Drive Charge Separation Research Articles

Surface-iodine-grafting-induced local electric field drives charge separation of CdBiO2Br for excellent photocatalytic antibiotic degradation

Photocatalytic technology has demonstrated immense potential in environmental purification, particularly in the degradation of antibiotics that pose serious health risks. However, the rapid recombination of photogenerated charge carriers and the limited light absorption capacity, which are major obstacles, severely hinder the photocatalytic degradation activity. Herein, the surface halogen-grafting strategy is introduced to address these issues in a Sillén-related layer-structured photocatalyst, CdBiO2Br (CBOB). The iodine-grafted CBOB nanosheets deliver a superb tetracycline hydrochloride photodegradation efficiency of 72.36 % within one hour, which is 2.44 times higher than that of the original CBOB. Additionally, they demonstrated a broad-spectrum degradation capability against various antibiotics and phenolic pollutants, such as ciprofloxacin, chloramphenicol hydrochloride, bisphenol A, highlighting their practical and industrial value. Mechanistic studies indicated that the high electronegativity of iodine could induce a surface dipole polarization effect to form localized electric fields that efficiently drive the separation of photogenerated electrons and holes, and, on the other hand, improve light absorption, which profoundly boosts the photocatalytic degradation process, as corroborated by experimental and theoretical results collectively. This work provides a foundation for understanding the highly electronegative halogen grafting-induced surface local electric field and offers new insights for the future design of high-performance environmental purification materials.

(Invited) Charge Separation and Stabilisation in Photocatalyst Materials for Solar Driven Water Splitting

In photocatalytic systems for solar energy conversion, a key challenge for efficient operation is the efficient separation and stabilisation of photogenerated charges, and thereby the minimisation of undesired recombination losses. This challenge is particularly great for photocatalytic systems because of the long charge lifetimes required to drive catalysis A further consideration is to minimise the energy losses required to drive this charge separation. My talk will cover examples of recent work from my group addressing aspects of this challenge. I will start by considering the role of polaron localisation / relaxation in driving charge separation in metal oxides, and quantification of the energetic loss associated with this polaron formation. Kinetic competion between polaron formation and ligand field state mediated charge recombination will be highlighted as the key challenge limiting the performance of many visible light absorbing metal oxides. The roles of metal oxide dielectric constant, surface facet energies, traps, ‘photocharging’ and heterojunctions in aiding charge separation in metal oxides will be discussed, drawing on examples from BiVO4 and SrTiO3. I will then go on to discuss charge separation and stabilisation in alternative photocatalyst materials, including organic semiconductor nanoparticles and metal-organic frameworks, highlighting the observation of remarkably long lived charge photogeneration in both systems and their relevance to the efficiency photocatalytic performance.

Cocreation of photogenerated electron and hole collectors on polymeric carbon nitride synergistically promotes carrier separation and reaction kinetics towards propelling photocatalytic hydrogen evolution

It is a challenging issue for the creation of photogenerated carrier collectors on the photocatalyst to drive charge separation and promote reaction kinetics in the photocatalytic reaction. Herein, based on one-step dual-modulation strategy, IrO2 nanodots are modified at the edge of polymeric carbon nitride (PCN) nanosheets and atomically dispersed Ir atoms are implanted in the skeleton of PCN to obtain a unique Ir-PCN/IrO2 photocatalyst. IrO2 nanodots and atomically dispersed Ir atoms act as hole and electron collectors to synergistically promote the carrier separation and reaction kinetics, respectively, thereby greatly improving the photocatalytic hydrogen evolution (PHE) performance. As a result, without adding additional cocatalyst, the PHE rate over the optimal Ir-PCN/IrO2-2% sample reaches up to 1564.4 μmol h−1 g−1 under the visible light irradiation, with achieving an apparent quantum yield (AQY) of 15.7% at 420 nm.

