Strategies for improving photocatalytic performance of g-C3N4 by modulating charge separation and current research status

Graphitic carbon nitride (g-C3N4) has been extensively investigated over the past decade for its potential utilizations in photocatalytic energy generation and pollutant degradation. To better meeting the requirements for practical utilizations, it is crucial to address the issue of poor charge separation properties in g-C3N4, which origin from the strong interactions in photogenerated electron-hole pairs. In this review, we summarized the pertinent studies on developing strategies to promote the charge separation properties of g-C3N4. The strategies can be categorized into two categories of promoting the surface migration of charge carriers and prolonging the lifetime of surface charge. Finally, we present potential challenges in promoting charge separation and offer feasible suggestions to face these challenges.

Designing step-scheme AgI decorated Ta2O5-x heterojunctions for boosted photodegradation of organic pollutants

Step-scheme (S-scheme) AgI decorated Ta2O5-x heterojunctions have been designed and synthesized via a combination of solvothermal and chemical deposition methods for enhanced visible-light harvesting and high-performance photocatalysis. The AgI nanoparticles showed great influences on the visible-light absorption and charge separation between AgI and Ta2O5-x microspheres. The experimental results indicated that the as-prepare AgI/Ta2O5-x composites achieved enhanced photocatalytic performance towards tetracycline degradation under visible light, and the AgI/Ta2O5-x-11 sample displayed the highest photocatalytic performance and the maximum rate constant of approximately 0.09483 min-1, which was 7.22 times that of Ta2O5-x microspheres and 2.56 times that of AgI, respectively. The highly enhanced photocatalytic performance was mainly attributed to the construction of S-scheme heterostructure and formation of oxygen vacancies in Ta2O5-x microspheres. In addition, the trapping experimental and DMPO spin-trapping ESR spectra confirmed the ⸱O2- and ⸱OH species as the main radicals during tetracycline degradation. Current work indicates an S-scheme tantalum-based composites for high-performance environmental photocatalysis.

Boosted Tetracycline and Cr(VI) Simultaneous Cleanup over Z-Scheme WO3/CoO p-n Heterojunction with 0D/3D Structure under Visible Light

In this study, a Z-Scheme WO₃/CoO p-n heterojunction with a 0D/3D structure was designed and prepared via a simple solvothermal approach to remove the combined pollution of tetracycline and heavy metal Cr(VI) in water. The 0D WO₃ nanoparticles adhered to the surface of the 3D octahedral CoO to facilitate the construction of Z-scheme p-n heterojunctions, which could avoid the deactivation of the monomeric material due to agglomeration, extend the optical response range, and separate the photogenerated electronhole pairs. The degradation efficiency of mixed pollutants after a 70 min reaction was significantly higher than that of monomeric TC and Cr(VI). Among them, a 70% WO₃/CoO heterojunction had the best photocatalytic degradation effect on the mixture of TC and Cr(VI) pollutants, and the removing rate was 95.35% and 70.2%, respectively. Meanwhile, after five cycles, the removal rate of the mixed pollutants by the 70% WO₃/CoO remained almost unchanged, indicating that the Z-scheme WO₃/CoO p-n heterojunction has good stability. In addition, for an active component capture experiment, ESR and LC-MS were employed to reveal the possible Z-scheme pathway under the built-in electric field of the p-n heterojunction and photocatalytic removing mechanism of TC and Cr(VI). These results offer a promising idea for the treatment of the combined pollution of antibiotics and heavy metals by a Z-scheme WO₃/CoO p-n heterojunction photocatalyst, and have broad application prospects: boosted tetracycline and Cr(VI) simultaneous cleanup over a Z-scheme WO₃/CoO p-n heterojunction with a 0D/3D structure under visible light.

Preparation of reed-based hydrothermal carbonized carbon photocatalyst and effective degradation of methylene blue and tetracycline

Hydrothermal carbonation carbon (HTCC) is a promising semiconductor material for the photocatalytic degradation of pollutants. However, the poor charge transfer capability of HTCC and the unclear mechanism of photocatalysis limit its practical application. In this study, a novel Z-type heterojunction photocatalyst of silver carbonate (Ag₂CO₃) and HTCC (Ag₂CO₃/HTCC) was developed for the degradation of methylene blue (MB) and tetracycline (TC) from wastewater using a hydrothermal- coprecipitation method. Compared to Ag₂CO₃ and HTCC, 40% Ag₂CO₃/HTCC had excellent photocatalytic activity and stability. The free radical scavenger experiments showed that •O₂^- and h⁺ were the main substances for the degradation of MB and TC. The intermediates formed during the photodegradation were identified by HPLC-MS, and a possible mechanism and pathway for the degradation of MB and TC by Ag₂CO₃/HTCC was proposed. This study provides a new idea for the synthesis of Z-type HTCC heterojunction with a high-photocatalytic efficiency and its photocatalytic mechanism.

