Charge Transfer Dynamics in Organic-Inorganic Hybrid Heterostructures-Insights by Vibrational-Sum Frequency Generation Spectroscopy

Organic-inorganic heterostructures play a pivotal role in modern electronic and optoelectronic applications including photodetectors and field effect transistors, as well as in solar energy conversion such as photoelectrodes of dye-sensitized solar cells, photoelectrochemical cells, and in organic photovoltaics. To a large extent, performance of such devices is controlled by charge transfer dynamics at and across (inner) interfaces, e.g., between a wide band gap semiconductor and molecular sensitizers and/or catalysts. Hence, a detailed understanding of the structure-dynamics-function relationship of such functional interfaces is necessary to rationalize possible performance limitations of these materials and devices on a molecular level. Vibrational sum-frequency generation (VSFG) spectroscopy, as an interface-sensitive spectroscopic technique, allows to obtain chemically specific information from interfaces and combines such chemical insights with ultrafast time resolution, when integrated as a spectroscopic probe into a pump-probe scheme. Thus, this minireview discusses the advantages and potential of VSFG spectroscopy for investigating interfacial charge transfer dynamics and structural changes at inner interfaces. A critical perspective of the unique spectroscopic view of otherwise inaccessible interfaces is presented, which we hope opens new opportunities for an improved understanding of function-determining processes in complex materials, and brings together communities who are devoted to designing materials and devices with spectroscopists.

Transfer of Sodium Ion across Interface between Na+-Selective Electrode Membrane and Aqueous Electrolyte Solution: Can We Use Nernst Equation If Current Flows through Electrode?

Electrochemical impedance and chronopotentiometric measurements with Na+-selective solvent polymeric (PVC) membranes containing a neutral ionophore and a cation exchanger revealed low-frequency resistance, which is ascribed to Na+ ion transfer across the interface between the membrane and aqueous solution. The attribution is based on the observed regular dependence of this resistance on the concentration of Na+ in solutions. The respective values of the exchange current densities were found to be significantly larger than the currents flowing through ion-selective electrodes (ISEs) during an analysis in non-zero-current mode. This fact suggests that the interfacial electrochemical equilibrium is not violated by the current flow and implies that the Nernst equation can be applied to interpret the data obtained in non-zero-current mode, e.g., constant potential coulometry.

Remote-Controllable Interfacial Electron Tunneling at Heterogeneous Molecular Junctions via Tip-Induced Optoelectrical Engineering

Molecular electronics enables functional electronic behavior via single molecules or molecular self-assembled monolayers, providing versatile opportunities for hybrid molecular-scale electronic devices. Although various molecular junction structures are constructed to investigate charge transfer dynamics, significant challenges remain in terms of interfacial charging effects and far-field background signals, which dominantly block the optoelectrical observation of interfacial charge transfer dynamics. Here, tip-induced optoelectrical engineering is presented that synergistically correlates photo-induced force microscopy and Kelvin probe force microscopy to remotely control and probe the interfacial charge transfer dynamics with sub-10 nm spatial resolution. Based on this approach, the optoelectrical origin of metal-molecule interfaces is clearly revealed by the nanoscale heterogeneity of the tip-sample interaction and optoelectrical reactivity, which theoretically aligned with density functional theory calculations. For a practical device-scale demonstration of tip-induced optoelectrical engineering, interfacial tunneling is remotely controlled at a 4-inch wafer-scale metal-insulator-metal capacitor, facilitating a 5.211-fold current amplification with the tip-induced electrical field. In conclusion, tip-induced optoelectrical engineering provides a novel strategy to comprehensively understand interfacial charge transfer dynamics and a non-destructive tunneling control platform that enables real-time and real-space investigation of ultrathin hybrid molecular systems.

Ultrafast Charge Transfer and Recombination Dynamics in Monolayer-Multilayer WSe2 Junctions Revealed by Time-Resolved Photoemission Electron Microscopy

The ultrafast carrier dynamics of junctions between two chemically identical, but electronically distinct, transition metal dichalcogenides (TMDs) remains largely unknown. Here, we employ time-resolved photoemission electron microscopy (TR-PEEM) to probe the ultrafast carrier dynamics of a monolayer-to-multilayer (1L-ML) WSe2 junction. The TR-PEEM signals recorded for the individual components of the junction reveal the sub-ps carrier cooling dynamics of 1L- and 7L-WSe2, as well as few-ps exciton-exciton annihilation occurring on 1L-WSe2. We observe ultrafast interfacial hole (h) transfer from 1L- to 7L-WSe2 on an ∼0.2 ps time scale. The resultant excess h density in 7L-WSe2 decays by carrier recombination across the junction interface on an ∼100 ps time scale. Reminiscent of the behavior at a depletion region, the TR-PEEM image reveals the h density accumulation on the 7L-WSe2 interface, with a decay length ∼0.60 ± 0.17 μm. These charge transfer and recombination dynamics are in agreement with ab initio quantum dynamics. The computed orbital densities reveal that charge transfer occurs from the basal plane, which extends over both 1L and ML regions, to the upper plane localized on the ML region. This mode of charge transfer is distinctive to chemically homogeneous junctions of layered materials and constitutes an additional carrier deactivation pathway that should be considered in studies of 1L-TMDs found alongside their ML, a common occurrence in exfoliated samples.

Distortion-Induced Interfacial Charge Transfer at Single Cobalt Atom Secured on Ordered Intermetallic Surface Enhances Pure Oxygen Production

In this work, atomic cobalt (Co) incorporation into the Pd2Ge intermetallic lattice facilitates operando generation of a thin layer of CoO over Co-substituted Pd2Ge, with Co in the CoO surface layer functioning as single metal sites. Hence the catalyst has been titled Co1-CoO-Pd2Ge. High-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy confirm the existence of CoO, with some of the Co bonded to Ge by substitution of Pd sites in the Pd2Ge lattice. The role of the CoO layer in the oxygen evolution reaction (OER) has been verified by its selective removal using argon sputtering and conducting the OER on the etched catalyst. In situ X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy demonstrate that CoO gets transformed to CoOOH (Co3+) in operando condition with faster charge transfer through Pd atoms in the core Pd2Ge lattice. In situ Raman spectroscopy depicts the emergence of a CoOOH phase on applying potential and shows that the phase is stable with increasing potential and time without getting converted to CoO2. Density functional theory calculations indicate that the Pd2Ge lattice induces distortion in the CoO phase and generates unpaired spins in a nonmagnetic CoOOH system resulting in an increase in the OER activity and durability. The existence of spin density even after electrocatalysis is verified from electron paramagnetic resonance spectroscopy. We have thus successfully synthesized intermetallic supported CoO during synthesis and rigorously verified the role played by an intermetallic Pd2Ge core in enhancing charge transfer, generating spin density, improving electrochemical durability, and imparting mechanical stability to a thin CoOOH overlayer. Differential electrochemical mass spectrometry has been explored to visualize the instantaneous generation of oxygen gas during the onset of the reaction.