Built-in electric field intensified by SPR effect to drive charge separation over Ag-AgI/Sb2S3 heterojunction for high-efficiency photocatalytic metronidazole mineralization

The photocatalytic activity of semiconductor is extremely limited due to the rapid recombination of photo-generated carriers and low utilization of solar energy. Herein, a novel Ag-AgI/Sb2S3 (A-S) heterojunction with superior photocatalytic activity was synthesized, in which the optimal Ag-AgI/Sb2S3 heterojunction exhibited 31 times photocatalytic activity of metronidazole removal than that of than that of Sb2S3. The excellent performance of the prepared photocatalysts could be attributed to the synergy of surface plasmon resonance effect by in situ-generated Ag nanoparticles and the built-in electric field of heterostructure, which would broaden the response range of solar light and accelerate the transfer of photogenerated carriers. Besides, characterization and theoretical calculations confirm Ag atoms acquire abundant electrons from Sb atoms of Sb2S3, allowing Ag+ to reduce to Ag0. Meanwhile, a portion of the Ag0 plasma-induced hot electrons reduce O2 to ·O2–, while the remaining portion flows to AgI and circulates in AgI and Ag0 to form a closed loop, thus eliminating the photo-corrosion of AgI. The stability of A-S photocatalyst was also confirmed by five cycle-tests. Furthermore, a reasonable photocatalytic mechanism was proposed by analyzing the intermediates of metronidazole. This work has implications for designing efficient photocatalysts using high-performance materials based on Ag nanoparticles.

Facets control charge separation during photoelectrochemical water oxidation with strontium titanate (SrTiO3) single crystals

The photoelectrochemical water oxidation activity of SrTiO3 crystals is determined by the variable work functions of different facets. This produces facet dependent semiconductor–liquid junctions that can drive charge separation in photocatalysts.

Enhancing photocatalytic CO2 reduction via a single-domain ferroelectric Z-scheme heterojunction of BiFeO3/CsPbBr3 inducing dual built-in electric fields

A Z-scheme heterojunction designed via hot injection method enables growth of nanocrystals on nanosheets. This single-domain design causes a polarization electric field within the nanosheets and an electric field at the heterojunction interface. Both drive charge separation enhancing CO2 reduction.

Triazinyl-graphdiyne induces electron directional migration to drive charge separation of CdS for photocatalytic hydrogen evolution

This work provides new insights for efficient H2 production by offering adsorption and reduction active sites. The study indicates that TA-GDY selectively triggers electron migration through the interaction between nitrogen atoms and acetylene bonds.

Effective photocatalytic hydrogen evolution by Ti3C2-modified CdS synergized with N-doped C-coated Cu2O in S-scheme heterojunctions

Photocatalytic hydrogen evolution through water splitting holds tremendous promise for converting solar energy into a clean and renewable fuel source. However, the efficiency of photocatalysis is often hindered by poor light absorption, insufficient charge separation, and slow reaction kinetics of the photocatalysts. In this study, we designed and synthesized a novel S-scheme heterojunction comprising Ti3C2 MXene, CdS nanorods, and nitrogen-doped carbon coated Cu2O (Cu2O@NC) core-shell nanoparticles. Ti3C2 MXene as a cocatalyst enhances the light absorption and charge transfer of CdS nanorods. Simultaneously, the core-shell Cu2O@NC nanoparticles establish a pathway for transferring photogenerated electrons and create a favorable band alignment for efficient hydrogen evolution. The synergistic effects of Ti3C2 MXene and Cu2O@NC on CdS nanorods result in multiple charge transfer channels and improved photocatalytic performance. The optimal hydrogen evolution rate of the Ti3C2-CdS-Cu2O@NC S-scheme heterojunction photocatalyst is 7.4 times higher than that of pure CdS. Experimental techniques and DFT calculations were employed to explore the structure, morphology, optical properties, charge dynamics, and band structure of the heterojunction. The results revealed that the S-scheme mechanism effectively suppresses the recombination of photogenerated carriers and facilitates the separation and migration of photogenerated electrons and holes to the reaction sites. Furthermore, Ti3C2 MXene provides abundant active sites essential for accelerating the surface H2-evolution reaction kinetics. The Cu2O@NC core-shell nanoparticles with a large surface area and high stability are closely adhered to CdS nanorods and establish an S-scheme internal electric field with CdS nanorods to drive charge separation. This investigation provides valuable insights into the rational design of CdS-based photocatalysts, enabling efficient hydrogen production by harnessing the robust kinetic driving force provided by the S-scheme heterojunctions.

