Light-harvesting Semiconductor Research Articles

Hafnium dinitride/poly(9-vinylcarbazole) heterojunction: Structural, electronic, optical and vibrational properties. A DFT and DFPT study

Photosensitivity of semiconducting polymers can be enhanced by blending donor and acceptor polymers to optimize photoinduced charge separation. With a motivation to understand microscopic aspects of HfN2 and PVK relevant to optoelectronic properties of transition metal-dinitride/semiconductor interfaces, we examined their structural, electronic, optical and vibrational properties using first-principles calculations based on density functional theory (DFT) and density functional perturbation theory (DFPT) levels of theory. We computed a large direct energy gap of 3.751 eV for PVK and an indirect narrow band gap of 1.396 eV in HfN2. The gap value computed for the composite structure, HfN2/PVK, was 0.958 eV. The heterojunction displayed an indirect energy gap. The charge transfer between HfN2 and PVK results in a polarized field within the interface region, which will benefit the separation of photogenerated carriers. The calculated density of electronic states, Lowdin charge transfer and charge density difference certify that this proposed layered nanoheterojunction is an excellent light-harvesting semiconductor. These findings indicate that the conjugated polymer PVK is a promising candidate as a non-noble metal co-catalyst for HfN2 photocatalysis. It also provides useful information for understanding the observed enhanced photocatalytic mechanisms in experiments.

(Invited) Carbon Nitride Layers As Light-Harvesting Semiconductors for Photoelectrochemical Cells

Photoelectrochemical cells (PECs) have been developed as environmentally friendly systems that can directly utilize photogenerated electron-hole pairs for water splitting, fuel production, conversion of carbon dioxide, and pollutant degradation. Most reports on the photocatalytic or PEC hydrogen (H2) evolution via water splitting have focused on the H2 reduction half-reaction by generating on the photoanode oxygen or using sacrificial agents to consume the generated h+. Lately, much effort has been invested into synthesizing valuable chemicals on the photoanode while retaining the production of H2 on the cathode.Over the past few years, polymeric carbon nitrides (CN) have attracted widespread attention due to their outstanding electronic properties, which have been exploited in various applications, including photo- and electro-catalysis, heterogeneous catalysis, CO2 reduction, water splitting, light-emitting diodes, and PV cells. CN comprises only carbon and nitrogen, and it can be synthesized by several routes. Its unique and tunable optical, chemical, and catalytic properties, alongside its low price and remarkably high stability to oxidation (up to 500 °C), make it a very attractive material for photoelectrochemical applications. However, few reports regarded CN utilization in PECs due to the difficulty in acquiring a homogenous CN layer on a conductive substrate and our lack of a basic understanding of the intrinsic layer properties of CN.This talk will introduce new approaches to growing CN layers with altered properties on conductive substrates for photoelectrochemical applications. The growth mechanism and their chemical, photophysical, electronic, and charge transfer properties will be discussed. I will show the utilization of PEC with a CN-based photoanode as a stable and efficient platform for the oxidation of water or organic molecules to oxygen or added-value chemicals, respectively, with hydrogen co-production.

ZnO-based Cu metal-organic framework (MOF) nanocomposite for boosting and tuning the photocatalytic degradation performance.

Although metal-organic frameworks (MOFs) are a viable choice for photocatalysts with large surface area and tunable pore structure, the rapid recombination of excited photogenerated charges results in low activity towards photodegradation. Aiming at improving the photocatalytic activities of MOFs, different strategies to incorporate MOF with light-harvesting semiconductors have been developed. In this research, we report an effective photocatalyst designed by incorporating Cu-MOF with ZnO for the photocatalytic degradation of Rose Bengal exhibiting excellent degradation efficiency of 97.4% in 45min under natural sunlight with catalyst dosage of 320mg/L. The optical, morphology and surface characteristics of the prepared nanocomposite were studied using scanning electron microscopy (SEM-EDX), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (PXRD), Brunauer-Emmett-Teller (BET) analysis, thermogravimetric (TGA) analysis, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and ultraviolet diffused reflectance spectroscopy (UV-DRS) techniques. Further studies showed that the degradation followed first-order kinetics with a rate constant of 0.077869min-1. The degradation mechanism was investigated by photoluminescence (PL) study, XPS, zeta potential and quenching experiment in presence of different scavengers. Meanwhile, the fabricated composite displayed good recovery and reuse properties up to 5 cycles as revealed by XRD analysis proving itself a potential MOF-based photocatalyst towards environmental remediation process.

