鉍基光催化劑在太陽能轉換中的合理設計

關鍵詞：光催化；鉍；異質結；電荷分離；應用

1 Introduction

Addressing environmental threats and energy shortages hasbecome an urgent and serious problem worldwide. As anabundant clean energy source， solar energy is an important wayto alleviate environmental pollution and energy scarcity.Photocatalytic technology， as an effective means to harness solarenergy， holds broad application prospects in areas likeenvironmental treatment， energy conversion， organic synthesis，and biomedical applications. In 1972， Japanese scientistspublished an article describing photolysis on the surface ofsingle-crystal TiO2 under UV irradiation， a phenomenon knownas the “Honda-Fujishima Effect”， which marked a new chapterin photocatalytic research 1. An increasing number of researchershave devoted themselves to the research and application ofphotocatalytic technology 2–4. Photocatalytic reactions involvethe steps of light uptake， formation and separation ofphotoexcited e?/h+ pairs， generation of active species， redoxreaction， and maintenance of the catalytic cycle. Photocatalystsstimulate electrons by absorbing light energy to form electronholepairs， which then react with adsorbed substances to generateactive species and trigger redox reactions to purify pollutants，synthesize and transform substances. Its application field extendsfrom photohydrogen production and pollutant degradation toorganic synthesis， air purification， CO2 reduction 5–7， N2 fixation 8，H2 evolution 9–11， and H2O2 production 12，13. The synthesis ofefficient photocatalysts including metallic oxides （TiO2 14–16，ZnO 17，18， In2O3 19）， nitride （Ta3N5 20， g-C3N4 21–25）， metalsulfides（CdS 26–33， MoS2 34）， and carbides （CNT 35， graphene 36–38，MXenes 39）， has entered a substantial development stage. Atpresent， the creation of advanced semiconductor photocatalystsaimed at alleviating energy crises and environmental problemshas aroused wide enthusiasm for exploration 40–42.

Concurrently， research on Bi-based photocatalysts hasincreasingly captured attention （Fig. 1） 43–47. As a group IIIAelement， bismuth exhibits narrow bandgap properties due to thehybridization of its 6s and O 2p orbitals， which facilitates rapidcharge migration and enables bismuth-based photocatalysts tohave an excellent visible light response with excellentphotocatalytic performance. Common bismuth-basedphotocatalysts include bismuth halide oxides， bismuthmolybdate， bismuth metal， bismuth vanadate， bismuth oxide，bismuth sulfide， and so on. Despite the immense versatility ofthese bismuth-based photocatalysts， their photocatalytic activityis not up to industrial standards. Based on this， researchers haveexplored strategies to increase photocatalytic performance byadjusting the bismuth oxidation state and lattice structure tobetter separate photoexcited electron-hole pairs. Bismuth-basedcatalysts are being developed to create new photocatalyticmaterials， increase the activity and scope of photocatalyticreactions， and overcome the current obstacles of low lightharvestingefficiency， poor selectivity， and high cost. Reasonableapproaches to Bi-based catalysts are crucial for the developmentof environmentally friendly， efficient， and affordable materialsthat can address current challenges in energy and environmentalsustainability. Enhancing the efficacy of bismuth-basedphotocatalysts for practical use necessitates a thoroughunderstanding of their structure， synthesis， and structuralmodification methods.

Earlier evaluations of bismuth-based photocatalysts havebeen notably narrow in various aspects. For example， theirstrategies do not encompass every synthetic pathway， they fallshort of thoroughly examining every potential catalytic andreaction pathway， and they lack a comprehensive investigationof the possible uses for bismuth-based photocatalysts in differentapplications. Previous assessments might not have adequatelyhighlighted the significance of sophisticated characterizationmethods in grasping the features and operations of bismuthbasedphotocatalysts， or they might have merely cited restrictedcharacterization approaches. Earlier studies might haveinadequately explored optimizing bismuth-based photocatalysts’effectiveness via materials engineering.

Aiming to offer a more thorough and detailed view， thisanalysis delves deeper into the mentioned elements， with thegoal of creating a more useful guide for studying and applyingbismuth-based photocatalysts. As shown in Fig. 2， this reviewarticle summarizes （1） the classification of bismuth-basedphotocatalysts， such as Sillén structures， Aurivillius structures，and bismuth element; （2） synthesis methods for bismuth-basedphotocatalysts， including hydrothermal/solvothermal methods，chemical precipitation， solid-state reaction， and other methods;（3） structural modifications of bismuth-based photocatalysts，（including defect engineering， heteroatom doping， morphologycontrol， SPR effect， and heterojunctions）; （4） applications inphotocatalysis （such as CO2 reduction， water splitting， N2fixation， H2O2 production， NOx removal， and selective organicsynthesis）; and （5） in situ characterization techniques. Lastly，discussions ensue about the difficulties and prospectivedirections in this field of intense study. This review aspires to actas a guide for the design of efficient and green photocatalyticreaction systems using bismuth-based materials， exploring abenign and sustainable future.

2 Bi-based photocatalysts

Photocatalysts based on bismuth typically react to visiblelight， making them suitable for uses like environmentalpurification and solar-powered energy conversion. A diversearray of bismuth-based photocatalysts exists， encompassingstratified bismuth-based substances like Sillén-structuralbismuth halide oxide with Aurivillius-structural Bi2MO6 （M =W， Mo， Cr）， bismuthine， BiVO4， Bi2S3， and Bi2O3.

2.1 Layered Bi

Layered bismuth-based materials have a laminar structureconsisting of [Bi2O2]2+ sheets and dispersed anionic or/andcationic groups， and show great potential for catalyticconversion from solar energy， attributed to their distinctivecrystal configurations， diverse composition， extensive atomiccoordination， and advantageous hybrid electron band structure.The structure of layered bismuth-based materials is classifiedinto Sillén structure and Aurivillius structure by intercalationunit. As shown in Fig. 3a， the Sillén structural bismuth-basedphotocatalysts consist of PbO-type blocks [Bi2O2] or [Bi3O4+n]with halogen atoms interspersed. As shown in Fig. 3b， theAurivillius structural bismuth-based photocatalysts arecomposed of alternating fluorite layers of [Bi2O2]2+ and nABO3sandwiches.

2.1.1 Sillén-structural

The field of photocatalysis has been captivated by Sillénstructuredbismuth-based photocatalysts due to their distinctivestratified composition and potent photo-oxidation capabilities 48–51.Commonly， these structures are made up of layers of bismuthoxygen，featuring [Bi2O2] or [Bi3O4+n] units， and are layeredwith halogen atoms in single， double， or triple forms. Typically，the structural equations used are [Bi2O2][Xm] or [Bi3O4+n][Xm]（X = I， Br， Cl; m = 1， 2， 3）. BiOX （X = F， Cl， Br， I） emerges as asignificant V-VI-VII ternary material molecule due to its opticalproperties 52–54. Each BiOX compound exhibits a tetragonalrutile formation， marked by alternating [Bi2O2] layers withhalogen atom double layers 55. Nonetheless， challenges likeminimal carrier separation， limited mobility， and a scarcity ofactive sites continue to impede oxyhalide development.Scientists incorporated a high-entropy alloy into the Sillénstructure， facilitating the creation of an extremely thin halideform， ABiO2X （A = Ca， Cd， Sr， Pb， Ba; X = Br， Cl， I），distinguished by its significant stability and highly active sites.By transforming the initial [X]? bilayer to a single layer， thismethod notably shortens the travel routes of h+ and e? bysubstantially decreasing the space between layers 56.