Portable Visual Photoelectrochemical Biosensor Based on a MgTi2O5/CdSe Heterojunction and Reversible Electrochromic Supercapacitor for Dual-Modal Cry1Ab Protein Detection.

Reversible electrochromic supercapacitors (ESCs) have attracted considerable interest as visual display screens. The use of ESCs in combination with a photoelectrochemical (PEC) biosensor promises to improve the detection efficiency. Herein, a visual PEC biosensor is developed by introducing a circuit module between a PEC-sensing platform (PSP) and a reversible ESC for Cry1Ab protein detection. In PSP, a type II MgTi2O5/CdSe heterojunction effectively drives charge separation by their cross-matched band gap structures, generating an amplified photocurrent. Next, the circuit module is designed to connect the PSP and ESC, realizing the signal conversion from photocurrent to voltage. ESC, as a visual display screen, produces reversible color changes with different voltages. As the concentration of Cry1Ab increases, the photocurrent decreases due to the specific binding between the aptamer and Cry1Ab in PSP, while the color of the reversible ESC changes from green to blue. To improve the integrity of the device, a portable PEC biosensor is further constructed via three-dimensional printing for dual-modal Cry1Ab protein detection, thus collecting both PEC and visual signals. The linear ranges are 0.3-3000 ng mL-1 for PEC mode and 1-1000 ng mL-1 for visual mode. This work presents a portable, efficient, sensitive, and visualized detection system, providing an important reference for practical visualization applications.

Built-in electric field intensified by photothermoelectric effect drives charge separation over Z-scheme 3D/2D In2Se3/PCN heterojunction for high-efficiency photocatalytic CO2 reduction

It is a challenging issue to further drive charge separation through the oriented design of Z-scheme heterojunction in the exploitation of cost-effective photocatalytic materials. In this contribution, the unique Z-scheme 3D/2D In2Se3/PCN heterojunction is developed through implanting In2Se3 microspheres on PCN nanosheets using an in situ growth technique, which acquires the effective CO generation activity from photocatalytic CO2 reduction (CO2R). The CO yield of 4 h in the CO2R reaction over the optimal In2Se3/PCN-15 sample reaches up to 11.40 and 2.41 times higher than that of individual PCN and In2Se3, respectively. Such greatly enhanced photocatalytic performance is primarily the improvement of photogenerated carrier separation efficiency. To be more specific, the formed built-in electric field is significantly intensified by producing the temperature difference potential between In2Se3 and PCN owing to the photothermoelectric effect of In2Se3, which actuates the high-efficiency separation of photogenerated charge carriers along the Z-scheme transfer path in the In2Se3/PCN heterojunction. The effective strategy of enhancing the built-in electric field to drive photogenerated charge separation proposed in this work opens up an innovative avenue to design Z-scheme heterojunction applied to high-efficiency photocatalytic reactions, such as hydrogen generation from water splitting, CO2R, and degradation of organic pollutants.

