Dye-sensitized System Research Articles

Dye-Sensitized Photocatalysis: Hydrogen Evolution and Alcohol-to-Aldehyde Oxidation without Sacrifical Electron Donor.

There is a growing interest in developing dye-sensitized photocatalytic systems (DSPs) to produce molecular hydrogen (H2 ) as alternative energy source. To improve the sustainability of this technology, we replaced the sacrificial electron donor (SED), typically an expensive and polluting chemical, with an alcohol oxidation catalyst. This study demonstrates the first dye-sensitized system using a diketopyrrolopyrrole dye covalently linked to 2,2,6,6-tetramethyl-1-piperidine-N-oxyl (TEMPO) based catalyst for simultaneous H2 evolution and alcohol-to-aldehyde transformation operating in water with visible irradiation.

Molten salt synthesis of 1T/2H mixed phase MoS2 for boosting photocatalytic H2 evolution via Schottky junction under EY-sensitized system

Clean H2 fuel obtained from the photocatalytic water splitting to hydrogen reaction could efficiently alleviate current energy crisis and the concomitant environmental pollution problems. Therefore, it is desirable to search for a highly efficient photocatalytic system to decrease the energy barrier of water splitting reaction. Herein, the 1T/2H mixed phase MoS2 sample with Schottky junction between contact interfaces is developed through molten salt synthesis for photocatalytic hydrogen production under a dye-sensitized system (Eosin Y-TEOA-MoS2) driven by the visible light. In mixed phase MoS2 sample, the photogenerated electrons of 2H-phase MoS2 migrated to the 1T-phase MoS2 are difficult to jump back because of the existence of Schottky barrier, which greatly suppresses the quenching of EY and therefore results in an enhanced hydrogen evolution performance. Therefore, the optimized MoS2 sample (MoS2-350) has an initial hydrogen evolution rate of 213 μmol h−1 and corresponding apparent quantum yield of 36.1 % at 420 nm, far higher than those of pure Eosin Y. It is strongly confirmed by the steady‐state/time-resolved photoluminescence (PL) spectra and transient photocurrent response experiments. With the assistance of Density functional theory (DFT) calculation, the function of Schottky junction in photocatalytic hydrogen evolution reaction is well explained. In addition, a new and universal method (SVM curve) of judging oxidation or reduction quenching for photosensitizers is proposed.

Nickel-decorated graphite phase carbon nitride photocatalysts coupled with carbazole-based calixarene dyes for high-efficiency visible light-driven hydrogen evolution

The development of artificial devices that mimic the highly efficient and sophisticated photosystems found in nature is a subject worthy of in-depth study. Two calixarene D-π-A dyes, C4-CZSTP and C4-CZBTP, containing an N-methylcarbazole group as a secondary auxiliary electron donor, are synthesized on the basis of our previously reported calixarene dye C4-BTP. These dyes are then cooperated with Ni-deposited g-C3N4 to prepare ternary photocatalysts, denoted as C4-CZSTP/Ni/g-C3N4, C4-CZBTP/Ni/g-C3N4 and C4-BTP/Ni/g-C3N4, and their photocatalytic performances are evaluated under visible light irradiation. The optimized 4.0 wt% C4-CZSTP/Ni/g-C3N4 sample gives a remarkable H2 evolution rate of 5552 μmol g−1 h−1, representing one of the highest records among the reported analogous systems based on g-C3N4. And after an extended test of 49 h, a total H2 yield of 63.57 mmol g−1 is obtained, corresponding to a turnover number (TON) based on C4-CZSTP moles (TONC4-CZSTP) of 3180. This performance can be attributed to the enhanced light-harvesting and photoelectron conversion of the as-prepared calixarene dyes, the adjusted energy levels and the effective cooperation of the three components. The optimal performance of C4-CZSTP/Ni/g-C3N4 among the three dye-sensitized systems is due to its higher lowest unoccupied molecular orbital (LUMO) value, i.e. stronger electronic driving force from the excited dye to g-C3N4. This study provides an interesting example for the design and preparation of a low-cost, high-efficiency and noble metal-free photocatalytic system for solar fuel production.