Bandlike Transport and Charge-Carrier Dynamics in BiOI Films.

Following the emergence of lead halide perovskites (LHPs) as materials for efficient solar cells, research has progressed to explore stable, abundant, and nontoxic alternatives. However, the performance of such lead-free perovskite-inspired materials (PIMs) still lags significantly behind that of their LHP counterparts. For bismuth-based PIMs, one significant reason is a frequently observed ultrafast charge-carrier localization (or self-trapping), which imposes a fundamental limit on long-range mobility. Here we report the terahertz (THz) photoconductivity dynamics in thin films of BiOI and demonstrate a lack of such self-trapping, with good charge-carrier mobility, reaching ∼3 cm2 V-1 s-1 at 295 K and increasing gradually to ∼13 cm2 V-1 s-1 at 5 K, indicative of prevailing bandlike transport. Using a combination of transient photoluminescence and THz- and microwave-conductivity spectroscopy, we further investigate charge-carrier recombination processes, revealing charge-specific trapping of electrons at defects in BiOI over nanoseconds and low bimolecular band-to-band recombination. Subject to the development of passivation protocols, BiOI thus emerges as a superior light-harvesting semiconductor among the family of bismuth-based semiconductors.

Homogeneous–Heterogeneous Hybrid Artificial Photosynthesis Induced by Organic Semiconductors with Controlled Surface Architectures

AbstractPhotocatalysis is considered an effective approach for converting CO2 into high‐value‐added chemicals. However, practical implementation of this technology is limited by the efficiency and stability of photocatalysis. Herein, an interfacial control strategy is proposed to optimize the homogeneous‐heterogeneous hybrid photocatalysis by enhancing the interaction between light‐harvesting semiconductors (LHS) and molecular active centers (MAC). Based on this strategy, self‐assembled organic semiconductors with controlled surface architectures are constructed using 1,6‐bis(phenylethynyl)pyrene building blocks to act as LHS. Combining with the classical MAC, an excellent CO2 photoreduction performance is achieved with a CO turnover number of > 2980 maintaining long‐term stability with a selectivity of > 90%, and an apparent quantum yield of > 2.3%. Theoretical calculations combined with in situ and transient spectroscopy studies reveal that the optimized biphase interface dominates the synergy between the homogeneous and heterogeneous photocatalysts. This strategy and the proposed mechanism of interactions will contribute to the design of future artificial photosynthesis systems.

Surface selenation engineering on metal cocatalysts for highly efficient photoreduction of carbon dioxide to methanol

Solar-driven reduction of CO2 and H2O to methanol is highly desirable owing to multiple advantages of green and renewable liquid fuels, whose efficiency and selectivity are restricted by sluggish charge kinetics and unfavorable surface reaction dynamics in conversional semiconductor photocatalysts. This study proposes a surface selenation strategy on metal cocatalysts to improve the photocatalytic performance in CO2-to-CH3OH conversion. Electron microscopy observation shows the formation of a stacked structure with crystalline Rh core sandwiched between semiconductor substrate and amorphous selenated Rh (RhSe) shell after the treatment. The combination of experimental characterizations with theoretical simulations reveals that Rh core smoothens the migration of photoelectrons from light-harvesting semiconductor to RhSe shell, while the selenation of Rh shell not only reduces the rate-limiting barriers required for CH3OH production, but also inhibits the occurrence of side H2 evolution. Enabled by such a design, significantly enhanced photocatalytic activities (41.2 μmol gcat-1 h-1) and selectivties (91.2%) in CH3OH generation are achieved by the Rh@RhSe semi-core-shell cocatalysts, 12.7 and 6.4 times as high as that of pristine Rh, respectively. Moreover, the generalized strategy can be extended to other metal cocatalysts for improved CO2-to-CH3OH transformation, which opens a new avenue for powering the future with liquid sunshine.

