Photocatalytic Water Reduction Research Articles

Simultaneously Tuning the Defects and Surface Properties of Ta3N5 Nanoparticles by Mg-Zr Codoping for Significantly Accelerated Photocatalytic H2 Evolution.

The simultaneous control of the defect species and surface properties of semiconducting materials is a crucial aspect of improving photocatalytic performance, yet it remains challenging. Here, we synthesized Mg-Zr-codoped single-crystalline Ta3N5 (Ta3N5:Mg+Zr) nanoparticles by a brief NH3 nitridation process, exhibiting photocatalytic water reduction activity 45 times greater than that of pristine Ta3N5 under visible light. A coherent picture of the relations between the defect species (comprising reduced Ta, nitrogen vacancies and oxygen impurities), surface properties (associated with dispersion of the Pt cocatalyst), charge carrier dynamics, and photocatalytic activities was drawn. The tuning of defects and simultaneous optimization of surface properties resulting from the codoping evidently resulted in the generation of high concentrations of long-lived electrons in this material as well as the efficient migration of these electrons to evenly distributed surface Pt sites. These effects greatly enhanced the photocatalytic activity. This work highlights the importance and feasibility of improving multiple properties of a catalytic material via a one-step strategy.

Aggrandizing the Photoactivity of ZnO Nanorods toward N2 Reduction and H2 Evolution through Facile In Situ Coupling with NixPy

The photocatalytic water and nitrogen reduction in the presence of solar light gives rise to an alternative route for sustainable green energy. This novel approach can resolve the difficulty of greenhouse gas emanation from fossil fuels and global warming and energy crisis. Making headway for developing more robust and efficient photocatalytic systems still remains an enormous challenge toward industrial usage on a large scale. To meet this challenge, herein we report hydrothermally prepared ZnO nanorods (NRs) coupled with nickel phosphide (NiₓPy) via a low-temperature phosphidation method. NiₓPy acts as a robust co-catalyst, which increases the visible light absorption efficacy and photocatalytic performance of ZnO NRs toward nitrogen reduction and hydrogen evolution. The nanorod morphology of the ZnO-NiₓPy hybrid is revealed by transmission electron microscopy (TEM) analysis while the formation of the Ni–P moiety and its interaction with ZnO NRs are verified by X-ray photoelectron spectroscopy (XPS) analysis. Besides, the excellent photogenerated charge separation and transfer efficiency are further confirmed using photoluminescence (PL), electrochemical impedance spectroscopy (EIS), and transient analysis, which are the main aspects for activity enhancement in the nickel phosphide-modified ZnO NRs. The optimized ZnO-NiₓPy nanocomposite exhibits an outstanding ammonia production rate of 2304.43 μmol h–¹ g–¹ without using any organic scavenger or precious noble metal. On the other hand, this catalyst also exhibits stupendous performance with a H₂ generation rate of 10 481 μmol h–¹ g–¹ using methanol as a hole scavenger. This magnificent result can be attributed to (i) Ni–P sites acting as active reaction sites for nitrogen and proton adsorption and activation, (ii) efficacious charge carrier segregation, and (iii) proficient charge transfer to the surface of nickel phosphide. This finding offers a new avenue for developing noble metal-free phosphide-based hybrids toward highly efficient photocatalytic reactions.

Primary photocatalytic water reduction and oxidation at an anatase TiO2 and Pt-TiO2 nanocrystalline electrode revealed by quantitative transient absorption studies

Quantitative assessments of electron and hole transfer dynamics with water on anatase nanocrystalline TiO2 films were conducted by employing a series of transient absorption spectrometers. For water reduction reactions, both conduction band and trapped electrons decay with two different single exponential components with the difference in time scale of 6∼7 orders of magnitudes. The faster reaction occurs in 8–16 ns, while the slower component shows a lifetime of 0.1∼1.4 s. Pt nanoparticle deposition on the TiO2 surface switches the slower single exponential reaction to a stretched exponential with an accelerated half lifetime of 2–7 ms, indicating that this slower reaction is limited by the electron trapping-detrapping movements inside the TiO2. Water oxidation reactions occur multi-exponentially from 100 ns to 200 ms with a half lifetime of 10 μs. Pt on the TiO2 surface catalyses water oxidation, occurring from 25 ns with a half lifetime of 8 μs.

