Homophase Junction for Promoting Spatial Charge Separation in Photocatalytic Water Splitting

Structure-Shifting Intermediates During Nanoparticle Cation Exchange for the Retrosynthetic Construction of Intraparticle Heterophase Homojunctions

Heterophase homojunctions, which connect two compounds having different crystal structures but the same compositions, are important components of many nanoscale photocatalysts and electronic device systems because they integrate two electronically distinct materials with minimal lattice mismatch. Making heterophase homojunctions in high yield is challenging and has largely been limited to post-processing, aggregation, and modulated growth techniques. As a result, heterophase homojunctions within colloidal nanoparticles are rare, despite their potentially beneficial electronic characteristics. Here, we demonstrate a retrosynthetic pathway for rationally incorporating heterophase homojunctions into colloidal nanoparticles. Our approach leverages a structure-shifting intermediate that is applied during nanoparticle cation exchange. Specifically, partial cation exchange reactions of roxbyite Cu1.8S nanorods, which exhibit a distorted hexagonal close-packed (hcp) structure, with Ni2+ produce regions of Ni9S8 that have a distorted cubic close-packed (ccp) structure. The resulting hcp-Cu1.8S/ccp-Ni9S8 nanorods interface crystallographically aligned hcp and ccp regions and provide a synthetic entryway, through additional series of cation exchange reactions, to form derivative nanorods that maintain the hcp/ccp junction while further modifying composition. Using this approach, we demonstrate multistep retrosynthetic pathways to two distinct metal sulfide heterophase homojunctions, roxbyite-Cu1.8S (hcp)/digenite-Cu1.8S (ccp) and wurtzite-CdS (hcp)/zincblende-CdS (ccp). The CdS nanorods that incorporate a heterophase homojunction exhibit a single band gap that is intermediate in energy between those of the two individual phases. The ability to design synthetic pathways to heterophase homojunctions in colloidal nanoparticles is important for achieving synergistic and enhanced electronic and optical properties in nanoscale semiconductor systems.

Unveiling the role of heterophase nanostructure in MnO2-based colorimetric sensors for ascorbic acid detection

Nanozymes based on manganese oxide (MnO2) are demonstrated to be promising probes in colorimetric sensing applications. In this study, the r-MnO2/β-MnO2 heterophase nanostructure was simply prepared by a calcination process with controllable temperature. The characterization of the nanostructured material was confirmed by SEM, UV-vis spectroscopy, Raman, TGA-DSC, and XRD analysis. The r-MnO2/β-MnO2 exhibits a remarkably good catalytic activity in the oxidation process of 3,3',5,5'-tetramethylbenzidine (TMB) compared with the r-MnO2 or Mn2O3 nanostructure owing to its heterophase junctions. The enhanced performance of the colorimetric sensor for ascorbic acid (AA) detection was investigated using the r-MnO2/β-MnO2 heterophase nanostructure as probe. The r-MnO2/β-MnO2 material enhanced the monitoring of AA in the wide linear range from 1 µM to 50 μM with a limit of detection of 0.84 µM. This work presents a promising and straightforward approach for the construction of MnO2-based colorimetric sensor and their practical application in plant growth monitoring.

Facile Fabrication of Lilac-Like Multiple Self-Supporting WO3 Nanoneedle Arrays with Cubic/Hexagonal Phase Junctions for Highly Sensitive Ethylene Glycol Gas Sensors

Metal oxides with nanoarray structures have been demonstrated to be prospective materials for the design of gas sensors with high sensitivity. In this work, the WO3 nanoneedle array structures were synthesized by a one-step hydrothermal method and subsequent calcination. It was demonstrated that the calcination of the sample at 400 °C facilitated the construction of lilac-like multiple self-supporting WO3 arrays, with appropriate c/h-WO3 heterophase junction and highly oriented nanoneedles. Sensors with this structure exhibited the highest sensitivity (2305) to 100 ppm ethylene glycol at 160 °C and outstanding selectivity. The enhanced ethylene glycol gas sensing can be attributed to the abundant transport channels and active sites provided by this unique structure. In addition, the more oxygen adsorption caused by the heterophase junction and the aggregation of reaction medium induced by tip effect are both in favor of the improvement on the gas sensing performance.