Photocatalytic enhancement mechanisms for novel g-C3N4/PVK nanoheterojunction

The interactions between monolayer graphitic carbon nitride (g-C3N4) and conjugated polymer poly(9-vinylcarbazole) (PVK) have been explored. We investigated the enhanced photocatalytic mechanisms for the novel g-C3N4/PVK nanoheterojunction covering the state-of-the-art of DFT by performing rigorous DFT calculations combined with van der Waals corrections (GGA + vdW). The calculated band alignment between g-C3N4 monolayer and PVK monomer clearly reveals that the conduction band minimum and the valence band maximum of g-C3N4 monolayer are higher than those of the conjugated polymer PVK. This predicted band alignment ensures the photogenerated electrons easily migrate from the g-C3N4 monolayer to the PVK monomer, and will lead to high hydrogen-evolution reaction activity. The charge transfer between g-C3N4 monolayer and PVK results in a polarized field within the interface region, which will benefit the separation of photogenerated carriers. The calculated density of electronic states, Lowdin charge transfer and charge density difference certify that this proposed layered nanoheterojunction is an excellent light-harvesting semiconductor. These findings indicate that the conjugated polymer PVK is a promising candidate as a non-noble metal co-catalyst for g-C3N4 photocatalysis. It also provides useful information for understanding the observed enhanced photocatalytic mechanisms in experiments.

Intracellular InP quantum dots facilitate the conversion of carbon dioxide to value-added chemicals in non-photosynthetic bacteria

The semiconductor material with low bio-toxicity and broad light absorption region is crucial for semiconductor-bacteria photosynthetic biohybrid systems. Here we introduce the low-toxic InP/ZnSe/ZnS quantum dots (QDs) into the non-photosynthetic bacteria to produce value-added chemicals. The QDs with ideal biocompatibility act as the light-harvesting semiconductor in the photosynthetic biohybrid systems. The results indicate that the QDs can enter the interior of bacteria and effectively promote the conversion of CO2 to acetate. The electrons generated intracellular present the primary contribution in the conversion process compared with the extracellular electrons transferred through the electron mediator. The total productivity of photosynthesis products can reach ∼ 0.89 mmol·L−1·h−1 with a quantum efficiency (QE) of 6–8%. Assisted with the genetically engineered E.coli, the obtained acetate is then converted into a high-value-added product. The hybrid photosynthetic system that integrated the QDs and non-photosynthetic bacteria exhibits great potential for large-scale solar-to-chemical conversion applications and environmental protection.

Enhanced biomass growth of lactobacillus acidophilus with CdS-NPs under visible light irradiation: Semi-artificial biohybrid system

The bacterial biomass growth in a semi-artificial biohybrid system composed of cadmium sulfide nanoparticles (as a light-harvesting semiconductor) and Lactobacillus acidophilus (as the non-pathogenic electron acceptor bacterium) was investigated. The photo-induced electrons generated from cadmium sulfide nanoparticles within the culture medium efficiently enhanced the biomass growth (about 1.15-fold) and indicated a significant difference (P-value = 0.003 (<0.05)) after 48 h of incubation at 37 °C under white light-emitting diode lamp (12 W) compared to the dark control (i.e. culture grown without nanoparticles and light exposure). Scanning electron microscopy indicated the well-distributed spherical nanoparticles on the smooth surface of the bacterium. Energy-dispersive X-ray spectroscopy revealed the presence of nanoparticles on the bacterium cell surface. The findings of this study revealed that integrating cadmium sulfide nanoparticles with Lactobacillus acidophilus under visible light irradiation had a positive effect on bacterium growth.