Lateral Electron and Hole Hopping between Dyes on Mesoporous ZrO2: Unexpected Influence of Solvents with a Low Dielectric Constant

Lateral intermolecular charge transfer between photosensitizers on metal oxide substrates is important for the understanding on the overall working principles of dye-sensitized systems. Such studies usually concentrate on either hole or electron transfer separately and are conducted in solvents with a high dielectric constant (εs) that are known, however, to show a drastic decrease of the local dielectric constant close to the metal oxide surface. In the present study, both hole and electron hopping between organic donor-acceptor photosensitizers was experimentally investigated on PB6 dye-sensitized mesoporous ZrO2 films. The donor (close to the surface) and acceptor (away from surface) subunit of the PB6 dye were observed to be involved in hole and electron hopping, respectively. Hole and electron transfer kinetics were found to differ remarkably in high-εs solvents, but similar in solvents with εs < 12. This finding indicates that low-εs solvents maintain similar local dielectric constant values close to, and further away from, the semiconductor surface, which is different from the previously observed behavior of high dielectric constant solvents at a metal oxide interface.

Tuning metal-support interaction of NiCu/graphene cocatalysts for enhanced dye-sensitized photocatalytic H2 evolution

The modulation of metal-support interaction (MSI) is of particular interest in improving the catalytic performance of metal nanocatalysts. Herein, we adopt an alloying strategy to tune the MSI in graphene-supported Ni catalyst by introducing Cu. The NiCu/graphene cocatalysts (denoted as NxCyG) are facilely synthesized by directly calcinating the mixture of metal chlorides and graphene oxide for dye-sensitized photocatalytic hydrogen evolution reaction (HER). The results reveal the introduction of Cu significantly affects the MSI in NxCyG cocatalysts, and a reasonable Ni/Cu ratio (4:1) significantly enhances the MSI in N4C1G cocatalyst. Such an enhanced MSI facilitates the electron injection from excited dye to RGO and further transfer to the NiCu alloy, and leads to smaller size of NiCu alloy nanoparticles (NPs) with sufficient active sites. Moreover, the synergetic effect between Ni and Cu is conducive to optimize the hydrogen adsorption on NiCu surface. As a result, the N4C1G cocatalyst shows much improved electrocatalytic and photocatalytic HER performance compared with monometallic Ni/graphene and Cu/graphene. The apparent quantum yield (AQY) at 420 nm reaches up to 67.7 % in the dye-sensitization system. We believe that this work develops a facile and effective strategy to tune the MSI of metal/graphene catalysts for enhanced HER performance.

High‐Efficiency Photocatalytic Hydrogen Production by Nanorods NiMoO4 Supported NiCoP Nanosheets

Based on the structural characteristics of bimetallic phosphates, the rod‐like NiCoP/NiMoO4 heterojunction composite catalyst is successfully synthesized by hydrothermal method, which realizes the rapid transfer of photogenerated carriers across the heterojunction interface, thereby improving the performance of photocatalytic decomposition of water to produce hydrogen. Through photocatalytic experiments, the hydrogen evolution rate of the NiCoP/NiMoO4 is 61.4 μmol h−1, which is 2.93 times that of pure NiMoO4. Further study on the catalyst shows that this structure could greatly improve the utilization rate of sensitizer and hole sacrifice agent in the dye sensitization system, and the rate of photon‐generated carrier separation and transfer is significantly improved to achieve a good hydrogen evolution effect.

Non-noble copper ion anchored on NH2-MIL-101(Fe) as a novel cocatalyst with transient metal centers for efficient photocatalytic water splitting

The earth-abundant copper species anchored on the -NH2 of NH2-MIL-101(Fe) as noble-metal-free cocatalysts via transient CuII/CuI centers were obtained via a convenient synthesis technique to achieve excellent photocatalytic hydrogen evolution performance in the dye sensitization system. The prepared sample 6Cu-NM-101 exhibited the best hydrogen evolution activity of 5770.96 μmol·g−1·h−1, which was 4.06 times higher than the original NH2-MIL-101(Fe). A series of characterization tests, especially the CV and the Auger Cu LMM, demonstrated that the transient CuII/CuI center played a significant part in the novel ligand to linker metal charge transfer (LLMCT) process. In other words, the electrons could transfer from the lowest unoccupied molecular orbital (LUMO) energy level of the excited state Eosin Y (EY*) to the transient CuII/CuI center to further accelerate the carrier transfer and separation rate. Our work provided a more convenient and effective strategy to prepare various earth-abundant metal ions based on MOFs as low-cost cocatalysts via transient metal centers to boost the photocatalytic hydrogen evolution performance.