BiOBr is notably recognized for its ideal band gap between2.6 and 2.9 eV， a plentiful supply of raw materials， and notablestability. This method finds extensive application in reducingCO2， splitting water for H2 production， and degrading organicpollutants and so on. Nevertheless， the function of pure BiOBris typically reduced owing to the shift merging of photogeniccarriers. To enhance BiOBr’s photocatalytic efficiency， methodsinclude doping， defect engineering， and heterostructureconstruction. For example， Hua et al. presented highly dispersedS-scheme heterojunctions of MOF-BiOBr/Mn0.2Cd0.8S，demonstrating superior photocatalytic efficiency in carbondioxide reduction using the hydrothermal technique （Fig. 4a） 54.Within the MOF-BiOBr nanorods， the cauliflower-like MCS isevenly scattered， and the contact area between MCS and MOFBiOBris greatly increased. Fig. 4b shows that the optimal MOFBiOBr/MCS demonstrated remarkable performance in CO2reduction， enabling an impressive CO evolution rate （60.59μmol?h?1?g?1）. The CO2 reduction of the 40% MOF-BiOBr/MCSwas further elucidated using in situ DRIFT， as shown in Fig. 4c，where the characteristic vibrational bands at 1560 cm–1 reflectthe monodentate adsorption of CO2. DFT calculations yieldedwork functions of 7.8 and 4.6 eV for pure BiOBr and MCS，respectively， and determined that the electrons were transferredfrom the MCS in the high Fermi energy level to the MOF-BiOBrin the low EF. Merging the experimental findings and DFTcomputations， the MOF-BiOBr/MCS heterojunction’sphotogenerated charge transfer process aligns with the S-schememechanism （Fig. 4d）. Upon BiOBr’s proximate contact withMCS， MCS electrons will actively shift towards BiOBr untiltheir Fermi energies match. When balanced， the side of theMCS’s surface bears a positive charge， a result of electrondepletion， causing the electron depletion area and the edge of theMCS band to bend upwards. Conversely， the area near the BiOBrinterface is negatively charged， causing the electron accretionlayer and the edge of the BiOBr band to warp downwards.Consequently， an IEF forms at the juncture where BiOBrintersects with MCS， disrupting the uninterrupted electronmovement from MCS back into BiOBr. When exposed to light，e– ions gathered in BiOBr’s CB commonly merge with h+ ionsin MCS’s VB， owing to Coulombic forces and IEF influences.The S-scheme structure causes the e?/h+ pairs to be distant fromeach other， and the electrons clustered on the MCS surface reactwith carbon dioxide to generate carbon monoxide （Fig. 4e）.

In addition， quantum dots from semiconductors holdimmense promise in photocatalysis， extensively employed inhydrogen production， reducing carbon dioxide levels， andbreaking down pollutants， owing to their superior absorption ofvisible light， multi-exciton effect， surface effect， and modifiableenergy band characteristics. Primarily， QDs comprisemonolithic， group II-VI， III-V， I-III-VI， and chalcogenidequantum dots， each endowed with distinct characteristics.Lately， the focus on low-dimensional heterostructures， likequantum dot-nanosheet combinations， in photocatalytic studieshas been significant， due to their extraordinary interferentialproperties. Zan’s team achieved success in creating 0D g-C3N4quantum dots on the 2D BiOBr surface through hydrothermaltechniques， leading to the creation of CNQDs/BiOBr S-schemecomplexes that have a tightly-knit contact point 52. This structuremainly forms by reason of the interaction of heterocyclic π-electrons within CNQDs and BiOBr. Inspired by C-elementdoping to regulate the electronic structure of BiOX and bismuthcharacteristics， researchers have explored combining C dopingand Bi loading techniques applied to BOX photocatalysts. As amaterial with good lattice matching， the deposition of Biquantum dots on the BiOX surface has become a reasonable wayto boost BOX activity. For instance， He and colleagues havesuccessfully developed an innovative Bi quantum dots modifiedcarbon doped BiOCl catalyst and applied it to remove NOxpollutants from the air under visible light 57. Through thecombined impact of C atom doping and Bi quantum dot loading，this catalyst effectively enhanced light utilization and theefficiency of charge separation， demonstrating excellent NOremoval performance and stability. Experimental results revealthat the NO removal rate of the C/BOC/B catalyst reached53.0%， and it maintained high activity after multiple cycles ofuse. Atop the C/BOC/B， the doped C atoms can act as electronguiding channels， funneling the charge from Bi quantum dotstowards BiOCl. Bi quantum dots with plasma resonance effectscan serve as light-harvesting centers and electron donors， furtherpromoting the photocatalytic reaction.

BiOIs possess a slender band gap that supports not just lightstimulation but also enhances the creation of e–/h+ pairs. Thisslender band gap leads to a modest CB and a minor VB，culminating in reduced redox capacity. Bi and O moderatelyoverexpressed bismuth iodides （e.g.， Bi4O5I2 and Bi7O9I3） werefound to be able to appropriately reposition the VB and CB toincrease the bismuth halides’ redox capacity. They were found tobe effective in promoting photogenerated charge separationwhen composited with other materials. For example， He et al.prepared Bi7O9I3/carbon paper composites with a gradedstructure using the in situ annealing method 58. The gradedstructure provides more reaction sites， which is conducive to thegeneration of ·OH and the improvement of carrier separation，thus facilitating the photocatalytic performance. In the gradedBi7O9I3-CP， an IEF pointing to Bi7O9I3 emerges between theBi7O9I3 layers and carbon paper. Activated electrons onBi7O9I3’s CB were moved to CP in visible light， transforming O2into ·O2?. The transfer of this charge is potent enough to inhibitthe merging carrier complexation， thus providing many h+ in theVB of Bi7O9I3 conducive to oxidizing phenol and H2O.Oxidation outcomes from H2O lead to ·OH capable ofadditionally oxidizing phenol. The efficiency of theBi7O9I3/carbon paper in removing phenol in wastewatertreatment demonstrates the value of its application.

2.1.2 Aurivillius structural

The Aurivillius-structured bismuth photocatalysts arecomposed of alternating [Bi2O2]2+ layers consecutive with nnumber of [AnBnO3n+1]2? perovskite units， with the generalstructural formula [Bi2O2][AnBnO3n+1]， where A represents a 12-coordinate cation and B a minor 6-coordinate cation 59. The Asite is located at the center of the dodecahedron， typicallyoccupied by a mono-， di-， or trivalent atom， such as K+， Na+，Pb2+， Ca2+， Sr2+， Ba2+， Bi3+， Ln3+， etc. B is situated at the centerof the octahedron， typically occupied by atoms with d0 electrons，such as Ti4+， Nb5+， Ta5+， W6+， etc. When atoms without d0electrons occupy the B site， such as Fe3+， Ru4+， Cr3+， Ir4+， andMn4+， the structure may exhibit different properties. The interestin Bi-based Aurivillius-type photocatalytic materials stems fromtheir interesting electrical characteristics and high transitiontemperature. Bi2MO6 （where M = W， Mo， Cr） stands as the mostresearched 60， evidenced by its efficacy in breaking down organiccontaminants.