In-Sb Covalent Bonds over Sb2Se3/In2Se3 Heterojunction for Enhanced Photoelectrochemical Water Splitting

Antimony selenide is a promising P-type photocatalyst, but it has a large number of deep energy level defects, leading to severe carrier recombination. The construction of a heterojunction is a common way to resolve this problem. However, the conventional heterojunction system inevitably introduces interface defects. Herein, we employ in situ synthesis to epitaxially grow In2Se3 nanosheets on Sb2Se3 nanorods and form In-Sb covalent interfacial bonds. This petal-shaped heterostructure reduced interface defects and enhanced the efficiency of carrier separation and transport. In this work, the photocurrent density in the proposed Sb2Se3/In2Se3 photocathode is 0.485 mA cm-2 at 0 VRHE, which is 30 times higher than that of pristine Sb2Se3 and it has prominent long-term stability for 24 h without obvious decay. The results reveal that the synergy of the bidirectional built-in electric field constructed between In2Se3 and Sb2Se3 and the solid In-Sb interfacial bonds together build a high-efficiency transport channel for the photogenerated carriers that display enhanced photoelectrochemical (PEC) water-splitting performance. This work provides efficient guidance for reducing interface defects via the in situ synthesis and construction of interfacial bonds.

In-situ construction of TiO2 polymorphic junction nanoarrays without cocatalyst for boosting photocatalytic hydrogen generation

There are significant challenges in developing technologies for high-yield photocatalytic hydrogen production reactions. Current photocatalytic materials face three key problems: low utilization of light, rapid recombination of photogenerated electron-hole pairs, and a limited number of active sites during photocatalytic reactions. As a result, these materials only improve one or two of the three steps involved in photocatalytic hydrogen production reactions. Consequently, achieving simultaneous multifunctional synergy to enhance the efficiency of all three processes is difficult. Here, we report an in situ dissolution-recrystallisation approach to design and fabricate a three-dimensional TiO2 rutile/anatase (AE-TiO2) array photocatalytic material for photocatalytic hydrolysis applications. It is shown that the unique 3D nanoarray structure and in situ fabrication of the AE-TiO2 homojunction with synergistic effects among the components lead to an increase in light harvesting efficiency, charge transport separation efficiency and surface active sites, which remarkably improve the photocatalytic hydrolysis performance. The prepared AE-TiO2 homojunction materials realizes a maximal photoactivity of 4 μmol cm-2·h-1, which is 39 times larger than that of pure TiO2 rutile nanorods.

Molecular Level Modulation of Anthraquinone-containing Resorcinol-formaldehyde Resin Photocatalysts for H2 O2 Production with Exceeding 1.2 % Efficiency

Designing polymeric photocatalysts at the molecular level to modulate the photogenerated charge behavior is a promising and challenging strategy for efficient hydrogen peroxide (H₂ O₂ ) photosynthesis. Here, we introduce electron-deficient 1,4-dihydroxyanthraquinone (DHAQ) into the framework of resorcinol-formaldehyde (RF) resin, which modulates the donor/acceptor ratio from the perspective of molecular design for promoting the charge separation. Interestingly, H₂ O₂ can be produced via oxygen reduction and water oxidation pathways, verified by isotopic labeling and in situ characterization techniques. Density functional theory (DFT) calculations elucidate that DHAQ can reduce the energy barrier for H₂ O₂ production. RF-DHAQ exhibits excellent overall photosynthesis of H₂ O₂ with a solar-to-chemical conversion (SCC) efficiency exceeding 1.2 %. This work opens a new avenue to design polymeric photocatalysts at the molecular level for high-efficiency artificial photosynthesis.

A library of polytypic copper-based quaternary sulfide nanocrystals enables efficient solar-to-hydrogen conversion

Designing polytypic homojunction is an efficient way to regulate photogenerated electrons and holes, thereafter bringing desired physical and chemical properties and being attractive photocatalysts for solar-to-hydrogen conversion. However, the high-yield and controllable synthesis of well-defined polytypes especially for multinary chalcogenide - the fundamental factor favoring highly efficient solar-to-hydrogen conversion - has yet to be achieved. Here, we report a general colloidal method to construct a library of polytypic copper-based quaternary sulfide nanocrystals, including Cu₂ZnSnS₄, Cu₂CdSnS₄, Cu₂CoSnS₄, Cu₂MnSnS₄, Cu₂FeSnS₄, Cu₃InSnS₅ and Cu₃GaSnS₅, which can be synthesized by selective epitaxial growth of kesterite phase on wurtzite structure. Besides, this colloidal method allows the precise controlling of the homojunction number corresponding to the photocatalytic performance. The single-homojunction and double-homojunction polytypic Cu₂ZnSnS₄ nanocrystal photocatalysts show 2.8-fold and 3.9-fold improvement in photocatalytic hydrogen evolution rates relative to the kesterite nanocrystals, respectively. This homojunction existed in the polytypic structure opens another way to engineer photocatalysts.

