Photocatalytic Water Reduction Research Articles

A Pyridine Anchoring Strategy for Dye‐Sensitized Photocatalysis

AbstractAnchoring groups play an important role in the assembly of functional molecules onto semiconductor substrates for dye‐sensitized photocatalysis. In addition to directly influencing the adsorption stability, anchoring groups significantly impact the efficiency of electron or hole injection into the substrate, which is a crucial step for photocatalytic reactions. Most reported dye‐sensitized photocatalytic systems, including semiconductor nanoparticles and photoelectrochemical cells, are prepared using the carboxylic or phosphonic acid anchors. The systems prepared with these conventional anchors usually suffer from low adsorption stability in aqueous media. Alternatively, pyridine has emerged as an anchoring group of photosensitizers and molecular catalysts in the construction of photocatalytic systems. The resulting semiconductor nanoparticles and photoelectrodes show superior adsorption stability in aqueous media, providing a simple and efficient way to sensitize the metal or metal oxide substrate. This review focuses on the recent advances of such a pyridine anchoring strategy in the assembly of photosensitizers and molecular catalysts for photocatalytic applications, mainly including photocatalytic water oxidation, reduction, and splitting. The further exploration and understanding of the pyridine‐anchoring method are believed to boost the development of advanced dye‐sensitized photocatalytic devices and technologies.

Facile fabrication of heavily N-doped Zn0.67Cd0.33S nanocatalyst with congenital sulfur vacancies for efficient photocatalytic reduction of water and hexavalent chromium

Twinned zinc cadmium sulfide (ZnxCd1-xS) has been widely investigated for efficient Cr(VI) reduction and H2 production under light irradiation due to its inherent homojunctions. However, long-distance migration of charge carriers and relatively low conduction band level inevitably cause low photocatalytic reduction ability. Herein, we developed a DMF-involved one-step solvothermal strategy to introduce N heteroatoms into Zn0.67Cd0.33S crystals incorporated with dopant-induced S vacancies. The synergetic promotion of the elevated conduction band, coupled with the robust aggregation of interactants, effectively impeded the recombination behavior of charge carriers, leading to a notable increase in photocurrent density (∼2.4 times), electron density (∼2.8 times), and a higher photo-reducing potential. The optimal NZCS-10 demonstrated a superior photocatalytic Cr(VI) reduction efficiency of 99.32 % within 30 min, and rendered ca. 5.8- and 7.6-fold enhancement for H2 evolution rate under alkaline and acidic conditions, respectively. This study provided a universal strategy for gaining highly reductive transition metal sulfides with more active sites.

Oxidative and selective C–C cleavage of glycerol to glycolaldehyde with atom-like Cu on Cu-TiO2: Photocatalytic water reduction with concurrent glycerol oxidation in sunlight

Concurrent consumption of electrons and holes for the conversion of a biomass component to value added products represents a highly efficient and sustainable approach towards utilizing renewable energy, but difficult to achieve. The integration of hydrogen production with glycerol oxidation presents a novel and sustainable approach towards achieving a circular economy. In the current study, integration of atom-like Cu-clusters onto TiO2 substrate has been achieved using a facile photo-deposition technique (TC-PDO). Also, novel synthetic approaches have been employed to augment the surface coverage of Cu on TiO2 with atom-like clusters of Cu, either by borohydride treatment on TiO2 followed by Cu-deposition (TC-200) or oxygen-vacancy creation by UV illumination followed by Cu-deposition (TC-PDO). Increased dispersion and enhanced electronic integration of Cu with TiO2 lead to a corresponding increase in the efficiency of photocatalytic hydrogen evolution (13.8 mmol/h.g for TC-PDO at pH 9). Several atom-like Cu integrated with each TiO2 particle acts as photocatalytic reactor, and the same enhances electron-hole separation as well as activity. Sustainable aspect was also studied for TC-PDO up to 25 h at pH 9. Concurrently, glycerol oxidation displays the highest selectivity to C2 product (glycolaldehyde with 70 %) with a C–C cleavage. The investigation of this process holds significant potential for the extensive and simultaneous exploitation of electrons and holes in order to achieve water splitting and glycerol oxidation towards selective value-added products formation.