Engineering interface structures for heterojunction photocatalysts

Solar photocatalysis is the most ideal solution to global energy concerns and environmental deterioration nowadays. The heterojunction combination has become one of the most successful and effective strategies to design and manufacture composite photocatalysts. Heterojunction structures are widely documented to markedly improve the photocatalytic behavior of materials by enhancing the separation and transfer of photogenerated charges, widening the light absorption range, and broadening redox potentials, which are attributed to the presence of both build-in electric fields at the interface of two different materials and the complementarity between different electron structures. So far, a large number of heterojunction photocatalytic materials have been reported and applied for water splitting, reduction of carbon dioxide and nitrogen, environmental cleaning, etc. This review outlines the recent accomplishments in the design and modification of interface structures in heterojunction photocatalysts, aiming to provide some useful perspectives for future research in this field.

Construction of Highly Active Zn3In2S6 (110)/g-C3N4 System by Low Temperature Solvothermal for Efficient Degradation of Tetracycline under Visible Light

Herein, Zn3In2S6 photocatalyst with (110) exposed facet was prepared by low temperature solvothermal method. On this basis, a highly efficient binary Zn3In2S6/g-C3N4 was obtained by low temperature solvothermal method and applied to the degradation of tetracycline (TC). The samples of the preparation were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, UV–vis diffuse reflection spectroscopy, and photoluminescence spectroscopy. Furthermore, the degradation performance of photocatalysts on TC was investigated under different experimental conditions. Finally, the mechanism of Zn3In2S6/g-C3N4 composite material degrading TC is discussed. The results show that Zn3In2S6 and Zn3In2S6/g-C3N4 photocatalysts with excellent performance could be successfully prepared at lower temperature. The Zn3In2S6/g-C3N4 heterojunction photocatalyst could significantly improve the photocatalytic activity compared with g-C3N4. After 150 min of illumination, the efficiency of 80%Zn3In2S6/g-C3N4 to degrade TC was 1.35 times that of g-C3N4. The improvement of photocatalytic activity was due to the formation of Zn3In2S6/g-C3N4 heterojunction, which promoted the transfer of photogenerated electron–holes. The cycle experiment test confirmed that Zn3In2S6/g-C3N4 composite material had excellent stability. The free radical capture experiment showed that ·O2− was the primary active material. This study provides a new strategy for the preparation of photocatalysts with excellent performance at low temperature.

Recent advancement in Bi5O7I-based nanocomposites for high performance photocatalysts

Bi₅O₇I belongs to the family of bismuth oxyhalides (BiOX, X = Cl, Br, I), having a unique layered structure with an internal electrostatic field that promotes the separation and transfer of photo-generated charge carriers. Interestingly, Bi₅O₇I exhibits higher thermal stability compared to its other BiOX member compounds and absorption spectrum extended to the visible region. Bi₅O₇I has demonstrated applications in diverse fields such as photocatalytic degradation of various organic pollutants, marine antifouling, etc. Unfortunately, owing to its wide band gap of ∼2.9 eV, its absorption lies mainly in the ultraviolet region, and a tiny portion of absorption lies in the visible region. Due to limited absorption, the photocatalytic performance of pure Bi₅O₇I is still facing challenges. In order to reduce the band gap and increase the light absorption capability of Bi₅O₇I, doping and formation of heterostructure strategies have been employed, which showed promising results in the photocatalytic performance. In addition, the plasmonic heterostructures of Bi₅O₇I were also developed to further boost the efficiency of Bi₅O₇I as a photocatalyst. Here, in this review article, we present such recent efforts made for the advanced development of Bi₅O₇I regarding its synthesis, properties and applications. The strategies for photocatalytic performance enhancement have been discussed in detail. Moreover, in the conclusion section, we have presented the current challenges and discussed possible prospective developments in this field.

WS2/In2S3 composite photocatalyst for photocatalytic H2 generation and pollutant degradation

A Z-scheme WS2/In2S3 photocatalyst with a bi-layered sheet-like structure was designed to promote separation and transfer of photocarriers.

