Interfacial Charge Transfer Properties Research Articles

Doping of Colloidal Nanocrystals for Optimizing Interfacial Charge Transfer: A Double-Edged Sword.

Doping of colloidal nanocrystals offers versatile ways to improve their optoelectronic properties, with potential applications in photocatalysis and photovoltaics. However, the precise role of dopants on the interfacial charge transfer properties of nanocrystals remains poorly understood. Here, we use a Cu-doped InP@ZnSe quantum dot as a model system to investigate the dopant effects on both the intrinsic photophysics and their interfacial charge transfer by combining time-resolved transient absorption and photoluminescent spectroscopic methods. Our results revealed that the Cu dopant can cause the generation of the self-trapped exciton, which prolongs the exciton lifetime from 48.3 ± 1.7 to 369.0 ± 4.3 ns, facilitating efficient charge separation to slow electron and hole acceptors. However, hole localization into the Cu site alters their energetic levels, slowing hole transfer and accelerating charge recombination loss. This double-edged sword role of dopants in charge transfer properties is important in the future design of nanocrystals for their optoelectronic and photocatalytic applications.

Oxygen vacancies enhanced electrocatalytic water splitting of P-FeMoO4 initiated via phosphorus doping

Transition metal oxides (TMOs) are abundant and cost-effective materials. However, poor conductivity and low intrinsic activity limit their application in electrolyzed water catalysts. Herein, we prepared P-FeMoO4 in situ on nickel foam (P-FMO@NF) by phosphorylation-modified FeMoO4 to optimize its electrocatalytic properties. Interestingly, phosphorus doping is accompanied by the generation of oxygen vacancies and surface phosphates. Oxygen vacancies accelerated Mo dissolution during the oxygen evolution reaction (OER), leading to the rapid reconfiguration of P-FMO@NF to FeOOH and regulating the electronic structure of P-FMO@NF. The formation of phosphates is caused by the substitution of some molybdates with phosphates, which further increases the amount of oxygen vacancies. Hence, the OER overpotential of P-FMO@NF at a current density of 10 mA cm−2 is only 206 mV, and the hydrogen evolution reaction (HER) overpotential is 154 mV. It was assembled into a water splitting cell with a voltage of just 1.59 V at 10 mA cm−2 and shows excellent stability over 50 h. These excellent electrocatalytic properties are mainly attributed to the oxygen vacancies, which improve the interfacial charge transfer properties of the catalysts. This study provides new insights into phosphorus doping and offers a new perspective on the design of electrocatalysts.

Electrochemical Characterization of Artificial Solid Electrolyte Interphase Developed on Graphite Via ALD

During formation of Li-ion batteries, a ‘natural’ solid electrolyte interphase (SEI) is formed at the anode side by decomposition products of the electrolyte. The properties of the SEI are extremely decisive for the overall battery properties, such as rate capability and cycling stability. However, the SEI formation consumes Li, leading to so called ‘formation losses’ that can make up to 15% of the theoretical energy density of the battery. Several approaches have been presented to overcome formation losses while preserving excellent overall battery properties. Particularly, electrochemical prelithiation and the application of artificial SEIs prior cell assembly are considered to effectively reduced formation losses while improving the interfacial charge transfer properties and increasing cycling stability.Herein, the authors present an innovative approach of applying a multifunctional artificial SEI on anode material powders via consecutive atomic layer deposition (ALD) cycles. As a model system, graphite powder has been chosen to be modified and characterized.The resulting electrodes show substantially improved electrochemical performance in half cells and full cells, regarding initial capacity loss, CE and cycling stability. Furthermore, model electrodes consisting of a single layer of graphite particles were manufactured, which exclude the effects of a typical composite electrode and therefore reveal the intrinsic properties of the active material. Using this approach, the interfacial kinetics and the rate capability are investigated comparatively between pristine and ALD coated electrodes, revealing the impact of the artificial SEI on the materials level.

Polarization Modulation on Charge Transfer and Band Structures of GaN/MoS2 Polar Heterojunctions

As important third-generation semiconductors, wurtzite III nitrides have strong spontaneous and piezoelectric polarization effects. They can be used to construct multifunctional polar heterojunctions or quantum structures with other emerging two-dimensional (2D) semiconductors. Here, we investigate the polarization effect of GaN on the interfacial charge transfer and electronic properties of GaN/MoS2 polar heterojunctions by first-principles calculations. From the binding energy, the N-polarity GaN/MoS2 heterojunctions show stronger structural stability than the Ga-polarity counterparts. Both the Ga-polarity and N-polarity GaN/MoS2 polar heterojunctions have type-II energy band alignments, but with opposite directions of both the built-in electric field and interfacial charge transfer. In addition, their heterostructure types can be effectively modulated by applying in-plane biaxial strains on GaN/MoS2 polar heterojunctions, which can undergo energy band transitions from type II to type I. As a result, it provides a feasible solution for the structural design and integrated applications of hybrid 3D/2D polar heterojunctions in advanced electronics and optoelectronics.