One-step phosphorization preparation of gradient-P-doped CdS/CoP hybrid nanorods having multiple channel charge separation for photocatalytic reduction of water

The separation and transfer of photogenerated charge carriers are the crucial process in photocatalysis, and the realization of multiple charge separation and transfer routes in a single catalyst structure is very promising to achieve high-efficiency catalytic performance. We herein report a simple strategy to synthesize CdS/CoP hybrid nanorods (HNRs) via a one-step phosphorization treatment of the CdS/Co(OH)2 precursors, in which the gradient-P-doped-CdS NRs and CoP cocatalyst can be synchronously obtained (denoted as gP-CdS/CoP HNRs). The gradient P doping gradually reduced the band gap of CdS as well as elevated Fermi level with doping concentration up, resulting in the formation of a built-in electric field in the CdS NRs. The built-in electric field points from the surface towards the interior of the CdS NRs, which facilities the separation of photogenerated charge carriers in CdS and the transfer of electrons to the CdS/CoP interface. The transferred electrons are then captured by the CoP cocatalysts, leading to further separation of the charge carriers. Owing to the coupling of gradient-P-doped CdS nanorods with the CoP cocatalysts, the optimized gP-CdS/CoP HNRs exhibit remarkably enhanced photocatalytic water reduction performance, with a H2 production rate of 22.95 mmol g−1 h−1 which is 28.7 and 3.2 times higher than that of pristine CdS and gP-CdS, respectively. This work demonstrates the synergetic effects of charge carrier separation in the coupled nanostructure of the gradient-P-doped CdS NRs with the CoP cocatalyst, which provides a new platform for developing heterostructures with multiple charge separation and transfer routes for photocatalysis.

About Control: Kinetics in Molecule- based Photochemical Water Reduction Investigated by Transient IR Spectroscopy.

This review aims to promote the role of transient IR spectroscopy to investigate molecular-based photocatalytic water reduction. Examples are discussed in which this method has been successfully applied to elucidate reaction mechanisms. Focus is given to kinetic changes and their consequences when a photochemical water reduction system, which is functional and well understood in solution, is brought onto a metal oxide surface.

Synergy of semiconductor components of non-covalent functionalized (PdS doped)-G CdS NPs composite provide efficient photocatalytic water reduction under visible light

A new composite based on a few-layered pristine graphene, G, was prepared under mild conditions by adsorption of a functionalized pyrimidine derivative, Tren, followed by sequential precipitation of PdS and CdS NPs. The composite G-Tren-PdS-CdS showed high photocatalytic activity in water reduction providing a constant average H2 production of 2.3 mmol·g−1·h−1 during 24 h, which exceded 18.9 times that of the bare CdS. Structural analyses of the fresh and used phocatalyst revealed high robustness of the material. A systhematic study of the structure and optical properties of the materials revealed the pyrimidine-G moiety, G-Tren, behaves as a semiconductor with a band gap which can be finelly tuned through the control of the amount of adsorbed pyrimidine. The study also allowed to propose a reliable mechanism for the photocatalytic process. The excellent performance of the photocatalyst, in which CdS acts as electron provider for the hydrogen reduction and PdS and G-Tren moieties as cocatalysts, is attributed mainly to the complementary of the ranges of visible light absorption of the components, G-Tren, CdS and PdS, and to the possible role played by the arene basic centers provided by the unaltered π-conjugated structure of G, as reduction active sites.

Bottom-up construction of reduced-graphene-oxide-anchored spinel magnet Fe2.02Ni1.01O3.22, anatase TiO2 and metallic Ag nanoparticles and their synergy in photocatalytic water reduction