While polytypic semiconductors are promising for solar-to-fuel applications, preparing homojunction nanomaterials has proven challenging. Here, authors obtain a library of polytypic copper-based quaternary sulfide nanocrystals by selective epitaxial growth of kesterite phase on wurtzite structure.

Constructing Anatase-Brookite TiO2 Phase Junction by Thermal Topotactic Transition to Promote Charge Separation for Superior Photocatalytic H2 Generation

Phase junctions of photocatalysts can promote the separation of photogenerated charge carriers for efficient utilization of the carriers. Construction of phase junctions and establishing their structure-performance relationship are still required. Herein, polycrystalline TiO₂ decahedral plates with different phases were synthesized by thermal treatment-induced topotactic transition of titanium oxalate crystals. The phase of TiO₂ evolved from pure anatase to anatase-brookite, anatase-brookite-rutile, and then to anatase-rutile, while the morphology of the decahedral plates was well maintained. The biphase anatase-brookite was found to be most efficient in photocatalytic hydrogen generation. Specifically, the hydrogen generation rate of the biphase anatase-brookite TiO₂ was nearly 2.4 times greater than that of the biphase anatase-rutile TiO₂. The spatially resolved surface photovoltage measurements indicate the more efficient separation of photogenerated charge carriers and thus greater photocatalytic activity of the former. This work provides a strategy for developing efficient phase-junction photocatalysts.

Unveiling the charge transfer dynamics steered by built-in electric fields in BiOBr photocatalysts

Construction of internal electric fields (IEFs) is crucial to realize efficient charge separation for charge-induced redox reactions, such as water splitting and CO₂ reduction. However, a quantitative understanding of the charge transfer dynamics modulated by IEFs remains elusive. Here, electron microscopy study unveils that the non-equilibrium photo-excited electrons are collectively steered by two contiguous IEFs within binary (001)/(200) facet junctions of BiOBr platelets, and they exhibit characteristic Gaussian distribution profiles on reduction facets by using metal co-catalysts as probes. An analytical model justifies the Gaussian curve and allows us to measure the diffusion length and drift distance of electrons. The charge separation efficiency, as well as photocatalytic performances, are maximized when the platelet size is about twice the drift distance, either by tailoring particle dimensions or tuning IEF-dependent drift distances. The work offers great flexibility for precisely constructing high-performance particulate photocatalysts by understanding charge transfer dynamics.

Recent advances and perspectives for solar-driven water splitting using particulate photocatalysts

The conversion and storage of solar energy to chemical energy via artificial photosynthesis holds significant potential for optimizing the energy situation and mitigating the global warming effect. Photocatalytic water splitting utilizing particulate semiconductors offers great potential for the production of renewable hydrogen, while this cross-road among biology, chemistry, and physics features a topic with fascinating interdisciplinary challenges. Progress in photocatalytic water splitting has been achieved in recent years, ranging from fundamental scientific research to pioneering scalable practical applications. In this review, we focus mainly on the recent advancements in terms of the development of new light-absorption materials, insights and strategies for photogenerated charge separation, and studies towards surface catalytic reactions and mechanisms. In particular, we emphasize several efficient charge separation strategies such as surface-phase junction, spatial charge separation between facets, and polarity-induced charge separation, and also discuss their unique properties including ferroelectric and photo-Dember effects on spatial charge separation. By integrating time- and space-resolved characterization techniques, critical issues in photocatalytic water splitting including photoinduced charge generation, separation and transfer, and catalytic reactions are analyzed and reviewed. In addition, photocatalysts with state-of-art efficiencies in the laboratory stage and pioneering scalable solar water splitting systems for hydrogen production using particulate photocatalysts are presented. Finally, some perspectives and outlooks on the future development of photocatalytic water splitting using particulate photocatalysts are proposed.

