Efficient Charge Carrier Transfer Research Articles

Facile strategy to fabricate Ni2P/g-C3N4 heterojunction with excellent photocatalytic hydrogen evolution activity

Transition metal phosphides are considered as the most prospective replacements for noble metal cocatalysts used for H2 evolution during photocatalytic water splitting. In this work, Ni2P/g-C3N4 composite photocatalyst was synthesized using a simple in-situ hydrothermal method by one step. Benefiting from the excellent light trapping, efficient transfer of charge carriers and strong stability of Ni2P nanoparticles, as well as the stable interface contact between Ni2P and g-C3N4, the Ni2P/g-C3N4 exhibit greatly enhanced H2 evolution performance during photocatalytic water splitting. The optimized H2 evolution rate can reach 3344 μmol h−1 g−1 over 17.5 wt% Ni2P/g-C3N4, which is 68.2 times greater than that of pure g-C3N4 and even much greater than that of 15 wt% Pt/g-C3N4. The apparent quantum efficiency (QE) is about 9.1% under 420 nm monochromatic. The enhancement mechanism was demonstrated in detail by transient photocurrent responses, photoluminescence spectra and electrochemical impedance spectroscopy. This work develops a facile strategy to fabricate transition metal phosphide/semiconductor heterojunction systems with potential application for photocatalytic H2 evolution.

MoS2/SnNb2O6 2D/2D nanosheet heterojunctions with enhanced interfacial charge separation for boosting photocatalytic hydrogen evolution

Semiconductor-based photocatalytic hydrogen evolution through water splitting is of great importance for tacking the ever-increasing energy concerns. Here, novel visible-light-driven MoS2/SnNb2O6 2D/2D nanosheet heterojunction for photocatalytic hydrogen evolution was designed and fabricated by a facile hydrothermal method. The as-prepared 2D/2D heterojunctions not only offer extended visible light absorption range but also accelerate the separation and transfer efficiency of charge carriers owing to the typical 2D interface. Therefore, the MoS2/SnNb2O6 2D/2D heterojunctions showed superior H2 evolution photocatalytic performance compared with bare MoS2 and SnNb2O6 nanosheet. The optimal H2 production rate of as-prepared MoS2/SnNb2O6 heterojunction was up to 4.3-fold and 5-fold as high as that of bare SnNb2O6 and MoS2. This works will provide a new perspective on the development of 2D/2D nanosheet heterojunction photocatalysts with high performance for water splitting.

Construction of mesoporous g-C3N4/TiO2 nanocrystals with enhanced photonic efficiency

Ease synthesis of mesoporous g-C3N4/TiO2 nanocrystals at various C3N4 contents (1–50 wt%) was performed by mixing tetrabutyl orthotitanate and g-C3N4 throughout sol-gel avenue with existence template. The photooxidation of the herbicide imazapyr was chosen as a probe molecule to explore the photocatalytic performance of the prepared photocatalysts compared to that of the commercially P-25 along with g-C3N4. TEM images exhibited that TiO2 nanoparticles have very small particle size in the range of 5–10 nm and g-C3N4 sheet-like nanoparticles possess characteristic for graphitic carbon nitride. The most efficient photocatalyst toward all the prepared samples was 10% g-C3N4/TiO2 nanocrystals. The photonic efficiency of 10% g-C3N4/TiO2 nanocrystals is 100 and 3 times greater than pure g-C3N4 and P-25, respectively. The construction of mesoporous TiO2 and g-C3N4 with heterogeneity produces a synergistic effect that provides an efficient charge carriers transfer. The highest photocatalytic efficiency can be explained by large surface area, narrow band gap energy, small particles size, and efficient charge carriers’ separation.

Nanocrystalline Boron-Doped Diamond as a Corrosion-Resistant Anode for Water Oxidation via Si Photoelectrodes.

Due to its high sensitivity to corrosion, the use of Si in direct photoelectrochemical (PEC) water-splitting systems that convert solar energy into chemical fuels has been greatly limited. Therefore, the development of low-cost materials resistant to corrosion under oxidizing conditions is an important goal toward a suitable protection of otherwise unstable semiconductors used in PEC cells. Here, we report on the development of a protective coating based on thin and electrically conductive nanocrystalline boron-doped diamond (BDD) layers. We found that BDD layers protect the underlying Si photoelectrodes over a wide pH range (1-14) in aqueous electrolyte solutions. A BDD layer maintains an efficient charge carrier transfer from the underlying silicon to the electrolyte solution. Si|BDD photoelectrodes show no sign of performance degradation after a continuous PEC treatment in neutral, acidic, and basic electrolytes. The deposition of a cobalt phosphate (CoPi) oxygen evolution catalyst ontothe BDD layer significantly reduces the overpotential for water oxidation, demonstrating the ability of BDD layers to substitute the transparent conductive oxide coatings, such as indium tin oxide (ITO)and fluorine-doped tin oxide(FTO), frequently used as protective layers in Si photoelectrodes.

