Photocatalytic Redox Reactions Research Articles

2D/0D Bi2MoO6 nanosheets/BN quantum dots photocatalysts with enhanced charge separation for efficient elimination of antibiotic

Photocatalytic activity is mainly limited by recombination of photogenerated carriers and insufficient active sites, which would restrain the efficiency of photocatalytic redox reactions. Herein, 2D/0D heterojunctions of Bi2MoO6 nanosheets/BN quantum dots (BM/BNQDs) were prepared via a simple hydrothermal method, which exhibited enhanced photocatalytic performance for the tetracycline (TC) degradation under visible-light irradiation. Combing small-size BNQDs in the average diameter of ~ 5 nm with BM nanosheets displayed tremendous advantages such as rapid charge transport and sufficient active sites. Notably, the as-synthesized BNQDs acting as the photoinduced hole extractors effectively dispersed photogenerated holes and electrons, facilitating the apparent rate constant exhibited by the BM/2BNQDs catalyst 3.1 times that of pure BM. Moreover, PL and electrochemical test results indicated that the separation and transfer of photogenerated charge carriers were enhanced after introducing BNQDs in BM nanosheets. And this work is expected to provide design ideas and insights for the development of efficient photocatalysts based on 2D/0D nanocomposites.

Robust Co9S8@CdIn2S4 Cage for Efficient Photocatalytic H2 Evolution

We construct here a highly active and stable Co9S8@CdIn2S4 heterostructure for photocatalytic H2 evolution under visible light. In a hierarchical hollow construction with closely connected heterogeneous shells, two photoactive sulfide semiconductors are incorporated rationally. This unique architecture can efficiently allow photoinduced charges to be separated and transferred and provide a broad surface area and exposure of rich sites for photocatalytic redox reactions. The Co9S8@CdIn2S4 hierarchical hollow heterostructure shows enhanced light-harvesting ability and remarkable catalytic activity because of its distinctive structural and compositional advantages. The photocatalyst shows a high hydrogen evolution rate of 4604 μmol h–1 g–1 without any noble metal as a cocatalyst under 1.5 AM simulated light irradiation. In this work, using the two-dimensional ultrathin subunits with distinct structural and compositional benefits, we show how this material can be interfaced successfully to give intriguing photophysical properties.

Synergetic effects of Bi5+ and oxygen vacancies in Bismuth(V)-rich Bi4O7 nanosheets for enhanced near-infrared light driven photocatalysis

The appropriate energy level position of photocatalysts dominates the photocatalytic redox reaction and utilization efficiency of solar energy for wastewater treatment. Herein, we report a near-infrared (NIR) light driven Bi5+-rich Bi4O7 photocatalyst, achieving a greatly enhanced photocatalytic activity for pollutant removal compared with Bi3+-replenished Bi2O3. Density functional theory calculations show the formation of an intermediate band in the Bi4O7 structure because of the hybridization of O 2p and Bi 4s orbits. The formation of the intermediate band not only narrows the band gap but also improves the optical absorption property and separation efficiency of the photoinduced carriers. The existence of the oxygen vacancies (OVs) in the Bi4O7 nanosheets ensures high carriers’ concentration, which is verified by the Hall effect test. The synergetic effects of the OVs and Bi5+ greatly accelerate the separation efficiency of the photogenerated carriers. Consequently, the Bi4O7 nanosheets exhibit enhanced NIR light driven photocatalytic activity for the degradation of rhodamine B and ciprofloxacin compared with the bulk Bi2O3. This study paves the way to the design of highly efficient NIR light-responsive Bi-based photocatalysts for environmental purification.

Quantitative Single-Particle Fluorescence Imaging Elucidates Semiconductor Shell Influence on Ag@TiO2 Photocatalysis.

The understanding of the structure-reactivity relationship is helpful for the nanocatalyst (NC) design. However, though precisely parse, this information is challenging due to the heterogeneity of NCs and the complex mechanism of energetic charge carrier (e-/h+ pairs) generation and transfer within the catalysts upon light irradiation. Here, the effect of the semiconductor shell on the photocatalytic redox reaction is probed at the single-Ag@TiO2 NC level with single-molecule imaging. By engineering the TiO2 shell thickness, catalytic activities of the NCs are precisely controlled and quantitatively measured to show a parabolic-like distribution with increasing TiO2 thickness. Besides, the varied activity among different NCs and the dynamic activity fluctuation of single NCs during continuous redox conversion are observed. Mathematical analysis indicates that the TiO2 layer affects the activity of the core-shell NCs by simultaneously affecting the fate of photo-induced e-/h+ pairs and hot electrons generated at the Ag core. This work sheds light on molecular-scale elucidation of the impact of metal-semiconductor NC structures on their reactivities.