Construction of double-functionalized g-C3N4 heterojunction structure via optimized charge transfer for the synergistically enhanced photocatalytic degradation of sulfonamides and H2O2 production

Herein, supporting g-C₃N₄ embedded with benzene-ring (BCN) on P-modified g-C₃N₄ (PCN) successfully synthesized the homogeneous photocatalyst BCN/PCN (PBCN) via a simple thermal polymerization reaction. Under blue-light (LED) irradiation, the optimized PBCN (0.448 min^-1) demonstrated excellent photocatalytic performance, attaining over 74 times the degradation rate for sulfisoxazole (SSZ) in contrast to non-functionalized g-C₃N₄ (CN, 0.006 min^-1). Theoretical calculations revealed that the substitution of heterocyclic rings in the g-C₃N₄ triazine networks with benzene-rings enabled them to serve as electron donors, while promoting photoinduced spatial charge dissociation. Further, the carrier PCN tended to serve as electron acceptors to form electron-rich corner-phosphorous sites. Reactive species experiments demonstrate that the O₂˙^- and h⁺ constituted the primary photocatalytic mechanism of SSZ degradation. The potential SSZ degradation routes were predicted based on the transformation products via mass spectrometry. Finally, the composite materials also exhibited excellent photocatalytic activity in the conversion of solar energy to chemical energy (H₂O₂). This study guides the rational modification of g-C₃N₄-based semiconductors to achieve green energy production and beneficial ecological applications.

MoC/MAPbI3 hybrid composites for efficient photocatalytic hydrogen evolution

Metal halide perovskites, such as iodine methylamine lead (MAPbI₃), have received extensive attention in the field of photocatalytic decomposition of HI for hydrogen evolution, due to their excellent photoelectric properties. In this paper, a new MAPbI₃-based composite, MoC/MAPbI₃, was synthesized. The results show that 15 wt% MoC/MAPbI₃ has the best hydrogen production performance (38.4 μmol h^-1), which is approximately 24-times that of pure MAPbI₃ (1.61 μmol h^-1). With the extension of the catalytic time, the hydrogen production rate of MoC/MAPbI₃ reached 165.3 μmol h^-1 after 16 h due to the effective separation and transfer of charge carriers between MoC and MAPbI₃, showing excellent hydrogen evolution rate performance under visible light. In addition, the cycling stability of MoC/MAPbI₃ did not decrease in multiple 4 h cycle tests. This study used the non-precious metal promoter MoC to modify MAPbI₃, and provides a new idea for the synthesis of efficient MAPbI₃-based composite catalysts.

Photoelectrochemical Water-Splitting Using CuO-Based Electrodes for Hydrogen Production: A Review

The cost-effective, robust, and efficient electrocatalysts for photoelectrochemical (PEC) water-splitting has been extensively studied over the past decade to address a solution for the energy crisis. The interesting physicochemical properties of CuO have introduced this promising photocathodic material among the few photocatalysts with a narrow bandgap. This photocatalyst has a high activity for the PEC hydrogen evolution reaction (HER) under simulated sunlight irradiation. Here, the recent advancements of CuO-based photoelectrodes, including undoped CuO, doped CuO, and CuO composites, in the PEC water-splitting field, are comprehensively studied. Moreover, the synthesis methods, characterization, and fundamental factors of each classification are discussed in detail. Apart from the exclusive characteristics of CuO-based photoelectrodes, the PEC properties of CuO/2D materials, as groups of the growing nanocomposites in photocurrent-generating devices, are discussed in separate sections. Regarding the particular attention paid to the CuO heterostructure photocathodes, the PEC water splitting application is reviewed and the properties of each group such as electronic structures, defects, bandgap, and hierarchical structures are critically assessed.

Boosting photoelectrochemical efficiency by near-infrared-active lattice-matched morphological heterojunctions

Photoelectrochemical catalysis is an attractive way to provide direct hydrogen production from solar energy. However, solar conversion efficiencies are hindered by the fact that light harvesting has so far been of limited efficiency in the near-infrared region as compared to that in the visible and ultraviolet regions. Here we introduce near-infrared-active photoanodes that feature lattice-matched morphological hetero-nanostructures, a strategy that improves energy conversion efficiency by increasing light-harvesting spectral range and charge separation efficiency simultaneously. Specifically, we demonstrate a near-infrared-active morphological heterojunction comprised of BiSeTe ternary alloy nanotubes and ultrathin nanosheets. The heterojunction's hierarchical nanostructure separates charges at the lattice-matched interface of the two morphological components, preventing further carrier recombination. As a result, the photoanodes achieve an incident photon-to-current conversion efficiency of 36% at 800 nm in an electrolyte solution containing hole scavengers without a co-catalyst.