In addition， semiconductors were modified to improve theirphotocatalytic properties. As an example， Xu and colleaguesemployed a hydrothermal technique for the creation ofCeO2/Bi2MoO6 composite catalysts 61. When the CeO2/Bi2MoO6samples were irradiated with UV light， the binding energiesbetween Bi 4f and Mo 3d moved further to higher values， whilethe Ce 3d’s binding energy decreased. This change wasattributed to the movement of carriers created by light throughthe CeO2/Bi2MoO6 interface to CeO2， indicating a shift inphotogenerated electron transport from the oxide semiconductorBi2MoO6’s CB to the reduced semiconductor CeO2’s VB.Experimental data reveals that in photocatalysis， reducing CO2to CO using CeO2/Bi2MoO6 surpasses the efficiency of pureBi2MoO6. This phenomenon results from the synergisticcooperation of the S-scheme complex and Ce3+/Ce4+ ionbridging， leading to not just better light uptake but also bettercharge separation and redox functionalities.

The SPR effect enables Bi NPs on the Bi2MoO6 surface toenhance light absorption， generating and transferring hotelectrons， while simultaneously reducing carrier recombinationtime. However， its photocatalytic capabilities are hindered dueto insufficient reduction. Reduced α-MnS and oxidized Bi2MoO6enable S-scheme heterojunction formation， attributed to theirappropriate bandgaps and intrinsic properties， thus addressingdeficiencies in photosynthesis and reduction capability （Fig. 5a） 62.Bi was reduced into plasmonic Bi NPs， causing the nanorods toshrink to a size of 50 nm （Fig. 5b）. The nanoconfinement effectof Bi NPs promoted the CO2 reduction. The CO2 adsorptiongraph reveals that α-MnS exhibits higher CO2 adsorptioncapacity than γ-MnS （Fig. 5c）. This is linked to DETA’sprotonation reaction， creating positively charged ammoniumions and numerous alkaline functional groups on the α-MnSsurface， thus binding to the acidic gas CO2. As depicted in Fig.5d， DFT simulation and theoretical calculations yield the Fermienergy levels as α-MnS， Bi and Bi2MoO6 in descending order.When Bi2MoO6 combines with α-MnS to form a heterojunction，electrons undergo thermodynamic migration from α-MnS toBi2MoO6 until the EF is equilibrated. With the loss of electronsin α-MnS and the occurrence of positive actions， the band bendsupwards， resulting in a positively charged contact interface.Meanwhile， Bi2MoO6 absorbs electrons and performs negativework， causing its band to bend downwards， leading to anegatively charged contact interface. Thus， an IEF emerges atthe junction point of the two semiconductors. Upon excitationby visible light of Bi2MoO6 and α-MnS， e? in the VB migrateinto the CB， resulting in an enhanced recombination rate of theelectron-hole pairs. The SPR effect of metallic Bi coupled withthe Bi2MoO6’s elevated Fermi energy level， cause manyelectrons to transition from Bi to Bi2MoO6. These electronsrecombine with α-MnS’s h+， resulting in the retention of α-MnS’s e?. This indicates that the S-scheme heterostructureeffectively reduces electron recombination with holes， indirectlypreserving many e? in α-MnS and h+ in Bi2MoO6， enabling theirparticipation in the PRCC reaction.

2.2 Bismuth element

Lately， the field of materials science has become drawn to 2Dmetallocene substances of atomic thickness， owing to theirsuperior photocatalytic qualities. 63. As shown in Fig. 3c，Bismuthene is a two-dimensional layered metal with hexagonallattices， excellent electron mobility， low toxicity， and highstability. Bismuthene has photocatalytic characteristics such asnarrow band gaps and a high specific surface area. As anillustration， Ozer et al. reported using 2D bismuthene as aphotocatalyst in liquid-phase organic transformations. The initialsynthesis of layered bulk Bi involved a surfactant-aidedchemical reduction technique， where the compound BiCl3 wasreduced using a borane-tert-butylamine compound in oleylamine（OAm） at a temperature of 120 °C. Subsequently， the bismuthwas extracted from the mass through a one-hour ultrasonic probehomogenizer， creating a dark grey suspension of the bismuthenenanosheet. As an effective photocatalyst， the 2D bismuthenestands out in its capability for photooxidizing isomeric alkanesunder diverse reaction conditions， including indoor light， dark，and outdoor low temperature. Possible charge transfer betweenthe surface of benzene and bismuth ruthenium was investigatedbased on Bader charge analysis， which may promote substrateactivation 64. Additionally， bismuthene promotes interfacialcharge transfer in Z-scheme heterojunctions to enhancephotoactivity. Zhang et al. designed layered metallocenes toefficiently facilitate the assembly of a C3N4/bismuthene/BiOClcomplex. Bismuthene acts as an intermediary in enhancing thefunctionality and stability in photocatalytic processes， with thephotocatalytic response for CO2 reduction exhibitingexceptional charge conductivity， numerous metal-semiconductorjunctions， and narrower charge diffusion areas 65.

2.3 BiVO4

Within many photocatalysts， BiVO4 is recognized for itseffectiveness in water purification and energy applications，attributed to its stable crystal structure， superior efficiency inlight quantum and electron movement 66. BiVO4 stands out as aneffective n-type semiconductor candidate for these applications，attributed to its narrow bandgap （2.4–2.8 eV）， non-toxicity， highchemical and photostability， and ability to absorb sunlight.BiVO4 exhibits different crystalline phases， namely tetragonalscheelite （ts-BiVO4）， monoclinic scheelite （ms-BiVO4），orthorhombic pucherite （op-BiVO4）， and tetragonal zircon （tz-BiVO4）. Among these， ms-BiVO4 has been widely investigateddue to its lower and more suitable bandgap， and betterphotocatalytic properties under visible light irradiation.However， pure BiVO4 exhibits modest photocatalytic efficiency，prompting researchers to enhance its performance by addingmetals or metal oxides （Ag， Co3O4， Cu， Pt， Na） to its surface ofBiVO4 to improve photocatalysis. For instance， Philo andcolleagues synthesized ultrathin 2D m-BiVO nanosheets withnarrowed V ― O and extended Bi ― O bonds through Naintroduction， optimizing samples with excellent photocatalyticOER performance （Fig. 6a） 67.

2.4 Bi2S3

Bi2S3， characterized by its limited band gap of 1.3 eV andsuperb absorption rate， has shown great promise forphotocatalysis research due to its effective electron transmissionand responsiveness to the entire visible light region. Recently，researchers have paid significant attention to the photocatalyticactivity of materials based on Bi2S3 70. However， pure Bi2S3photocatalysts have suboptimal performance due to their poorstability and tendency for electron-hole pair recombination.Therefore， Bi2S3 is often combined with other semiconductors（BiOBr 71， Fe2O3 72， CdS 68， g-C3N4 73） to enhance photocatalyticperformance. As shown in Fig. 6b， Liu and colleaguessynthesized a biomimetic sea urchin-like Bi2S3/CdS （BCS-UL）Z-scheme heterojunction for complete degradation of glyphosateand conversion into CO2 （731 μmol?h?1?g?1） 68.