Molecule assembly of heterostructured TiO2@BiOCl via fenton-like reaction for enhanced solar energy conversion

In this work, we applied the fenton-like reaction to fabricate TiO2 nanocrystalline, and employed molecular self-assembly with TiO2 and BiOCl to form composite photocatalyst with cookie-like nano-heterostructure for enhanced solar energy to hydrogen. The TiO2@BiOCl photocatalyst with cookie-like nano-heterostructure exhibited excellent photocatalytic performance for the photocatalytic hydrogen evolution under sunlight irradiation compared to the single TiO2 or BiOCl. Besides, the TiO2@BiOCl photocatalyst displayed outstanding reusability and stability in the photocatalytic reaction. Meanwhile, the possible mechanisms of cookie-like nano-heterostructure formation and photocatalytic redox reaction have been proposed. What's more, this work demonstrated a new perspective for the design and mass preparation of TiO2-based composite photocatalytic materials at atmospheric condition, which demonstrated that this method has enormous potential to achieve mass preparation of efficient photocatalyst for photocatalytic application.

Anatase TiO2 with Co-exposed (001) and (101) Surface-Based Photocatalytic Materials for Energy Conversion and Environmental Purification.

Anatase TiO2 with co-exposed (001) and (101) surfaces [(001)-(101)-TiO2 ], as a semiconductor photocatalyst in crystal plane control engineering, has become a research hotspot in environmental purification and energy conversion due to its strong physicochemical stability, non-toxic and harmless, and low production cost. This review briefly introduces the basic principles and influencing factors of [(001)-(101)-TiO2 ]. On this basis, the effect of heterostructures formed by different materials and modification methods on its photocatalytic activity are elaborated in detail. Mainly formed heterostructures mentioned in this review include oxygen vacancy/Ti3+ -[(001)-(101)-TiO2 ] heterostructures, noble metal-[(001)-(101)-TiO2 ] heterostructures, metal sulfide-[(001)-(101)-TiO2 ] heterostructures, metal oxide-[(001)-(101)-TiO2 ] heterostructures and carbon material-[(001)-(101)-TiO2 ] heterostructures. The light absorption range and charge separation mechanism of (001)-(101)-TiO2 after modification are discussed. Moreover, the application of photocatalytic redox reaction in simulating photosynthesis to prepare new energy (hydrogen evolution and CO2 reduction), environmental purification and sterilization is introduced in detail. Finally, various measures of designing (001)-(101)-TiO2 nanostructures for further applications in energy production and environmental remediation are discussed.

Controlling Plasmon-Induced Photocatalytic Redox Reactions on WO3 Nanowire/AgNPs Substrates via Defect Engineering

Using surface plasmons as a catalyst for surface reactions has been of great interest in recent years. Local surface plasmon resonance excitation has been shown to accelerate the rate of chemical r...

Pyridinic N–reduced graphene oxide and ZnO composite synergistically enhance photocatalytic performance

This paper reports the synthesis of nitrogen-doped reduced graphene oxide (N-rGO)/zinc oxide (ZnO) composite photocatalysts by one-step hydrothermal method. Taking urea as the nitrogen source, ammonia is released at high temperature, and reduced graphene oxide (rGO) has a strong adsorption effect on ammonia, which is more conducive to improving the nitrogen doping efficiency in the nitrogen doping process. The results of the dye degradation test showed that when the nitrogen content was 4 wt%, the photocatalytic degradation ability of the catalyst was the best, which was 88% higher than that of pure zinc oxide. The X-ray photoelectron spectroscopy (XPS) test showed that the catalytic effect of the sample dominated by pyridinic nitrogen was significantly improved. Besides, fluorescence tests have confirmed that highly conductive rGO can simultaneously capture or transfer photoelectrons within the system, thereby enhancing the photoelectron migration and reducing the private carrier recombination efficiency. N-rGO could act as an active electron accelerator, induce and enhance synergistic coupling, promote photocatalytic redox reactions, and dramatically improve the photocatalytic ability of modified graphene-based composite.