Embedding Plasmonic Metal into Heterointerface of MOFs-Encapsulated Semiconductor Hollow Architecture for Boosting CO2 Photoreduction

Coupling hollow semiconductor with metal-organic frameworks (MOFs) holds great promise for constructing high-efficient CO₂ photoreduction systems. However, energy band mismatch between them makes it difficult to exert their advantages to maximize the overall photocatalytic efficiency, since that the blockage of desirable interfacial charge transfer gives rise to the enrichment of photoelectrons and CO₂ molecules on the different locations. Herein, an interfacial engineering is presented to overcome this impediment, based on the insertion of plasmonic metal into the heterointerfaces between them, forming a stacked semiconductor/metal@MOF photocatalyst. Experimental observations and theoretical simulations validate the critical roles of embedded Au in maneuvering the charge separation/transfer and surface reaction: (i) bridges the photoelectron transfer from hollow CdS (H-CdS) to ZIF-8; (ii) produces hot electrons and shifts them to ZIF-8; (iii) induces the formation of ZIF-8 defects in promoting the CO₂ adsorption/activation and transformation to CO with low energy barriers. Consequently, the as-prepared H-CdS/Au@ZIF-8 with optimal ZIF-8 thickness exhibits distinctly boosted activity and superb selectivity in CO production as compared with H-CdS@ZIF-8 and other counterparts. This work provides protocols to take full advantages of components involved for enhanced solar-to-chemical energy conversion efficiency of hybrid artificial photosynthetic systems through rationally harnessing the charge transfer between them.

Amplification-Free, High-Throughput Nanoplasmonic Quantification of Circulating MicroRNAs in Unprocessed Plasma Microsamples for Earlier Pancreatic Cancer Detection

Pancreatic ductal adenocarcinoma (PDAC) is a deadly malignancy that is often detected at an advanced stage. Earlier diagnosis of PDAC is key to reducing mortality. Circulating biomarkers such as microRNAs are gaining interest, but existing technologies require large sample volumes, amplification steps, extensive biofluid processing, lack sensitivity, and are low-throughput. Here, we present an advanced nanoplasmonic sensor for the highly sensitive, amplification-free detection and quantification of microRNAs (microRNA-10b, microRNA-let7a) from unprocessed plasma microsamples. The sensor construct utilizes uniquely designed -ssDNA receptors attached to gold triangular nanoprisms, which display unique localized surface plasmon resonance (LSPR) properties, in a multiwell plate format. The formation of -ssDNA/microRNA duplex controls the nanostructure-biomolecule interfacial electronic interactions to promote the charge transfer/exciton delocalization processes and enhance the LSPR responses to achieve attomolar (10^-18 M) limit of detection (LOD) in human plasma. This improve LOD allows the fabrication of a high-throughput assay in a 384-well plate format. The performance of nanoplasmonic sensors for microRNA detection was further assessed by comparing with the qRT-PCR assay of 15 PDAC patient plasma samples that shows a positive correlation between these two assays with the Pearson correlation coefficient value >0.86. Evaluation of >170 clinical samples reveals that oncogenic microRNA-10b and tumor suppressor microRNA-let7a levels can individually differentiate PDAC from chronic pancreatitis and normal controls with >94% sensitivity and >94% specificity at a 95% confidence interval (CI). Furthermore, combining both oncogenic and tumor suppressor microRNA levels significantly improves differentiation of PDAC stages I and II versus III and IV with >91% and 87% sensitivity and specificity, respectively, in comparison to the sensitivity and specificity values for individual microRNAs. Moreover, we show that the level of microRNAs varies substantially in pre- and post-surgery PDAC patients (n = 75). Taken together, this ultrasensitive nanoplasmonic sensor with excellent sensitivity and specificity is capable of assaying multiple biomarkers simultaneously and may facilitate early detection of PDAC to improve patient care.

Observation of Multi-Directional Energy Transfer in a Hybrid Plasmonic-Excitonic Nanostructure

Hybrid plasmonic devices involve a nanostructured metal supporting localized surface plasmons to amplify light-matter interaction, and a non-plasmonic material to functionalize charge excitations. Application-relevant epitaxial heterostructures, however, give rise to ballistic ultrafast dynamics that challenge the conventional semiclassical understanding of unidirectional nanometal-to-substrate energy transfer. Epitaxial Au nanoislands are studied on WSe₂ with time- and angle-resolved photoemission spectroscopy and femtosecond electron diffraction: this combination of techniques resolves material, energy, and momentum of charge-carriers and phonons excited in the heterostructure. A strong non-linear plasmon-exciton interaction that transfers the energy of sub-bandgap photons very efficiently to the semiconductor is observed, leaving the metal cold until non-radiative exciton recombination heats the nanoparticles on hundreds of femtoseconds timescales. The results resolve a multi-directional energy exchange on timescales shorter than the electronic thermalization of the nanometal. Electron-phonon coupling and diffusive charge-transfer determine the subsequent energy flow. This complex dynamics opens perspectives for optoelectronic and photocatalytic applications, while providing a constraining experimental testbed for state-of-the-art modelling.

Facile Fabrication of Ni9 S8 /Ag2 S Intertwined Structures for Oxygen and Hydrogen Evolution Reactions

Here, we report the fabrication of the unique intertwined Ni₉ S₈ /Ag₂ S composite structure with hexagonal shape from their molecular precursors by one-pot thermal decomposition. Various spectroscopic and microscopic techniques were utilized to confirm the Ni₉ S₈ /Ag₂ S intertwined structure. Powder X-ray Powder Diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis suggest that there is an enrichment of Ni₉ S₈ phase in Ni₉ S₈ /Ag₂ S. The presence of Ag₂ S in Ni₉ S₈ /Ag₂ S improves the conductivity by reducing the interfacial energy and charge transfer resistance. When Ni₉ S₈ /Ag₂ S is employed as an electrocatalyst for electrochemical oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activity, it requires a low overpotential of 152 mV for HER and 277 mV for OER to obtain the geometrical current density of 10 mA cm^-2 , which is definitely superior to that of its components Ni₉ S₈ and Ag₂ S. This work provides a simple design route to develop an efficient and durable electrocatalyst with outstanding OER and HER performance and the present catalyst (Ni₉ S₈ /Ag₂ S) deserves as a potential candidate in the field of energy conversion systems.

