorption. Finally, the remaining challenges in the field, including mechanistic understanding, reactor design, economic consideration, and potential solutions, are critically discussed by us.