Near-infrared-driven photoelectrocatalytic oxidation of urea on La-Ni-based perovskites

The solar-driven photoelectrocatalytic oxidation of urea is an efficient and costly way to produce hydrogen and purify wastewater synergistically. However, it is still a big challenge facing to the semiconductor-catalyst photoanode with high utilization of sunlight. Herein, we report a series ofLa-Ni-based Ruddlesden-Popper phases perovskite materials (Lan+1NinO3n+1, n = 1, 2 and 3) for photoelectrocatalytic oxidations of urea. Such La-Ni -based perovskite materials act simultaneously as a light-harvesting semiconductor and as the electrocatalyst for urea oxidation and all of Lan+1NinO3n+1 catalysts display a strong near-infrared (NIR) light absorption (≥ 800 nm). The La4Ni3O10 catalyst exhibits the best activity towards urea oxidation reaction (UOR) with an applied potential of 1.54 V vs. RHE at 1 mA cm-2, compared to that of 1.64 V and 1.56 V vs. RHE for La2NiO4 and La3Ni2O7, respectively, and 44.7 μmol cm-2h-1 of N2 generation rate under NIR light irradiation at the potential of 1.62 V vs. RHE, which is 2.5 times and 1.8 times of that for La2NiO4 and La3Ni2O7, respectively. The unique structure of La4Ni3O10 accelerates the separation and transport of electrons and holes, which is beneficial for the diffusion and adsorption of urea molecules in photoanode.

Emerging Stacked Photocatalyst Design Enables Spatially Separated Ni(OH)2 Redox Cocatalysts for Overall CO2 Reduction and H2 O Oxidation.

Construction of photocatalytic systems with spatially separated dual cocatalysts is considered as a promising route to modulate charge separation/transfer, promote surface redox reactivities, and prevent unwanted reverse reactions. However, past efforts on the loading of spatially separated double-cocatalysts are limited to hollow structured semiconductors with inner/outer surface and monocrystalline semiconductors with different exposed facets. To overcome this limitation, herein, enabled by a unique stacked photocatalyst design, a facile and versatile strategy for spatial separation of redox cocatalysts on various semiconductors without structural and morphological restriction is demonstrated. The smart design begins with the deposition of light-harvesting semiconductors on reduced graphene oxide (rGO) nanosheets, followed with the coverage of Ni(OH)2 outer layer. The ternary photocatalysts exhibit superior activities and stabilities of H2 O oxidation and selective CO2 -to-CO reduction, remarkably surpassing other counterparts. The origin of the enhanced performance is attributed to the synergistic interplay of rGO@Ni(OH)2 reduction cocatalysts surrounding the semiconductors and Ni(OH)2 oxidation cocatalysts directly supported by the semiconductors, which mitigates the charge recombination, supplies highly active and selective sites for overall reactions, and preserves the semiconductors from photocorrosion. This work presents a new approach to regulating the position of dual cocatalysts and ameliorating the net efficiency of photoredox catalysis.

Photochemical H2 Evolution from Bis(diphosphine)nickel Hydrides Enables Low-Overpotential Electrocatalysis.

Molecules capable of both harvesting light and forming new chemical bonds hold promise for applications in the generation of solar fuels, but such first-row transition metal photoelectrocatalysts are lacking. Here we report nickel photoelectrocatalysts for H2 evolution, leveraging visible-light-driven photochemical H2 evolution from bis(diphosphine)nickel hydride complexes. A suite of experimental and theoretical analyses, including time-resolved spectroscopy and continuous irradiation quantum yield measurements, led to a proposed mechanism of H2 evolution involving a short-lived singlet excited state that undergoes homolysis of the Ni-H bond. Thermodynamic analyses provide a basis for understanding and predicting the observed photoelectrocatalytic H2 evolution by a 3d transition metal based catalyst. Of particular note is the dramatic change in the electrochemical overpotential: in the dark, the nickel complexes require strong acids and therefore high overpotentials for electrocatalysis; but under illumination, the use of weaker acids at the same applied potential results in a more than 500 mV improvement in electrochemical overpotential. New insight into first-row transition metal hydride photochemistry thus enables photoelectrocatalytic H2 evolution without electrochemical overpotential (at the thermodynamic potential or 0 mV overpotential). This catalyst system does not require sacrificial chemical reductants or light-harvesting semiconductor materials and produces H2 at rates similar to molecular catalysts attached to silicon.