Enhanced Pd/a-WO3 /VO2 Hydrogen Gas Sensor Based on VO2 Phase Transition Layer

The utilization of clean hydrogen energy is becoming more feasible for the sustainable development of this society. Considering the safety issues in the hydrogen production, storage, and utilization, a sensitive hydrogen sensor for reliable detection is essential and highly important. Though various gas sensor devices are developed based on tin oxide, tungsten trioxide, or other oxides, the relatively high working temperature, unsatisfactory response time, and detection limitation still affect the extensive applications. In the current study, an amorphous tungsten trioxide (a-WO₃ ) layer is deposited on a phase-change vanadium dioxide film to fabricate a phase transition controlled Pd/a-WO₃ /VO₂ hydrogen sensor for hydrogen detection. Results show that both the response time and sensitivity of the hydrogen sensor are improved greatly if increasing the working temperature over the transition temperature of VO₂ . Theoretical calculations also reveal that the charge transfer at VO₂ /a-WO₃ interface becomes more pronounced once the VO₂ layer transforms to the metal state, which will affect the migration barrier of H atoms in a-WO₃ layer and thus improve the sensor performance. The current study not only realizes a hydrogen sensor with ultrahigh performance based on VO₂ layer, but also provides some clues for designing other gas sensors with phase-change material.

Facet Dependent Activity of Fe(III) Species Modified TiO2 for Simulated Sunlight Driven Fenton-like Reactions

TiO₂ crystals with different exposed facets are synthesized and modified facilely by depositing Fe(III) species. With more (101) facets exposed, the photoactivity of Fe-TiO₂ is obviously enhanced with peroxymonosulfate (PMS) as oxidant. The degradation rate for 20 ppm Bisphenol A (BPA) on Fe-TiO₂ (101) can achieve 0.219 min^-1, ∼8.5 times faster than that of pure TiO₂ under simulated sunlight irradiation. Photoelectrochemical measurements and density functional theory (DFT) calculations confirm that the interfacial charge transfer (IFCT) on Fe-TiO₂ (101) is stronger than that on Fe-TiO₂ (001) and a faster Fe(III)/Fe(II) transformation rate can be therefore achieved. As a result, the generation of ·OH and ¹O₂ will be accelerated with more (101) facets exposed, thus obtaining better photoactivity. Under the Fe-TiO₂/PMS/Light system, BPA can be effectively degraded in a wide pH range or in the presence of multiple inorganic anions. After five cycles, 100% BPA can still be degraded within 60 min. The study provides new photocatalysts design strategy based on Fe(III)/Fe(II) redox for PMS based photocatalytic oxidation.

High-Performance Visible-Near-Infrared Single-Walled Carbon Nanotube Photodetectors via Interfacial Charge-Transfer-Induced Improvement by Surface Doping

Single-walled carbon nanotubes (SWCNTs) are considered to be promising candidates for next-generation near-infrared (NIR) photodetectors due to their extraordinary electrical and optical properties. However, the low separation efficiency of photogenerated carriers limits the full utilization of the potential of pristine SWCNTs as photoactive materials. Herein, we report a novel high-performance visible-NIR SWCNT-based photodetector via interfacial charge-transfer-induced improvement by Au nanoparticle (AuNP) surface doping. Under 1064 nm light illumination, the as-fabricated AuNP/SWCNT photodetector exhibits an excellent photoelectrical performance with a responsivity of 2.16 × 10⁵ A/W and a high detectivity of 1.82 × 10¹⁴ Jones, which is three orders of magnitude higher than that of the SWCNT photodetector under the same conditions. Importantly, the interfacial charge transfer between AuNPs and SWCNTs has been first investigated using Raman shift statistics at room temperature. Experimental results indicate that the interfacial charge transfer induced by AuNP doping can reduce the Fermi level of SWCNTs and effectively improve the generation and transport of photogenerated carriers, thereby enhancing the photoelectric performance of SWCNT-based photodetectors. We believe that our results not only demonstrate a facile route to improve the performance of SWCNT-based photodetectors but also provide a novel methodology to characterize the interfacial charge transfer between dopants and SWCNTs.

Enhanced Charge Separation for Efficient Photocatalytic H2 Production by Long-Lived Trap-State-Induced Interfacial Charge Transfer

Photogeneration of charge carriers in semiconductors provides the scientific fundamental for photocatalytic water splitting. However, an ongoing challenge is the development of a new mechanism promoting charge carrier separation. Here we propose a trap-state-induced interfacial charge-transfer transition mechanism (TSICTT), in which electrons in long-lived trap states recombine with holes on the valence band (VB) of the semiconductor, thus prolonging the electron lifetime. We demonstrate this concept in the Sr₄Al₁₄O₂₅:Eu²⁺, Dy³⁺/CdS (SAO/CdS) heterostructure, where trapped electrons with a lifetime of up to several hours in the SAO persistent luminescence phosphor (PLP) can continuously consume holes on the VB of CdS nanoparticles (NPs). We discover that the interfacial interaction and the work function difference between SAO and CdS are crucial for the TSICTT, which finally contributes to the improved H₂ production from 34.4 to 1212.9 μmol g_CdS^-1 h^-1 under visible-light irradiation. This model introduces a new strategy to manipulate charge carrier transport for the effective utilization of solar energy.

Metal-Organic Framework-Derived Tubular In2O3-C/CdIn2S4 Heterojunction for Efficient Solar-Driven CO2 Conversion

Realizing high-efficiency solar-driven CO₂ reduction to chemicals and fuels requires high-performance photocatalysts with high utilization efficiency of solar light, efficient charge separation and transfer, and robust adsorption capacity for CO₂. In this work, a tubular In₂O₃-C/CdIn₂S₄ (IOC/CIS) ternary heterojunction with an intimate interfacial contact was fabricated by pyrolysis of In-MIL-68 and subsequent solvothermal synthesis of CdIn₂S₄. The construction of a heterojunction promotes the separation and transfer efficiency of photogenerated carriers. The introduction of carbon not only accelerates the interfacial charge migration but also enhances light absorption and CO₂ adsorption. The resulting 5IOC/CIS sample presents a remarkably improved photocatalytic CO₂ reduction activity with a CO generation rate of 2432 μmol g^-1 h^-1, much higher than that of the In₂O₃/CdIn₂S₄ (IO/CIS) heterojunction (1906 μmol g^-1 h^-1). Furthermore, the type II charge transfer mechanism of the heterojunction was confirmed by the electron paramagnetic resonance characterization. This work provides new insight into the design and preparation of a highly efficient hollow heterojunction for photocatalytic applications.

Investigation on the Mechanism of Radical Intermediate Formation and Moderate Oxidation of Spiro-OMeTAD by the Synergistic Effect of Multisubstituted Polyoxometalates in Perovskite Solar Cells

Conventional oxidation of 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD) by air would bring various drawbacks for perovskite solar cells (PSCs), such as low power conversion efficiency (PCE) and poor stability. Here, a series of heteroatom-substituted Keggin-type polyoxometalates (POMs), H₄PMo₁₁VO₄₀ (PMo₁₁V), H₅PMo₁₀V₂O₄₀ (PMo₁₀V₂), and H₆PMo₉V₃O₄₀ (PMo₉V₃) are prepared and applied as p-type dopants to realize quantitative and controllable oxidation of Spiro-OMeTAD under an inert condition. The possible mechanism and electron donor regions in the oxidation of Spiro-OMeTAD are investigated using two-dimensional nuclear magnetic resonance (NMR) spectra and the relationship between POM structures and the oxidation degree of Spiro-OMeTAD is proposed. In addition, the synergistic effect of heteroatoms makes V₂-substituted PMo₁₀V₂ exhibit appropriate oxidation of Spiro-OMeTAD and promoted the highest efficient hole extraction as well as the decreased charge recombination. Therefore, the champion device doped with PMo₁₀V₂ shows a PCE of 20.41% and a superior open circuit voltage (V_oc) of 1.133 V, surpassing that of the pristine device (18.61%). This work presents a fresh perspective to the controllable oxidation of Spiro-OMeTAD employing economical inorganic POM dopants, which would promote the commercialization of PSCs.