Efficient piezo-photocatalysis of 0D/2D α-Fe2O3/Bi2WO6: Synergy of weak force-driven piezoelectric polarization and Z-scheme junction

Photocatalysis shows huge potential in environmental purification, but suffers from fast photocharge recombination and finite photoabsorption. Piezoelectric polarization is perceived as a promising approach to drive charge separation, but it always relies on the energy-guzzling ultrasonic vibration. Herein, a piezo-photocatalytic system integrating dual electric fields constructed by weak force-driven piezoelectric polarization and Z-scheme junction is developed in 0D/2D α-Fe2O3/Bi2WO6. The introduction of low-frequency water flow-induced piezoelectric polarization field accelerates the migration of bulk photoexcited carriers of polar Bi2WO6, and forming Z-scheme junction with intimate interface guarantees the spatial separation of interfacial charges and strong visible light response. Benefiting from these merits, water flow-triggered α-Fe2O3/Bi2WO6 delivers a superb tetracycline hydrochloride photodegradation efficiency of 82% within 20 min, which outperforms related piezo-photocatalysts in previous reports, even those driven by high-frequency ultrasound. KPFM and DFT calculations provide forceful evidence for the Z-scheme transfer pathway between α-Fe2O3 and Bi2WO6. Additionally, the synergetic effect of constructing the Z-scheme junction and introducing piezoelectric polarization is well confirmed by PFM, COMSOL simulation, ESR and photoelectrochemical characterization. This work offers a novel strategy to design the piezo-photocatalytic system and maybe realize the in-situ treatment of sewage taking full advantage of hydrodynamic characteristics.

Analysis of charge trapping and long lived hole generation in SrTiO3 photoanodes

Long lived hole generation in SrTiO3 is observed herein using transient absorption spectroscopy, even in the absence of applied bias to drive charge separation.

Crystal facet and Na-doping dual engineering ultrathin BiOCl nanosheets with efficient oxygen activation for enhanced photocatalytic performance.

Photocatalytic oxidation (PCO) based on semiconductors offers a sustainable and promising way for environmental remediation. However, the photocatalytic performance currently suffers from weak light-harvesting ability, rapid charge combination and a lack of accessible reactive sites. Ultrathin two-dimensional (2D) materials are ideal candidates to overcome these problems and become hotpots in the research fields. Herein, we demonstrate an ultrathin (<4 nm thick) Na-doped BiOCl nanosheets with {001} facets (Na-BOC-001) fabricated via a facile bottom-up approach. Because of the synergistic effect of highly exposed active facets and optimal Na doping on the electronic and crystal structure, the Na-BOC-001 showed an upshifted conduction band (CB) with stronger reduction potential for O2 activation, more defective surface for enhanced O2 adsorption, as well as the highest visible-light driven charge separation and transfer ability. Compared with the bulk counterparts (BOC-010 and BOC-001), the largest amount of active species and the best photocatalytic performance for the tetracycline hydrochloride (TC) degradation were achieved for the Na-BOC-001 under visible-light irradiation, even though it had slightly weaker visible-light absorption ability. Moreover, the effect of the Na doping and crystal facet on the possible pathways for TC degradation was investigated. This work offers a feasible and economic strategy for the construction of highly efficient ultrathin 2D materials.

Improving the Stability of Silicon Nanowires During Photoelectrochemical Hydrogen Generation with Zinc 1T‐Phase Molybdenum Disulfide

AbstractSemiconductor photoelectrodes directly convert sunlight into stored chemical energy. In photoelectrochemical (PEC) devices, this photoconversion process relies on the junction between the semiconductor and catalyst to drive charge separation and generate electron/hole charge carriers. The growth of native oxides (SiOx) on the surface of semiconductors during device operation induces charge carrier recombination and photodegradation, which limit the operation lifetime of PEC devices. Likewise, the commercialization of photoelectrochemical devices is hindered by the use of expensive, rare precious metal catalysts such as platinum to enhance hydrogen evolution kinetics. This work demonstrates how drop casting zinc 1T‐phase molybdenum disulfide (Zn 1T‐MoS2) onto silicon nanowires (SiNWs) generates an interface that overcomes these challenges. This Zn 1T‐MoS2/SiNWs junction drives hydrogen evolution under acidic conditions (0.5 M H2SO4) comparably to platinum‐modified SiNWs (Pt/SiNWs) with a positive overpotential of 164 mV at 10 mA cm−2 and low Tafel slope of 42 mV dec−1. Compared to the bare SiNWs, the Zn 1T‐MoS2/SiNWs junction retains roughly 66% more photocurrent density and reduces SiOx growth by 16% after 24 h of continuous electrolysis. By developing a deep understanding of the catalyst‐semiconductor interface, photoelectrochemical devices may be effectively designed to maintain their stability over a lifetime of operation.