Protoenzymes: The Case of Hyperbranched Polymer-Scaffolded ZnS Nanocrystals.

Enzymes are biological catalysts that are comprised of small-molecule, metal, or cluster catalysts augmented by biopolymeric scaffolds. It is conceivable that early in chemical evolution, ancestral enzymes opted for simpler, easier to assemble scaffolds. Herein, we describe such possible protoenzymes: hyperbranched polymer-scaffolded metal-sulfide nanocrystals. Hyperbranched polyethyleneimine (HyPEI) and glycerol citrate polymer-supported ZnS nanocrystals (NCs) are formed in a simple process. Transmission electron microscopy (TEM) analyses of HyPEI-supported NCs reveal spherical particles with an average size of 10 nm that undergo only a modest aggregation over a 14-day incubation. The polymer-supported ZnS NCs are shown to possess a high photocatalytic activity in an eosin B photodegradation assay, making them an attractive model for the study of the origin of life under the “Zn world” theory dominated by a photocatalytic proto-metabolic redox reaction network. The catalyst, however, could be easily adapted to apply broadly to different protoenzymatic systems.

Simultaneous photocatalytic degradation of ibuprofen and H2 evolution over Au/sheaf-like TiO2 mesocrystals

Considerable effort has been devoted to the efficient degradation of pharmaceuticals and personal care products (PPCPs), while the chemical energy in these processes has been widely overlooked. In this study, we demonstrated the simultaneous hydrogen production and ibuprofen degradation through heterogeneous photocatalysis. By anchoring Au nanoparticles (NPs) on the (101) surface of sheaf-like TiO2 mesocrystals with [001] orientation, efficient charge separation is achieved, which is essential for the photocatalytic redox reactions. XPS analysis showed that the binding energies of Ti 2p and O 1s indicated no shift after Au addition. Peaks observed at 81.8 and 85.5 eV due to Au 4f7/2 and Au 4f5/2 of metallic gold on the surface of Au/meso-TiO2, confirmed the formation of Au NPs. The as-synthesized anatase TiO2 mesocrystals are composed of small nanocrystals with a size of 8 nm and exhibit the uniform sheaf-like morphology along [001] orientation. As expected, the 1 wt% Au/TiO2 mesocrystals shows the largest photocurrent density, highest H2-evolution rate, and fastest photodegradation rate of ibuprofen under simulated sunlight irradiation among all the studied catalyst. Furthermore, the effect of solution pH, common anions (Cl−, NO3−, and SO42−) and cations (Na+, K+, and Ca2+) on photocatalytic H2 evolution and degradation of ibuprofen were individually investigated and discussed. A mechanism for the simultaneous photocatalytic hydrogen generation and degradation of ibuprofen has also been proposed. This work opens up new opportunities for the development of energy efficient techniques for PPCPs degradation.

Boosting Visible-Light Photocatalytic Redox Reaction by Charge Separation in SnO2 /ZnSe(N2 H4 )0.5 Heterojunction Nanocatalysts.

In this work, environmentally friendly photocatalysts with attractive catalytic properties are reported that have been prepared by introducing SnO2 quantum dots (QDs) directly onto ZnSe(N2H4)0.5 substrates to induce advantageous charge separation. The SnO2/ZnSe(N2H4)0.5 nanocomposites could be easily synthesized through a one‐pot hydrothermal process. Owing to the absence of capping ligands, the attached SnO2 QDs displayed superior photocatalytic properties, generating many exposed reactive surfaces. Moreover, the addition of a specified amount of SnO2 boosted the visible‐light photocatalytic activity; however, the presence of excess SnO2 QDs in the substrate resulted in aggregation and deteriorated the performance. The spectroscopic data revealed that the SnO2 QDs act as a photocatalytic mediator and enhance the charge separation within the type II band alignment system of the SnO2/ZnSe(N2H4)0.5 heterojunction photocatalysts. The separated charges in the heterojunction nanocomposites promote radical generation and react with pollutants, resulting in enhanced photocatalytic performance.

UiO-66-NH2 Metal-Organic Frameworks with Embedded MoS2 Nanoflakes for Visible-Light-Mediated H2 and O2 Evolution.