Magic Doping and Robust Superconductivity in Monolayer FeSe on Titanates

The enhanced superconductivity in monolayer FeSe on titanates opens a fascinating pathway toward the rational design of high-temperature superconductors. Utilizing the state-of-the-art oxide plus chalcogenide molecular beam epitaxy systems in situ connected to a synchrotron angle-resolved photoemission spectroscope, epitaxial LaTiO3 layers with varied atomic thicknesses are inserted between monolayer FeSe and SrTiO3, for systematic modulation of interfacial chemical potential. With the dramatic increase of electron accumulation at the LaTiO3/SrTiO3 surface, providing a substantial surge of work function mismatch across the FeSe/oxide interface, the charge transfer and the superconducting gap in the monolayer FeSe are found to remain markedly robust. This unexpected finding indicate the existence of an intrinsically anchored "magic" doping within the monolayer FeSe systems.

Interfacial Charge Transport in 1D TiO2 Based Photoelectrodes for Photoelectrochemical Water Splitting

1D nanostructured photoelectrodes are promising for application as photoelectrochemical (PEC) devices for solar energy conversion into hydrogen (H₂ ) owing to the optical, structural, and electronic advantages. Titanium dioxide (TiO₂ ) is the most investigated candidate as a photoelectrode due to its good photostability, low production cost, and eco-friendliness. The obstacle for TiO₂ 's practical application is the inherent wide bandgap (UV-lights response), poor conductivity, and limited hole diffusion length. Here, a comprehensive review of the current research efforts toward the development of 1D TiO₂ based photoelectrodes for heterogeneous PEC water splitting is provided along with a discussion of nanoarchitectures and energy band engineering influences on interfacial charge transfer and separation of 1D TiO₂ composited with different dimensional photoactive materials. The key focus of this review is to understand the charge transfer processes at interfaces and the relationship between photogenerated charge separation and photoelectrochemical performance. It is anticipated that this review will afford enriched information on the rational designs of nanoarchitectures, doping, and heterojunction interfaces for 1D TiO₂ based photoelectrodes to achieve highly efficient solar energy conversion.

Isolated Cobalt Ions Embedded in Magnesium Oxide Nanostructures: Spectroscopic Properties and Redox Activity

Atomic dispersion of dopants and control over their defect chemistry are central goals in the development of oxide nanoparticles for functional materials with dedicated electronic, optical or magnetic properties. We produced highly dispersed oxide nanocubes with atomic distribution of cobalt ions in substitutional sites of the MgO host lattice via metal organic chemical vapor synthesis. Vacuum annealing of the nanoparticle powders up to 1173 K has no effect on the shape of the individual particles and only leads to moderate particle coarsening. Such materials processing, however, gives rise to the electronic reduction of particle surfaces, which-upon O2 admission-stabilize anionic oxygen radicals that are accessible to UV/Vis diffuse reflectance and electron paramagnetic resonance (EPR) spectroscopy. Multi-reference quantum chemical calculations show that the optical bands observed mainly originate from transitions into 4 A2g (4 F), 4 T1g (4 P) states with a contribution of transitions into 2 T1g , 2 T2g (2 G) states through spin-orbit coupling and gain intensity through vibrational motion of the MgO lattice or the asymmetric ion field. Related nanostructures are a promising material system for single atomic site catalysis. At the same time, it represents an extremely valuable model system for the study of interfacial electron transfer processes that are key to nanoparticle chemistry and photochemistry at room temperature, and in heterogeneous catalysis.

Resistive Switching in Few-Layer Hexagonal Boron Nitride Mediated by Defects and Interfacial Charge Transfer

We present resistive switching (RS) behavior of few-layer hexagonal boron nitride (h-BN) mediated by defects and interfacial charge transfer. Few-layer h-BN is grown by metal-organic chemical vapor deposition and used as active RS medium in Ti/h-BN/Au structure, exhibiting clear bipolar RS behavior and fast switching characteristics about ∼25 ns without an initial electroforming process. Systematic investigation on microstructural and chemical characteristics of the h-BN reveals that there are structural defects such as homoelemental B-B bonds at grain boundaries and nitrogen vacancies, which can provide preferential pathways for the penetration of Ti^x+ ions through the h-BN film. In addition, the interfacial charge transfer from Ti to the h-BN is observed by in situ X-ray photoelectron spectroscopy. We suggest that the attractive Coulomb interaction between positively charged Ti^x+ ions and the negatively charged h-BN surface as a result of the interfacial charge transfer facilitates the migration of Ti^x+ ions at the Ti/h-BN interface, leading to the facile formation of conductive filaments. We believe that these findings can improve our understanding of the fundamental mechanisms involved in RS behavior of h-BN and contribute a significant step for the future development of h-BN-based nonvolatile memory applications.

Defects-Induced In-Plane Heterophase in Cobalt Oxide Nanosheets for Oxygen Evolution Reaction

Cobalt oxides as efficient oxygen evolution reaction (OER) electrocatalysts have received much attention because of their rich reserves and cheap cost. There are two common cobalt oxides, Co₃ O₄ (spinel phase, stable but poor intrinsic activity) and CoO (rocksalt phase, active but easily be oxidatized). Constructing Co₃ O₄ /CoO heterophase can inherit both characteristic features of each component and form a heterophase interface facilitating charge transfer, which is believed to be an effective strategy in designing excellent electrocatalysts. Herein, an atomic arrangement engineering strategy is applied to improve electrocatalytic activity of Co₃ O₄ for the OER. With the presence of oxygen vacancies, cobalt atoms at tetrahedral sites in Co₃ O₄ can more easily diffuse into interstitial octahedral sites to form CoO phase structure as revealed by periodic density functional theory computations. The Co₃ O₄ /CoO spinel/rocksalt heterophase can be in situ fabricated at the atomic scale in plane. The overpotential to reach 10 mA cm^-2 of Co₃ O₄ /CoO is 1.532 V, which is 92 mV smaller than that of Co₃ O₄ . Theoretical calculations confirm that the excellent electrochemical activity is corresponding to a decline in average p-state energy of adsorbed-O on the Co₃ O₄ /CoO heterophase interface. The reaction Gibbs energy barrier has been significantly decreased with the construction of the heterophase interface.