Reduced-Dimensional α-CsPbX3 Perovskites for Efficient and Stable Photovoltaics

Summary Inorganic CsPbX3 perovskites have gained great attention owing to their excellent thermal stability and carrier transport properties. However, the power conversion efficiency (PCE) of solution-processed CsPbX3 perovskite solar cells is still far inferior to that of their hybrid analogues. Insufficient film thickness and undesirable phase transition are the two major obstacles limiting their device performance. Here, we show that by adopting a new precursor pair, cesium acetate (CsAc) and hydrogen lead trihalide (HPbX3), we were able to overcome the notorious solubility limitation for Cs precursors to fabricate high-quality CsPbX3 perovskite films with large film thickness (∼500 nm). We further introduced a judicious amount of phenylethylammonium iodide (PEAI) into the system to induce reduced-dimensional perovskite formation. Unprecedentedly, the resulting quasi-2D perovskites significantly suppressed undesirable phase transition and thus reduced the film's trap density. Following this approach, we reported a state-of-the-art PCE to date, 12.4%, for reduced-dimensional α-CsPbI3 perovskite photovoltaics with greatly improved performance longevity.

Unraveling the Growth of Hierarchical Quasi-2D/3D Perovskite and Carrier Dynamics.

The construction of hybrid perovskites (both two-dimensional (2D) and three-dimensional (3D)) has attracted intensive research interest recently. Here, a facile, two-step consecutive deposition method was developed for the first time to grow a hierarchical quasi-2D/3D perovskite superstructure, with an oriented quasi-2D ((BA)2(MA)n-1PbnI3n+1) perovskite nanosheet (NS) perpendicularly aligned on 3D perovskites. The superstructures are found to be mixtures of multiple perovskite phases, with n = 2, 3, 4 and 3D perovskite; however, the n value was naturally increased from top to the bottom, which is distinct from many other works. We found that the concentration gradient, namely, the initial ratio and amount of BAI/MAI, collectively contributes to the spatially confined nucleation and growth of oriented quasi-2D superstructure perovskite on 3D perovskites. An efficient charge carrier transfer was demonstrated from small-n to large-n phases in this perovskite superstructure, indicating a different type of energy funnel from top to bottom.

ZnO1−x/carbon dots composite hollow spheres: Facile aerosol synthesis and superior CO2 photoreduction under UV, visible and near-infrared irradiation

For the first time, ZnO1−x/carbon dots composite hollow spheres (denoted ZnO1−x/C) have been synthesized via a single-step aerosol process and employed for CO2 photoreduction over the whole UV–vis-NIR spectrum. The effects of the precursor component ratio and synthesis temperature on the physicochemical properties of the composites are systematically investigated to maximize CO2 conversion efficiency. Under UV–vis–NIR light, the best performing sample had an average CO production rate of 60.77 μmol g−1 h−1, which is about 54.7 times higher than that of pristine ZnO, and 11.5 times higher than that of commercial TiO2 (Degussa-P25). More importantly, whereas ZnO and Degussa-P25 are photocatalytically inactive, the photocatalytic response of the ZnO1−x/C composite was successfully achieved under NIR illumination alone, with an average CO production rate of 15.98 μmol g−1 h−1. The realization of NIR-driven CO2 photoreduction with enhanced photocatalytic activity benefits from 1) the hollow structure, which allows multiple internal reflections of light for enhanced light absorption; 2) the oxygen deficiency of ZnO1−x and the deposited carbon, which enable efficient charge carrier transfer and improved CO2 adsorption; and 3) the strong NIR absorption of ZnO1−x/C, in which ZnO1−x is excited by absoring the up-converted photoluminescence emissions (410–560 nm) of the carbon dots.

Разработка и исследование фотовольтаических структур на основе слоев квантовых точек PbS с различными лигандами

AbstractThe photoconductivity of condensates of lead-sulfide quantum dots (QDs)—QD solids—with various organic ligands is studied. It is demonstrated that the QD solid photoconductivity increases exponentially with a reduction in length of ligand molecules and does not depend on their chemical structures, since it is governed by hopping transport of charge carriers. In contrast, the photocurrent in photovoltaic ITO/PEDOT: PSS/PbS/ZnO/Al elements depends on the ligand structure, since this structure sets the positions of QD energy levels and thus affects the efficiency of charge carrier transfer to electrodes. The difference between mechanisms of generation of photoconductivity and photovoltaic currents is discussed.