2D/2D Heterojunction systems for the removal of organic pollutants: A review

Photocatalysis is considered to be an effective way to remove organic pollutants, but the key to photocatalysis is finding a high-efficiency and stable photocatalyst. 2D materials-based heterojunction has aroused widespread concerns in photocatalysis because of its merits in more active sites, adjustable band gaps and shorter charge transfer distance. Among various 2D heterojunction systems, 2D/2D heterojunction with a face-to-face contact interface is regarded as a highly promising photocatalyst. Due to the strong coupling interface in 2D/2D heterojunction, the separation and migration of photoexcited electron-hole pairs are facilitated, which enhances the photocatalytic performance. Thus, the design of 2D/2D heterojunction can become a potential model for expanding the application of photocatalysis in the removal of organic pollutants. Herein, in this review, we first summarize the fundamental principles, classification, and strategies for elevating photocatalytic performance. Then, the synthesis and application of the 2D/2D heterojunction system for the removal of organic pollutants are discussed. Finally, the challenges and perspectives in 2D/2D heterojunction photocatalysts and their application for removing organic pollutants are presented.

Assembly of CaIn2S4 on Defect-Rich BiOCl for Acceleration of Interfacial Charge Separation and Photocatalytic Phenol Degradation via S-Scheme Electron Transfer Mechanism

The novel 2D/2D S-scheme heterostructure of BiOCl nanosheets coupled with CaIn2S4 nanosheets (CaIn2S4/BiOCl-SOVs), which contains surface oxygen vacancies (SOVs), has been successfully prepared by high-temperature calcination combined with a solvothermal synthetic strategy. Under visible-light irradiation, the apparent rate constant (Kapp/mim−1) for phenol degradation on the 1 wt% CaIn2S4/BiOCl-SOVs photocatalyst is about 32.8 times higher than that of pure BiOCl. The superior performance was attributed to the synergistic effect between the SOVs, CaIn2S4, and BiOCl, which can effectively narrow the bandgap and accelerate the interfacial charge separation of CaIn2S4/BiOCl-SOVs heterojunctions. Subsequently, it significantly promotes the generation of superoxide radicals (O2−), hydroxyl radicals, and h+, which participate in the photodegradation process of phenol. The catalyst still maintained a relatively high activity after repeated tests as a demonstration of its photostability. This work successfully proposed an efficient method to design a new 2D/2D S-scheme heterostructure with SOVs as possible photocatalysts in the field of environmental remediation.

Boosted charge transfer in dual Z-scheme BiVO4@ZnIn2S4/Bi2Sn2O7 heterojunctions: Towards superior photocatalytic properties for organic pollutant degradation

A core-shell structured dual Z-scheme ternary photocatalyst BiVO₄@ZnIn₂S₄/Bi₂Sn₂O₇ was fabricated via hydrothermal and heat-circumfluence strategy. With ZnIn₂S₄ serving as a bridge to connect BiVO₄ and Bi₂Sn₂O₇, the developed ternary catalyst displayed boosted charge transfer and spatial separation capabilities. The effect of mass ratios of BiVO₄@ZnIn₂S₄ and Bi₂Sn₂O₇ on photodegradation efficiency under visible light irradiation was explored. The optimal ternary heterojunction photocatalyst possessed remarkable photocatalytic rate constant for Rhodamine B (RhB) degradation, which was 63 and 12 times higher than that of BiVO₄ and Bi₂Sn₂O₇, respectively. In addition, the as-prepared ternary photocatalyst had good universality. Notably, the novel dual Z-scheme photocatalysts could improve the separating/transferring efficiency and reduction/oxidation ability of charge carriers. Meanwhile, the hierarchical structure offered sufficient reaction sites for photodegradation. This work provides a new insight into the rational design of ternary dual Z-scheme photocatalysts.

Phase transformation and heterojunction construction of bismuth oxyiodides by grinding-assisted calcination in the presence of thiourea and their photoactivity

Bismuth-rich oxyhalides are promising photocatalysts due to their special layered structure and adjustable band gap energy. In this work, a series of bismuth oxyiodides were fabricated by grinding-assisted calcination in the presence of thiourea, where grinding-induced mechanical force could accelerate the decomposition reaction and thiourea could prohibit the crystal particles from growing due to coordination action. The combined effect of grinding and thiourea could decrease the temperature of phase transformation of bismuth oxyiodides. Among these, heterojunction Bi4O5I2/Bi5O7I containing uniform flower-like microspheres assembled by ultra-thin nanosheets exhibited the highest photocatalytic activity and favorable stability for the degradation of the antibiotic tetracycline under visible light irradiation. This work could provide a good reference for the design of bismuth-rich oxyhalide heterojunction for photocatalytic applications.