Coupling Niobia Nanorods with a Multicomponent Carbon Network for High Power Lithium-Ion Batteries

High power lithium-ion batteries require highly conductive electrodes. For an active electrode material that has limited electron conductivity, it is critical to build a carbon network that is not only highly conductive itself but also highly compatible with the electroactive material for efficient interfacial charge transfer. Herein, we design a multicomponent carbon network that is optimized for electrical coupling with the electroactive Nb₂O₅ nanorods for efficient electron injection. The self-support electrode is constructed by using 0D polypyrrole-derived (Ppy) carbon nanoparticles as glue to bind the Nb₂O₅ nanorods with 1D carbon nanotubes (CNTs) and 2D graphene nanosheets (GNSs). The 0D carbon nanoparticles also cross-link 1D CNTs with 2D GNSs, which can effectively prevent the GNSs from aggregation and form the 3D CNT/GNS network that provides continuous electronic and ionic pathways. This 3D Nb₂O₅@C self-support electrode exhibits a high discharge capacity of 246.3 mA h g^-1 at 0.5 C and 100 mA h g^-1 at 20 C and excellent Coulombic efficiency of 99.98% at 20 C. Even increasing the mass loading to 7.1 mg cm^-2, the Nb₂O₅@C electrode can still reach a discharge capacity of 172.4 mA h g^-1 at 0.5 C after 100 cycles. A high power density of 1043 W kg^-1 can be achieved at an energy density of 104.3 W h kg^-1 based on the electrode weight, which is among the highest values demonstrated so far for Nb₂O₅ electrodes. The results pave the way toward practical applications of Nb₂O₅ anodes in high-power lithium-ion batteries.

Interfacial charge-transfer Mott state in iridate-nickelate superlattices

We investigate [Formula: see text]/[Formula: see text] superlattices in which we observe a full electron transfer at the interface from Ir to Ni, triggering a massive structural and electronic reconstruction. Through experimental characterization and first-principles calculations, we determine that a large crystal field splitting from the distorted interfacial [Formula: see text] octahedra surprisingly dominates over the spin-orbit coupling and together with the Hund's coupling results in the high-spin (S = 1) configurations on both the Ir and Ni sites. This demonstrates the power of interfacial charge transfer in coupling lattice, charge, orbital, and spin degrees of freedom, opening fresh avenues of investigation of quantum states in oxide superlattices.

Strain-Engineered van der Waals Interfaces of Mixed-Dimensional Heterostructure Arrays

van der Waals (vdWs) heterostructures have provided a platform for nanoscale material integrations and enabled promise for use in optoelectronic devices. Because of the ultrastrength of two-dimensional materials, strain engineering is considered as an effective way to tune their band structures and further tailor the interface performance of vdWs heterostructures. However, the less-constrained vdWs interfaces make the traditional strain technique via lattice-mismatched growth infeasible. Here, we report a strategy to construct mixed-dimensional heterostructure arrays with periodically strain-engineered vdWs interfaces utilizing one-dimensional semiconductor-induced nanoindentation. Using monolayer MoS₂ (1L-MoS₂)/ZnO heterostructure arrays as a model system, we demonstrate inhomogeneous built-in strain gradient at the heterointerfaces ranging from 0 to 0.6% tensile. Through systematic optical characterization of the hybrid structures, we verify that strain can improve the interfacial charge transfer efficiency. Consequently, we observe that the photoluminescence (PL) emission of 1L-MoS₂ at strained interfaces is dramatically quenched more than 50% with respect to that at unstrained interfaces. Furthermore, we confirm that the strain-optimized interfacial carrier behavior is attributed to the reduction of interfacial barrier height, which originated from the strain-dependent Fermi level of 1L-MoS₂. These results demonstrate that strain provides another degree of freedom in tuning the vdWs interface performance and our method developed here should enable flexibility in achieving more sophisticated vdWs integration via strain engineering.

Modulation of Acceptor Position in Organic Sensitizers: The Optimization of Intramolecular and Interfacial Charge Transfer Processes

With respect to cyanoacryclic acid as the traditional acceptor and anchoring group in dye-sensitized solar cells, the introduction of stronger electron-deficient groups has greatly expanded the scope of molecular design and increased their efficiencies for organic dyes. In this contribution, benzothiadiazole (BTD) was selected as a representative acceptor to illustrate the influence of its position on the photophysical properties and corresponding photovoltaic performances. Through the insertion of BTD in different positions of the framework with the same composition units and orders, four sensitizers were designed and synthesized. The structure-property relationship demonstrated the preference of the suitable position of an electron acceptor for optimization of the spectra response and interfacial charge transfer. In our system, dye LI-96 with BTD in the middle of the conjugated bridge showed the broadest spectrum and achieved the best photovoltaic performance (8.25%), which may pave a new way to design or optimize the efficient sensitizers by rational design of the acceptor position.

In Situ Formation of WO3-Based Heterojunction Photoanodes with Abundant Oxygen Vacancies via a Novel Microbattery Method

Non-stoichiometric ratio semiconductor materials have exhibited excellent performance in energy conversion and storage fields. However, the hydrogen treatment method that is commonly used to introduce oxygen vacancies is expensive and dangerous. In this paper, a novel microbattery method using Zn powder and Fe powder as reductant has been developed to synthesize the oxygen vacancy modified WO_{3- x} films and oxygen-deficient heterojunction films (ZnWO_{4- x}/WO_{3- x} and Fe₂O_{3- x}/WO_{3- x}) at room temperature. The as-prepared WO_{3- x} and ZnWO_{4- x}/WO_{3- x} heterojunction films exhibit improved photoelectrochemical performance. It is worth noting that this microbattery method can quickly introduce oxygen vacancies into semiconductor materials, including powders and films at room temperature.

Interaction-Dependent Interfacial Charge-Transfer Behavior in Solar Water-Splitting Systems

Dual-band-gap systems are promising for solar water splitting due to their excellent light-harvesting capability and high charge-separation efficiency. However, a fundamental understanding of interfacial charge-transfer behavior in the dual-band-gap configuration is still incomplete. Taking CdS/reduced graphene oxide (CdS/RGO) nanoheterojunctions as a model solar water splitting system, we attempt here to highlight the interaction-dependent interfacial charge-transfer behavior based on both experimental observations and theoretical calculations. Experimental evidence points to charge transfer at the CdS-RGO interface playing a dominant role in the photocatalytic hydrogen production activity. By tuning the degree of reduction of RGO, the interfacial interaction, and, thereby, the charge transfer can be controlled at the CdS-RGO interface. This observation is supported by theoretical analysis, where we find that the interfacial charge transfer is a balance between the effective single-electron- and hole-transfer probability and the surface free electron and hole concentration, both of which are related to the surface potential and tailored by interfacial interaction. This mechanism is applicable to all systems for solar water splitting, providing a useful guidance for the design and study of heterointerfaces for high-efficiency energy conversion.

Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu2O Polyhedra Arising from Tunable Interfacial Band Alignment

ZnS particles were grown over Cu₂O cubes, octahedra, and rhombic dodecahedra for examination of their facet-dependent photocatalytic behaviors. After ZnS growth, Cu₂O cubes stay photocatalytically inactive. ZnS-decorated Cu₂O octahedra show enhanced photocatalytic activity, resulting from better charge carrier separation upon photoexcitation. Surprisingly, Cu₂O rhombic dodecahedra give greatly suppressed photocatalytic activity after ZnS deposition. Electron paramagnetic resonance spectra agree with these experimental observations. Time-resolved photoluminescence profiles provide charge-transfer insights. The decrease in the photocatalytic activity is attributed to an unfavorable band alignment caused by significant band bending within the Cu₂O(110)/ZnS(200) plane interface. A modified Cu₂O-ZnS band diagram is presented. Density functional theory calculations generating plane-specific band energy diagrams of Cu₂O and ZnS match well with the experimental results, showing that charge transfer across the Cu₂O(110)/ZnS(200) plane interface would not happen. This example further illustrates that the actual photocatalysis outcome for semiconductor heterojunctions cannot be assumed because interfacial charge transfer is strongly facet-dependent.

Fe2TiO5 as an Efficient Co-catalyst To Improve the Photoelectrochemical Water Splitting Performance of BiVO4

Fe₂TiO₅ was synthesized via the solvothermal method and adopted as co-catalyst to improve the photoelectrochemical (PEC) water splitting performance of BiVO₄ photoanode. After surface modification by Fe₂TiO₅, the BiVO₄/Fe₂TiO₅ photoanode shows a 300 mV cathodic shift in onset potential and 3 times enhancement in photocurrent, which delivers a photocurrent density of 3.2 mA/cm² at 1.23 V vs reverse hydrogen electrode. Systematic optical, electrochemical, and intensity-modulated photocurrent spectroscopy characterizations were performed to explore the role of Fe₂TiO₅ and reveal that the enhanced PEC performance is mainly caused by the surface passivation effect of Fe₂TiO₅.

Efficiently Improving the Stability of Inverted Perovskite Solar Cells by Employing Polyethylenimine-Modified Carbon Nanotubes as Electrodes

Inverted perovskite solar cells (PSCs) have been becoming more and more attractive, owing to their easy-fabrication and suppressed hysteresis, while the ion diffusion between metallic electrode and perovskite layer limit the long-term stability of devices. In this work, we employed a novel polyethylenimine (PEI) modified cross-stacked superaligned carbon nanotube (CSCNT) film in the inverted planar PSCs configurated FTO/NiO _x/methylammonium lead tri-iodide (MAPbI₃)/6, 6-phenyl C61-butyric acid methyl ester (PCBM)/CSCNT:PEI. By modifying CSCNT with a certain concentration of PEI (0.5 wt %), suitable energy level alignment and promoted interfacial charge transfer have been achieved, leading to a significant enhancement in the photovoltaic performance. As a result, a champion power conversion efficiency (PCE) of ∼11% was obtained with a V_oc of 0.95 V, a J_sc of 18.7 mA cm^-2, a FF of 0.61 as well as negligible hysteresis. Moreover, CSCNT:PEI based inverted PSCs show superior durability in comparison to the standard silver based devices, remaining over 85% of the initial PCE after 500 h aging under various conditions, including long-term air exposure, thermal, and humid treatment. This work opens up a new avenue of facile modified carbon electrodes for highly stable and hysteresis suppressed PSCs.

Highly Efficient Performance and Conversion Pathway of Photocatalytic CH3SH Oxidation on Self-Stabilized Indirect Z-Scheme g-C3N4/I3--BiOI

A self-stabilized Z-scheme porous g-C₃N₄/I^3--containing BiOI ultrathin nanosheets (g-C₃N₄/I^3--BiOI) heterojunction photocatalyst with I₃^-/I^- redox mediator was successfully synthesized by a facile solvothermal method coupling with light illumination. The structure and optical properties of g-C₃N₄/I^3--BiOI composites were systematically characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, X-ray photoelectron spectroscopy, N₂ adsorption/desorption, UV-vis diffuse reflectance spectrum, and photoluminescence. The g-C₃N₄/I^3--BiOI composites, with a heterojunction between porous g-C₃N₄ and BiOI ultrathin nanosheets, were first applied for the photocatalytic elimination of ppm-leveled CH₃SH under light-emitting diode visible light illumination. The g-C₃N₄/I^3--BiOI heterojunction with 10% g-C₃N₄ showed a dramatically enhanced photocatalytic activity in the removal of CH₃SH compared with pure BiOI and g-C₃N₄ due to its effective interfacial charge transfer and separation. The adsorption and photocatalytic oxidation of CH₃SH over g-C₃N₄/I^3--BiOI were deeply explored by in situ diffuse reflectance infrared Fourier transform spectroscopy, and the intermediates and conversion pathways were elucidated and compared. Furthermore, on the basis of reactive species trapping, electron spin resonance and Mott-Schottky experiments, it was revealed that the responsible reactive species for catalytic CH₃SH composition were h⁺, ^•O₂^-, and ¹O₂; thus, the g-C₃N₄/I^3--BiOI heterojunction followed an indirect all-solid state Z-scheme charge-transfer mode with self-stabilized I₃^-/I^- pairs as redox mediator, which could accelerate the separation of photogenerated charge and enhance the redox reaction power of charged carriers simultaneously.

In-Situ Synthesis of Hydrogen Titanate Nanotube/Graphene Composites with a Chemically Bonded Interface and Enhanced Visible Photocatalytic Activity

Hydrogen titanate nanotube (HTT)/graphene nanocomposites are synthesized by hydrothermal reduction of graphene oxide (GO) and simultaneous preparation of nanotubular HTT via an alkaline hydrothermal process. By using this facile in-situ compositing strategy, HTT are densely supported upon the surface of graphene sheets with close interface contacts. The as-prepared HTT/graphene nanocomposites possess significantly enhanced visible light catalytic activity for the partial oxidation of benzylic alcohols. The amount of graphene has significant influence on catalytic activity and the optimal content of graphene is 1.0 wt %, giving a normalized rate constant k of 1.71 × 10⁻³ g/m²·h, which exceeds that of pure HTT and HTT/graphene-1.0% mixed by a factor of 7.1 or 5.2. Other than the general role of graphene as a high-performance electron acceptor or transporter, the observed enhancement in photocatalytic activity over HTT/graphene can be ascribed to the improved interfacial charge migration from enhanced chemical bonding (Ti–C bonds) during the in-situ compositing process. The formation of Ti–C bonds is confirmed by XPS analysis and the resulting enhanced separation of photoinduced charge carriers is demonstrated by electrochemical impedance spectra and transient photocurrent response.