Paramagnet spinel NiFe2O4 nanocrystals were synthesized using the hydrothermal route and were put in contact with anatase TiO2 PC500; the mixture of semi-conductors was employed as substrate to deposit reduced Graphene Oxide (rGO) sheets synthetised via Hummers modified method. Several rGO mass amounts ‘x’ were deposited (x = 0, 5, 10, 15% and 20%). These steps of synthesis were followed by surface decoration with Ag metallic particles using the photodeposition method, which led to different heterojunction nanocomposite materials. The XRD analysis proved that the NiFe2O4 has a spinel crystalline structure and the anatase TiO2 has a tetragonal system; these crystalline systems were conserved after the deposition of rGO. The electronic properties were studied experimentally using the UV-Vis DRS illustrating a hierarchical gap for the spinel NiFe2O4 and an indirect gap for TiO2; these analyses revealed that the electronic and structural properties are affected when the rGO is deposited. The SEM images proved the formation of the heterojunction and the good mixing of NiFe2O4 and TiO2 with the rGO. The EDX allowed attributing a chemical formula for the spinel structure, Fe2.02Ni1.01O3.22. Furthermore, the photocatalytic properties are influenced by the specific area of the photocatalyst; for that the DLS analysis was carried out to estimate the average aggregated grain seize. At the end, the composites were evaluated for their enhanced adsorptive performance and photocatalytic activity under visible light irradiation to degrade Methylene Blue (MB). In addition, the deposit of metallic Ag showed great improvement in the photocatalytic performances over pristine composites.

Tuning the Optoelectronic Properties of Hybrid Functionalized MIL-125-NH2 for Photocatalytic Hydrogen Evolution.

Metal-organic frameworks (MOFs) constructed with mixed ligands have shown great promise in the generation of materials with improved sorption, optical, and electronic properties. With an experimental, spectroscopic, and computational approach, herein, we investigated how the incorporation of different functionalized ligands within the structure of MIL-125-NH2 affects its performance in photocatalytic water reduction. We found that multiligand incorporation within the MOF structure has an impact on the light absorption spectrum and the electronic structure. These combined modifications improve the photocatalytic performance of MIL-125-NH2, thereby increasing the rate of hydrogen evolution reaction. Of the four nanoparticle/MOF photocatalytic systems tested, we showed that the Pt/MIL-125-NH2/(OH)2 system (Pt nanoparticle plus MIL-125-NH2 with amino and dihydroxyl functionalized ligands) outperforms its counterpart Pt/MIL-125-NH2 system, attributed to the enhanced p-π conjugation between the lone pairs of O atoms and their aromatic ligands resulting in a red-shifted absorption spectrum and greater spatial distribution of electron density.

High-Throughput Screening of Earth-Abundant Water Reduction Catalysts toward Photocatalytic Hydrogen Evolution.

Noble-metal photosensitizers and water reduction co-catalysts (WRCs) still present the highest activity in homogeneous photocatalytic hydrogen production. The search for earth-abundant alternatives is usually limited by the time required to screen new catalyst combinations; however, here, we utilize newly designed and developed high-throughput photoreactors for the parallel synthesis of novel WRCs and colorimetric screening of hydrogen evolution. This unique approach allowed rapid optimization of photocatalytic water reduction using the organic photosensitizer Eosin Y and the archetypal cobaloxime WRC [Co(GL1)2pyCl], where GL1 is dimethylglyoxime and py is pyridine. Subsequent combinatorial synthesis generated 646 unique cobalt complexes of the type [Co(LL)2pyCl], where LL is a bidentate ligand, that identified promising new WRC candidates for hydrogen production. Density functional theory (DFT) calculations performed on such cobaloxime derivative complexes demonstrated that reactivity depends on hydride affinity. Alkyl-substituted glyoximes were necessary for hydrogen production and showed increased activity when paired with ligands containing strong hydrogen-bond donors.

Synthesis of Y2Ti2O5S2 by thermal sulfidation for photocatalytic water oxidation and reduction under visible light irradiation

Synthesis of Y2Ti2O5S2 by thermal sulfidation for photocatalytic water oxidation and reduction under visible light irradiation

Highly efficient H2 generation over Cu2Se decorated CdS0.95Se0.05 nanowires by photocatalytic water reduction