Fe(III) modified BiOCl ultrathin nanosheet towards high-efficient visible-light photocatalyst

To pursue high photocatalytic activity in visible-light region, the Fe(III) modified BiOCl ultrathin nanosheet (Fe(III)@BOC NS) has been firstly synthesized via a facile solvothermal approach. The morphological and compositional characterizations reveal that the thickness of the as-prepared Fe(III)@BOC NS is less than 5 atomic layer with the exposure of active [001] facets. And the Fe(III) doping and surface grafting result in a 0.58eV down-shift of the BiOCl conduction band minimum extending the light absorption from ultraviolet light to visible light region, as well as a promoted interfacial charge transfer upon visible light irradiation. Meanwhile, the Fe(III) modification introduces unique active sites on the surface of ultrathin nanosheet facilitating the surface reaction. Moreover, both of the appropriate self-induced electric field of [001] facets, and the shortened charge carrier diffusion length of ultrathin nanosheet enhance the separation and transfer efficiency of charge carriers. Therefore, by taking those advantages, we experimentally demonstrate that the Fe(III)@BOC NS is a high-efficiency visible-light photocatalyst for both of environment remediation and water splitting.

Fabrication of TiO2/CdS/Ag2S Nano‐Heterostructured Photoanode for Enhancing Photoelectrochemical and Photocatalytic Activity under Visible Light

AbstractWide optical absorption window, long lived and high charge carrier transfer efficiency are the prime characteristics towards the accomplishment of remarkably efficient photocatalysts. In this context, we have synthesized TiO2/CdS/Ag2S core/shell/shell (CSS) heterostructures by using ion‐exchange method. The photoelectrochemical (PEC) measurements unveiled appreciably increased PEC water splitting performance of the CSS heterostructure (used as photoanodes), as indicated by high photocurrent density of ∼ 7.6 mA/cm2 at 1.0 V versus Ag/AgCl and low photocurrent onset potential of ∼ 0.1 V as compared to pristine TiO2 nanorods. Mott‐Schottky analysis evident the p‐n junction features of heterostructures that strengthen the separation of electron and holes. We have investigated the exciton dynamics with photoluminescence (PL) and time correlated single photon counting (TCSPC) studies. The diminished PL intensity and shorten average lifetime (τavg) in CSS heterostructures is a clear manifestation of suppression of charge carrier recombination as well as efficient charge carrier transfer from CdS (shell) to TiO2 (core). Thus, both factors prolonged and efficient transfer of visible light driven charge carriers led to phenomenal photoactivity of TiO2/CdS/Ag2S based CSS heterostructures towards the degradation of toxic organic pollutants and PEC water oxidation. We anticipate that the architecture of CSS hetrostructures will provide a new paradigm in the realm of photocatalysis.

Ferrites boosting photocatalytic hydrogen evolution over graphitic carbon nitride: a case study of (Co, Ni)Fe2O4 modification

The charge carrier separation and surface catalytic redox reactions are of primary importance as elementary steps in photocatalytic hydrogen evolution. In this study, both of these two processes in photocatalytic hydrogen evolution over graphitic carbon nitride (g-C3N4) were greatly promoted with the earth-abundant ferrites (Co, Ni)Fe2O4 modification. CoFe2O4 was further demonstrated to be a better modifier for g-C3N4 as compared to NiFe2O4, due to the more efficient charge carrier transfer as well as superior surface oxidative catalytic activity. When together loading CoFe2O4 and reductive hydrogen production electrocatalyst Pt onto g-C3N4, the obtained Pt/g-C3N4/CoFe2O4 photocatalyst achieved visible-light (λ > 420nm) hydrogen production rate 3.5 times as high as Pt/g-C3N4, with the apparent quantum yield reaching 3.35% at 420nm.