Pt Particle Size Affects Both the Charge Separation and Water Reduction Efficiencies of CdS-Pt Nanorod Photocatalysts for Light Driven H2 Generation.

Decreasing the metal catalyst size into nanoclusters or even single atom is an emerging direction of developing more efficient and cost-effective photocatalytic systems. Because the catalyst particle size affects both the catalyst activity and light driven charge separation efficiency, their effects on the overall photocatalytic efficiency are still poorly understood. Herein, using a well-defined semiconductor-metal heterostructure with Pt nanoparticle catalysts selectively grown on the apexes of CdS nanorods (NRs), we study the effect of the Pt catalyst size on light driven H2 generation quantum efficiency (QEH2). With the increase of the Pt catalyst size from 0.7 ± 0.3 to 3.0 ± 0.8 nm, the QEH2 of CdS-Pt increases from 0.5 ± 0.2% to 38.3 ± 5.1%, by nearly 2 orders of magnitude. Transient absorption spectroscopy measurement reveals that the electron transfer rate from the CdS NR to the Pt tip increases with the Pt diameter following a scaling law of d5.6, giving rise to the increase of electron transfer efficiency at larger Pt sizes. The observed trend can be understood by a simplified kinetic model that assumes the overall efficiency is the product of the quantum efficiencies of charge separation (including hole transfer, electron transfer, and hole scavenging) and water reduction steps, and for CdS-Pt NRs, the quantum efficiencies of electron transfer and water reduction steps increase with the Pt sizes. Our findings suggest the importance of improving the quantum efficiencies of both charge separation and catalysis in designing efficient semiconductor-metal hybrid photocatalysts, especially in the regime of small metal particle sizes.

Conjugated π Electrons of MOFs Drive Charge Separation at Heterostructures Interface for Enhanced Photoelectrochemical Water Oxidation.

Photoanode material with high efficiency and stability is extensively desirable in photoelectrochemical (PEC) water splitting for green/renewable energy source. Herein, novel heterostructures is constructed via coating rutile TiO2 nanorods with metal organic framework (MOF) materials UiO-66 or UiO-67 (UiO-66@TiO2 and UiO-67@TiO2 ), respectively. The π electrons in the MOF linkers could increase the local electronegativity near the heterojunction interface due to the conjugation effect, thereby enhancing the internal electric field (IEF) at the heterojunction interface. The IEF could drive charge transfer following Z-scheme mechanism in the prepared heterostructures, inducing photogenerated charge separation efficiency increasing as 156% and 253% for the UiO-66@TiO2 and UiO-67@TiO2 , respectively. Correspondingly, the UiO-66@TiO2 and UiO-67@TiO2 enhanced the photocurrent density as approximate two- and threefolds compared with that of pristine TiO2 for PEC water oxidation in universal pH electrolytes. This work demonstrates an effective method of regulating the IEF of heterojunction toward further improved charge separation.

Long-lived and disorder-free charge transfer states enable endothermic charge separation in efficient non-fullerene organic solar cells

Organic solar cells based on non-fullerene acceptors can show high charge generation yields despite near-zero donor–acceptor energy offsets to drive charge separation and overcome the mutual Coulomb attraction between electron and hole. Here, we use time-resolved optical spectroscopy to show that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer states that occurs over hundreds of picoseconds at room temperature, three orders of magnitude slower than comparable fullerene-based systems. Upon free electron–hole encounters at later times, both charge-transfer states and emissive excitons are regenerated, thus setting up an equilibrium between excitons, charge-transfer states and free charges. Our results suggest that the formation of long-lived and disorder-free charge-transfer states in these systems enables them to operate closely to quasi-thermodynamic conditions with no requirement for energy offsets to drive interfacial charge separation and achieve suppressed non-radiative recombination.