All-in-one ultrathin nanoporous ZnIn2S4 with ameliorated photoredox capability: harvesting electron–hole pairs in cooperative hydrogen and benzaldehyde production

All-in-one ultrathin porous ZnIn2S4 toward photocatalytic water reduction and benzyl alcohol oxidation for the co-production of hydrogen fuels and value-added benzaldehyde.

Effect of Additions of Mixtures of Sodium Sulfite and Formic and Acetic Acid on the Photocatalytic Reduction of Water with Cadmium Sulfide Suspensions

Effect of Additions of Mixtures of Sodium Sulfite and Formic and Acetic Acid on the Photocatalytic Reduction of Water with Cadmium Sulfide Suspensions

Effect of Additions of Mixtures of Sodium Sulfite and Formic and Acetic Acid on the Photocatalytic Reduction of Water with Cadmium Sulfide Suspensions

The photochemical reduction of water with cadmium sulfide suspensions containing mixtures of sodium sulfite and formic and acetic acid solutions has been studied by electromotive force measurements and gasometry. The sulfite ions are oxidized on the anode in the sulfite–acetate suspensions; the sulfite ions and formic acid, in the sulfite–formate solutions. Hydrogen peroxide was found among the products of the photochemical reaction; it is involved in the oxidation of the radicals of sulfite ions and molecular sulfur of CdS particles and prevents the reduction of water.

ZrO2 modification of homogeneous nitrogen-doped oxide MgTa2O6−xNx for promoted photocatalytic water splitting

Homogeneous nitrogen-doped oxides are of wide visible light utilization for promising photocatalytic water splitting to produce hydrogen, but currently the poor charge separation severely limits their photocatalytic performances. In this work, a homogeneous nitrogen-doped tunneled oxide of MgTa2O6−xNx with an absorption edge of 570 nm was selected as a prototype to investigate the influence of ZrO2 modification on the charge separation as well as photocatalytic performance. It is interesting to observe that the formation of the reduced tantalum species, regarded as recombination centers, in the MgTa2O6−xNx sample could be effectively inhibited via the surface passivation with ZrO2 nanoparticles, based on which the photocatalytic water reduction and oxidation half-reaction activities could be remarkably promoted. Together with modification of the deposited Pt cocatalyst, the optimized H2 evolution rate over Pt-ZrO2/MgTa2O6−xNx (Zr/Ta = 0.10) photocatalyst was almost 4.5 times as high as that of the pristine Pt-MgTa2O6−xNx sample free of ZrO2 modification, whose apparent quantum yield at 420 nm (± 15 nm) achieved herein was superior to those of other reported homogeneous nitrogen-doped photocatalysts. The improved charge separation probably attributes to the introduction of Zr–O–Ta bond after ZrO2 modification, which is helpful to stabilize the tantalum species at more cationic state and inhibit the formation of the reduced tantalum species. This work extends the application territory of ZrO2 modification to the homogeneous nitrogen-doped oxide photocatalysts, and demonstrates its feasibility and effectiveness for remarkably enhanced photocatalytic water splitting performance.

Novel Triangulenes: Computational Investigations of Energy Thresholds for Photocatalytic Water Splitting.

Organic materials with Inverted Singlet-Triplet (INVEST) gaps are interesting for their potential use in photocatalytic small molecule transformations such as the entirely solar-driven water splitting reaction. However, only a few INVEST emitters are thermodynamically able to split water requiring a first singlet excited dark state, S1 , above 1.27 or 1.76 eV, and absorption near solar the maximum, 2.57 eV. These requirements and the INVEST character are key for achieving a long-lived photocatalyst for water splitting. The only known INVEST emitters that conform to these criteria are large triangular boron carbon nitrides with unknown synthesis pathways. Using ADC(2), a quantum-mechanical method, we describe three triangulenes. 3 a is a cyano azacyclopenta[cd]phenalene derivative while 3 b and 3 c are cycl[3.3.3]azine derivatives. 3 b has a previously undescribed disulfide bridge. Overall, 3 a fulfills requirements for photocatalytic four-electron reduction of water while the S1 states of 3 b and 3 c are likely slightly low for the two-electron reduction process. By analyzing impacts of ligands, we find that there are guidelines describing how S1 -S5 energies and oscillator strengths, T1 energies, and ΔES1T1 gaps are affected, requiring deep-learning algorithms for which studies will be presented by us in due time. The impact of ground-state geometries, solvation effects, as well as reduced-cost ADC(2) algorithms on our findings are also discussed.