Hydrogen evolution from water splitting by means of a photocatalytic approach is an ideal future energy source and free of fossil reserves, in contrary photocatalytic O2 evolution remains a bottleneck due to high over potential and low efficiency. For reasonable use of solar light, photocatalysts must be sufficiently stable and efficient toward harvesting of sunlight from both theoretical and practical viewpoints. In this regard, here we have prepared MoS2-modified UiO-66-NH2 MOF through a facile hydrothermal technique and evaluated its efficiency toward photocatalytic H2 and O2 evolution by water splitting in the presence of sacrificial agents. A couple of similar type of analyses have been studied previously; however, this analysis represents a diverse scientific approach on the basis of interfacial contact toward reveal the actual potential of nanoflakes MoS2 as well as UiO-66-NH2. In this regard the as-synthesized photocatalyst was well-characterized by XRD, FTIR, UV-vis diffuse reflectance spectra, FESEM, HRTEM, XPS, and BET analysis techniques, which provide sufficient evidence toward successful synthesis of the pristine materials and efficacious anchorage of MoS2 on the active surface of UiO-66-NH2 by the ionic interaction between Zr-O and S/Mo. Among the synthesized photocatalysts (3 wt %) MoS2/UiO-66-NH2 shows the optimum outcome toward H2 and O2 evolution, i.e., 512.9 μmol/h (4.37 times greater than bare UiO-66-NH2) and 263.6 μmol/h (4.25 and 11.32 times greater than bare UiO-66-NH2 and MoS2, respectively). The superior performance obtained by the composite is due to the synergistic effect of pristine UiO-66-NH2 and MoS2 which proceeds through a type-II interband alignment for the facile channelization of excitons. This investigation will bestow a beneficial blue-print to construct challenging photocatalysts and to find out the paramount performance toward photocatalytic water redox reaction.

CdS nanoparticles modified Ni@NiO spheres as photocatalyst for oxygen production in water oxidation system and hydrogen production in water reduction system

The exploration of bifunctional catalysts to produce both hydrogen and oxygen will accelerate the progress of overall water-splitting. This paper introduces a facile method to fabricate bifunctional photocatalysts toward the water splitting by doping nano CdS with S vacancies on Ni@NiO nano-spheres. A serious of photocatalytic catalysts were developed and subsequently characterized, the two half-reactions of photocatalytic water splitting were studied in detail by using sacrificial agents, the hydrogen output at 41699 μmol g−1 h−1 (AQE 28.5%, λ = 420 nm) while the evolution of O2 is as high as 55120 μmol g−1 h−1 (AQE 35.7%, λ = 450 nm). It is a strategy that the presence of Ni@NiO can trigger the S vacancy in CdS to act as an effective electron trap, the p-n junction of NiO-CdS, as well as the Schottky barrier between NiO and Ni, can change the dynamic characteristics of photogenerated electrons and prolong their life. These prolonged photogenerated electrons can initiate stable and effective photocatalytic redox reactions.

Visible-light-driven integrated organic synthesis and hydrogen evolution over 1D/2D CdS-Ti3C2Tx MXene composites

Exploiting solar energy to photocatalyze integrated selective conversion of bioethanol into fine chemicals and hydrogen (H2) is a promising avenue in response to current energy and environmental crises. Herein, we have reported the mild synthesis of a binary heterostructure of CdS-Ti3C2Tx MXene (CdS-MX) combining one-dimensional (1D) CdS nanowires (NWs) with two-dimensional (2D) MXene structure, and its application in photocatalytic coupling redox reaction of H2 evolution and selective conversion of bioethanol into 1,1-diethoxyethane (DEE) under acidic conditions. The results reveal that the synergistic effect of the intimate interfacial contact and matched energy level alignment between electrically conductive 2D MXene and semiconductor 1D CdS NWs benefits the separation and migration of charge carriers. Furthermore, CH(OH)CH3 has been proven to be the pivotal radical intermediate during photoredox process. This work is anticipated to stimulate further interest on rational construction of MXene-semiconductor based composites toward photoredox coupling organic synthesis and H2 evolution in a sustainable manner.