Deposition of hydrophilic Ti3C2Tx on a superhydrophobic ZnO nanorod array for improved surface-enhanced raman scattering performance

BackgroundSuperhydrophobic substrate modifications are an effective way to improve SERS sensitivity by concentrating analyte molecules into a small surface area. However, it is difficult to manipulate low-volume liquid droplets on superhydrophobic substrates.ResultsTo overcome this limitation, we deposited a hydrophilic Ti3C2Tx film on a superhydrophobic ZnO nanorod array to create a SERS substrate with improved analyte affinity. Combined with its interfacial charge transfer properties, this enabled a rhodamine 6G detection limit of 10−11 M to be achieved. In addition, the new SERS substrate showed potential for detection of biological macromolecules, such as microRNA.ConclusionCombined with its facile preparation, the SERS activity of ZnO/Ti3C2Tx suggests it may provide an ultrasensitive environmental pollutant-monitoring and effective substrate for biological analyte detection.Graphical

Two-Step Process of a Crystal Facet-Modulated BiVO4 Photoanode for Efficiency Improvement in Photoelectrochemical Hydrogen Evolution.

The photoactivity of nanoporous bismuth vanadate (BiVO4, BVO) photoanodes that were fabricated by a two-step process (electrodeposition and then thermal conversion) in photoelectrochemical (PEC) hydrogen (H2) evolution can be enhanced about 1.44-fold by improving the constitutive ratio of (111̅), (061), and (242̅) crystal facets. The PEC characterization was carried out to investigate the factors altering the performance, which revealed that the crystal facet modulation could improve the photoactivity of the BVO photoanodes. In addition, the orientation-controlled BVO thin-film electrodes are introduced as evidence that the present crystal facet modulation is the positive effect for BVO photoanodes in PEC. The investigation of energy band structures and interfacial charge carrier dynamics of the BVO photoanodes reveals that the crystal facet modulation could result in a shorter lifetime of charge carrier recombination and larger band bending at the interface between BVO and electrolytes. This outcome could improve the charge separation and charge transfer efficiencies of BVO photoanodes, promoting the efficiency of PEC H2 evolution. Moreover, this crystal facet modulation can combine with co-catalyst decoration to further improve the solar-to-hydrogen efficiency of BVO photoanodes in PEC. This study presents a potential strategy to promote the PEC activity by crystal facet modulation and important insights into the interfacial charge transfer properties of semiconductor photoelectrodes for the application in solar fuel generation.

Promoted interfacial charge transfer by coral-like nickel diselenide for enhanced photocatalytic hydrogen evolution over carbon nitride nanosheet

Seeking an efficient and non-precious co-catalyst for g-C3N4 (CN) remains a great demanding to achieve high photocatalytic hydrogen generation performance. Herein, a composite photocatalyst with high efficiency was prepared by modifying CN with coral-like NiSe2. The optimal hydrogen evolution rate of 643.16 μmol g−1 h−1 is from NiSe2/CN-5 under visible light. Superior light absorption and interfacial charge transfer properties including suppressed photogenerated carrier recombination and efficient separation of photogenerated electron-hole pairs have been observed, which account for the enhanced photocatalytic performance of CN.

Improved charge transfer by binary Ni3S2-NiS2 for boosting the photocatalytic hydrogen generation over g-C3N4

Developing efficient, cheap and robust photocatalysts for hydrogen evolution is promising to solve the issues of energy shortage and environmental degradation. In this work, binary Ni3S2-NiS2 was in-situ grown on g-C3N4 (CN) through a one-pot hydrothermal method. The optimal hydrogen evolution activity (1206.6 μmol g-1h−1) was achieved from Ni3S2-NiS2/CN-3 with excellent stability under visible light irradiation. Improved light absorption and superior interfacial charge transfer properties including suppressed photogenerated carrier recombination and efficient separation of photogenerated electron-hole pairs have been observed, which account for the enhanced photocatalytic performance.