Nanostructured Mg substituted Mn-Zn ferrites: A magnetic recyclable catalyst for outstanding photocatalytic and antimicrobial potentials

With recently increasing the environmental problems and expected energy crisis, it is necessary to synthesis a low-cost, efficient, and UV-light responsive photocatalyst for contaminants' degradation. The nanostructured spinel ferrite Mn_0.5Zn_0.5-xMg_xFe₂O₄ NPs (x = 0.0, 0.125, 0.25, 0.375 and 0.50) were synthesized via the sol-gel method. The crystallite size was lied in nano regime ranging from 21.8 to 36.5 nm. The surface chemical composition of the Mn_0.5Zn_0.5-xMg_xFe₂O₄ NPs was investigated via XPS analysis. Mossbauer spectra showed that the peaks were shifted to higher values of the maximum magnetic field as the Mg content increased, indicating that the crystallinity is enhanced while the crystal size is decreased. Also, various parameters such as the photocatalyst dose, dyes concentration, pH, point of zero charge, and the metals leaching were studied. The point of zero charge (PZC) has found at pH = 2.38. The Mn_0.5Zn_0.125Mg_0.375Fe₂O₄ NPs showed an excellent UV-assisted photocatalytic activity against Chloramine T (90 % removal efficiency) and Rhodamine B (95 % removal efficiency) after 80 min as compared to pure Mn_0.5Zn_0.5Fe₂O₄ ferrite NPs. Besides, it a recyclable catalyst at least four times with a negligible reduction of photocatalytic activity with slight elements leaching. Furthermore, the Mn_0.5Zn_0.25Mg_0.25Fe₂O₄ NPs showed a high antimicrobial activity towards pathogenic bacteria and yeats.

Green and controllable synthesis of one-dimensional Bi2O3/BiOI heterojunction for highly efficient visible-light-driven photocatalytic reduction of Cr(VI)

BiOI nanosheets have been successfully deposited on the porous Bi₂O₃ nanorobs via a one-pot precipitation method. The physicochemical features of the as-prepared materials were characterized in detail by a series of techniques, and the results revealed that BiOI nanosheets were evenly distributed on the porous Bi₂O₃ nanorobs. Because of higher photogenerated electron-hole pairs separation efficiency and the larger specific surface area compared to the pristine Bi₂O₃ and BiOI, the 50%Bi₂O₃/BiOI composite exhibited significantly enhanced photocatalytic activity for Cr(VI) reduction under visible light irradiation, and the reduction rate constant was 0.02002 min^-1, which was about 27.4 and 2.6 times higher than that of pure Bi₂O₃ (0.00073 min^-1) and BiOI (0.00769 min^-1), respectively. Moreover, the 50%Bi₂O₃/BiOI composite also possessed the excellent photochemical stability and recyclability, thereby facilitating its wastewater treatment application.

AgI loading BiOI composites with enhanced photodegradation efficiency for bisphenol A under simulated solar light

Bismuth oxyiodide (BiOI) is a narrow band gap semiconductor which can be driven by visible irradiation. In order to efficiently separate photo-generated carriers and utilization of visible light, a facile solvothermal approach was used to synthesize a novel AgI loading BiOI 3D hierarchical composite (AgI-BiOI). The AgI-BiOI with Ag and Bi molar ratio of 1:8 (AgI-BiOI (1-8)) showed great enhancement for photocatalytic degradation of bisphenol A (BPA) with pseudo-first degradation rate constant about 3.7 or 14.5 times than that of pristine BiOI or AgI under simulated solar light. This synergistic enhancement for BPA degradation on AgI-BiOI(1-8) is mainly ascribed to enhancing the light absorption intensity and accelerating photo-generated carriers separation due to the formation of AgI-BiOI heterojunction. Free radical quenching experiments proved that positive holes (h⁺) and superoxide (O₂^•-) radicals were dominantly responsible for the degradation of BPA rather than singlet oxygen (¹O₂) or hydroxyl radicals (•OH). The AgI-BiOI(1-8) hardly showed any ecotoxicity to C. elegans through lethal experiments. The luminance bacteria acute toxicity of degradation intermediates of BPA increased before 30 min then reduced significantly with reaction. The good durability and environmental-friendly characteristics make AgI-BiOI(1-8) catalyst to be a good solar light-driven candidate.

