Improved Charge Separation Research Articles

Photocatalytic β-O-4 bond cleavage in lignin models and native lignin through CdS integration on titanium oxide photocatalyst under visible light irradiation

Converting lignin, a key sustainable biopolymer, into valuable oxygen-containing compounds is a significant challenge. To address such a challenge, photocatalytic self-transfer hydrogenolysis strategy is employed utilizing a CdS(x%)/TiO2 heterojunction photocatalyst, with minimal CdS loading on TiO2. The CdS(3 %)/TiO2 catalyst, under blue light, dehydrogenates HCα–OH groups, transferring hydrogen to Cβ–O bonds, cleaving β-O-4 ether bonds in lignin model compounds yielding over 95 % phenols and acetophenones. It utilizes glyceryl moieties as a hydrogen source, yielding ∼ 24 % of diverse lignin monomer derivatives from teak lignin. Improved charge separation in the CdS(3 %)/TiO2 catalyst is revealed by electrochemical and spectral analyses and exhibits delayed charge carrier recombination. Scavenging studies confirm a type II charge transfer mechanism and support visible-light-driven lignin fragmentation. The present photocatalytic process offers a promising, cost-effective approach for converting lignin into valuable aromatic compounds, advancing renewable biomass-derived chemicals.

Surface sulphur vacancies of CdS/MIL-68(In)–NH2 for Boosting Visible-Light photocatalytic hydrogen evolution

The incorporation of sulfur vacancies (SVs) during the synthesis of CdS represents an essential approach for enhancing its photocatalytic hydrogen production performance. The CdS/MIL-68(In)–NH2 photocatalytic material with SVs was successfully synthesized via a facile two-step method and employed for visible light photocatalytic hydrogen evolution. Among various loading ratios, the CdS(30)/MIL-68(In)–NH2 composite exhibited remarkable photocatalytic performance, demonstrating hydrogen evolution rates 3.7 times and 7.3 times higher than those of pure CdS and mechanically mixed CdS with MIL-68(In)–NH2, respectively, while maintaining significant photocatalytic stability. The incorporation of MIL-68(In)–NH2 effectively enhanced the dispersion of CdS particles, thereby increasing the availability of active sites for efficient photocatalytic reactions. Moreover, the presence of SVs on the surface of CdS served as electron trapping levels, significantly improving charge separation efficiency. This study not only provides novel insights into fabricating composite photocatalysts but also explores a defect engineering regulation strategy.

Fabrication of nanostructured AgCl-coated Ag2SO4 photocatalysts for efficient antibiotic degradation

Binary AgCl-coated Ag2SO4 nanostructures were fabricated via a facile chemical deposition method and evaluated by the visible-light photodegradation of highly toxic antibiotic tetracycline. The experimental results revealed that AgCl was successfully deposited onto the surface of Ag2SO4 particles, and the as-prepared Ag2SO4@AgCl composites exhibited significantly improved photocatalytic activity under visible light in comparison with Ag2SO4 and AgCl nanoparticles. Notably, the obtained Ag2SO4@AgCl-31 composite emerged as the most effective photocatalyst, achieved a maximum rate constant of 0.08215 min−1 under visible light, which was 3.22 times of Ag2SO4 and 2.42 times that of AgCl, respectively. Furthermore, the as-prepared Ag2SO4@AgCl-31 sample also demonstrated a good photocatalytic stability. The enhanced photocatalytic degradation performance for the Ag2SO4@AgCl composites could be primarily attributed to the formation of the core–shell heterostructures for improved charge separation.