Synthesis of hierarchical tandem double Z-scheme heterojunctions for robust photocatalytic H2 generation

The design of advanced structures for semiconductor photocatalysts is one of the most tremendous approaches to stimulate their conversion performance from solar energy to chemical energy. Nonetheless, it is challenging to harvest the full solar spectrum in semiconductor photocatalysts for excessive activity. Herein, a hierarchical CuCo2O4/graphdiyne/CuO nanocages (CCOGC NCs) tandem heterojunction was once effectively synthesized by means of liquid–liquid interface method for the first time and a facile calcination method. The graphdiyne nanosheets (GDY NSs) not only function as a cost-effective cocatalyst but also act as a bridge to connect two light-harvesting semiconductors into a tandem heterojunction where the CuCo2O4 (CCO) NCs and CuO Quantum dots (QDs) absorb visible light on both sides efficiently, coupling with the GDY NSs into a double Z-scheme photocatalyst system. Benefiting from the novel hierarchical tandem nanocages structure, the optimized CCOGC NCs photocatalyst manifests high activity under visible light irradiation with the hydrogen (H2) generation rate of 9617 µmol h−1 g−1, which is much enhanced compared with those of the CCO NCs and CCO/GDY NCs (CCOG NCs), as well CCO/CuO (CCOC NCs).

Significantly Enhanced Photocatalytic CO2 Reduction by Surface Amorphization of Cocatalysts.

Rational phase engineering of reduction cocatalyst offers a promising route to modulate the photocatalytic activity and selectivity in the conversion of CO2 to chemical feedstocks. However, it remains a great challenge to choose a suitable phase given that high-crystallinity phase is more conducive to the charge transfer and separation, while amorphous phase is more favorable for the adsorption and activation of CO2 molecules. To resolve this dilemma, herein, with Pd as a well-defined model, a surface amorphization strategy has been developed to fabricate crystalline@amorphous semi-core-shell cocatalysts based on the transformation of outer layer atoms of crystalline cocatalysts to disorder phase. According to the theoretical and experimental analysis, in the heterostructured cocatalysts, crystalline core shuttles the photoexcited electrons from light-harvesting semiconductor to amorphous shell due to its strong electronic coupling with both components. Meanwhile, amorphous shell provides efficient active sites for preferential activation and conversion of CO2 and suppression of undesirable proton reduction. Benefiting from the synergistic effects between crystalline core and amorphous shell, the optimized heterophase cocatalyst with suitable thickness of amorphous shell achieves superior CO (22.2 µmol gcat-1 h-1 ) and CH4 (38.1µmol gcat-1 h-1 ) formation rates with considerable selectivity and high stability in comparison with crystalline and amorphous counterparts.

Accelerating directional charge separation via built-in interfacial electric fields originating from work-function differences

In this work, a hierarchical porous SnS2/rGO/TiO2 hollow sphere heterojunction that allows highly-efficient light utilization and shortening distance of charge transformation is rationally designed and synthesized. More importantly, an rGO interlayer is successfully embedded between the TiO2 hollow sphere shells and outermost SnS2 nanosheets. This interlayer functions as a bridge to connect the two light-harvesting semiconductors and acts as a hole injection layer in the tandem heterojunction. The induced built-in electric fields on both sides of the interface precisely regulate the spatial separation and directional migration of the photo-generated holes from the light-harvesting semiconductor to the rGO hole injection interlayer. These synergistic effects greatly prolong the lifetime of the photo-induced charge carriers. The optimized tandem heterojunction with a 2 wt% rGO loading demonstrate enhanced visible-light-driven photocatalytic activity for Rhodamine B (RhB) dye degradation (removal rate: 97.3%) and Cr(VI) reduction (removal rate: 97.09%). This work reveals a new strategy for the rational design and assembly of hollow-structured photocatalytic materials with spatially separated reduction and oxidation surfaces to achieve excellent photocatalytic performance.