Engineering of anatase/rutile TiO2 heterophase junction via in-situ phase transformation for enhanced photocatalytic hydrogen evolution

Constructing effective interphase boundary is one of the efficient approaches for improving photocatalytic performances of semiconductor materials. In this work, an anatase/rutile-TiO₂ (AR-TiO₂) heterophase junction with appropriate carbon content was successfully fabricated via an in-situ phase transformation process. The phase transformation started from the inner core of the nanoparticles and the area of phase interface between anatase and rutile was carefully controlled by regulating the activation temperature. The well-established type-II band alignment between two TiO₂ phases with residual carbon as additional charge transfer intermediary which significantly improved the light-harvesting and photoinduced electron-hole pair separation. As a result, the optimal AR-TiO₂-550 catalyst (without adding commonly used Pt as co-catalyst) remarkably enhanced photocatalytic H₂ generation (201 μmol h^-1 g^-1), which was about 12-fold to that of P25. The AR-TiO₂-550 heterophase junction also showed long-term stability under simulated solar light irradiation. This research provides a new phase engineering route for developing high-efficient photocatalysts.

Nb2O5-Based Photocatalysts

Photocatalysis is one potential solution to the energy and environmental crisis and greatly relies on the development of the catalysts. Niobium pentoxide (Nb2O5), a typically nontoxic metal oxide, is eco-friendly and exhibits strong oxidation ability, and has attracted considerable attention from researchers. Furthermore, unique Lewis acid sites (LASs) and Brønsted acid sites (BASs) are observed on Nb2O5 prepared by different methods. Herein, the recent advances in the synthesis and application of Nb2O5-based photocatalysts, including the pure Nb2O5, doped Nb2O5, metal species supported on Nb2O5, and other composited Nb2O5 catalysts, are summarized. An overview is provided for the role of size and crystalline phase, unsaturated Nb sites and oxygen vacancies, LASs and BASs, dopants and surface metal species, and heterojunction structure on the Nb2O5-based catalysts in photocatalysis. Finally, the challenges are also presented, which are possibly overcome by integrating the synthetic methodology, developing novel photoelectric characterization techniques, and a profound understanding of the local structure of Nb2O5.

A microfluidic cathodic photoelectrochemical biosensor chip for the targeted detection of cytokeratin 19 fragments 21-1

A microfluidic chip integrated with a microelectrode and a cathodic photoelectrochemical (PEC) biosensor for the ultrasensitive detection of non-small cell lung cancer cytokeratin fragments based on a signal amplification strategy was designed. The mechanism for signal amplification is developed based on the p-n junction of AgI/Bi₂Ga₄O₉, with dissolved O₂ as an electron acceptor to produce the superoxide anion radical (˙O₂^-) as the working microelectrode. By combining this with a novel superoxide-dismutase-loaded honeycomb manganese oxide nanostructure (SOD@hMnO₂) as the co-catalyst signal amplification label, ˙O₂^- can be catalyzed by SOD via a disproportionation reaction to produce O₂ and H₂O₂; then, hMnO₂ is able to trigger the decomposition of H₂O₂ to generate O₂ and H₂O. Therefore, the increased O₂ promotes the separation of electron-hole pairs via consuming more electrons, leading to an effective enhancement of the cathodic PEC behavior. Under optimum conditions, with the cytokeratin 19 fragments 21-1 (CYFRA 21-1) as the targeted detection objects, the microfluidic cathodic PEC biosensor chip exhibited excellent linearity from 0.1 pg mL^-1 to 100 ng mL^-1, with a detection limit of 0.026 pg mL^-1 (S/N = 3). The exciting thing that this work offers is a new strategy for the detection of other important cancer biomarkers for disease diagnosis and prognosis.

Carbon quantum dots-TiO2 nanocomposite as an efficient photocatalyst for the photodegradation of aromatic ring-containing mixed VOCs: An experimental and DFT studies of adsorption and electronic structure of the interface

In this work, we have developed and optimized TiO₂ nanoparticles decorated with carbon quantum dots to examine its potential use in the photocatalytic oxidation of aromatic ring containing gas-phase mixed volatile organic compounds, e.g., benzene, toluene, and p-xylene. Carbon quantum dots decorated TiO₂ demonstrated good photodegradation efficiency in contrast to pure TiO₂ under UV-vis light illumination. For example, with 0.5 wt% carbon quantum dots decorated on TiO₂, 64 % of the mixed volatile organic compounds were photodegraded, while pure TiO₂ only exhibited 44 % of the photodegradation efficiency. Also, the carbon quantum dots (0.5 wt%)/TiO₂ nanocomposite demonstrated considerable photocatalytic activity within the visible region. On the other hand, pure TiO₂ remained inactive within the visible region. The density functional theory study of the carbon quantum dots/TiO₂ interface revealed that C 2p states of carbon quantum dots incorporated new energy states around the Fermi level near the lowest conduction band. This might be accountable for the improved charge separation process and better conductivity of the photogenerated electrons. The improved photocatalytic performance of the carbon quantum dots/TiO₂ nanocomposites can be attributed to good light harvesting within the UV-vis region, charge separation, and adsorption capability.