(Invited) Ultrafast Spectroscopy of Nanostructured Plasmonic and Energy Conversion Materials

Ultrafast processes that occur upon the absorption of photons by a material frequently play important roles in the efficiency of a desired energy conversion or photocatalytic outcome. For example, energetic carriers produced upon photoexcitation of plasmonic systems can quickly dissipate energy through electron scattering processes on femtosecond timescales, followed by electron-phonon coupling on picosecond timescales. Once the carriers are thermalized, less energy is available to drive a charge separation or photocatalytic process. At the same time, there are number of handles with which to improve the efficiency of photoinduced processes, and ultrafast spectroscopy can be used to characterize that improvement. Nanostructuring, for example, can produce higher efficiency of photocatalytic processes as free charges are more likely to reach a surface following photoexcitation, and local anisotropies at the surface can drive electromagnetic field enhancements and the greater production of “hot” electrons. Hybridization of differing materials is also a powerful handle with which to manipulate or improve charge separation and photocatalytic outcomes. New materials can lead to greater robustness under illumination, as well as lead to changes in dissipation processes. In this talk, I describe recent efforts to characterize novel nanostructures of interest for plasmonics and energy conversion via ultrafast spectroscopy, as well as explore the ultrafast dissipation processes in refractory plasmonic nanomaterials. Particular attention is paid to dissipation of energy to the local environment of the nanoparticles as well as to ultrafast electron scattering processes. Hybrid nanostructures are also explored. Work performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Enhanced Visible-Light-Driven Photocatalytic Overall Splitting of Pure Water in a Porous Microreactor.

Photocatalytic overall splitting of pure water (H2O) without sacrificial reagent to hydrogen (H2) and oxygen (O2) holds a great potential for achieving carbon neutrality. Herein, by anchoring cobalt sulfide (Co9S8) as cocatalyst and cadmium sulfide (CdS) as light absorber to channel wall of a porous polymer microreactor (PP12), continuous violent H2 and O2 bubbling productions from photocatalytic overall splitting of pure H2O without sacrificial reagent is achieved, with H2 and O2 production rates as high as 4.41 and 2.20 mmol h-1 gcat.-1 respectively. These are significantly enhanced than those in the widely used stirred tank-type reactor in which no O2 is produced and H2 production rate is only 0.004 mmol h-1 gcat.-1. Besides improved charge separation and interaction of H2O with photocatalyst in PP12, bonding interaction of Co9S8 with PP12 creates abundant catalytic active sites for simultaneous productions of H2 and O2, thus leading to the significantly enhanced H2 and O2 bubbling productions in PP12. This offers a new strategy to enhance photocatalytic overall splitting of pure H2O without sacrificial reagent.

(Invited) Exciton Dissociation in Mixed-Dimensionality SWCNT-Based Heterojunctions

Quantum-confined semiconductors provide highly tunable optical and electrical properties for a wide variety of emerging applications. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have shown tremendous potential in applications ranging from digital logic, biological imaging, quantum information processing, photovoltaics, and thermoelectric energy harvesting. Energy harvesting applications rely critically upon the creation of tailored interfaces that enable the movement of energetic species (excitons, electrons, holes) in specified directions. In this talk, I will discuss the use of a variety of rationally designed “mixed-dimensionality” heterojunctions between s-SWCNTs and transition metal dichalcogenide (TMDC) monolayers to harvest visible/near-infrared photons and convert these excitations into long-lived charge-separated states. Type-II heterojunctions between s-SWCNTs and TMDCs enable rapid exciton dissociation and microsecond charge-separated state lifetimes. We have developed new TMDC/SWCNT/TMDC trilayer architectures that enable charge transfer cascades for improving charge separation yield and lifetime. These trilayer architectures allow us to probe out-of-plane carrier diffusion in the nanotube layer and the degree to which bound interfacial excitons impact the photophysics in these mixed-dimensionality heterostructures. I will also discuss the methodology by which we attempt to quantify the photoinduced exciton dissociation quantum yields in these nanoscale heterostructures. Using several heterostructure results, we are attempting to narrow in on appropriate ways to quantify the local dielectric constant and exciton size in both the SWCNT and TMDC components, each of which impact the estimated charge carrier yields. Taken together, these studies provide fundamental insights into the links between ground-state thermodynamics and excited-state dynamics for model nanoscale heterojunctions used to harvest solar energy and produce long-lived charge-separated states.