Remarkable SERS Activity Observed from Amorphous ZnO Nanocages

Enhancement of the semiconductor-molecule interaction, in particular, promoting the interfacial charge transfer process (ICTP), is key to improving the sensitivity of semiconductor-based surface enhanced Raman scattering (SERS). Herein, by developing amorphous ZnO nanocages (a-ZnO NCs), we successfully obtained an ultrahigh enhancement factor of up to 6.62×10⁵ . This remarkable SERS sensitivity can be attributed to high-efficiency ICTP within a-ZnO NC molecule system, which is caused by metastable electronic states of a-ZnO NCs. First-principles density functional theory (DFT) simulations further confirmed a stronger ICTP in a-ZnO NCs than in their crystalline counterparts. The efficient ICTP can even generate π bonding in Zn-S bonds peculiar to the mercapto molecule adsorbed a-ZnO NCs, which has been verified through the X-ray absorption near-edge structure (XANES) characterization. To the best of our knowledge, this is the first time such remarkable SERS activity has been observed within amorphous semiconductor nanomaterials, which could open a new frontier for developing highly sensitive and stable SERS technology.

Molecular Arrangement and Charge Transfer in C60/Graphene Heterostructures

Charge transfer at the interface between dissimilar materials is at the heart of electronics and photovoltaics. Here we study the molecular orientation, electronic structure, and local charge transfer at the interface region of C₆₀ deposited on graphene, with and without supporting substrates such as hexagonal boron nitride. We employ ab initio density functional theory with van der Waals interactions and experimentally characterize interface devices using high-resolution transmission electron microscopy and electronic transport. Charge transfer between C₆₀ and the graphene is found to be sensitive to the nature of the underlying supporting substrate and to the crystallinity and local orientation of the C₆₀. Even at room temperature, C₆₀ molecules interfaced to graphene are orientationally locked into position. High electron and hole mobilities are preserved in graphene with crystalline C₆₀ overlayers, which has ramifications for organic high-mobility field-effect devices.

Construction and Nanoscale Detection of Interfacial Charge Transfer of Elegant Z-Scheme WO3/Au/In2S3 Nanowire Arrays

Elegant Z-scheme WO3/Au/In2S3 nanowire arrays were precisely constructed through a facile step-by-step route. Surface potential change on pristine or In2S3-Au coated WO3 single nanowire under dark and illumination detected through a Kelvin probe force microscopy (KPFM) technique indicates that the vectorial holes transfer of In2S3 → Au → WO3 should occur upon the excitation of both WO3 and In2S3. In such charge transfer processes, the embedded Au nanoparticles in the heterojunction systems act as a charge mediator for electrons in the conduction band of WO3 and holes in the valence band of In2S3. The strong charge carrier separation ability of this structure will finally enhance the oxidation ability of WO3 with high concertation of photogenerated holes and, further, leave the free electrons in the In2S3 with long surviving time. Therefore, the unique Z-scheme WO3/Au/In2S3 heterostructure shows great visible-light activity toward photocatalytic reduction of CO2 in the presence of water vapor into renewable hydrocarbon fuel (methane: CH4).

Construction and Nanoscale Detection of Interfacial Charge Transfer of Elegant Z-Scheme WO3/Au/In2S3 Nanowire Arrays

Elegant Z-scheme WO3/Au/In2S3 nanowire arrays were precisely constructed through a facile step-by-step route. Surface potential change on pristine or In2S3-Au coated WO3 single nanowire under dark and illumination detected through a Kelvin probe force microscopy (KPFM) technique indicates that the vectorial holes transfer of In2S3 → Au → WO3 should occur upon the excitation of both WO3 and In2S3. In such charge transfer processes, the embedded Au nanoparticles in the heterojunction systems act as a charge mediator for electrons in the conduction band of WO3 and holes in the valence band of In2S3. The strong charge carrier separation ability of this structure will finally enhance the oxidation ability of WO3 with high concertation of photogenerated holes and, further, leave the free electrons in the In2S3 with long surviving time. Therefore, the unique Z-scheme WO3/Au/In2S3 heterostructure shows great visible-light activity toward photocatalytic reduction of CO2 in the presence of water vapor into renewable hydrocarbon fuel (methane: CH4).

Photoelectrochemical Water Splitting System--A Study of Interfacial Charge Transfer with Scanning Electrochemical Microscopy

Fast charge transfer kinetics at the photoelectrode/electrolyte interface is critical for efficient photoelectrochemical (PEC) water splitting system. Thus, far, a measurement of kinetics constants for such processes is limited. In this study, scanning electrochemical microscopy (SECM) is employed to investigate the charge transfer kinetics at the photoelectrode/electrolyte interface in the feedback mode in order to simulate the oxygen evolution process in PEC system. The popular photocatalysts BiVO4 and Mo doped BiVO4 (labeled as Mo:BiVO4) are selected as photoanodes and the common redox couple [Fe(CN)6](3-)/[Fe(CN)6](4-) as molecular probe. SECM characterization can directly reveal the surface catalytic reaction kinetics constant of 9.30 × 10(7) mol(-1) cm(3) s(-1) for the BiVO4. Furthermore, we find that after excitation, the ratio of rate constant for photogenerated hole to electron via Mo:BiVO4 reacting with mediator at the electrode/electrolyte interface is about 30 times larger than that of BiVO4. This suggests that introduction of Mo(6+) ion into BiVO4 can possibly facilitate solar to oxygen evolution (hole involved process) and suppress the interfacial back reaction (electron involved process) at photoanode/electrolyte interface. Therefore, the SECM measurement allows us to make a comprehensive analysis of interfacial charge transfer kinetics in PEC system.

Peripheral Hole Acceptor Moieties on an Organic Dye Improve Dye-Sensitized Solar Cell Performance

Investigation of charge transfer dynamics in dye-sensitized solar cells is of fundamental interest and the control of these dynamics is a key factor for developing more efficient solar cell devices. One possibility for attenuating losses through recombination between injected electrons and oxidized dye molecules is to move the positive charge further away from the metal oxide surface. For this purpose, a metal-free dye named E6 is developed, in which the chromophore core is tethered to two external triphenylamine (TPA) units. After photoinduced electron injection into TiO2, the remaining hole is rapidly transferred to a peripheral TPA unit. Electron-hole recombination is slowed down by 30% compared to a reference dye without peripheral TPA units. Furthermore, it is found that the added TPA moieties improve the electron blocking effect of the dye, retarding recombination of electrons from TiO2 to the cobalt-based electrolyte.