Light and electric field induced unusual large-scale charge separation in hybrid semiconductor objects

Separation of electric charges is the most crucial phenomenon in natural photosynthesis, and is also extremely important for many artificial energy conversion systems based on semiconductors. The usual roadblock in this context is the fast recombination of electrons and holes. Here we demonstrate that the synergy of light and electric fields allows separating very efficiently electric charges over an unusually large distance in TiO2. The generated internal electric field can also be used to shuttle electrons simultaneously to the two opposite sides of a hybrid TiO2-polyaniline object. This counterintuitive behavior is based on the combination of the principles of bipolar electrochemistry and semi-conductor physics.

Novel Bi2+xWO6 p-n Homojunction Nanostructure: Preparation, Characterization, and Application for a Self-Powered Cathodic Photoelectrochemical Immunosensor

Synthesizing novel cathodic photoactive materials with high photoelectrochemical (PEC) performance is urgently important for the development of photocathodic sensors. Herein, a novel photocathode material, Bi self-doped Bi₂WO₆ (i.e., Bi_2+xWO₆) p-n homojunction, is prepared via a simple ethylene glycol-assisted solvothermal reduction for the first time. Compared with pristine Bi₂WO₆, Bi_2+xWO₆ possesses a narrower band gap and higher light harvesting ability. Among the synthesized materials, Bi_2.1WO₆ exhibits the highest photocurrent response, which is 23 times that of pure Bi₂WO₆ because of the synergistic effect of doped Bi and the p-n homojunction. The open circuit potential, "V-shaped" Mott-Schottky plot, linear sweep voltammetry curve, and transient photocurrent demonstrate the p-n homojunction characteristics of the material well. By using the Bi_2+xWO₆ p-n homojunction as the photocathode for sensing and the plasmonic WO₃/Au composite as the photoanode for signal amplification, a new self-powered membraneless PEC immunosensor is established for a highly sensitive detection of human epididymal protein 4. This study offers a new idea for designing novel photocatalysts with satisfactory performance, and the Bi_2+xWO₆ p-n homojunction is expected to act as a promising PEC platform for developing various self-powered biosensors.

Toward enhanced photocatalytic activity of graphite carbon nitride through rational design of noble metal-free dual cocatalysts

The g-C3N4-MoS2-M(OH)x ternary heterostructures were designed and fabricated for the first time. The embedding of noble-metal-free MoS2-M(OH)x dual cocatalysts over g-C3N4 nanosheets led to obvious synergistic effect for improving the transport as well as utilization efficiency of photo-generated charge carriers. Consequently, the optimal ternary heterostructure (g-C3N4-MoS2-Ni(OH)2) exhibited photocatalytic hydrogen production activity 4.5 times larger than the sum of the photocatalytic HER activity of g-C3N4-MoS2 and g-C3N4-Ni(OH)2. More significantly, even in the absence of the sacrificial agent, the g-C3N4-MoS2-Ni(OH)2 ternary heterostructure exhibited a photocatalytic HER activity of 0.3 mmol h-1 g-1 with considerable H2O2 production under UV-visible light.

Engineering the Morphology and Crystal Phase of 3 D Hierarchical TiO2 with Excellent Photochemical and Photoelectrochemical Solar Water Splitting

Owing to their unique characteristics, hierarchical TiO₂ nanostructures have several advantages in solar-fuel production. In this work, a single-step approach has been developed to control both the crystal phase and morphology of TiO₂ with 3 D urchin-like structure via a surfactant-free solvothermal route. The growth of 3 D hierarchical structure with phase-engineered band alignment, the role of the H₂ O/HCl ratio, and fine-tuning of the reaction parameters are investigated systematically. An optimum ratio of anatase (41 %) to rutile (59 %) in the mixed-phase TiO₂ (AR-2) results in excellent photocatalytic H₂ generation activity of 5753 μmol g^-1 after 5 h of irradiation with apparent quantum yields of 20.9 % at 366 nm and 4.5 % at 420 nm. The superior performance of AR-2, attributed to efficient separation of charge carriers through the phase junction, is apparent from the transient photocurrent response and photoluminescence studies. The 3 D urchin-like pure rutile TiO₂ (R-1) composed of nanorods shows enhanced photocatalytic activity compared with pure anatase and pure rutile TiO₂ nanoparticles, and this demonstrates the role of morphology. The best-performing mixed-phase 3 D TiO₂ shows excellent durability up to 25 h and is shown to produce 3522 μmol g^-1 of H₂ under natural sunlight, which highlights its potential for long-term application.