A novel approach of polychiral single walled carbon nanotubes functionalization for heterojunction solar cell

This study introduces an innovative method for functionalizing polychiral single-walled carbon nanotubes (f-SWCNTs) by depositing thin films onto n-type crystalline silicon (n-Si), thereby creating efficient heterojunction solar cells. The approach capitalizes on the effective charge separation facilitated by the built-in potential resulting from the functionalization of polychiral SWCNTs, leading to enhanced electron transfer from f-SWCNTs to Si driven by functional groups. The resulting f-SWCNT/Si heterojunction device demonstrates remarkable performance, attributed to the high optical absorption and improved charge separation and transfer mechanisms of the f-SWCNT film. Moreover, by adjusting the concentration of f-SWCNTs, the film properties and overall device performance have been customized, achieving a notable power conversion efficiency of 12 %. These discoveries serve as a foundation for the advancement of highly desirable, efficient, cost-effective, and broadband high-performance f-SWCNT/Si heterojunction solar cells

Engineering S-scheme mCN@mPDIP molecular heterojunction with highly efficient interface charge transfer for photocatalytic aerobic oxidation synthesis

The construction of S-scheme heterojunctions, which offers a promising approach for spatially separating photogenerated charge carriers with high redox potentials and multimolecular activation, represents a viable modification strategy in photocatalytic applications. However, the prevalent insufficient contact areas between two components result in low interface charge transfer efficiency, thereby impeding the photocatalytic performance of such heterostructures. Herein, we address this limitation by introducing a unique mCN@mPDIP molecular heterojunction through a pH-triggered molecule self-assembly eutectoid technique, enabling intimate interface contact and promoting highly efficient interfacial charge transfer following an S-scheme mechanism. Consequently, the mCN@mPDIP molecular heterojunction achieves significantly improved charge separation efficiency and higher concentration of active carriers compared to typical bCN-bPDIP bulk heterojunction and nCN/nPDIP nano heterojunction. Combined with the effective sulfide activation on mPDIP sites and O2 activation on mCN sites, the resulting mCN@mPDIP demonstrates outstanding activity in the photocatalytic aerobic oxidation of sulfides into sulfoxides without any redox mediators.

Cu2O Photocathodes with Bidirectional CuSCN Layers to Improve Charge Separation for Enhanced Photoelectrochemical Water Splitting

Cu₂O Photocathodes with Bidirectional CuSCN Layers to Improve Charge Separation for Enhanced Photoelectrochemical Water Splitting

Rational design and construction of direct Z-scheme ternary heterojunction photocatalyst AgBr/CoWO4/Ag for efficient environmental remediation

The indiscriminate discharge of micropollutants (e.g., dyes, antibiotics, industrial additives, etc.) represents a significant risk to human health, and the removal of these substances from water bodies has become a prominent area of research within the field of environmental remediation. A simple hydrothermal-precipitation-photoreduction method was employed to synthesize novel Z-scheme heterojunction photocatalysts of AgBr/CoWO4/Ag. The catalysts demonstrated remarkable degradation capabilities with regard to a range of micropollutants present in wastewater. Of the catalysts tested, 5AgBr/CoWO4/Ag exhibited the highest degradation rates, reaching 98.58% for Rhodamine B, 86.82% for tetracycline hydrochloride, and 95.60% for 2-mercaptobenzothiazole within 60 min. In particular, the reaction kinetic rate of 5AgBr/CoWO4/Ag towards Rhodamine B degradation (k2 = 0.26278 L mg−1·min−1) is 9 times that of AgBr (k2 = 0.02953 L mg−1·min−1) and 113 times that of CoWO4 (k2 = 0.00233 L mg−1·min−1), which serves to highlight the exceptional photocatalytic activity of the material. The experimental data and subsequent analysis indicated that the enhanced photocatalytic performance can be attributed to two factors: firstly, the electron mediation by Ag nanoparticles leading to improved charge separation efficiency, and secondly, the formation of Z-scheme heterojunctions between AgBr and CoWO4. The cyclic tests provided confirmation of the excellent stability and recyclability of the AgBr/CoWO4/Ag photocatalysts. It is anticipated that this study will facilitate the development of novel methods for the degradation of refractory micropollutants and provide insights into environmental remediation, thereby contributing to the sustainable development of society.