2.5 Bi2O3

Bi2O3 has a wide range of light absorption （2.1–2.8 eV） and isconsidered one of the most effective photocatalysts because iteffectively captures visible light for photocatalysis and generatese?/h+ pairs. Bi2O3 photocatalytic oxidation degradescontaminants and enhances antimicrobial efficacy. Bi2O3 hasseveral crystal structures： α， β， γ， δ， ε， and ω， each with uniqueproperties. Among them， α-Bi2O3 and δ-Bi2O3 represent thestable stages at low and high temperatures， respectively， and β-Bi2O3 and γ-Bi2O3 are the suitable phases. For the primaryquartet of Bi2O3 polymorphs， the range of phase stabilitytemperatures is denoted， α-Bi2O3 falls under 1000 K， β-Bi2O3stays from 603 to 923 K， γ-Bi2O3 from 773 to 912 K， and δ-Bi2O3lies between 1002 and 1097 K. The standard constanttemperature state of bismuth oxide is the monoclinic γ-Bi2O3featuring a twisted polymer layer composed of five bismuthatoms， encased in octahedral-like units. Temperatures exceeding710 °C shift δ-Bi2O3 away from α-Bi2O3， known for its irregularconfiguration and uneven OVs. The creation process of δ-Bi2O3resembles that β-Bi2O3 with its oxygen-rich states. Similarly， γ-Bi2O3 has a cubic form， but its instability makes independentproduction challenging， often necessitating the presence ofvarious oxides or other metals. As depicted in Fig. 6c， Lei andcolleagues discovered that inserting oxygen vacancies into α-Bi2O3 and β-Bi2O3 significantly improves photocatalyticefficiency 69. These OVs facilitate the transformation of carriersand intermediates into active sites. Additionally， the crystalformation and oscillatory voltages together manage the electrontransfer process. The varying surface defect configurations of α-Bi2O3 and β-Bi2O3 contribute to higher reactant activation andcharge exchange efficiency， with β-Bi2O3 exhibiting tunnelingstructural defects compared to α-Bi2O3’s sawtooth design. Lu etal. 74 enhanced the photocatalytic efficiency of Bi2O3 by creatinga Bi2O3/Bi2SiO5 compound through a straightforwardcalcination process. Under artificial solar radiation， theefficiency of Bi2O3/Bi2SiO5 heterojunction photocatalysts inbreaking down organic contaminants was markedly greatercompared to α-Bi2O3 alone. Increased catalytic activity isassociated with enhanced surface area， improved contact angles，the presence of β-Bi2O3， and the development of p-nheterostructures.

3 Synthesis strategies

The method of synthesis is widely recognized for its impacton the structural design， dimensions， and specific surface area ofbismuth-based photocatalysts， crucially affecting their uptakeand photocatalytic properties. Furthermore， this hasconsequences for ecological conditions， the scale of synthesis，production expenses， and safety measures. As shown in Fig. 7，hydrothermal/solvothermal， chemical precipitation， and solidstatereactions are commonly used to prepare Bi-basedphotocatalysts.

3.1 Hydrothermal/solvothermal method

Nowadays， hydrothermal/solvothermal approaches areextensively employed in the development of bismuth-basedphotocatalysts， to synthesize the materials in high-temperatureand high-pressure aqueous solutions. Hydrothermal/solvothermalmethods possess advantages in controlling over the facets，dimensions， surface flaws， structure， and size of Bi-basedphotocatalysts through modifications in pH， solvent type，reaction duration， and thermal conditions. A clear drawback ofthe hydrothermal/solvothermal approach is its reducedefficiency and safety hazards related to environments with hightemperatures and pressure. Hydrothermal/solvothermalprocesses carry the risk of significant leakage of nanoparticlesinto water， and solvothermal methods also run the risk ofhazardous solvent emissions. Recently， Zhao et al. effectivelydeveloped a range of BS/BVO/MCS S-scheme heterojunctionvia the microwave hydrothermal technique 75. As shown in Fig.7a， the close linkage between Mn0.5Cd0.5S-DETA nanoparticlesand BiVO4 nanosheets. Such tight bonding facilitates themovement of photogenerated carriers over the boundary， andfosters the disentanglement of e? and h+. In contrast to the pureBiVO4 （14.11 μmol?h?1?g?1）， the ternary composite displays amore effective rate in reducing CO2 （44.74 μmol?h?1?g?1）.Additionally， the impact of pH and reacting temperature on theefficiency of photocatalyzing was examined. By selectingdifferent solvent， the morphology of the product can beeffectively controlled， thereby achieving precise tuning of thematerial’s properties. As shown in Fig. 7b， Bi/BiOBrnanoflowers with OVs were created following a straightforwardsolvothermal chemical reduction possess with water/ethanol assolvent 76. Xin’s team added Bi（NO3）3·5H2O and KBr to amixture of ethanol and water in varying volume proportions andstirred the mixture. Then keep at 180 °C for 24 h. Solidprecipitates were collected through centrifugation. Finally， theBi/BiOBr nanoflowers were obtained by thoroughly washing theprecipitates with ultrapure water and anhydrous ethanol，followed by drying. Researchers observed that as the ethanolcontent increased， the microstructure of BiOBr transformedfrom nanosheets to nanoflowers， accompanied by the formationof OVs and metallic Bi. When exposed to constant airflow andvisible light， Bi/BiOBr nanoflowers exhibited superior NOphotocatalytic conversion capabilities. The enhancement inperformance and the reduction in NO2 generation are credited tothe collaborative interaction of metallic bismuth and oxygenvacancies. The collaborative effect enhances the kinetics ofcharge carriers， fosters the production of ROS， and broadens thematerial’s light response range. As the investigation intohydrothermal/solvothermal techniques progresses， expectationsare high for their pivotal in creating and using bismuth-basedphotocatalysts， propelling the advancement of photocatalytictechnology.

3.2 Chemical precipitation method

The chemical deposition method typically involvesthoroughly mixing materials and then collecting the productafter allowing the mixture to settle. The precipitation process issimple and can be used to synthesize vacant bismuth oxide bysolution method with precipitants or reducing agents. Fig. 7cillustrates， Li’s team synthesized Bi-MOF via a solvothermalmethod， then added NH4Br for halogenation treatment， andsynthesized BiOBr/C nanosheets after annealing 77.Subsequently， BiOBr/C was diluted in water， with varying levelsof NaBH4 solution introduced， and then incubated at ambienttemperature for 2 h. The obtained sediment was then passedthrough filtration and then dried to achieve a Bi@BiOBr/Cconcentration. Introducing NaBH4 aimed at in situ reduction，reducing Bi2+ to metallic Bi， which then deposited on theBiOBr/C surface. The optimized photocatalyst demonstrated aneradication efficiency reaching 69.5% for NO with visible light，a rate that was 3.5-fold higher than that achievable using soleBiOBr. Graphitized carbon acts as a bridge connecting BiOBrand Bi nanoparticles， while the combination of metal Bi and OVsenhances light absorption and aids in producing ·O2?， whichresults in an effective selective oxidation of NO. In conclusion，the chemical deposition method offers many advantages in thepreparation of bismuth-based photocatalysts. For example，compared to some preparation methods that require hightemperatureprocessing， chemical precipitation is usuallyperformed at lower temperatures， which helps to save energy andreduce possible thermal damage. Moreover， the chemicaldeposition method can achieve uniform deposition of thecatalyst on the substrate， which is very favorable to the stabilityand performance consistency of the catalyst.