Nanoarchitectonics of uniformly distributed noble-metal-free CoP in g-C3N4 via in-situ fabrication for enhanced photocatalytic and electrocatalytic hydrogen production

Developing an efficient, robust and low-budget bifunctional material for hydrogen evolution is a daunting challenge, which has attracted plenty of researchers’ interest. Herein, we report the non-noble metal bifunctional g-C3N4/CN/CoP catalysts in situ formed for both photocatalytic and electrocatalytic hydrogen production through a convenient phosphorization process. It was found that the optimal g-C3N4(90)/CN300/CoP3 composite not only had the best photocatalytic hydrogen production rate (29.78 mmol·g−1·h−1) in the dye sensitization system, but also had an excellent electrocatalytic hydrogen evolution activity, with an overpotential of 221 mV at 10 mA·cm−2. The XRD, FT-IR, TG, et al. confirm the close contact between ZIF-Co and g-C3N4 in the ideal g-C3N4/ZIF-Co precursor could expediently achieve the uniformly distributed CoP to accelerate electron transfer and reduce the recombination rate of charge. This work can provide a novel strategy to fabricate non-noble metal dual-functional materials for photocatalytic and electrocatalytic hydrogen production to solve energy crises and environmental further.

System Engineering Enhances Photoelectrochemical CO2 Reduction

The photoelectrochemical carbon dioxide reduction reaction (PEC-CO2RR) allows us to convert solar power to chemical energy with photosensitive materials. Semiconductor-based, plasmon-assisted, and dye-sensitized systems have been extensively investigated in the PEC-CO2RR. Beyond the remarkable progress in materials science, it is expected to realize new systems for satisfying the needs of both research discoveries and industry applications. In this Perspective, we summarize the latest progress in the field of the PEC-CO2RR, focusing on enhancing efficiencies via matching photoelectron flux, charge transfer rate, and mass transport rate. Based on the principles for the state-of-the-art PEC-CO2RR, new designs of the system engineering strategies on photoelectrode fabrication, reactor design, electrolyte optimization, and membrane selection are proposed to enhance selectivity and stability of the PEC-CO2RR.

Activating atomically dispersed Co–N/C sites on g-C3N4 nanosheets via incorporating sulfur enables efficient visible light H2 evolution

The inert Co–N/C sites on g-C3N4 nanosheets (CNs) can be effectively activated via incorporating S for efficiently catalyzing visible light H2 evolution in a dye-sensitized system.

Hollow tubular Co9S8 grown on In2O3 to form S-scheme heterojunction for efficient and stable hydrogen evolution

In this work, an innovative Co9S8/In2O3 composite photocatalyst was prepared by an in-situ grown method. After the composite catalyst loaded with Co9S8 on In2O3 effectively solved the self-agglomeration effect of In2O3 prepared by the method in this work and significantly improved the efficiency of hydrogen production. The useless electrons and holes in Co9S8/In2O3 are removed more effectively than in the other catalyst, so that more highly reducible electrons are involved in the reduction reaction of hydrogen evolution, which is due to the formation of S-scheme heterojunction. The cyclic hydrogen production experiment confirmed that the stability of the composite photocatalyst is excellent, and its 5 h hydrogen production amount reached 277.77 μmol. In this work, a simple synthetic method for the synthesis of composite photocatalysts and new photocatalysts for the photocatalytic hydrogen evolution of dye-sensitized systems (Eosin Y) is presented.

Ultra-stable dye-sensitized graphene quantum dot as a robust metal-free photocatalyst for hydrogen production

Covalently attaching a visible light absorbing dye to the graphene quantum dot (GQD) was proven as a viable method to prepare single component metal-free photocatalyst for solar hydrogen production. Despite promising aspects with earlier reported Rhodamine dye attached GQD system, however, quick deterioration of its HER activity within several hours seriously limits its practical application. We have challenged this stability issue by combining the N/S hetero atom doping in GQD structure and molecular structural elaboration of organic dye. By employing 2,3-diaminophenazine (Phz) attached NS-co-doped GQD (Phz-NS_GQD) as a single component photocatalyst in this work, we could demonstrate a dye-sensitized system which sustained over two days without any deterioration (complete linearity up to 50 h was measured) of its HER activity. Higher Apparent Quantum Yield (AQY) values of 49% and 47.7% at 400, 450 nm wavelength respectively were well supported to its improved catalytic efficiency for H2 production. It was shown that the N,S–co doping and the highly efficient photoinduced electron transfer to GQD core facilitated by phenazine dye are responsible for the improved HER efficiency as well as outstanding photostability.