Effect of Formic and Acetic Acid Additives on the Photocatalytic Reduction of Water by Cadmium Sulfide Particles

Effect of Formic and Acetic Acid Additives on the Photocatalytic Reduction of Water by Cadmium Sulfide Particles

Preparation strategy of bimetallic MOF hollow photocatalysts for hydrogen evolution

Hollow structure materials have been proved to be excellent photocatalysts due to their advantages in the separation of electron-hole pairs. However, the synthesis of such materials is complex, time-consuming, and poor versatility. In this study, Cu-DHTP/ZIF-67 (DHTP = 2,5-dihydroxyterephthalic acid) hollow materials containing both Co and Cu were designed and synthesized by a simple one-step etching and ligand exchange strategy. It is worth noting that the bimetallic hollow catalyst can be synthesized in the presence of both Cu(Ⅱ) and DHTP within 5 min at room temperature. The synthesized hollow bimetallic catalyst has improved photoresponse performance and electron-hole pair separation ability, which results in the best catalytic performance compared with DHTP/ZIF-67, Cu/ZIF-67, and ZIF-67 when used in the photocatalytic water splitting reaction. The hydrogen evolution yield of Cu-DHTP/ZIF-67 can reach 39.7 mmol g−1, which is 6 times than that of pure ZIF-67. This study demonstrates the promising potential of Cu-DHTP/ZIF-67 photocatalyst in the field of photocatalytic water reduction and hydrogen evolution, and provides a new idea for designing and preparing more hollow structure bimetallic photocatalysts with excellent performance.

Nanoparticulate Perovskites for Photocatalytic Water Reduction.

SrTiO3 and BaTiO3 nanoparticles (NPs) were activated using H2O2 or aqueous HNO3, and pristine and activated NPs were functionalized with a 2,2'-bipyridine phosphonic acid anchoring ligand (1), followed by reaction with RuCl3.3H2O and bpy, RhCl3.3H2O and bpy, or RuCl3.3H2O. The surface-bound metal complex functionalized NPs were used for the photogeneration of H2 from water, and their activity was compared to related systems using TiO2 NPs. The role of pH during surface complexation was found to be important. The NPs were characterized using Fourier transform infrared (FTIR) and solid-state absorption spectroscopies, thermogravimetric analysis mass spectrometry (TGA-MS), and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and the dihydrogen generation was analyzed using gas chromatography-mass spectrometry (GC-MS). Our findings indicate that extensively functionalized SrTiO3 or BaTiO3 NPs may perform better than TiO2 NPs for water reduction.

Saturated Ti-coordinated {001} facets-dependent photocatalytic water reduction over NH2-MIL-125(Ti) sheets: Observation and unraveling of facets effect

Facets-dependent photocatalytic H2 evolution rates of inorganic semiconductors have been well recognized, but it remains unclear for metal-organic frameworks (MOFs). Herein, we controllably prepared a series of NH2-MIL-125(Ti) MOF sheets with different ratios of {001}/{111} facets exposure, on which the photocatalytic H2 evolution rates from water are demonstrated to be almost linearly increased with enhanced proportion of {001} facets. Notably, the optimized activity of ca. 2640 μmol h-1 g-1 is by far superior to all the NH2-MIL-125(Ti)-based photocatalysts reported. The discovery of facets-dependent photocatalytic water splitting performance (i.e. facets effect) was unravelled to mainly result from their distinct charge separation abilities according to the experimental facts that the saturated six-coordinated Ti atoms (Ti6c) in the {001} facets can more efficiently accept and store the photogenerated electrons by the reduction of Ti4+ into Ti3+ with respect to the low-coordinated Ti atoms (Ti4c and Ti5c) in the {111} facets.

Enhanced Photocatalytic H2 Evolution Performance of the Type-II FeTPPCl/Porous g-C3N4 Heterojunction: Experimental and Density Functional Theory Studies.