NiSe2/Cd0.5Zn0.5S as a type-II heterojunction photocatalyst for enhanced photocatalytic hydrogen evolution

A designed type-II heterojunction photocatalyst, NiSe2/Cd0.5Zn0.5S (NiSe2/CZS), was successfully synthesized and it exhibits outstanding photocatalytic hydrogen evolution performance. The optimal loading amount of NiSe2 on Cd0.5Zn0.5S is 13 wt %, and the corresponding hydrogen production rate is approximately 121.01 mmol g−1 h−1 under visible light. The heterojunction structure between Cd0.5Zn0.5S and NiSe2 promoted the separation of photogenerated electron-hole pairs, effectively suppressed the photogenerated carrier recombination and endowed the material with excellent interfacial charge transfer properties, thus improving the photocatalytic performance.

DYE-CATALYST INTERACTIONS IN A WATER-SPLITTING SYSTEM: A FIRST-PRINCIPLES INVESTIGATION OF INTERFACIAL STRUCTURES BASED ON COUMARIN343/[FeFe](mcbdt)(CO)6/NiO

Organic dyes and molecular catalysts co-adsorbed on a metal oxide substrate are widely employed for water-splitting photoelectrodes. In this manuscript, we employ first-principles calculations to investigate interactions between a coumarin dye and a molecular catalyst on the nickel oxide substrate in an experimentally verified water-splitting photocathode. The nanoscopic interfacial structures and interfacial charge transfer properties of dye/catalyst/metal oxide hybrid systems in the water-splitting photocathode are elucidated. In particular, the intermolecular hydrogen bond and the substrate-to-catalyst electron transfer stabilize the interfacial structure. The present study helps the fundamental understanding of water-splitting photoelectrodes co-adsorbed with organic dyes and molecular catalysts.

Catalytic Mismatching of CuInSe2 and Ni3Al Demonstrates Selective Photoelectrochemical CO2 Reduction to Methanol

Photoelectrochemical catalysts are often plagued by ineffective interfacial charge transfer or nonideal optical conversion properties. To overcome this challenge, strategically pairing a catalytica...

Dye-catalyst interactions in a water-splitting system: a first-principles investigation of interfacial structures based on coumarin343/[FeFe](mcbdt)(CO)6/NiO

Organic dyes and molecular catalysts co-adsorbed on a metal oxide substrate are widely employed for water-splitting photoelectrodes. In this manuscript, we employ first-principles calculations to investigate interactions between a coumarin dye and a molecular catalyst on the nickel oxide substrate in an experimentally verified water-splitting photocathode. The nanoscopic interfacial structures and interfacial charge transfer properties of dye/catalyst/metal oxide hybrid systems in the water-splitting photocathode are elucidated. In particular, the intermolecular hydrogen bond and the substrate-to-catalyst electron transfer stabilize the inter­facial structure. The present study helps the fundamental understanding of water-splitting photo­electrodes co-adsorbed with organic dyes and molecular catalysts.

Oxidized SWCNT and MWCNT as co-catalysts of polymeric carbon nitride for photocatalytic hydrogen evolution

Polymeric carbon nitride (PCN) has been found as a promising photocatalyst for hydrogen evolution from water splitting. However, disadvantages limit its commercial applications. Therefore, modification of PCN in order to increase the hydrogen evolution rate and stability of the photocatalyst are extensively studied. In this contribution, we compare the influence of oxidation and the number of walls in carbon nanotubes (SWCNT and MWCNT) on the photocatalytic performance of PCN. It was found that oxidation of both types of CNT enhanced a photocurrent response, interfacial charge transfer and separation properties leading to the enhanced hydrogen evolution rate. Interestingly, MWCNT were found to enhance the charge transfer and separation properties more significantly. However, this material affected inferior co-catalytic behavior. We suggest it was related to the lower number of reactive electrons migrating to the surface reaction sites because of transferring to the inner tubes. SWCNT influence ~12-fold increase in H2 evolution rate, while MWCNT influenced ~8-fold enhancement in comparison to pristine PCN.