Electrolyte and interfacial engineering for self-formation of In₂S₃/In₂O₃ heterojunction with an enhanced photoelectrochemical activity

Electrolyte and interfacial engineering of photoanodes has become a crucial approach to enhance photoelectrochemical water-splitting performance. By optimizing both the electrolyte and the interfacial properties of the photoanode, the synergistic effects of the heterojunction and electrolyte can be leveraged to improve efficiency. In this work, we explore the crucial role of electrolyte and interfacial engineering in optimizing photoanode performance. By engineering an In2O3 film in a polysulfide electrolyte to form an in-situ In₂S₃/In₂O₃ heterojunction interface, we demonstrate a significant increase in anodic photocurrent compared to traditional aqueous Na2SO4 or NaOH electrolytes. This enhancement is attributed to both the thermodynamically favorable oxidation of sulfide and sulfite, and the unique electronic structure developed at the In₂S₃/In₂O₃ junction, which significantly improves charge separation and overall performance. Based on this finding, an efficient In₂S₃/In₂O3 heterojunction interface through optimal anion exchange with sulfur was developed, leading to a maximum photocurrent density of 5.57 mA cm−2 in the polysulfide electrolyte. In addition, detailed structural, optical, and photoelectrochemical properties of the In₂S₃/In₂O₃ interface providing insights into the mechanisms underlying the enhanced water-splitting performance have also been discussed.

Improving light harvesting and charge carrier separation enabling enhanced photoelectrochemical hydrogen production by Sb2S3-decorated TiO2 nanotube arrays on porous Ti-photoanodes

The Ag + ions doped TiO2 nanotube arrays fabricated on Ti mesh through an electrochemical anodization process, and n/n heterostructure formed by coating the TiO2 nanotube array photoanode with an n-type stibnite (Sb2S3) layer improve photoelectrochemical water splitting reaction by enhancing light harvesting and charge carrier separation. Sb2S3 is chosen because of its suitable band gap position and strong visible light response. The Ag + ion incorporated TiO2 nanotube array coated with Sb2S3 exhibits a photocurrent density of 6.5 mA cm−2 vs. RHE, whereas bare TiO2 nanotube array coated with Sb2S3 and pristine TiO2 nanotube array exhibit photocurrent densities of 3.6 and 0.27 mA cm−2, respectively, under the same conditions, which is nearly 1.8 and 24 times greater than the TiO2 nanotube array coated with Sb2S3 and pristine TiO2 nanotube array photoanodes. The applied bias photon-to-current efficiency of Ag + ion incorporated TiO2 nanotube array coated with Sb2S3 is 3.6% and the improved Photoelectrochemical performance of Ag + ion doped TiO2 nanotube array coated with Sb2S3 is attributable to higher conductivity of TiO2 nanotube array caused by increased oxygen vacancies and broad optical activity of Sb2S3. The 1-dimensional TiO2 nanotube array device structure in the Ti mesh improves charge separation and light harvesting while reducing photogenerated charge carrier recombination and the potential of this novel device structure for advanced energy conversion applications is highlighted in this study.

Well-synthesized carbon dots from flower of Cassia fistula and its hydrothermally grown heterojunction photocatalyst with zinc oxide (CDs@ZnO-H400) for photocatalytic degradation of ciprofloxacin and paracetamol

Pharmaceutical residues in aquatic environments have the potential to induce bacterial resistance, subsequently complicating disease treatment and posing a substantial hazard to public health. This study demonstrates the production of carbon dots from Cassia fistula flowers using a hydrothermal process. The obtained carbon dots were then mixed with zinc oxide to create CDs@ZnO-H400. This heterojunction photocatalyst exhibited notable enhancement in photocatalytic performance due to improved charge separation efficiency at the interface. The CDs@ZnO-H400 catalyst effectively removed two medications, ciprofloxacin and paracetamol. Ciprofloxacin showed maximum photocatalytic efficiency, with 99.8 % degradation, while paracetamol exhibited a degradation efficiency of 75.1 %. The stability of the synthesized photocatalyst was evaluated across four usage cycles using EDX-SEM, FTIR, and XRD techniques. The CDs@ZnO-H400 heterostructure photocatalyst maintained strong photoactivity even after the fourth cycle of usage.