3.3 Solid-state reaction method

The solid-phase synthesis technique is employed to producecatalytic materials through blending， pulverizing， and drying thereactants， then proceeding with heating or roasting. Duringsynthesizing heterojunction nanostructures using the solid-statemethod， this technique improves the exact surface area throughthe regulation of the annealing temperature， eliminating the needfor solvents or agents that manage shape controlling. The solidstatemethod is a time-saving， labor-saving， and environmentallyfriendly process that yields high-quality results， capable ofproducing superior products on a large scale. A drawback of thesolid-state technique lies in the uniformity of the reactionsubstances， complicating their inter-material diffusion andpotentially causing diverse morphological patterns in theresulting nanoparticle. According to Fig. 7d， Xu’s teamsynthesized Bi4NbO8Cl/g-C3N4 photocatalysts using ball millingand calcination， enabling tight connections between interfaces.The notable enhancement in photocatalytic efficiency ofBi4NbO8Cl/g-C3N4 composite materials compared to a singlematerial stem from the beneficial two-dimensional/twodimensionalform， coupled with developing type IIheterojunctions， which exhibit interfacial impacts， aiding theseparation of charges 78.

3.4 Other synthetic methods

In addition to the aforementioned synthesis methods，researchers often combine other synthetic operations to createbismuth-based photocatalysts， such as microwave hydrothermalmethods and sol-gel techniques. For instance， Zhang andcolleagues employed the microwave hydrothermal method forthe synthesis of Bi2O3-MoO3 composites 79， while Li et al.utilized the sol-gel technique combined with calcination tosynthesize CuFe2O4/Bi2O3 composites 80. Generally， selecting aprocess for creating photocatalysts should be based on thedesired attributes of the product. Different synthesis methodscome with their own set of pros and cons， and choosing the mostappropriate one should align with the specific requirements ofthe application. Solvothermal technology may be advantageousfor obtaining larger specific surface areas and porous structures.When precise control over structure and form is necessary， thesolvent technique might be more suitable. If maintaining controlover morphology and structure is essential， employing the refluxmethod might be preferable. The choice of synthesis methodultimately depends on various factors including cost， efficiency，environmental impact， and the characteristics and intended useof the photocatalyst.

4 Structural regulation methods

Bi-based photocatalysts are constrained by their bandgaps，with typically wide bandgaps limiting absorption to theultraviolet region. To increase photocatalytic activity under solarirradiation， reducing the band gap using strategies such asdoping， forming alloys， or creating heterostructures extends theabsorption range into the visible region. Moreover， the surfacearea and morphology of photocatalysts greatly affectperformance， with higher surface areas providing more activesites and specific morphologies such as nanosheets ornanoparticles enhancing light uptake and charge separation，essential for optimizing photocatalytic performance. Effectivecharge separation is crucial for preventing electron-holerecombination and enhancing photocatalytic activity， which canbe achieved by forming heterojunctions， introducing defects ordopants， and designing surface states. Although bismuth-basedmaterials are generally chemically stable， their stability andlifetime under operating conditions can be further enhanced bysurface modifications such as coating with protective layers orusing stable supports. Limited light uptake and facile bonding ofe?/h+ pairs in pure Bi-based semiconductors contribute to theirunderactive photocatalytic performance in practicalenvironments and energy application. The crystal and electronicstructures of semiconductors profoundly impact photocatalyticproperties. Regarding crystal structure， the size， form， andcrystal surface of bismuth-based photocatalysts are closelyrelated， and modulating microstructure can enhancephotocatalyst performance. Additionally， electronic structureinfluences light absorption， charge exchange， thermodynamicdriving force， and kinetic energy barriers. Researchers aim toenhance photocatalytic efficiency by adjusting electron structurethrough defect engineering， heteroatom doping， morphologycontrol， SPR effect， and heterostructure construction.

4.1 Defect engineering

Defect engineering is a key method for understanding andenhancing the efficiency of Bi-based photocatalysts. Defects atthe surface or interface of a material can alter its functional，electronic， and geometrical structure， thereby impacting itsphotocatalytic activity. For instance， oxygen defects areprevalent in semiconductor photocatalysts， resulting in nonstoichiometriccharge imbalances that facilitate electron-holepair separation and enhance photocatalytic efficiency.Specifically， oxygen vacancies （OVs） play a crucial role byinteracting with co-catalyst functional groups， thereby boostingsurface reaction efficiency. Contrary to the belief that defectshinder photocatalytic performance， recent research suggests thatdefect engineering can enhance effectiveness by controllingdefect quantity and nature， and understanding their role in thephotocatalytic process. Surface OVs， for example， can narrowthe semiconductor bandgap， extend light absorption edges， andprovide defect energy levels that lower the energy threshold forinterfacial charge transfer， thereby enhancing the feasibility ofcreating S-scheme heterojunctions. Surface OVs can also act asactive sites for CO2 adsorption and activation， significantlyboosting semiconductor photocatalytic efficiency in CO2conversion. Moreover， OVs synergize with S-schemeheterostructures to improve light absorption and spatiallysegregate charge carriers with optimal redox capacities， therebyfacilitating catalytic reactant generation （Fig. 8a） 81. Forexample， Guan et al. synthesized OVs-rich BiOBr-（0 01）/Bi2SiO5/Bi S-scheme heterogeneous photocatalysts， whereOVs and metallic Bi act as electron traps to promote S-schemeheterojunction formation， notably enhancing CO2 reductionperformance （Fig. 8b） 82. Similarly， Bariki et al. demonstratedthat oxygen defects in Bi2W2O9 lattice and ternaryheterojunction formation greatly enhance photo-induced chargemigration， segregation， and photocatalytic efficiency ofheterostructured materials （Fig. 8c） 83. These vacancies createtrap energy levels that delay charge-carrier recombination andfacilitate rapid electron channeling， thereby enhancing overallphotocatalytic performance.