Cu-clusters nodes of 2D metal-organic frameworks as a cost-effective noble-metal-free cocatalyst with high atom-utilization efficiency for efficient photocatalytic hydrogen evolution

Several 2D nanosheets of porphyrin MOFs with various transition-metal clusters as metal nodes were prepared via a simple solvothermal method to apply in the photocatalytic hydrogen evolution, in which the hydrogen production rate of the optimal NS-Cu was as high as 15.39 mmol g−1 h−1. A series of experimental technologies especially cyclic voltammetry (CV) and Mott-Schottky (M-S) had been adopted to investigate the charge-transfer property of photo-generated electron-hole pairs, it was found that the uniformly dispersed Cu-clusters nodes in the original 2D MOFs played a key role in the electron transfer process, that was, the photo-generated electron transferred from excited state eosin-Y to the Cu-clusters nodes for the efficient hydrogen evolution. The excellent photocatalytic performance could be attributed to the reversible oxidation-–reduction process of CuII/CuI, which had excellent electron-receiving and electron-outputting capabilities. Our results provided a novel avenue to adapt the uniformly dispersed metal nodes in the original MOFs as cost-effective noble-metal-free cocatalysts with very high atom-utilization efficiency to improve the photocatalytic hydrogen evolution performance in dye-sensitized system.

Dye-Sensitized Fe-MOF nanosheets as Visible-Light driven photocatalyst for high efficient photocatalytic CO2 reduction

Dye-sensitized system holds great potential for the development of visible-light-responsive photocatalysts not only because it can enhance the light absorption and charge separation efficiency of the systems but also because it can tune the band structure of catalysts. Herein, two-dimensional (2D) Fe-MOF nanosheets (Fe-MNS) with a LUMO potential of 0.11 V (vs. RHE) was prepared. Interestingly, it has been found that when the 2D Fe-MNS catalyst was functionalized with visible-light-responsive [Ru(bpy)]32+ as a dye-sensitizer, the electrons from the [Ru(bpy)]32+ can effectively inject into the 2D Fe-MNS, which resulted in a negative shift of the LUMO potential of the 2D Fe-MNS to −0.15 V (vs. RHE). Consequently, the [Ru(bpy)]32+/Fe-MNS catalytic system exhibits a sound photocatalytic CO2-to-CO activity of 1120 μmol g-1h−1 under visible-light-irradiation. The photocatalytic CO production was further ameliorated by regulating the electronic structure of the 2D Fe-MNS by doping Co ions, achieving a remarkable photocatalytic activity of 1637 μmol g-1h−1. This work further supports that the dye-sensitized system is an auspicious strategy worth exploring with different catalysts for the development of visible-light-responsive photocatalytic systems.

Visualizing Oxidative Stress Level for Timely Assessment of Ischemic Stroke via a Ratiometric Near-Infrared-II Luminescent Nanoprobe.

Ischemic stroke (IS) characterized with high morbidity and mortality rates is considered as one of the most dangerous brain diseases. The timely assessment of IS is crucial for making a clinical decision due to the severity of IS featured with time-dependence. Herein, we develop a highly reactive oxygen species (HROS)-responsive ratiometric near-infrared-II (NIR-II) nanoprobe based on a dye-sensitized system between IR-783 dye and lanthanide-doped nanoparticles. Once intravenously injected into the mice, the probe is rapidly accumulated at a lesion site by recognizing the activated endothelial cell or impaired blood-brain barrier (BBB) in the ischemic area and further responds to HROS, thereby allowing in vivo imaging of the oxidative stress level. The probe is not only able to discriminate the salvageable ischemic tissue from infarcted stroke core by visualizing the enriched degree of the probe at the lesion site but also can grade the salvageable ischemic tissue by analyzing the oxidative stress level. In addition, the ischemia area was clearly delineated by NIR-II luminescence imaging after cerebral ischemia for 30 min, which is significantly earlier than with the magnetic resonance imaging (MRI) method, thereby providing a practical tool for the timely assessing of IS.

Activate Fe3S4 Nanorods by Ni Doping for Efficient Dye-Sensitized Photocatalytic Hydrogen Production.

Developing suitable catalysts capable of receiving injected electrons and possessing active sites for hydrogen evolution reaction (HER) is the key to building an efficient dye-sensitized system for hydrogen production. Fe3S4 is generally regarded as an inferior HER catalyst among the metal sulfide family, mainly due to its weak surface adsorption toward H atoms. In this work, we demonstrate a facile metal-organic framework-derived method to synthesize uniform Fe3S4 nanorods and active them for HER by Ni doping. Our experimental results and theoretical calculations reveal that Ni doping can greatly modify the electronic structure of Fe3S4 nanorods, improving their electron conductivity and optimizing their surface adsorption energy toward H atoms. Sensitized by a commercial organic dye (eosin-Y), 1%Ni-doped Fe3S4 nanorods display a high H2 production rate of 3240 μmol gcat-1 h-1 with an apparent quantum yield of 12% under 500 nm wavelength, which is significantly higher than that of pristine Fe3S4 and even higher than that of 1% Pt-deposited Fe3S4. The working mechanism of this dye-sensitized system is explored, and the effect of Ni-doping concentration has been studied. This work presents a facile strategy to synthesize metal-doped sulfide nanocatalysts with greatly enhanced activity toward photocatalytic H2 production.