Morphology engineering of WO3/BiVO4heterojunctions for efficient photocatalytic water oxidation

Four types of nanostructural WO3/BiVO4 heterojunctions were designed by depositing a BiVO4 layer onto WO3 scaffolds with different morphologies and investigated to understand the influence of heterojunction morphology on their photoelectrochemical performances. The architecture of WO3 scaffolds was controlled by adjusting the solvents used in their solvothermal synthesis. The solvent was found to exert a great influence on the architecture of the obtained WO3 scaffolds. Methanol leads to the formation of block-like nanostructures and ethanol facilitates the formation of nanorods, while isopropyl alcohol and ethylene glycol give rise to the formation of triangular nanowalls and 3D hierarchical nanostructures, respectively. Monoclinic WO3 nanostructures were obtained via removal of ammonium or structural water by annealing the samples obtained at 510 °C in air. A BiVO4 layer was then deposited on the monoclinic WO3 scaffolds with different morphologies by spin-coating to form WO3/BiVO4 heterojunction films, which show significantly enhanced photocurrents compared to their corresponding bare WO3 scaffolds. Among these WO3/BiVO4 nanostructure heterojunctions, the one based on a rod-like WO3 nanostructure shows the best photoelectrochemical (PEC) performance, which can be ascribed to its optimal rod-like morphology, for the construction of WO3/BiVO4 heterojunction regarding efficient charge carrier transfer and collection, while the other one based on WO3 nanowalls exhibits better stability and a higher photocurrent in a long-run test, indicating that morphology engineering significantly influences the activity and stability of WO3-based heterojunctions.

Au-decorated GaOOH nanorods enhanced the performance of direct methanol fuel cells under light illumination

We reported for the first time that GaOOH nanorods, which were prepared using a facile chemical precipitation method, may display noticeable photocatalytic activities toward methanol oxidation under light illumination, a significant revelation for demonstrating their use as the anode photocatalyst in the half-cell reaction of direct methanol fuel cells (DMFCs). The GaOOH nanorods were further decorated with Au nanoparticles to endow them with increasingly pronounced charge separation, which conduced to a remarkable enhancement in the photocatalytic performance. Time-resolved photoluminescence spectroscopy was employed to depict the charge transfer event across the interface of GaOOH/Au, from which a correlation between photocatalytic efficiency and interfacial charge dynamics was realized. By incorporating GaOOH nanorods into the traditional Pt-catalyzed half-cell reaction, a 12.7% increase in anodic current generation can be attained under light illumination, demonstrating the promising potential of GaOOH as a practical anode photocatalyst in DFMCs. A further enhancement in methanol oxidation current up to 70.8% can be achieved by employing Au-decorated GaOOH nanorods, which was attributed to the efficient charge carrier transfer rendered by the Au decoration. The current study delivers both fundamental and practical importance as it broadens the scope of electrocatalysts for fuel cells, specifically by introducing a new class of highly efficient photocatalysts for promoting electrochemical reactions.

ChemInform Abstract: Artificial Photosynthesis over Graphene—Semiconductor Composites. Are We Getting Better?

AbstractReview: 137 refs.

Ionic liquid self-combustion synthesis of BiOBr/Bi24O31Br10 heterojunctions with exceptional visible-light photocatalytic performances.

Heterostructured BiOBr/Bi24O31Br10 nanocomposites with surface oxygen vacancies are constructed by a facile in situ route of one-step self-combustion of ionic liquids. The compositions can be easily controlled by simply adjusting the fuel ratio of urea and 2-bromoethylamine hydrobromide (BTH). BTH serves not only as a fuel, but also as a complexing agent for ionic liquids and a reactant to supply the Br element. The heterojunctions show remarkable adsorptive ability for both the cationic dye of rhodamine B (RhB) and the anionic dye of methylene orange (MO) at high concentrations, which is attributed to the abundant surface oxygen vacancies. The sample containing 75.2% BiOBr and 24.8% Bi24O31Br10 exhibits the highest photocatalytic activity. Its reaction rate constant is 4.0 and 9.0 times that of pure BiOBr in degrading 50 mg L(-1) of RhB and 30 mg L(-1) of MO under visible-light (λ > 400 nm) irradiation, respectively, which is attributed to the narrow band gap and highly efficient transfer efficiency of charge carriers. Different photocatalytic reaction processes and mechanisms over pure BiOBr and heterojunctions are proposed.

Ruthenium cation substitutional doping for efficient charge carrier transfer in organic/inorganic hybrid solar cells

Solution-processed organic/inorganic hybrid solar cells have emerged as a new platform for low-cost optoelectronics. At the heart of photovoltaic devices lies the matching of a junction, which requires the suitable energy level alignment of n-type and p-type semiconductors. Incorporating foreign ions into bulk semiconductors has been largely employed for many decades, yet electronically active doping in energy level control of the hybrid bulk heterojunctions has been rarely involved and the demonstration of robust functional optoelectronic devices had thus far been elusive. Herein, we introduce Ru ions into TiO2 to decorate the energy level of the acceptor to gain better energy level alignment between the donor and acceptor. By reducing the ‘excess’ energy offset between the n-type and p-type semiconductors, the electron transfer becomes faster, thus leading to a notable enhancement in power conversion efficiency, i.e., from 2.20% to 2.89%. The results demonstrate that the energy level can be controlled effectively by the versatile Ru dopants. This work opens an effective route for accelerating the charge carrier transfer at the interface and achieving high-performance organic/inorganic hybrid optoelectronic devices.