The planarization of side chain in carbazole sensitizer and its effect on optical, electrochemical, and interfacial charge transfer properties

The energy level is the critical property of the organic dye, determining the light-harvesting region of the dye, as well as the thermodynamic possibilities and the efficiencies of multiple interfacial charge transfer processes in DSSCs. In our previous work, the planarization of D-π-A skeleton can efficiently and selectively optimize the energy levels, leading to the improvement of the light-harvesting capability and the interfacial charge transfer processes. However, whether the planarization of side chain will also produce similarly superior influence is still unclear. Thus, by the planarization of side chain, two novel carbazole isomers are developed and evaluated, while the planarization reaction is also investigated by nuclear magnetic resonance spectroscopy and theoretical calculation. With the planarization of side chain in donor part, the HOMO levels of two dyes are slightly lifted while their LUMO levels remain at the same level, leading to the slightly expansion of their light-harvesting region. However, the interfacial charge transfer processes cannot be efficiently improved by this planarization due to its irregular and negligible effect, which is far different from the influence caused by the planarization of D-π-A skeleton. Consequently, our finding demonstrates that although the planarization of side chain in donor part has irregular influence on interfacial charge transfer processes, it still can selectively uplift the HOMO level and further improve the light-harvesting capability, which can be applied as the useful complement to the planarization of D-π-A skeleton for the development of organic dye in the future.

Atomistic understanding on molecular halide perovskite/organic/TiO2 interface with bifunctional interfacial modifier: A case study on halogen bond and carboxylic acid group

Organic interfacial modifiers such as the bifunctional molecules have been successfully incorporated in halide perovskite solar cells to boost the optoelectronic performance. However, the atomistic understanding on the structures and properties of the bifunctional molecule-mediated tri-layer perovskite interface remains elusive. In this manuscript, we employ first principles calculations to provide a nanoscopic view on the structures and properties of the perovskite/organic/electron transporting layer (ETL) tri-layer, based on a case study of the MA4PbI6/bromoacetate/TiO2 system. The calculation reveals the simultaneous coupling of the bifunctional molecule with TiO2 and the halide perovskite material. The organic interfacial modifier is suggested to provide a rich platform for molecular engineering the halide perovskite interface to improve the interfacial charge transfer properties in perovskite solar cells. This work provides a foundation in the convergence to a fundamental understanding of the more sophisticated organic molecule-modified perovskite interfaces in halide perovskite-based optoelectronic devices.

Creating Scalable Faradaic Carbon Nanotube Electrodes with Mild Chemical Oxidation

The advancement of novel materials to ameliorate interfacial charge transfer properties will drastically improve energy storage, heterogeneous catalysis, and various other electrochemical applications. Here we discuss a facile method that can utilize the Faradaic capabilities of residual iron nanoparticle catalysts that are captured within Multi-walled Carbon Nanotubes (MWNT) post-synthesis, thereby correcting the difficulties associated with creating hybrid nanocomposite electrodes. Non-purified MWNTs, experience a chemical oxidation procedure in an acidic environment with KMnO4 to partly “unzip” the MWNTs and reveal the redox-active iron nanoparticles to the electrolyte. A consistent redox peak affiliated with the Fe2+/3+ transition is achieved during the MWNT oxidation procedure yielding a ~350% improvement in capacitance (>300 F g-1) when compared to purified MWNT electrodes (70 F g-1). While these materials solely may be applicable as energy storage electrodes, the integration of redox species within an inert carbon electrode will also provide new opportunities to accelerate heterogeneous charge transfer reactions.

Efficient Photodoping of Graphene in Perovskite–Graphene Heterostructure

AbstractGraphene has been shown to be a prospective platform for active photonic devices with exotic optical properties. Employed in these devices, the graphene photodoping mechanism would allow for the remote spatiotemporal doping control by means of illumination, not restricted by the physical gate electrodes. This paper reports on the efficient graphene photodoping in graphene‐CH3NH3PbI3 perovskite heterostructure recently spotlighted for photodetector applications. To maximize the photoresponse, the heterostructure is optimized by systematically introducing additional layers of the self‐assembled monolayer of octadecyltrichlorosilane molecules, MoO3, and poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate for improving interfacial charge transfer properties. The photodoping amount estimated from the photocurrent measurements is 2.7 × 1012 cm−2 at zero gate bias—more than twice higher than previously observed in graphene‐perovskite heterostructures. Without external gate bias, the estimated graphene Fermi energy varies from 0.37 to 0.43 eV due to photodoping, which is sufficient to operate graphene‐based active photonic devices at mid‐infrared frequencies, and similar to that typically achievable with the conventional electrostatic gating. Furthermore, this work highlights the missing yet important aspect of the characterization method for the phototransistors with a 2D channel, resulting from the mismatch between the units of the current and power densities.