It is of great significance to improve the photocatalytic performance of g-C3N4 by promoting its surface-active sites and engineering more suitable and stable redox couples. Herein, first of all, we fabricated porous g-C3N4 (PCN) via the sulfuric acid-assisted chemical exfoliation method. Then, we modified the porous g-C3N4 with iron(III) meso-tetraphenylporphine chloride (FeTPPCl) porphyrin via the wet-chemical method. The as-fabricated FeTPPCl-PCN composite revealed exceptional performance for photocatalytic water reduction by evolving 253.36 and 8301 μmol g-1 of H2 after visible and UV-visible irradiation for 4 h, respectively. The performance of the FeTPPCl-PCN composite is ∼2.45 and 4.75-fold improved compared to that of the pristine PCN photocatalyst under the same experimental conditions. The calculated quantum efficiencies of the FeTPPCl-PCN composite for H2 evolution at 365 and 420 nm wavelengths are 4.81 and 2.68%, respectively. This exceptional H2 evolution performance is because of improved surface-active sites due to porous architecture and remarkably improved charge carrier separation via the well-aligned type-II band heterostructure. Besides, we also reported the correct theoretical model of our catalyst through density functional theory (DFT) simulations. It is found that the hydrogen evolution reaction (HER) activity of FeTPPCl-PCN arises from the electron transfer from PCN via Cl atom(s) to Fe of the FeTPPCl, which forms a strong electrostatic interaction, leading to a decreased local work function on the surface of the catalyst. We suggest that the resultant composite would be a perfect model for the design and fabrication of high-efficiency heterostructure photocatalysts for energy applications.

Reduced TiO2 with prolonged electron lifetime for improving photocatalytic water reduction activity

The reduction of anatase TiO2 with NaBH4 under argon atmosphere at a high temperature resulted in a longer electron lifetime and a larger electron population. The reduced gray anatase sample with disorder layer showed a higher evolution rate of H2 (130.2 μmol h−1 g−1) compared to pristine TiO2 (24.1 μmol h−1 g−1) in the presence of Pt co-catalyst in an aqueous glucose solution under exposure to ultraviolet light (λ ⩽ 400 nm). Ti3+ and oxygen vacancy defects were proposed to exist in the reduced TiO2. A continuum tail forms above the valence band edge top as a result of these two defects, which contribute to the lattice disorder. This is presumably also the case with the conduction band, which has a continuum tail composed of mid-gap states as a result of the defects. The Ti3+ and oxygen vacancy defects operate as shallow traps for photoexcited electrons, thereby preventing recombination. Since the defects are primarily located at the surface, i.e. in the disorder layer, the photoexcited electrons in shallow traps hence become readily available for the reduction of H3O+ into H2. The prolonged electron lifetime increases the photoexcited electron population in the reduced TiO2, resulting in enhanced water reduction activity.

Single phase of perovskite YTaON2 synthesized for photocatalytic water reduction and oxidation under visible-light irradiation

Single-phase perovskite-structured YTaON2 with a band gap of 2.1 eV was obtained for the first time by an enhanced nitriding kinetics method and its dual functionalities of water reduction and oxidation were proved.

Layered β‐ZrNBr Nitro‐Halide as Multifunctional Photocatalyst for Water Splitting and CO2 Reduction

AbstractDeveloping mixed‐anion semiconductors for solar fuel production has inspired extensive interest, but the nitrohalide‐based photocatalyst is still in shortage. Here we report a layered nitro‐halide β‐ZrNBr with a narrow band gap of ca. 2.3 eV and low defect density to exhibit multifunctionalities for photocatalytic water reduction, water oxidation and CO2 reduction under visible‐light irradiation. As confirmed by the results of electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations, the formation of anion vacancies in the nitro‐halide photocatalyst was inhibited due to its relatively high formation energy. Furthermore, performance of β‐ZrNBr can be effectively promoted by a simple exfoliation into nanosheets to shorten the carrier transfer distance as well as to promote charge separation. Our work extends the territory of functional photocatalysts into the nitro‐halide, which opens a new avenue for fabricating efficient artificial photosynthesis.

Layered β-ZrNBr Nitro-Halide as Multifunctional Photocatalyst for Water Splitting and CO2 Reduction.