Anchoring Cu2O onto WO3 by adsorption-ascorbic acid reduction: A p-n junction with improved photocatalytic and photoelectrochemical properties

Tungsten trioxide is a promising photoelectric material owing to its abundant structures and unique properties, but the serious photogenerated charge recombination limits its photocatalytic and photoelectrochemical performance. Herein, 0D Cu2O was anchored onto 3D WO3 in-situ via a facile adsorption-ascorbic acid reduction method at room temperature. The coupling of WO3 with Cu2O produces improved charge separation efficiency, thereby the photocatalytic degradation rate and transient photocurrent of optimal WO3/Cu2O are 2.7 and 1.6 times higher than those of WO3, respectively. The enhanced performance is ascribed to the formation of a p-n heterojunction, which was demonstrated by experimental characterizations and theoretical calculation.

Dual S-scheme MoS2/ZnIn2S4/Graphene quantum dots ternary heterojunctions for highly efficient photocatalytic hydrogen evolution

The layered chalcogenide ZnIn2S4 (ZIS) exhibits photo-stability and a tunable band gap but is limited in photocatalytic applications, such as hydrogen (H2) production, due to rapid carrier recombination and slow charge separation. To overcome these limitations, we have synthesized a ternary MoS2/ZIS/graphene quantum dots (GQDs) heterojunction, wherein MoS2 and GQDs are strategically attached to ZIS interlaced nanoflakes, enhancing light absorption across the 500–1500 nm range. This heterojunction benefits from dual S-scheme interfaces between MoS2-ZIS and ZIS-GQDs, establishing directed internal electric fields (IEFs). These IEFs accelerate the transfer of photoinduced electrons from the conduction bands of MoS2 and GQDs to the valence band of ZIS, promoting rapid recombination with holes and facilitating efficient catalytic reactions with plentiful photoinduced electrons stemmed from the conduction band of ZIS. As a result, the photocatalytic H2 production rate of the MoS2/ZIS/GQDs heterojunction is measured at 21.63 mmol h−1 g−1, marking an increase of 36.7 times over pure ZIS. This research provides valuable insights into designing novel heterojunctions for improved charge separation and transfer for solar energy conversion applications.

Efficient methane oxidation to formaldehyde via photon–phonon cascade catalysis

The oxidation of methane to value-added chemicals provides an opportunity to use this abundant feedstock for sustainable petrochemistry. Unfortunately, such technologies remain insufficiently competitive due to a poor selectivity and a low yield rate for target products. Here we show a photon–phonon-driven cascade reaction that allows for methane conversion to formaldehyde with an unprecedented productivity of 401.5 μmol h−1 (or 40,150 μmol g−1 h−1) and a high selectivity of 90.4% at 150 °C. Specifically, with a ZnO catalyst decorated with single Ru atoms, methane first reacts with water to selectively produce methyl hydroperoxide via photocatalysis, followed by a thermodecomposition step yielding formaldehyde. Single Ru atoms, serving as electron acceptors, improve charge separation and promote oxygen reduction in photocatalysis. This reaction route with minimized energy consumption and high efficiency suggests a promising pathway for the sustainable transformation of light alkanes.

Boosting Acidic Water Oxidation on Hematite Photoanodes with Synergistic Effects of Ce‐Doped Co3O4 Nanoparticles

AbstractPhotoelectrochemical (PEC) water splitting is pivotal for addressing the clean and renewable energy demands. However, developing low‐cost, efficient, and robust non‐precious metal catalysts for the acidic oxygen evolution reaction (OER) remains a significant challenge. In this study, we introduced Ce to modulate the electronic structure of Co3O4, enhancing the stability of Co3O4 without compromising its activity. Ce‐doped Co3O4 nanoparticles were electrodeposited onto Ti‐doped hematite surfaces (Ce : Co3O4/Fe2O3), significantly enhancing the PEC performance for water splitting under acidic conditions. These catalysts achieved high photocurrent densities and exhibited prolonged stability. Functioning as p‐type semiconductors, the Ce : Co3O4 nanoparticles not only boosted light absorption but also formed a p‐n heterojunction with the Ti‐doped hematite. This heterojunction generated a built‐in electric field that facilitated the separation and transfer of photogenerated carriers, thereby improving charge separation efficiency. Additionally, these nanoparticles expanded the active surface area of hematite and served as a co‐catalyst, markedly accelerating the OER kinetics. The Ce : Co3O4/Fe2O3 photoanode achieved an impressive photocurrent density of 1.08 mA cm−2 at 1.23 VRHE in acidic media, demonstrating its enhanced activity and stability.