Simultaneous probing of nanocrystal (NC)-ligand interaction-induced charge transfer/transport properties at the electron donor (lead selenide NC)/acceptor (zinc oxide) functional interface

Understanding correlation between interfacial charge transfer and transport at the electron donor/acceptor functional interface has remained elusive despite its impact on the optoelectronic devices because the conventional vertically stacked diode structure as well as energetic and morphological disorder at the interface complicates analysis of the interfacial region. We fabricated a test platform to enable simultaneous probing of charge transfer and transport at the nanocrystal (NC)/metal oxide interface. Using the transmission line method (TLM) in a ZnO/PbSe/ZnO test structure, we measured the ZnO/PbSe contact resistance from which interfacial charge transfer and transport properties can be correlated in relation to interfacial energetics depending on the size of PbSe NC. The interfacial transfer-transport test probe was validated by comparing size dependent energy level offset derived from the contact resistance value with the established trend. Importantly, by altering the chemical nature of the molecules attached to the NC surface, we discovered that charge transport properties including mobility in the PbSe channel away from the interface are extended into the interfacial region, enabling correlation between carrier diffusion and the electrical mobility at the electron donor/acceptor interface. With a very low mobility in the interface region, charge transfer associated with carrier diffusion is suggested to reflect the nature of energetic disordered interfacial region rather than the electrical mobility.

Understanding charge trapping/detrapping at the zinc oxide (ZnO)/MAPbI3 perovskite interface in the dark and under illumination using a ZnO/perovskite/ZnO test platform.

We fabricated a zinc oxide (ZnO)/methylammonium lead iodide (MAPbI3) perovskite/ZnO field effect transistor (FET) test platform device through which ZnO/perovskite interfacial contact properties can be probed in the dark and under illumination. Using pulsed laser deposition, highly conductive (0.014 Ω cm) ZnO source and drain electrodes were fabricated allowing for the investigation of the interfacial charge transfer properties through current-voltage characteristics of a ZnO/perovskite/ZnO FET. With a bottom-contact FET device, gate voltage dependent current hysteresis in the drain current-gate voltage curves was probed at low temperature to minimize the effect of ion migration on electronic charge transport in the perovskite layer. Under illumination, importantly, ZnO/perovskite electrical contact properties were significantly altered due to electronic energy barrier change at the interface arising from the detrapping of electrons from the ZnO/perovskite interface, resulting in an enhanced dark current and a suppressed photocurrent. The origin of current hysteresis in the ZnO/perovskite/ZnO FET device is discussed relating it to interfacial charging/discharging associated with ultraviolet (UV)-induced oxygen adsorption/desorption. The results presented herein demonstrate that interfacial electronic properties at the donor (perovskite)/acceptor (ZnO) interface can be altered by photoinduced carrier trapping/detrapping, providing insights that UV-induced persistent photoconduction in transition metal oxide electron transport layers including ZnO may be contributing to the current hysteresis observed in the perovskite photovoltaic devices.

Plasmonic stimulated photocatalytic/electrochemical hydrogen evolution from water by (001) faceted and bimetallic loaded titania nanosheets under sunlight irradiation

This study demonstrates the synthesis of TiO2 nanosheets (TNST) exhibiting high percentage (54%) of exposed (001) facets and decorated with co-deposited gold and platinum nanoparticles (NPs). The as synthesized as well as modified sheets are then investigated for the photo-reduction of water under sunlight irradiation using different alcohols as hole-scavengers. Structural and morphological studies manifest the formation of well-defined sheet structure of TiO2 with average dimensions of 55 nm × 46 nm. The measured d spacing of 0.34 nm and 0.23 nm corresponds to (101) and (001) facets of anatase TiO2. The presence of Au and Pt was confirmed by electron diffraction and X-ray photoelectron spectroscopy. TiO2 nanosheets (TNST's) with 0.4 and 0.75 wt % of Au and Pt (Au0.4/Pt0.75-TNSTs), respectively were found to exhibit higher efficiency. Furthermore, photo-electrochemical (PEC) water splitting studies revealed the increase in photocurrent from 2 mA at 0.5 V to 4.5 mA at 0.65 V for Au0.4/Pt0.75-TNST photoelectrodes, which otherwise was 0.15 mA for the bare TNST photoelectrodes at the same potential. The observed PEC performance was well supported by impedance analysis, wherein Au0.4/Pt0.75-TNSTs (338 Ω) are found to exhibit lower resistance in comparison to bare TNST (737 Ω), which could be due to brilliant photo conductance and interfacial charge transfer properties of Au0.4/Pt0.75-TNSTs. Photocatalytically the Au0.4/Pt0.75-TNST produced ∼143 μmol h1 of H2 gas under direct sunlight irradiation, which is found equivalent to 16.1% apparent quantum efficiency.