Marigold shaped mesoporous composites Bi2S3/Ni(OH)2 with n-n heterojunction for high efficiency photocatalytic hydrogen production from water decomposition

The abundant active sites of catalysts are effective means to regulate the directional migration of photogenic carriers and improve the catalytic activity of photocatalysts. The Ni(OH)2 with three-dimensional nano-flower structure was successfully prepared. The three-dimensional nanofloral structure weakened the van der Waals forces between the nanosheets, inhibited the interlamellar superposition, and thus exposed more active sites to improve the activity of photocatalytic decomposition water to hydrogen production. When the addition amount of Bi2S3 is 10%, a solution of triethanolamine at pH 10 after 5 h of simulated sunlight irradiation, in the dye sensitization system the hydrogen production amount of the composite catalyst Bi2S3/Ni(OH)2 is 119 μmol, which is about 12 times that of pure Bi2S3 (12 μmol) and about 2.5 times that of pure Ni(OH)2 (48 μmol). Microstructure, optical, physicochemical properties and structural tests of the photocatalysts are investigated by Brunauer-Emmett-Teller (BET), Ultraviolet–visible (UV–Visible), Photoluminescence (PL) and Electrochemical techniques. Results prove that Bi2S3/Ni(OH)2–10% nanocomposite catalysts increase specific surface area and visible light utilization. The n-n heterogeneous junction is formed, which reduces the composite rate of electrons and holes, and improves photocatalytic split of water to produce hydrogen under visible light irradiation.

Enhanced stability and activity of Cu–BTC by trace Ru3+ substitution in water photolysis for hydrogen evolution

Constructing stable, efficient and cost-effective cocatalysts is of great significance for photocatalytic H2 evolution in a dye-sensitization system.

Tuning the Photo-electrochemical Performance of RuII -Sensitized Two-Dimensional MoS2.

Covalently tethering photosensitizers to catalytically active 1T-MoS2 surfaces holds great promise for the solar-driven hydrogen evolution reaction (HER). Herein, we report the preparation of two new RuII -complex-functionalized MoS2 hybrids [RuII (bpy)2 (phen)]-MoS2 and [RuII (bpy)2 (py)Cl]-MoS2 . The influence of covalent functionalization of chemically exfoliated 1T-MoS2 with coordinating ligands and RuII complexes on the HER activity and photo-electrochemical performance of this dye-sensitized system was studied systematically. We find that the photo-electrochemical performance of this RuII -complex-sensitized MoS2 system is highly dependent on the surface extent of photosensitizers and the catalytic activity of functionalized MoS2 . The latter was strongly affected by the number and the kind of functional groups. Our results underline the tunability of the photovoltage generation in this dye-sensitized MoS2 system by manipulation of the surface functionalities, which provides a practical guidance for smart design of future dye-sensitized MoS2 hydrogen production devices towards improved the photofuel conversion efficiency.

Ab-Initio Investigation on Dye Conformer Structures and the Interplay between Conformation and Multilayer Aggregation on TiO2 toward Solar Cell Application

Self-assembled monolayers (SAMs) have many applications such as the solar cells and water-spitting systems, which mimic the photosynthetic process in the plants and serve as an effective alternative to provide clean energy from the sun. Dye molecules that are self-assembled on semiconductor substrates (usually TiO2) play essential roles in the photon-to-electron conversion process. However, many structural aspects and structure-properties of dyes in SAMs remain elusive, and the conformer structures of dye molecules and their effects on the properties of SAMs are frequently neglected. In fact, dye molecules can adopt multiple conformer structures on the TiO2 substrate due to the existence of flexible alkyl chains and non-rigid backbones, which might affect the structures and properties of the dye-sensitized systems. In this research, we investigate the structures and effects of different conformers via the first principles calculations, employing an organic azo dye 4-[2-[4-(diethyl-amino)phenyl]diazenyl]-benzoic acid, which combines good transparency, aesthetic quality and electricity-generation properties. We find that various conformers of the organic dye could reside stably on the TiO2 substrate, and the conformer effects are especially dominant when the dye aggregation model is considered. The relationship between the azo dye conformation and the azo dye aggregation is explained. Our calculations demonstrate that including the conformer effects leads to diverse structures and properties of the azo dye/TiO2 systems, which could be important for the devices containing the dye/TiO2 systems such as the dye-sensitized solar cells, smart windows and dye-sensitized water splitting systems.