Photocatalytic redox reactions with metalloporphyrins

Metalloporphyrinoids are utilized as efficient sensitizers and catalysts in photosynthesis and the reverse reaction that is respiration. Because metalloporphyrinoids show strong absorption in the visible region and redox active, metalloporphyrinoids are also suited as photoredox catalysts for photo-driven redox reactions using solar energy. In particular, metalloporphyrins are utilized as pivotal components to mimic the structure and function of the photosynthetic reaction center. Metalloporphyrins are used as photoredox catalysts for hydrogen evolution from electron and proton sources combining hydrogen evolution catalysts. Metalloporphyrins also act as thermal redox catalysts for photocatalytic reduction of CO2 with photoredox catalysts. Metalloporphyrins are also used as dual catalysts for a photoredox catalyst for oxygenation of substrates with H2O and a redox catalyst for O2 reduction when dioxygen is used as a two-electron oxidant and H2O as an oxygen source, both of which are the greenest reactants. Free base porphyrins can also be employed as promising photoredox catalysts for C–C bond formation reactions.

Fabrication and characterization of ZnTiO3/Zn2Ti3O8/ZnO ternary photocatalyst for synergetic removal of aqueous organic pollutants and Cr(VI) ions

Highly efficient photocatalysts have great development prospects in wastewater treatment, especially in the degradation of organic pollutants and reduction of inorganic heavy metal ions. Herein, a Z-scheme ZnTiO3/Zn2Ti3O8/ZnO ternary photocatalyst was prepared by the solvothermal-calcination method and the influence of the content of tetrabutyl titanate precursor and different reaction temperature on the crystal phase structures, photoelectrochemical properties and photocatalytic activities of the samples were investigated. Due to its unique Z-scheme structure and suitable band gap position, which is favorable for the efficient migration and separation of photo-generated electrons and holes and the improvement of photocatalytic redox reaction capability, the samples show excellent performance for the degradation of organic pollutants and reduction of heavy metal Cr(VI) ions. Based on a series of characterization analyses, a possible Z-scheme photocatalytic mechanism is proposed. This work provides a simple preparation method for fabrication of multivariate heterojunction photocatalyst for degradation of organic pollutants and removal of heavy metal ions.

Construction of two-dimensionally relative p-n heterojunction for efficient photocatalytic redox reactions under visible light

Precisely modulating the interfacial contact of heterojunction photocatalysts and regulating the spatial transfer of photoexcited charge carriers are the core factors to promote the photocatalytic efficiency for energy and environmental applications. Herein, novel 2D/2D relative p-n heterojunction CdS/g-C3N4 with Z-scheme (or S-scheme) was fabricated by in-situ growing ultrasmall 2D CdS nanoflakes on the surface of 2D g-C3N4 nanosheets. The optimized 2D/2D heterojunction structure with proper band configuration and suitable heterojunction interface can improve photoexcited charge carriers separation spatially and transfer to the ideal reaction sites. Therefore, 2D/2D CdS/g-C3N4 composites show high photocatalytic reduction (H2 evolution) and oxidation (selective oxidation of 5-hydroxymethylfurfural into 2,5-diformylfuran) activities, which are about 26.8 times and 3.51 times higher than individual g-C3N4 and are about 4.4 times and 2.39 times higher than individual CdS, respectively. The mechanisms including direction and inner impetus of the photoexcited charge separation and transfer in the composite photocatalyst are proposed and verified by experimental and computational analyses. This investigation highlights the promising scope to intelligently regulate the spatial separation and transfer of photoexcited charge carriers and rationally construct new relative p-n junction or Z-scheme photocatalysts for energy and environmental applications.

Exciton Channel of Photoactivation for Redox Reactions on the Surface of 2D ZnO Nanostructures

The resonance excitation of excitons was used for the first time to activate photocatalytic redox reactions on ZnO. The sample activity was tested using the photoactivated oxygen isotope exchange (POIE), which can be used to predict both oxidation and reduction reactions. When the excitation energy is transferred to the surface by an electrically neutral exciton, then the main losses in transferring the photogenerated e–/h+ pairs due to recombination and to overcoming the surface potential barrier are strongly reduced. The radiative decay of an exciton on the ZnO surface is excluded by a 2D structure ZnO/ZnO1–x/O–, in which the exciton decays nonradiatively into a pair of long-lived (up to 8 × 103 s) electron and hole local states, on which in its turn a chemical reaction can take place. These centers decay at 450 K, emitting green luminescence. The exciton resonance excitation allows one to obtain the order-of-magnitude higher values of the efficiency of redox reaction compared to the case of interband excitation.