The development of efficient co-catalysts for promoting solar-driven water splitting to hydrogen (H2) energy conversion is of increasing importance but still a challenging scheme. In the present work, a noble-metal-free copper selenide (Cu2Se) is primarily evaluated the possibility of functioning as a co-catalyst for enhancing photocatalytic H2 evolution activity by virtue of density functional theory (DFT) calculations. Then, the photocatalysts CdS0.95Se0.05 nanowires (NWs) decorated with Cu2Se nanoparticles (NPs) as co-catalyst are designed and successfully fabricated via a hydrothermal method. Under visible light (λ ≥ 400 nm) illumination, the as-prepared Cu2Se/CdS0.95Se0.05 nanocomposites loading with 20 mol% of Cu2Se NPs exhibits the highest photocatalytic activity with an H2 generation rate of 570.7 μmol·h−1 and a corresponding apparent quantum efficiency (AQE) of 31.26%, which is about 7.1 and 27.4 times greater than that of pristine CdS0.95Se0.05 and CdS NWs, respectively. Theoretical calculations and experimental measurements demonstrate that the excellent activity of the hybrid catalysts is ascribed to the formation of Ohmic-type heterojunctions between CdS0.95Se0.05 semiconductor and semi-metallic Cu2Se, which can not only facilitate the charge carriers separation and transportation but also improve the surface H2-evolution kinetics.

Substituted 2,4-Di(pyridin-2-yl)pyrimidine-Based Ruthenium Photosensitizers for Hydrogen Photoevolution under Red Light

The photocatalytic reduction of water to form hydrogen gas (H2) is a promising approach to collect, convert, and store solar energy. Typically, ruthenium tris(bipyridine) and its many derivatives are used as photosensitizers (PSs) in a variety of photocatalytic conditions. The bis(terpyridine) analogues, however, have only recently gained attention for this application because of their poor photophysical properties. Yet, by the introduction of electron-donating or -withdrawing groups on the terpyridine ligands, the photophysical and electrochemical properties can be considerably improved. In this study, we report a series of nonsymmetric 2,6-di(pyridin-2-yl)pyrimidine ligands with peripheral pyridine substituents in different positions and their corresponding ruthenium(II) complexes. The presence of the pyrimidine ring stabilizes the lowest unoccupied molecular orbital, leading to a red-shifted emission and prolonged excited-state lifetimes as well as higher luminescence quantum yields compared to analogous terpyridine complexes. Furthermore, all complexes are easier to reduce than the previously reported bis(terpyridine) complexes used as PSs. Interestingly, the pyridine substituent in the 4-pyrimidine position has a greater impact on both the photophysical and electrochemical properties. This correlation between the substitution pattern and properties of the complexes is further investigated by using time-dependent density functional theory. In hydrogen evolution experiments under blue- and red-light irradiation, all investigated complexes exhibit much higher activity compared to the previously reported ruthenium(II) bis(terpyridine) complexes, but none of the complexes are as stable as the literature compounds, presumably because of an additional decomposition pathway of the reduced PS competing with electron transfer from the reduced PS to the catalyst.

Multi‐technical study of copper oxide on graphitic carbon nitride and its role in the photocatalytic reactions

AbstractBased on a facile three‐step preparation method, CuO/g‐C3N4 photocatalysts have been successfully synthesized by exfoliation of a bulk graphitic carbon nitride and subsequent hydrothermal decoration with CuO nanoparticles. Several advanced scientific tools were used to characterize crucial features of the prepared materials, in particular, to understand the role of copper oxide in photocatalytic reactions. CuO/g‐C3N4 reached a remarkably increased light‐induced photocatalytic reduction of water to hydrogen as well as oxidation of formaldehyde to CO2, in comparison with a neat g‐C3N4. The presence of CuO co‐catalyst resulted in more than 10 times higher H2 production rate. The increase of photocatalytic efficiency was observed also in the case of HCHO oxidation, but the effect of CuO addition was less impressive. Moreover, it has been evidenced that the durability of photocatalyst raised with an increasing amount of copper oxide. Simultaneously, the copper modified composites exhibited an improved photoelectrocatalytic (PEC) performance, confirmed by the photocurrent density measurements. In the following paper we described our findings concerning the role of copper oxide in CuO/g‐C3N4 composites.