Constructing magnetic and high-efficiency AgI/CuFe2O4 photocatalysts for inactivation of Escherichia coli and Staphylococcus aureus under visible light: Inactivation performance and mechanism analysis

Magnetic materials usually exhibit advanced performance in many areas for their easy separating and recycle ability. In this study, silver iodide/copper ferrite (AgI/CuFe₂O₄) catalysts with excellent magnetic property were successfully synthesized and characterized by a series of techniques. Two typical bacteria Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were applied to estimate the photocatalytic inactivation performance of obtained AgI/CuFe₂O₄ catalysts. Results revealed that the AgI/CuFe₂O₄ (12.5% AgI) composite could absolutely inactivate 3 × 10⁹ CFU/mL E. coli and 2.7 × 10⁸ CFU/mL S. aureus cells severally in 50 min and 40 min under visible light irradiation, which showed a much higher photo-disinfection activity than monomers. Transmission electron microscopy was used to study the biocidal action of this nanocatalyst, the results confirmed that the treated E. coli cells were damaged, the nanocatalyst permeated into cells and resulting in death of cells. Besides, it was found that the destruction of bacterial membrane together with substantial leaked potassium ion (K⁺) which caused by the photo-generated reactive species superoxide radical (O₂^-) and holes (h⁺) could be the direct disinfection principles. For a deep insight into practical applications, the influences of different catalyst concentrations and reaction pH were also taken into discussion in details. The overall results indicated the novel photocatalyst with strong redox capacity and outstanding reusability can be widely employed in bacteria elimination.

Construction of a high-performance photocatalytic fuel cell (PFC) based on plasmonic silver modified Cr-BiOCl nanosheets for simultaneous electricity production and pollutant removal

The development of high-performance photocatalytic fuel cells (PFCs) is seriously hampered by poor light utilization rates and low charge carrier transfer efficiency. Herein, we have experimentally obtained plasmonic Ag modified Cr-BiOCl (Cr-BOC/Ag) with high visible light photocatalytic activity and provided direct evidence for the substantially enhanced catalytic activity in metal-semiconductor photocatalysts. The experimental results revealed that the Cr doping and Ag modification could not only extend the photo absorption of BiOCl from the UV to the visible light region but could also greatly increase the generation and transfer rate of charge carriers because of its narrowed band gap and the localized surface plasmon resonance (LSPR) effect of metallic Ag. Under visible light irradiation, the Cr-BOC/Ag showed a remarkable enhancement in the PFC performance when the optimum contents of Cr doping and Ag loading was 14.4% and 4%, respectively. The trapping experiments and multiple characterizations demonstrated that the advantageous combination of the Cr doping effect and SPR effect induced by the Ag nanoparticles is responsible for the high generation rate of oxidative species and effective charge carrier transfer. By using RhB as fuel, approximately 75.1% color removal efficiency and 8.38% coulombic efficiency were obtained under visible light irradiation for 240 min, which are higher than those of MO and TC. In addition, the Jsc and Voc of the Cr-BOC/Ag photoanode were measured to be 0.0073 mA cm-2 and 0.543 V.

Construction of 2D heterojunction system with enhanced photocatalytic performance: Plasmonic Bi and reduced graphene oxide co-modified Bi5O7I with high-speed charge transfer channels

The efficient electron-hole charge pair separation, ultra-fast electron migration and excellent light harvest capacity are essential for semiconductor photocatalyst with superior photocatalytic performance. In this study, we constructed layered 2D/2D heterojunction composite of Bi@Bi₅O₇I/rGO (BiBGOI) through a facile surface charge mediated self-assembly strategy. The unique 2D/2D heterostructure with face to face contact can increase the contact area and generate a large amount of charge transfer nanochannels in the interfacial heterojunction, resulting in the enhancement of photocatalytic activity. Addition of semimetal Bi can enhance light absorption, and the local electromagnetic field dominated by SPR effect is favorable for photoinduced charge pair separation. The novel composite showed superior photocatalytic performance for decomposing levofloxacin (LVFX), which was attributed to the unique 2D/2D structure and SPR effect. The enhanced mineralization ability of the novel composite was ascribed to the strong oxidization ability of photoinduced holes, further evaluating high charge pair separation efficiency. In addition, the strong adsorption capacity of rGO for LVFX molecules can enable active radicals transfer into the surface to decompose it. This work will shed light on constructing 2D/2D heterojunction system assisted with SPR effect for the practical application in removal of organic pollutants.

Novel one-step combustion synthesis of BiOBr:Yb3+,Er3+/AgBr upconversion heterojunction photocatalysts with enhanced vis/NIR photocatalytic activities

A simple combustion method was reported to prepare a novel BiOBr:Yb³⁺,Er³⁺/AgBr (BYE/AgBr) heterojunction photocatalyst.

Ultrafine Au nanoparticles anchored on Bi2MoO6 with abundant surface oxygen vacancies for efficient oxygen molecule activation

Au NPs were anchored on Bi₂MoO₆ with rich SOVs to improve O₂ activation for photocatalytic degradation of phenol and dye.