Designing CdS-Based Ternary Heterostructures Consisting of Co-Metal and CoOx Cocatalysts for Photocatalytic H2 Evolution under Visible Light.

Heterostructure formation is an effective method used for designing photocatalysts that solve problems caused by photoexcited charge recombination phenomena associated with the photocatalytic water redox reaction. This work reports a new Co-metal-incorporated ternary heterostructured photocatalyst, CdS/CoOx/Co-metal, which enhanced charge separation to increase photocatalytic H2 evolution 30.5-fold in comparison to pure CdS under visible light. This work demonstrates for the first time the effect of the Co metal on photocatalytic H2 evolution using the CdS/CoOx/Co-metal ternary heterostructure. In the ternary heterostructure, Co metal and CoOx act as photogenerated electron- and hole-capturing cocatalysts, respectively. Results from photoelectrochemical studies along with photocatalytic H2 evolution data proved the enhancement of charge transfer and separation in the CdS/CoOx/Co-metal heterostructure due to the addition of Co metal and CoOx. Hence, the synergistic charge separation improvement achieved by the combination of CoOx and the Co metal with CdS produced a photocatalytic H2 evolution rate of 9.54 μmol/h, which is the highest reported H2 evolution rate for a CdS-based system under l sun solar irradiance (>420 nm) to the best of our knowledge.

The significant effect of photo-catalyzed redox reactions on the immobilization of chromium by hematite

Naturally occurring semiconducting metal (oxyhydr)oxides (MeOx) are ubiquitous geosorbents and geocatalysts, regulating the geochemical behaviors of the chemicals in the environment. The interactions between these MeOx and redox-sensitive elements may involve both the photo-catalytic redox reactions and surface adsorption, and the interplay of the two processes, therefore, makes the interactions rather complex and may have unique environmental effects. The complex interactions between Cr (Cr(III) or Cr(VI)) and hematite under simulated sunlight irradiation were systematically studied under different geochemical conditions (pH, light irradiation, beginning Cr species, etc.). The batch experimentals, X-ray photoelectron spectroscopy (XPS), transmitted electron microscopy (TEM), and energy-dispersive spectroscopy (EDS) were used to determine the concentration and distribution of Cr species (Cr(III), Cr(VI), or the sum of the two) in the aqueous and solid phases. Here we found that both the oxidation of Cr(III) and the reduction of Cr(VI) were accelerated by hematite under sunlight irradiation, resulting in the enhanced immobilization of Cr on hematite. The reduction and oxidation efficiencies were pH-dependent and more evident under acidic condition. In the Cr (III) reaction systems, the reactions can be characterized by (1) the adsorption of aqueous Cr(III) (Cr(III)aq) to hematite surfaces, (2) the oxidation of adsorbed Cr(III) (Cr(III)ad) to Cr(VI)ad by the photogenerated holes on hematite, i.e., heterogeneous oxidation, (Bargar et al., 1997) the accumulation of Cr(III)ad on hematite enhanced by Cr(VI)ad, and even the formation of Cr-riched precipitation, which was observed by TEM, and (3) the homogeneous oxidation of Cr(III)aq to Cr(VI)aq by the hydroxyl radicals, generating through interaction between O2 and the photogenerated electrons (more evident under acidic environment, e.g., pH 3). In the Cr(VI) reaction systems, the reduction of Cr(VI)ad by photogenerated electrons from hematite was also noticeable, with the yielded Cr(III) being fixed on the surfaces. Although the reactions in reduction systems were less extensive than those in the oxidation systems, the reduction reactions would reduce the environmental toxicity of Cr species; while in the oxidation systems, inspite of the higher removal efficiency, the oxidation processes actually increased the environmental risk of Cr. Our results clearly show that hematite could either oxidize Cr(III) or reduce Cr(VI), influencing the speciation and immobilization of Cr under sunlight irradiation. In this term, the natural semiconducting MeOx (e.g., hematite) on Earth's surfaces should play a significant role in affecting the environmental behaviors and toxicities of the redox-sensitive elements (e.g., Cr) through adsorption and photo-catalytic redox processes.