In situ fabrication of 2D Ti3C2/1T&2H-MoS2/TiO2 photocatalyst for sulfamethazine degradation

Sunlight-driven photocatalysis is a very appealing approach for solving the current energy crisis because it will not cause secondary pollution. Titanium dioxide (TiO2) has attracted much attention in the field of photocatalysis due to its simple preparation, environmental friendliness and easy modification. However, reducing the band gap and expanding the light absorption range to reduce the carrier recombination rate of TiO2 is still a huge challenge. Herein, we report a two-dimensional (2D) Ti3C2/1 T&2 H-MoS2/TiO2 photocatalyst obtained by in situ growth of a 1 T&2 H-MoS2/TiO2 Z-scheme heterojunction on a highly conductive Ti3C2 MXene surface. The spectral response range of the resulting Ti3C2/1 T&2 H-MoS2/TiO2 (Ti3C2:1 T&2 H-MoS2:TiO2= 1:10:10) photocatalyst was broadened to 696 nm and the band gap was reduced to 1.83 eV, which was responsible for the strong light-harvesting capacity and improved charge separation. The activity of the Ti3C2/1 T&2 H-MoS2/TiO2 photocatalyst was evaluated by photocatalytic degradation of sulfamethazine (SMZ). The degradation rate of SMZ reached 99.8 % after 90 min of illumination. These results demonstrate that the combination of Ti3C2 MXene and heterojunction is a feasible strategy to enhance the light-harvesting capacity and charge transport in photocatalytic reaction.

Fabrication of high photocatalytic activity photocatalysts with TiO2 supported on lamellar BiOCl

A lamellar nanostructure consisting of anatase TiO2/BiOCl was effectively developed while employing the approach of solvothermal. The samples underwent comprehensive analysis of their phase structures, morphologies, surface areas, optical properties, and electronic states using various techniques including x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) method, UV–vis diffuse reflectance spectra (UV–vis DRS), Fourier transform infrared spectroscopy (FTIR), and photoluminescence spectroscopy(PL). The photoelectrochemical data demonstrated that the incorporation of anatase TiO2 nanoparticles into the BiOCl lamellar structure significantly improved its photocatalytic efficiency to degrade Malachite Green (MG) in UV light, surpassing the performance of both pure BiOCl and anatase TiO2. Additionally, the study delved into the photocatalytic mechanism responsible for this enhanced performance. The superior photocatalytic efficiency of the anatase TiO2/BiOCl composites can be ascribed to higher surface area, smaller crystallite size, stronger light absorption and improved charge separation efficiency.

Research on the fabrication of nanorod-structured phorphyrin/g-C3N4 as photocatalyst materials for the removal of Cr6+ ions in an aqueous solution

Due to their potential to expand the photon energy harvesting zones and improve charge separation, two semiconductors together have been shown to be a promising technique to increase the photocatalytic activity of photocatalysts. Through the use of monomeric porphyrin molecules and g-C3N4 nanomaterials, self-assembly of the hybird g-C3N4@porrphyrin nanorod materials under CTAB surfactant-assited conditions was demonstrated in this study. SEM, EDS, FTIR, and UV-vis were used to characterize the generated hybrid material. Under simulated sunlight irradiation, the hybrid mateial’s photocatalytic behaviour was examined for the photoreduction of Cr6+ ions. After 100 minutes of reaction time under the simulated solar spectrum, the results showed that the hybrid material demonstrated high photocatalytic performance against, with clearance percentage of almost 100%. The prepared photocatalyst was also highly stable with the reduction of Cr6+ removal efficiency of less than 10% after 4 cycles of photocatalytic testing.