4.2 Heteroatom doping

By introducing external heteroatoms， intermediate states canbe generated within the band gap or electronic structures can beadjusted， enhancing the efficiency of photocatalysts. Thisapproach is commonly used to optimize traditionalphotocatalysts 84. Effective strategies involve doping with metalor non-metal ions to adjust the configuration and physicalproperties of bismuth-based catalysts， enabling them to absorbvisible light. Incorporating dopants has the potential to alter theelectronic configuration， leading to different energy states andreducing the band gap by introducing additional energy levels.For example， S/F doping can enhance Bi2WO6’s light absorptioncapability by narrowing its band gap 85， thereby influencing itsenergy band configuration and photoelectric performance，facilitating the separation and migration of electron-hole pairs.Adjusting the S/F ratio allows for the modulation of surfaceoxygen vacancies. Moreover， transition metal element doping iswidely applied to modify electronic configurations and enhancenitrogen adsorption and activation. For example， Yang andcolleagues investigated the impact of Co doping on thephotocatalytic nitrogen reduction reactions （PNRR） 86 ofBi2MoO6. Their research revealed that Co doping alters theelectronic structure of Bi2MoO6， activating Bi sites andintroducing new Co active centers， thereby establishing a dualactivesite system that enhances PNRR. The photocatalyticefficiency of Bi2MoO6 in nitrogen reduction exhibits a volcanoshapeddependence on the Co doping level， peaking at a 3%-Codoping ratio before gradually decreasing （Fig. 9a）. Thisunderscores the critical role of Co doping concentration inoptimizing Bi2MoO6’s photocatalytic performance. Fig. 9billustrates that 3% Co-Bi2MoO6 achieves a higher ammoniaproduction rate in a pure nitrogen environment， indicatingexcellent selectivity in converting nitrogen to ammonia whileminimizing side reactions with gases such as oxygen. In situFTIR spectra indicate that prolonged illumination progressivelyincreases the intensity of NH3 and NH4+ peaks， suggestingsuccessful nitrogen is successfully activation and conversioninto ammonia on the surface of Co-Bi2MoO6 （Fig. 9c）.Additionally， DMPO-induced electron spin resonance （ESR）spectra demonstrate that under visible light irradiation， 3% Co-Bi2MoO6 generates superoxide radicals （·O2?） in argon， air， andnitrogen atmospheres （Fig. 9d–f）. The production of ·O2? by 3%Co-Bi2MoO6 increases with light exposure time in both argonand nitrogen environments， with a more pronounced increaseobserved in the presence of oxygen. This indicates that 3% Co-Bi2MoO6 effectively catalyzes water oxidation of water undervisible light， generating oxygen and ·O2?， with enhancedefficiency in oxygen-containing environments.

4.3 Morphology control

The surface structures of bismuth-based materials are key tothe adsorption ability of photo-emitted electrons in reactingmolecules and their hopping characteristics. Additionally， themorphology determines the transfer path of photo-createdcarriers and affects the separation of light-generated electronholes to a degree. Adjusting the morphology of Bi-basedmaterials can effectively enhance photocatalytic efficiency bymodifying the band gap， optimizing the division of photocreatedcharges， and increasing the specific surface area toenhance redox reactions. Therefore， morphology factors are vitalin enhancing the photocatalytic performance. Appropriate 2Dnanostructures can be used as building blocks to constructhierarchical 3D morphologies. For example， Gu et al.synthesized Bi-MOF-M via a solvothermal method and grewBi2MoO6 on its surface in situ， constructing Bi-MOF-M/BMOchemical bonding heterojunction 81. Furthermore， 3D layeredphotocatalysts can demonstrate more advantages compared to2D nanosheets， thereby enhancing photocatalytic performance.For instance， Chen et al. synthesized 3D Bi/Bi4O5I2microspheres having abundant OVs using one-pot solvothermaltechnique and investigated the effect of reaction time on metalBi content 87. As shown in Fig. 10a， compared with thetraditional precipitation and hydrothermal methods， the ethyleneglycol solvothermal method not only guides the self-assembly ofnanosheets into microspheres with increased reaction sites，enhanced light absorption， and quicker interfacial chargedivision but also effectively introduces oxygen vacancies，enhancing the material’s conductivity and carrier separationefficiency. As shown in Fig. 10b， Bi-S2’s photocatalytic reactionrate is much larger than the peaks of Bi-P （Preparation of BiOIby precipitation strategy） and Bi-H （Hydrothermal technique），demonstrating the photocatalysts’ superior functioning withabundant OVs and moderate metal Bi content. In addition， Bi/Bi4O5I2 has good stability and sulfur tolerance. Fig. 10cdemonstrates the photocatalytic process of Bi/ Bi4O5I2. When Biwas introduced to form the inherent electric field betweenBi4O5I2 and Bi， and the combination of the OVs and theintegrated electric field facilitated carrier separation，significantly enhancing the photocatalytic activity，demonstrating excellent Hg removal efficiency， high stability，good sulfur tolerance and nitric oxide tolerance.

4.4 The surface plasma resonance （SPR） effect

Precious metals like gold， silver， and platinum can boost theefficiency of transferring and separating carriers created throughthe SPR effect， but their applications are limited due to their highprices 88. Photocatalysts based on non-precious plasma metals（NNPMs） stand out as distinct substitutes for precious metalphotocatalysts， attributed to their abundant resources， costeffectiveness，and mass industrialization capabilities 89.Research has revealed that inexpensive metal Bi displays SPReffects akin to those of precious metals， suggesting its use as apotential alternative to enhance the performance ofphotocatalysts. Bi’s SPR phenomenon creates anelectromagnetic field that drives electrons from the BiOCl toelevated excited states and aggregates at the valence bandminimum （VBM）. Electrons in an excited state subsequentlytravel to BiOCl’s CB， and those at middle energy shift to Bi. Thismovement aids in dividing photogenerated e?/h+ pairs， boostingthe production of free radicals. The SPR electric field generatedby Bi promotes carrier separation， while the Bi nanoparticlesserve as electron traps， speeding up this process. The enhancedelectromagnetic field increases light uptake， promoting chargeseparation， reducing charge complexation， and promoting theformation and reaction of surface reactions intermediates，thereby hastening the rate of photocatalytic reactions.Researchers have exploited the Bi SPR effect to improve thephotoelectrochemical function of Bi-based catalysts 90–93.

As an illustration， Xu and colleagues developed a newBi/BiVO4/g-C3N4 photocatalyst through an in situ reductiontechnique. Bi， a plasma metal， serves as a conduit for electronconduction，bridging BiVO4 and g-C3N4， forming a Z-schemecomplexes that greatly facilitate the isolation of photoexcitedelectrons and holes （Fig. 11a） 91. In addition， the Z-schemeheterostructure along with Bi’s SPR effect substantiallyenhances the effectiveness of charge carrier separation and PECfunctionality. Zhang et al. found that Bi/Bi2WO6 hybridphotocatalysts exhibited good photocatalytic properties. TheSPR phenomenon in Bi nanorods， aids in the dispersal andmovement of photons， thus increasing ROS production （e.g.， ·O2?and ·OH radicals）， responsible for NO oxidation （Fig. 11b） 92.