Self-cleaning isotype g-C3N4 heterojunction for efficient photocatalytic reduction of hexavalent uranium under visible light

Photocatalysis is a promising method to eliminate hexavalent uranium (U(Ⅵ)) and recycle it from wastewater. However, most of researched photocatalysts are metal-contained, inactive in visible light, and inconvenient to recycle, which unfortunately impedes the further utilization of photocatalytic technology in U(Ⅵ) pollution treatment. Herein, g-C₃N₄ isotype heterojunction with interpenetrated tri-s-triazine structure (ipCN) was prepared by inserting urea into the interlayer of tri-s-triazine planes of thiourea-derived g-C₃N₄ and in-site thermal treating. The synthesized nanocomposites were used to convert soluble U(Ⅵ) ions into U(Ⅳ) sediment under visible light. Experimental and characterization results reveal that ipCN possess larger BET surface area, more negative-charged surface, higher U(Ⅵ) adsorption capability, and more efficient mass diffusion and charges transfer properties. With these excellent characteristics, nearly 98% U(Ⅵ) could be removed within 20 min over ipCN_5:1 and 92% photoreduction efficiency could also be kept after 7 cycle uses, which were equal to or even superior than most reported metal-based photocatalysts. It is also proven that the configuration of U(Ⅵ) and photogenerated ·O₂^- play a significant role in the photocatalytic U(Ⅵ) reduction process, with (UO₂)_x(OH)_y^2x-y are more prone to be adsorbed and the photoinduced process of ·O₂^- will steal electrons from photocatalysts. Furthermore, with the self-generated ·O₂^- and H₂O₂, a green and facile regeneration process of photocatalysts was proposed This work provides a promising scheme to extract U(Ⅵ) from the perspectives of photocatalysts exploitation, photocatalytic reduction, and photocatalysts regeneration, which is meaningful for the sustainable U(Ⅵ) resource recovery and U(Ⅵ) pollution purification.

Self-Assembly Hydrothermal Synthesis of Silverton-Type Polyoxometalate-Based Photocatalysts for Enhanced Degradation

The synthesis of some complex polyoxometalates (POMs) is critical to develop potential photocatalysts with high catalytic activity and selectivity. Here, we address this challenge by a hydrothermal self-assembly route to obtain a novel POM-based Co₄W₆O₂₁(OH)₂·4H₂O with a hierarchical microsphere structure. The Co₄W₆O₂₁(OH)₂·4H₂O crystallizes in the cubic space group Im3̅ with cell parameters: a = b = c = 12.878 Å, α = β = γ = 90°, and Z = 4. The structure is further characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis spectroscopy, thermogravimetric analysis, and Fourier transform infrared spectra. After depositing Ag₂O nanoparticles on the 3D Co₄W₆O₂₁(OH)₂·4H₂O microsphere by photochemical synthesis, the Co₄W₆O₂₁(OH)₂·4H₂O/Ag₂O heterojunction presents enhanced photocatalytic activity for RhB compared with P25 and pristine Ag₂O. Moreover, we confirm the key role of holes for the Co₄W₆O₂₁(OH)₂·4H₂O/Ag₂O and put forward a possible mechanism for the photocatalytic degradation reaction.

Structural-Phase Catalytic Redox Reactions in Energy and Environmental Applications

The structure-property engineering of phase-based materials for redox-reactive energy conversion and environmental decontamination nanosystems, which are crucial for achieving feasible and sustainable energy and environment treatment technology, is discussed. An exhaustive overview of redox reaction processes, including electrocatalysis, photocatalysis, and photoelectrocatalysis, is given. Through examples of applications of these redox reactions, how structural phase engineering (SPE) strategies can influence the catalytic activity, selectivity, and stability is constructively reviewed and discussed. As observed, to date, much progress has been made in SPE to improve catalytic redox reactions. However, a number of highly intriguing, unresolved issues remain to be discussed, including solar photon-to-exciton conversion efficiency, exciton dissociation into active reductive/oxidative electrons/holes, dual- and multiphase junctions, selective adsorption/desorption, performance stability, sustainability, etc. To conclude, key challenges and prospects with SPE-assisted redox reaction systems are highlighted, where further development for the advanced engineering of phase-based materials will accelerate the sustainable (active, reliable, and scalable) production of valuable chemicals and energy, as well as facilitate environmental treatment.