Observation of Ground- and Excited-State Charge Transfer at the C60/Graphene Interface

We examine charge transfer interactions in the hybrid system of a film of C60 molecules deposited on single-layer graphene using Raman spectroscopy and Terahertz (THz) time-domain spectroscopy. In the absence of photoexcitation, we find that the C60 molecules in the deposited film act as electron acceptors for graphene, yielding increased hole doping in the graphene layer. Hole doping of the graphene film by a uniform C60 film at a level of 5.6 × 10(12)/cm(2) or 0.04 holes per interfacial C60 molecule was determined by the use of both Raman and THz spectroscopy. We also investigate transient charge transfer occurring upon photoexcitation by femtosecond laser pulses with a photon energy of 3.1 eV. The C60/graphene hybrid exhibits a short-lived (ps) decrease in THz conductivity, followed by a long-lived increase in conductivity. The initial negative photoconductivity transient, which decays within 2 ps, reflects the intrinsic photoresponse of graphene. The longer-lived positive conductivity transient, with a lifetime on the order of 100 ps, is attributed to photoinduced hole doping of graphene by interfacial charge transfer. We discuss possible microscopic pathways for hot carrier processes in the hybrid system.

Molecular-structure Control of Ultrafast Electron Injection at Cationic Porphyrin-CdTe Quantum Dot Interfaces

Charge transfer (CT) at donor (D)/acceptor (A) interfaces is central to the functioning of photovoltaic and light-emitting devices. Understanding and controlling this process on the molecular level has been proven to be crucial for optimizing the performance of many energy-challenge relevant devices. Here, we report the experimental observations of controlled on/off ultrafast electron transfer (ET) at cationic porphyrin-CdTe quantum dot (QD) interfaces using femto- and nanosecond broad-band transient absorption (TA) spectroscopy. The time-resolved data demonstrate how one can turn on/off the electron injection from porphyrin to the CdTe QDs. With careful control of the molecular structure, we are able to tune the electron injection at the porphyrin-CdTe QD interface from zero to very efficient and ultrafast. In addition, our data demonstrate that the ET process occurs within our temporal resolution of 120 fs, which is one of the fastest times recorded for organic photovoltaics.

Investigation of regeneration kinetics in quantum-dots-sensitized solar cells with scanning electrochemical microscopy

A fast quantum dots (QDs) regeneration process is necessary for highly efficient QDs-sensitized solar cells. Herein, CdSe and CdS QDs regeneration rates (kQD') in three redox electrolytes, which are triiodide and iodide ions (I3(-)/I(-)), Co(bpy)3(PF6)2 and Co(bpy)3(PF6)3 (Co(3+)/Co(2+)), and 1-methy-1-H-tetrazole-5-thiolate and its dimer (T2/T(-)), have been first investigated with scanning electrochemical microscopy (SECM). The results reveal that the kinetics of QDs regeneration depends on the nature of the QDs and the redox shuttles presented in QDSSCs. For QDs of CdSe and CdS, the regeneration rate (kQD') in the case of a T2/T(-)-based electrolyte is about two times larger than that of Co(3+)/Co(2+) and I3(-)/I(-). Additionally, the kQD' for CdSe is about two times larger than that of CdS in the same redox shuttle electrolyte, which could be due to a large driving force for the reaction between the exited state quantum dots (QD(+)) and redox electrolytes.

Tuning the localized surface plasmon resonance in Cu(2-x)Se nanocrystals by postsynthetic ligand exchange

Nanoparticles exhibiting localized surface plasmon resonances (LSPR) are valuable tools traditionally used in a wide field of applications including sensing, imaging, biodiagnostics and medical therapy. Plasmonics in semiconductor nanocrystals is of special interest because of the tunability of the carrier densities in semiconductors, and the possibility to couple the plasmonic resonances to quantum confined excitonic transitions. Here, colloidal Cu2-xSe nanocrystals were synthesized, whose composition was shown by Rutherford backscattering analysis and electron dispersive X-ray spectroscopy, to exhibit Cu deficiency. The latter results in p-type doping causing LSPRs, in the present case around a wavelength of 1100 nm, closely matching the indirect band gap of Cu2-xSe. By partial exchange of the organic ligands to specific electron trapping or donating species the LSPR is fine-tuned to exhibit blue or red shifts, in total up to 200 nm. This tuning not only provides a convenient tool for post synthetic adjustments of LSPRs to specific target wavelength but the sensitive dependence of the resonance wavelength on surface charges makes these nanocrystals also interesting for sensing applications, to detect analytes dressed by functional groups.

Charge carrier dynamics of photoexcited Co3O4 in methanol: extending high harmonic transient absorption spectroscopy to liquid environments

Charge carrier dynamics in Co3O4 thin films are observed using high harmonic generation transient absorption spectroscopy at the Co M2,3 edge. Results reveal that photoexcited Co3O4 decays to the ground state in 600 ± 40 ps in liquid methanol compared to 1.9 ± 0.3 ns in vacuum. Kinetic analysis suggests that surface-mediated relaxation of photoexcited Co3O4 may be the result of hole transfer from Co3O4 followed by carrier recombination at the Co3O4-methanol interface.

Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye-Semiconductor Interface

Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation.

Engineering the TiO2 -graphene interface to enhance photocatalytic H2 production

In this work, TiO2 -graphene nanocomposites are synthesized with tunable TiO2 crystal facets ({100}, {101}, and {001} facets) through an anion-assisted method. These three TiO2 -graphene nanocomposites have similar particle sizes and surface areas; the only difference between them is the crystal facet exposed in TiO2 nanocrystals. UV/Vis spectra show that band structures of TiO2 nanocrystals and TiO2 -graphene nanocomposites are dependent on the crystal facets. Time-resolved photoluminescence spectra suggest that the charge-transfer rate between {100} facets and graphene is approximately 1.4 times of that between {001} facets and graphene. Photoelectrochemical measurements also confirm that the charge-separation efficiency between TiO2 and graphene is greatly dependent on the crystal facets. X-ray photoelectron spectroscopy reveals that Ti-C bonds are formed between {100} facets and graphene, while {101} facets and {001} facets are connected with graphene mainly through Ti-O-C bonds. With Ti-C bonds between TiO2 and graphene, TiO2 -100-G shows the fastest charge-transfer rate, leading to higher activity in photocatalytic H2 production from methanol solution. TiO2 -101-G with more reductive electrons and medium interfacial charge-transfer rate also shows good H2 evolution rate. As a result of its disadvantageous electronic structure and interfacial connections, TiO2 -001-G shows the lowest H2 evolution rate. These results suggest that engineering the structures of the TiO2 -graphene interface can be an effective strategy to achieve excellent photocatalytic performances.

Direct correlation of electrochemical behaviors with anti-thrombogenicity of semiconducting titanium oxide films

Biomaterials-associated thrombosis is dependent critically upon electrochemical response of fibrinogen on material surface. The relationship between the response and anti-thrombogenicity of biomaterials is not well-established. Titanium oxide appears to have good anti-thrombogenicity and little is known about its underlying essential chemistry. We correlate their anti-thrombogenicity directly to electrochemical behaviors in fibrinogen containing buffer solution. High degree of inherent n-type doping was noted to contribute the impedance preventing charge transfer from fibrinogen into film (namely its activation) and consequently reduced degree of anti-thrombogenicity. The impedance was the result of high donor carrier density as well as negative flat band potential.