Artificial photosynthesis over graphene-semiconductor composites. Are we getting better?

Tremendous interest is devoted to fabricating numerous graphene (GR)-semiconductor composites toward improved conversion of solar energy, resulting from the observation that the photogenerated electrons from semiconductors (e.g., TiO2, CdS) can be readily accepted or shuttled in the two-dimensional (2D) GR sheet. Yet although the hunt is on for GR-semiconductor composite based photoredox applications that aim to exploit the remarkable electronic conductivity of GR, the work necessary to find out how it could best be harnessed to improve the photocatalytic performance of semiconductors remains scanty. In this review, we highlight a few problems associated with improving the photocatalytic performance of semiconductors via methodological coupling with GR. In particular, we address strategies for harnessing the structure and electronic conductivity of GR via strengthening the interfacial contact, optimizing the electronic conductivity of GR, and spatially optimizing the interfacial charge carrier transfer efficiency. Additionally, we provide a brief overview of assembly methods for fabricating GR-semiconductor composites with controllable film infrastructure to meet the requirements of practical photocatalytic applications. Finally, we propose that, only with the principle of designing and understanding GR-semiconductor composites from a system-level consideration, we might get better at imparting the power of GR with unique and transformative properties into the composite system.

The Crucial Influence of Fullerene Phases on Photogeneration in Organic Bulk Heterojunction Solar Cells

The conjugated polymer, poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (pBTTT‐C16), allows a systematic tuning of the blend morphology by varying the acceptor type and fraction, making it a well‐suited structural model for studying the fundamental processes in organic bulk heterojunction solar cells. To analyze the role of intercalated and pure fullerene domains on charge carrier photogeneration, time delayed collection field (TDCF) measurements and Fourier‐transform photocurrent spectroscopy (FTPS) are performed on pBTTT‐C16:[6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) solar cells with various stoichiometries. A weak influence of excess photon energy on photogeneration along with a photogeneration having a weaker field dependence at increasing fullerene loading is found. The findings are assigned to a dissociation via thermalized charge transfer (CT) states supported by an enhanced electron delocalization along spatially extended PC61BM nanophases that form in addition to a bimolecular crystal (BMC) for PC61BM rich blends. The highly efficient transfer of charge carriers from the BMC into the pure domains are studied further by TDCF measurements performed on non‐intercalated pBTTT‐C16:bisPC61BM blends. They reveal a field dependent charge generation similar to the 1:4 PC61BM blend, demonstrating that the presence of pure acceptor phases is the major driving force for an efficient, field independent CT dissociation.

Solution-growth and optoelectronic properties of ZnO:Cl@ZnS core–shell nanowires with tunable shell thickness

Arrays of vertically aligned ZnO:Cl@ZnS core–shell nanowires (NWs) were grown by a facile low-cost, high-yield and seed-free two-step process. These NWs were used to demonstrate the potential of 3D electrodes based on core–shell heterostructures to enhance charge carrier separation and transfer. With this goal in mind, the photocurrent density of ZnO:Cl@ZnS NWs was characterized as a function of the shell thickness. Although no significant variations in the absorption and photoluminescence spectra were found with the presence of the shell, the photocurrent measured from the core–shell NWs was highly enhanced with respect to bare ZnO:Cl NWs. These photocurrent variations are associated with the control of the band bending in the core–shell NW surface, which modifies the efficiency of charge carrier transfer between the NW and the electrolyte.

Efficient charge carrier transfer from m-LPPP to C 60 derivatives

The improvement of dissociation of singlet excitons by electron or hole capturing species like C 60 is essential for efficient photovoltaic cells based on conjugated systems. We investigate the charge transfer in blends of methyl-substituted ladder type poly-(paraphenylene) (m-LPPP) with new well soluble C 60 derivatives, which are characterized by sidegroups of different electronic properties (donor, acceptor nature). It is shown, that the electronic properties of the C 60 sidegroups strongly influence the efficiency of the charge transfer in these blends and hence significant different behaviours in the photocurrent and photoinduced absorption is observed.