Surface-state-induced upward band bending in P doped g-C3N4 for the formation of an isotype heterojunction between bulk g-C3N4 and P doped g-C3N4: photocatalytic hydrogen production

The staggered type heterojunction with g-C₃N₄ based nanomaterials has received much attention owing to its change in chemical potential between two semiconductors.

O-MoS2/Mn0.5Cd0.5S composites with enhanced activity for visible-light-driven photocatalytic hydrogen evolution

The reaction mechanism of O-MoS₂/Mn_0.5Cd_0.5S for photocatalytic hydrogen evolution is put forward and the satisfactory hydrogen production rate of the optimized composite is superior to most of the Mn–Cd–S based catalysts reported.

Band engineering of mesoporous TiO2 with tunable defects for visible-light hydrogen generation

A limited light absorption range and low electron transfer efficiency have always been the major problems in semiconductor photocatalysis.

Construction of a triple sequential junction for efficient separation of photogenerated charges in photocatalysis

A triple sequential junction providing a continuous charge separation and transfer channel was successfully fabricated by rational combining the anatase/rutile TiO₂ heterophase and rutile/rutile TiO₂ homophase junctions.

Surface Assistant Charge Separation in PEC Cu2S-Ni/Cu2O Cathode

Fabrication of a high efficiency photocathode is a challenging issue in photoelectrocatalysis (PEC). In this work, a Cu₂S-Ni/Cu₂O photocathode was constructed via electrodeposition followed by a two-step overlayer deposition procedure including direct-current magnetron sputtering (DCMS) and ion exchange reaction. We found that the presence of Ni in the inner-layer could not only affect the morphology but also enhance the formation rate of the outer-layer Cu₂S. The XPS results indicate that the Ni exist as NiO_x instead of Ni⁰. The photocurrent of Cu₂S-Ni/Cu₂O achieved 2 times of it on the pristine Cu₂O. The charge dynamic characterizations, including electrochemical impedance spectroscopy (EIS), Tafel slopes, and photoluminescence (PL) spectra, demonstrated that the Ni can promote the hydrogen evolution reaction follow the Heyrovsky reaction, while Cu₂S shows a crucial role on the surface charge separation. At last, surface photovoltage microscopy (SPVM) technology was used to reveal the function of each overlayer. It gives direct evidence for the charge transportation pathway in the system and explains the function of each component.

Versatile Functional Porous Cobalt-Nickel Phosphide-Carbon Cocatalyst Derived from a Metal-Organic Framework for Boosting the Photocatalytic Activity of Graphitic Carbon Nitride

Metal-organic framework-templated g-C₃N₄-NiCoP₂-porous carbon (PC) ternary hybrid nanomaterials were designed by taking full advantage of the metal-organic framework (MOF) derivative in the photocatalytic reaction for the first time. The MOF-templated porous structure could prevent the stacking of the carbon nitride nanosheet, and the carefully designed NiCoP₂, possessing low electrocatalytic hydrogen evolution reaction (HER) overpotential and flat-band potential, could improve the separation as well as the utilization efficiency of photogenerated electron-hole pairs. Moreover, the ligand-templated porous carbon, acting as an interface mediator between g-C₃N₄ and the NiCoP₂ cocatalyst, could boost the charge carrier transport. Consequently, the optimal ternary g-C₃N₄-NiCoP₂-PC heterostructure exhibited enhanced photocatalytic HER performance and considerable H₂ evolution performance of 5.8 μmol/h/g under UV-visible light with stoichiometric H₂O₂ production even in pure water. This work took full advantage of the MOF derivative for improving the photocatalytic reaction activity and provided a method that can hopefully help in designing a novel high-performance catalyst for solar conversion.

A high-efficiency and stable cupric oxide photocathode coupled with Al surface plasmon resonance and Al2O3 self-passivation

A high-efficiency and stable CuO/Al/Al₂O₃ photocathode for photoelectrochemical water splitting has been successfully synthesized by a facile magnetron sputtering combined with spontaneous oxidation method.