Developing mixed-anion semiconductors for solar fuel production has inspired extensive interest, but the nitrohalide-based photocatalyst is still in shortage. Here we report a layered nitro-halide β-ZrNBr with a narrow band gap of ca. 2.3 eV and low defect density to exhibit multifunctionalities for photocatalytic water reduction, water oxidation and CO2 reduction under visible-light irradiation. As confirmed by the results of electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations, the formation of anion vacancies in the nitro-halide photocatalyst was inhibited due to its relatively high formation energy. Furthermore, performance of β-ZrNBr can be effectively promoted by a simple exfoliation into nanosheets to shorten the carrier transfer distance as well as to promote charge separation. Our work extends the territory of functional photocatalysts into the nitro-halide, which opens a new avenue for fabricating efficient artificial photosynthesis.

Enhanced charge separation in dual Z-scheme Au decorated LaFeO3-g-C3N4-BiFeO3 system for efficient H2 production

The artificial Z-scheme system that mimics natural photosynthesis is encompassed of two semiconductors which generates two sets of charge carriers at different locations under solar irradiation. Researchers are trying to improve the efficacy of these systems by engineering highly stable redox couples, enhancing their light absorption capability, and discovering new cocatalysts. Herein, we report the fabrication of Au decorated LaFeO3-g-C3N4-BiFeO3 (Au/LFO-CN-BFO) catalyst as dual Z-scheme system for photocatalytic water splitting reactions. The photocatalytic performances of Au/LFO-CN-BFO, LFO-CN-BFO, LaFeO3, g-C3N4 and BiFeO3 were compared and the mechanism on the symmetric double Z-scheme Au/LFO-CN-BFO photocatalytic system was proposed. The results display that Au/LFO-CN-BFO has more superior photocatalytic performance compared to its counter parts (LaFeO3, g-C3N4 and BiFeO3), which is due to the symmetric double Z-scheme nature of photocatalyst. The as-fabricated Au/LFO-CN-BFO photocatalyst reveals high performance for photocatalytic water reduction to evolve H2 (i.e., 698.4 µmol h-1g−1) under visible light irradiation (λ ≥ 420 nm), which is far greater compared to that of its counterparts. As confirmed from various observations, the improved H2 evolution performance of the dual Z-scheme system is accredited to the improved light absorption and significantly promoted charge carrier’s separation. The design of this highly efficient dual Z-scheme composite photocatalyst would trigger the development of new artificial Z-scheme scheme systems that will solve the energy and associated environmental issues.

Gas bubbling exfoliation strategy towards 3D g-C3N4 hierarchical architecture for superior photocatalytic H2 evolution

Graphitic carbon nitride (g-C 3 N 4 ) shows a graphite-like layered structure, which provides a high theoretical value for solar-to-hydrogen evolution especially for a 2D nanostructure. However, conventional polycondensation induces a strong agglomeration and collapse of nanostructure, resulting in a relatively poor photocatalytic performance. To overcome this problem, we develop a gas bubbling exfoliation strategy with NH 4 Cl assistant to make ultrathin 2D g-C 3 N 4 nanosheets self-assembled into a 3D macroporous network on a large scale. The hierarchical structure significantly improves the specific surface area to 176.4 m 2 g −1 (11.6 times higher than the reference g-C 3 N 4 ), which allows a large water/g-C 3 N 4 interface for photocatalytic water reduction reaction. The ultrathin 2D g-C 3 N 4 nanosheets show a thickness of about 1.4 nm, which greatly suppress photoinduced carriers recombination and enhance charge transfer at the interface. Furthermore, the doping of N and Cl is achieved during synthesis. As a result, the resulting g-C 3 N 4 demonstrates a remarkable improvement in H 2 production of 12.89 mmol g -1 h −1 , which is 21 times higher than the g-C 3 N 4 obtained from the conventional condensation method. These explorations provide a facile guidance for the quasi 3D g-C 3 N 4 hierarchical architecture engineering even for various energy-related applications. • Ultrathin 2D g-C 3 N 4 nanosheets self-assembled into a 3D macroporous network. • Enhanced H 2 production of 12.89 mmol g -1 h −1 is achieved, which is 21 times higher. • The hierarchical structure improves the specific surface area to 176.4 m 2 g −1 . • The ultrathin 2D g-C 3 N 4 nanosheets show a thickness of about 1.4 nm.

Photocatalytic Reduction of Water by Particles of Cadmium Sulfide in a Solution of Sodium Sulfite

Photocatalytic Reduction of Water by Particles of Cadmium Sulfide in a Solution of Sodium Sulfite