4.5 Heterojunction

Within photocatalysis research， the severe merging of e? andh+ produced by light， along with the insufficient redox capabilityof single photocatalysts， leads to extremely low overallphotocatalytic reaction efficiency， a problem frequentlycriticized. In response to this challenge， a range of heterogeneousstructure methods have been developed 94–101. The use ofheterojunctions to bolster the efficacy of bismuth-basedphotocatalysts offers a comprehensive boost in theirfunctionality and efficiency， achieved by optimizing chargeseparation， extending the range of photoresponse， improvingstability and modulating electron transport properties. As shownin Fig. 12， the categorization of heterojunctions encompassestype I （Fig. 12a）， type II （Fig. 12b）， type III （Fig. 12c）， Z-scheme（Fig. 12d） 102，103， and S-scheme （Fig. 12e） 104–113 heterojunctionsbased on the composite mechanism. The widespread consensusis that the disadvantages of single-component photocatalysts canbe eliminated by synthesizing heterogeneous photocatalystsfeaturing unified functional parts， merging the benefits ofseparate parts. Therefore， researchers have fabricated variousheterojunctions to enhance how bismuth-containingphotocatalysts function. Depending on the type of material， theycan be classified as precious metal/bismuth-containingcomposites， semiconductor/bismuth-containing composites 114，heterojunctions between two bismuth-containingphotocatalysts 115，116， carbon-based materials/bismuthcontainingcomposites 117， and bismuth metal/bismuthcontainingsemiconductors 76. In type I heterojunctions， since theelevated VB of B compared to A’s and the decreased CB of Bover A’s， e? and h+ in A can accumulate in the price band and theprice band of B under illumination， preventing effectiveseparation. Thermal excitation is required to overcome band gapdiscontinuity when electrons are transferred fromsemiconductors with large band gaps to semiconductors withsmall band gaps. In type II heterojunctions， electrons andcavities can be transferred to lesser energy tiers in variedsubstances. Therefore， isolated e?/h+ pairs may participate inongoing redox processes. Effective overlapping of bands， whichallows the accumulation of electrons and holes across varioussubstances， is crucial for creating effective heterojunctionphotocatalysts. Within type III heterostructures， an overlappingregion of two semiconductor energy bands， yet they are notcompletely aligned. This banding forms a narrower band at theinterface， enabling electrons and holes to travel freely within theregion， thereby decreasing carrier recombination rates. In allsolid-state Z-scheme composites， the A-band cavities and Bbandelectrons migrate through the intermediate conductors andannihilate them， effectively separating the cavities in the A-bandand the B-band， thus reducing their recombination probability.The key to solid-state electronic media lies achieving dynamicequilibrium between the electronic contribution capacity andelectronic receptivity of the medium and maintaining relativestability during the reaction. Unlike the heterojunctionsmentioned above， S-scheme heterojunctions consist of oxidizingand reducing photocatalysts. The combination of electric fieldforce， Coulomb force， and band flexure promotes the merging ofphotogenic electrons in the oxidizing component withphotogenic pores in reductive components and prevents thetransfer of photogenic pores in oxidized components toFig. 12photogenic electrons in reductive components. This leads to asignificant reduction and oxidation potential for electrons andcavities， respectively. Because of these advantages， S-schemeheterojunctions has attracted wide attention in fields of hydrogenproduction， CO2 reduction， pollutant degradation， andsterilization.

The combination of physical adsorption and photocatalysis isadvantageous. Researchers are interested in bismuth-basedmaterials that bind to superconducting carbon materials. Theproperties of graphene include superior conductivity， largersurface area， mechanical and chemical solidity， and a flexiblemonolayer structure. When combined with semiconductorphotocatalysts， graphene not only absorbs organic pollutantswell but also boosts the isolation of carriers in catalysts， therebyimproving the overall performance of photocatalysis. In recentyears， many composite systems based on graphene and Bi-basedmaterials have been synthesized. Ghoreishian and colleaguesfound that coating CdS with a thin sliver of amorphous carbonbasedmaterial might effectively stabilize the surface and preventphotodegradation 118. By encapsulating Bi2S3@CdS compositesin reduced graphene oxide （RGO）， they discovered that theentire Bi2S3@CdS@RGO structure prevents photodegradationof CdS and enhances the separation of photogenerated carriers.RGO is an electron quencher that improves charge transfer，inhibits carrier recombination and creates more reaction sites forinteraction. In addition， carbon nanotubes have a uniquestructure with great specific surface area， are non-toxic，economical and porous. They can be used as multifunctionalnanomaterials， improving the adsorption capacity andphotoreactive function of semiconductor photocatalysts. Forexample， Dutta et al. synthesized CNT-mediated Ag-CuBi2O4/Bi2S3 laminar composites based on the S scheme withbetter photocatalytic performance than the original samples 119.Carbon nanotubes can act as electron quenchers， boost theprocess of transferring electrical charges， inhibit the merging ofgenerated photocarriers and develop additional reaction sites forphotocatalytic reactions.

Heterojunction configurations extend the light uptake rangeand optimize sunlight use by combining the advantages ofdifferent semiconductor materials. For example， Li et al.synergistically modulated S-scheme heterojunctions using afusion strategy and an integrated electric field， enhancingphotocatalytic efficacy 120. As shown in Fig. 13a， Bi19Br3S27/g-C3N4 photocatalysts feature C ― S bonding at the interface，employing a solvothermal in situ deposition strategy. Thephotoreduction behavior of the catalyst to CO2 was studied by insitu infrared spectroscopy （Fig. 13b）. DFT calculations exploredthe transfer routes of interfacial charges， revealing through EPRthat the S-scheme linking BBS and P-CN （Fig. 13c，d）. The e? inP-CN’s CB adheres to the h+ in BBS’s VB， whereas e? in BBS’sCB and the h+ of P-CN’s VB are involved in the redox process.Moreover， the robust interplay of C―S bonds between BBS andP-CN prevents the division of heterogeneous structure in thecatalytic process， thereby preserving relative stability.

5 Applications of Bi-based photocatalysts

Currently， the widespread application of Bi-basedphotocatalysts spans energy transformation and environmentalrestoration， including CO2 reduction， water splitting， N2 fixation，H2O2 production， NOx removal， and selective organic synthesis.

5.1 CO2 reduction

The threat of global warming due to rising atmospheric CO2can be addressed through artificial plant photosynthesis andphotocatalytic reduction of CO2. PRC can produce a variety oforganic compounds 121. The process of photocatalytic reductionof CO2 involves several steps， including the photocatalyst’selectron excitation by sunlight， the participation of electronmigration in the reduction reaction， and the participation ofseparated holes and electrons in the redox reaction 122. Factorsaffecting this process include the photocatalyst’s band gap，effectiveness in separating and transferring e?/h+ pairs， surfacecatalytic performance， and the presence of sacrificial agents. Theintroduction of OVs on the photocatalysts can promote thecreation of S-scheme heterostructure， improve redoxperformance， boost adsorption and activation of carbon dioxide，thereby increasing PRC activity 123.

Strategies to enhance CO2 reduction activities are improvedby preparing oxygen vacancies on photocatalysts. OVs serve aselectron capture and reaction sites. For instance， Guan et al.synthesized a pristine S-scheme BiOBr-（001）/Bi2SiO5/Biheterojunction and enriched it with solar thermal energy andultrasonic assistance 82. The presence OVs and metallic Bi aselectron traps is expected to facilitate the creation ofheterostructure， thus improving the performance of CO2reduction. The S-scheme heterostructure boosts thephotoelectron transfer efficiency and availability， enhancesstronger redox capability， and improves CO2 adsorption andactivation.

Moreover， Liu et al. constructed 2D nanosheets （BMOV） and2D bismuth composites （Bi/BMOV） for CO2 reduction （Fig.14a） 63. Fig. 14b–e show that the XPS spectra analysis ofBi/BMOVs samples before and after illumination reveal thetravel behavior and how elemental electronic densities on theBi/BMOVs surface fluctuate. Under LED irradiation， thephotogenerated electrons in the Bi/BMOVs migrate from BMOto bismuthene， while the electron densities of O and Moelements decrease. This migration behavior of photogeneratedelectrons helps improve charge segregation in Bi/BMOVssamples， thereby enhancing their photocatalytic capabilities inreducing CO2. The in situ DRIETS of Fig. 14f demonstrate thesignificance of carbon dioxide molecule activation bychemisorption on photocatalysts， which is an important processfor photocatalytic CO2 conversion （Fig. 14g）. Under lightexposure， the integration of OVs and bismuth alkene intensifiesthe photon-producing carriers of BMO. Bismuth’s superiorconductivity enables increased gathering of light-generatedelectrons on the photocatalyst’s exterior. Simultaneously， OVsmight significantly increase CO2 reaction initiation and thusgreatly enhance the rates of CO2 reduction.