Engineering self-doped surface defects of anatase TiO2 nanosheets for enhanced photocatalytic efficiency

Anatase TiO2 nanosheets with controlled locations of defects in the bulk or on the surface were successfully synthesized by finely adjusting the temperatures and amounts of added HF during hydrothermal synthesis. The TiO2 nanosheets with dominant surface defects exhibit much higher activity in photocatalytic water reduction to hydrogen than those with dominant bulk defects, unambiguously demonstrating the promotion effect of surface defects on the TiO2 photocatalysis. The surface defects can be act as the site both for trapping the photogenerated charges on the surface and for adsorbing and activating reactants, subsequently facilitating the photogenerated charges-participated surface reactions. These results offer a novel strategy to enhance the photocatalytic efficiency of TiO2 photocatalysts via engineering self-doped surface defects.

A cobalt complex of bis(methylthioether)pyridine, a new catalyst for hydrogen evolution

A new cobalt complex, [(btep)CoBr2], was produced by the reaction of CoBr2 and bis(methylthioether)pyridine (btep), and its structure has been determined by X-ray crystallography. [(btep)CoBr2] shows good activity for the electro- and photocatalytic reduction of water to H2. As an electrocatalyst, [(btep)CoBr2] can provide 591.9 mol of hydrogen per mole of catalyst per hour (mol H2/mol catalyst/h) from neutral water under an overpotential (OP) of 837.6 mV. As a co-catalyst in a photocatalytic system, together with CdS nanorods (CdS NRs) (0.14 mg mL−1) as a photosensitizer, and ascorbic acid (H2A) (0.12 M) as a sacrificial electron donor in an aqueous solution (pH 4.5), [(btep)CoBr2] can afford 9326.4 mol H2 per mole of catalyst over a 40 h irradiation with blue light (λmax = 469 nm). The highest apparent quantum yield (AQY) is ~ 25.5%. The catalytic mechanism for H2 production was investigated by several measurements and analysis.

Characterization and Evaluation of Copper Slag as a Bifunctional Photocatalyst for Alcohols Degradation and Hydrogen Production

With the goal of providing an economically viable method for reducing water pollution and health impact by the mezcal industry wastes, photocatalytic degradation of toxic alcohols using copper slag (CS) was evaluated, investigating the possibility for a concurrent hydrogen production. CS was characterized extensively by XRD, XRF, SEM–EDS, UV–Vis and electrochemical techniques to evaluate its properties as a photocatalyst. The slag consists of magnetite, fayalite and a silicate glass phase. CS showed a band gap of 2.75 eV, which is in the values range reported for fayalite, an n-type semiconductor with a clearly defined surface state. The band edge scheme for CS shows energy levels within the range required to perform the photocatalytic water reduction reaction, as well as the oxidation of the studied alcohols and their by-products. Experiments using UV and simulated solar light show that the efficiency of CS is higher in the visible range and that hydrogen production increases in the order methanol > propanol > isoamyl alcohol. An apparent quantum yield for methanol degradation of 40% under simulated solar light was obtained. This provides the prospect for a low cost and potential efficient photocatalyst for the oxidation of organic pollutants in industrial wastewater under solar radiation, with the simultaneous hydrogen generation.

Promoting the charge separation and photoelectrocatalytic water reduction kinetics of Cu2O nanowires via decorating dual-cocatalysts

Developing high activity and low-cost materials to produce hydrogen by the sustainable way of photoelectrochemical is key to social development. The abundance and inexpensive Cu2O has been received increasing research as its suitable energy level for photocatalytic water reduction. However, the fast charge recombination rate and the sluggish catalytic kinetics are the huge challenges facing the Cu2O photoreduction. Here, the highly reactive Cu2O@C-MoS2 photocathode is constructed by depositing dual-cocatalysts of the carbon layer and MoS2 nanosheets on Cu2O nanowires to realize efficient water reduction. An impressive carrier concentration of 6.59 × 1023 cm−3 is received, which is 2.78 times of the bare Cu2O, resulting in remarkable enhancement in photocurrent density of 3.34 times for the Cu2O@C-MoS2 photocathode. Moreover, the applied bias photon-to-current conversion efficiency of the bare Cu2O enhanced 4.5 times from 0.16 % to 0.72 % in the Cu2O@C-MoS2 photocathode. The analysis shows that the Cu2O as light absorber, the carbon layer as electron transfer promoter, and MoS2 nanosheets as catalytic sites, thus facilitating chrage separation and enhancing catalytic kinetics. This system paves a feasible strategy for designing other photoelectrodes to realize efficient charge separation and high catalytic activity.