Mass transport properties and applications of nanochannels

With the development of materials science, micro/nano processing technologies, and mass transport theories at the micro/nano scale, nanochannel-based technology has been receiving increasing attention. Nanochannels can be classified as biological and artificial. The size of a nanochannel is typically 1-100 nm, which greatly enhances the interaction between the channel surface and substances inside the channel, inducing several special mass transport properties such as ion selectivity, ionic current rectification, and resistive current pulse. Ion selectivity is caused by the electrostatic interaction between ions and the surface charge of the nanochannel, ionic current rectification arises from the asymmetric distribution of the electrochemical potential inside the nanochannel, and a resistive current pulse is generated by the blocking of the nanochannel during the transport of ions/molecules. By taking advantage of these mass transport properties, nanochannels can be applied to various fields. For example, gated ion transport can be realized by modifying the surface of the nanochannel with functional groups; single-molecule sensing can be realized using sub-nanoscale channels; the separation of ions, molecules, or nanoparticles can be realized by regulating the interaction between the nanochannel and transport substances; and various forms of energy, such as light, heat, and salt gradient, can drive charge separation within the nanochannel and be converted into electricity by harnessing the ion selectivity of the nanochannel. However, despite the achievements in nanochannel-based technology, in-depth exploration of the mass transport properties at the nanoscale and further expansion of its application remain to be realized. For the development of this technology, four major issues need to be addressed: fabrication of a single-atom-thick nanochannel/nanopore membrane, precise control of the nanochannel structure, effective regulation of the surface properties of the nanochannel, and enrichment and development of mass transport theories. Nanochannel-based technology requires interdisciplinary efforts at the intersection of chemistry, materials science, and nanotechnology, and shows good promise for solving basic problems in biology, environment, and energy. Herein, we briefly describe the mass transport properties of nanochannels and some emerging developments and applications, and finally provide a brief outlook on this field.

Direct Observation of Photoinduced Higher Oxidation States at a Semiconductor/Electrocatalyst Junction

Photoelectrochemical (PEC) water splitting devices using semiconductors and electrocatalysts rely on heterogeneous interfaces that drive charge separation, thus determining potential gradients that...

Influence of the neutron-skin effect on nuclear isobar collisions at energies available at the BNL Relativistic Heavy Ion Collider

The unambiguous observation of a Chiral Magnetic Effect (CME)-driven charge separation is the core aim of the isobar program at RHIC consisting of ${^{96}_{40}}$Zr+${^{96}_{40}}$Zr and ${^{96}_{44}}$Ru+${^{96}_{44}}$Ru collisions at $\sqrt {s_{\rm NN}}\!=\!200$ GeV. We quantify the role of the spatial distributions of the nucleons in the isobars on both eccentricity and magnetic field strength within a relativistic hadronic transport approach (SMASH, Simulating Many Accelerated Strongly-interacting Hadrons). In particular, we introduce isospin-dependent nucleon-nucleon spatial correlations in the geometric description of both nuclei, deformation for ${^{96}_{44}}$Ru and the so-called neutron skin effect for the neutron-rich isobar i.e. ${^{96}_{40}}$Zr. The main result of this study is a reduction of the magnetic field strength difference between ${^{96}_{44}}$Ru+${^{96}_{44}}$Ru and ${^{96}_{40}}$Zr+${^{96}_{40}}$Zr by a factor of 2, from $10\%$ to $5\%$ in peripheral collisions when the neutron-skin effect is included. Further, we find an increase of eccentricity by up to 10$\%$ when deformation is taken into account while neither the neutron skin effect nor the nucleon-nucleon correlations result into a significant modification of this observable with respect to the traditional Woods-Saxon modeling. Our results suggest a significantly smaller CME signal to background ratio for the experimental charge separation measurement in peripheral collisions with the isobar systems than previously expected.