Highly Promoted Photocatalytic Hydrogen Generation by Multiple Electron Transfer Pathways

A facile development of highly efficient photocatalytic nanostructures for solar fuel production via controlled cocatalyst deposition is described. 2 nm-Pt nanoparticles were selectively synthesized by radiolysis on polypyrrole (PPy), TiO2 or on both PPy-TiO2 (noted respectively (Pt-PPy)-TiO2, (Pt-TiO2)-PPy and Pt-(PPy-TiO2)). The PPy nanostructures not only act a photosensitizer, but also as a connection bridge between two light-harvesting semiconductors to form a p-n heterojunction, which absorbs UV and visible light efficiently. The designed heterojunction photocatalyst nanostructures (Pt-(PPy-TiO2)) exhibit broadened absorption to the visible region, long life-time of charge carriers and high photocatalytic activity for hydrogen production. This activity is markedly enhanced compared with that of (Pt-PPy)-TiO2 and (Pt-TiO2)-PPy, and is ascribed to more efficient multiple electron transfer pathways. Such designed structures provide promising ways for selected site deposition of metal nanoparticles on heterojunction semiconductors in high efficiency conversion from solar energy to solar fuel application.

Defining Intermediates of Nitrogenase MoFe Protein during N2 Reduction under Photochemical Electron Delivery from CdS Quantum Dots.

Coupling the nitrogenase MoFe protein to light-harvesting semiconductor nanomaterials replaces the natural electron transfer complex of Fe protein and ATP and provides low-potential photoexcited electrons for photocatalytic N2 reduction. A central question is how direct photochemical electron delivery from nanocrystals to MoFe protein is able to support the multielectron ammonia production reaction. In this study, low photon flux conditions were used to identify the initial reaction intermediates of CdS quantum dot (QD):MoFe protein nitrogenase complexes under photochemical activation using EPR. Illumination of CdS QD:MoFe protein complexes led to redox changes in the MoFe protein active site FeMo-co observed as the gradual decline in the E0 resting state intensity that was accompanied by an increase in the intensity of a new "geff = 4.5" EPR signal. The magnetic properties of the geff = 4.5 signal support assignment as a reduced S = 3/2 state, and reaction modeling was used to define it as a two-electron-reduced "E2" intermediate. Use of a MoFe protein variant, β-188Cys, which poises the P cluster in the oxidized P+ state, demonstrated that the P cluster can function as a site of photoexcited electron delivery from CdS to MoFe protein. Overall, the results establish the initial steps for how photoexcited CdS delivers electrons into the MoFe protein during reduction of N2 to ammonia and the role of electron flux in the photochemical reaction cycle.

Let a Hundred Polymers Bloom: Tunable Wetting of Photografted Polymer-Carbon Nitride Surfaces

Sunlight is widely used by plants to produce chemical energy fundamental for all life, a process that also serves as an inspiration for science. Recently, light-harvesting semiconductor materials, here, carbon nitrides, have been widely used in photochemistry to catalyze reactions under visible light illumination. We exploited photoactive carbon nitride thin films as a metal-free platform for one-step photografting from polymers on the surface. Here, we observe photoswitchable wetting properties of polystyrene-grafted carbon nitride, which are independent of any classical photoswitchable groups. Dense and thick polymer brush formation with a tunable thickness (up to 700 nm) was obtained, allowing for tailor surface properties. Furthermore, exploiting the reciprocal insolubility of water–dimethylacrylamide and pentane–styrene solutions, we provide a general method for the synthesis of centimeter-scale carbon nitride-based Janus asymmetric films. It is thought that the simple experimental design based on cheap and common chemicals provides a novel dimension of carbon nitride materials with advanced surface properties.

Visible Light‐driven Dehydrogenation of Benzylamine under Liberation of H2

AbstractThe visible light‐driven transformation of chemical compounds in combination with the liberation of H2 is highly attractive. Herein, we report on a photocatalyst that allows the acceptorless dehydrogenation of benzylamine. Upon light absorption, free charge carriers are generated and used for the concerted imine formation and liberation of H2. Our photocatalyst consists of CdS as a light harvesting semiconductor supported on colloidal metal‐organic framework crystallites. The decoration with co‐catalytic nickel nanoparticles promotes hydrogen evolution and, in addition, stabilizes the CdS component under irradiation.

Tri-functional molecular relay to fabricate size-controlled CoOx nanoparticles and WO3 photoanode for an efficient photoelectrochemical water oxidation

Heterojunction and element doping to couple light-harvesting semiconductors with catalytic materials have been widely employed for photoelectrochemical (PEC) water splitting.