Performance improvement strategies of CuWO4 photocatalyst for hydrogen generation and pollutant degradation

Abstract Recently, photocatalysis has attracted all-embracing attention as a sustainable and green approach owing to its promising applications in the deterioration of contaminants along with the transformation of solar energy into chemical energy. An efficient visible-light harnessing Copper tungstate photocatalyst has hooked excellent performance because of its peculiar features, including eco-friendliness, low cost, optimum band edge potentials, as well as stability under oxidative and acidic conditions. However, any review depicting the correlation between different morphologies, properties, and performance of CuWO4 is still absent. Therefore, this review highlighted the energy applications of CuWO4 photocatalyst, focusing on its morphological dimensions and strategies for photocatalytic performance enhancement. Various fabrication methods, such as hydrothermal, electrodeposition, spray-pyrolysis, co-precipitation, sonochemistry, and solid-state synthesis, are also discussed. Controllable synthesis of distinct morphological nanostructures with the effect of parameters like temperature, pH, solvent, reaction time, and concentration of precursors has been briefly summarized. Nano-structured CuWO4 was explored to show high photocatalytic activity than pristine CuWO4. Moreover, different revamp strategies, including doping of parent photocatalyst, loading of co-catalyst on photocatalyst surface, and heterojunction construction, are assumed for the efficient space charge isolation of photo charge carriers suppressing the rapid recombination of photo-generated electron-hole pairs for dynamic photocatalytic activity. The applications of CuWO4, including photocatalytic water reduction for H2 generation, photodegradation of dyes and antibiotics, and photoelectrochemical water splitting, have been highlighted. Finally, a brief, conclusive remark regarding present studies and unresolved challenges of nano-structured CuWO4 are put forth for future perspectives.

How to make use of methanol in green catalytic hydrogen production?

AbstractHydrogen production and storage as the core of novel hydrogen economy attract extensive attentions in both industry and academia. Methanol‐involved hydrogen production has been considered as an alternative strategy towards conventional methane stream reforming owing to its milder reaction condition and lower carbon footprint. Currently, it mainly includes three branches: (1) thermocatalytic methanol reforming, (2) photocatalytic methanol reforming, and (3) photocatalytic water reduction using methanol as a sacrificial agent. These branches with many similarities, however, are relatively independent in their development stages and research focuses, which seriously restricted the fundamental understanding of using methanol as both hydrogen carrier and catalytic accelerant in hydrogen production reactions. We summarize latest research progresses by expounding their respective advantages, common and individual problems, as well as methods and experiences being developed. The role of methanol and its feasibility in photocatalytic hydrogen production were also discussed from both reaction mechanism and system cost‐efficiency perspectives. Finally, frontiers of the three hydrogen production approaches are prospected, providing overall guidance for the potential application of economic and environmental sustainable methanol‐mediated hydrogen energy.

Single step combustion synthesis of novel Fe2TiO5/α-Fe2O3/TiO2 ternary photocatalyst with combined double type-II cascade charge migration processes and efficient photocatalytic activity

Heterogeneous photocatalytic route is an attractive approach to mitigate environmental and energy crisis. Herein, a new ternary Fe2TiO5/α-Fe2O3/TiO2 nanocomposite photocatalyst was fabricated by using a single-step combustion synthesis approach. Structures of the nanocomposite were characterized with various analytical techniques. The used fuels and calcination temperatures during the combustion synthesis processes were found to strongly affect the phase and morphology of the nanocomposite. The ternary photocatalyst displays significant visible light absorption, improved charge carrier separation and excellent photocatalytic activity under simulated solar light illumination in photocatalytic methylene blue degradation and water reduction. The OH and O2− radicals were identified spectroscopically as the reactive radicals. A double type-II and cascade mode of charge migration across different components in the ternary Fe2TiO5/α-Fe2O3/TiO2 photocatalyst was proposed. This study provides a rational and scalable single step synthesis of ternary composite material with excellent stability and simulated solar light induced photocatalytic activity.