5.2 Water splitting

One of the main challenges facing modern society is theenvironmental and energy crisis. Photocatalytic waterdecomposition is a method to directly use solar energy toproduce hydrogen fuel while reducing environmental pollution.Since the Honda-Fujishima effect was first reported， significantadvances have been made in photocatalytic research.Photocatalytic water decomposition occurs when photons withenergy levels matching or exceeding the photocatalyst’s bandgap cause electrons in the VB to move towards the CB. Electronsparticipate in reduction reactions， while holes facilitate oxidationreactions. The photocatalytic water decomposition processconsists of three basic stages： photon absorption by thephotocatalyst to generate electrons and holes， movement andseparation of charges at surface reaction sites， and reaction ofelectrons and holes with water to produce hydrogen andoxygen 124–127. The process of photocatalytic overall waterdecomposition to produce oxygen and hydrogen involves atransition from dissolved molecules to bubbles 128. Initially，oxygen forms on the photocatalyst surface and subsequentlydissolves in water. Once water becomes saturated with dissolvedoxygen， oxygen molecules aggregate and form small bubbles，which gradually increase in size with the addition of moreoxygen molecules. These bubbles are released from the watersurface when the buoyant force exceeds the adhesion force onthe catalyst surface. Hydrogen production follows a similarprocess. During overall hydrolysis， the environment becomes asaturated aqueous solution of dissolved oxygen， which can leadto undesired reverse and side reactions. Reverse reactionsinclude the reaction of dissolved oxygen with dissolvedhydrogen， adsorbed hydrogen and oxygen molecules， and coadsorbedhydrogen and oxygen molecules. Side reactionsprimarily involve oxygen molecules competing for electronswith adsorbed hydrogen， potentially generating superoxideradicals or hydrogen peroxide. These reverse and side reactionsreduce the efficiency of water splitting. The efficiency of overallwater decomposition is low due to thermodynamic and kineticchallenges， as well as the effects of dissolved oxygen， reverse，and side reactions. Therefore， combining hydrogen productionwith organic synthesis may offer more economic potential thanrelying solely on photocatalytic overall water decomposition.

Traditional photocatalysts face challenges such as severecarrier recombination， narrow light absorption range， and lowphotocatalytic activity， limiting their applications. Researchershave found that effective charge separation and transportinduced by S-scheme structure significantly enhance H2generation 129. To enhance water decomposition， photocatalystprovides reactive sites， improve charge segregation， and avertingcorrosion. Combining H2 generation with selective organicsynthesis presents a novel alternative to traditional watersplitting 130. Additionally， photocatalysts improve the surfaceredox kinetics by activating water molecules， thereby improvingthe catalytic conversion efficiency. For instance， Guo andcolleagues explored the use of pressure-induced phasetransformation to construct an in situ BiVO4/Bi0.6Y0.4VO4 Sschemeheterostructure， aiming to enhance overallphotocatalytic water splitting performance 131. Fig. 15aillustrates that the preparation of Bi0.6Y0.4VO4 solid solutionnanoparticles via precipitation-crystallization， followed bypressure-induced phases transformation tetragonal bismuthvanadate into a monoclinic phase， thereby forming theBiVO4/Bi0.6Y0.4VO4 composite photocatalyst. The study showedthat post high-pressure treatment， BYV solid solution surfaceexhibited monoclinic bismuth vanadate nanoparticlesapproximately 5 nm in diameter， achieving in situ heterojunctionstructure construction. Test results shown in Fig. 15bdemonstrate that the BiVO4/Bi0.6Y0.4VO4 heterojunctionexhibits significantly higher photocatalytic activity compared toBi0.6Y0.4VO4 solid solution alone. Fig. 15c displays the H2 andO2 generation rates of BiVO4/Bi0.6Y0.4VO4 heterojunctionduring three consecutive cycle tests， indicating good cyclingstability with no significant decline in H2 and O2 generationrates. The notable enhancement in overall photocatalytic watersplitting efficiency of BiVO4/Bi0.6Y0.4VO4 catalysts is mainlyattributed to their effective charge separation mechanism andenhanced light absorption capacity. Experimental outcomesdepicted in Fig. 15d， highlight how the unique energy bandstructure in S-scheme heterojunction causes photogeneratedelectrons to predominantly localize on the Bi0.6Y0.4VO4， whilephotogenerated holes migrate towards BiVO4 NPs， reducingcarrier recombination and extending carrier lifetime， therebyimproving photocatalytic activity. Additionally， the inherentelectric field formed at S-scheme complex interfaces furtherpromotes carrier movement and diffusion. Introduction ofBiVO4 nanoparticles enhances light absorption capability，broadens the spectral response range， and synergizes withBi0.6Y0.4VO4 solid solution， optimizing sunlight utilization forphotocatalytic processes. The nano porous structure alsoincreases the photocatalyst specific surface area， creating moreactive sites and improving catalytic activity.

5.3 N2 Fixation

Nitrogen plays a crucial role in the life of all organisms， yetdespite comprising approximately 78% of the atmosphere’svolume as N2， organisms cannot directly absorb it， relyingprimarily on NH4+ and NO3?. Recently， photocatalytic N2fixation has garnered significant interest due to its operationalsimplicity， mild reaction condition， and eco-friendliness. Thetechnique entails converting nitrogen into ammonia or similarnitrogen-based substances， using semiconductor materials ascatalysts in illuminated environments. Light exposure exciteselectrons from the VB of the semiconductor into the CB， creatingh+ in the VB. Excited electrons in the CB then participate inreduction reactions with reactants， while the holes in the VBengage in oxidation processes concurrently. When nitrogenmolecules are adsorbed onto the semiconductor’s surface，excited electrons and protons are generated， subsequentlyundergoing reduction to form ammonia.

As depicted in Fig. 16a， Chen et al. synthesizedAgBr/Bi4O5Br2 nanocomposites with excellent photocatalyticproperties for immobilizing N2. Fig. 16b–e illustrate theformation of AgBr on Bi4O5Br2 nanosheet layers. The Z-schememechanism facilitates directional electron migration betweenAgBr and Bi4O5Br2， enhancing charge separation. Under lightexposure， the NH3 production rate of the optimal AgBr/Bi4O5Br2heterojunction triples that of Bi4O5Br2 （Fig. 16f） 132.

5.4 H2O2 production

Hydroperoxide （H2O2） serves as a versatile oxidizing agentextensively utilized in processes such as disinfection， chemicalmanufacturing， sewage management， bleaching， and more 133.Additionally， it serves as a consistent carrier of energy. Thephotocatalytic generation of H2O2 presents a strategy that isenergy-efficient， eco-friendly， and cost-effective 134–136. Thecreation of H2O2 through photocatalysis， utilizing water andoxygen as primary reactants and solar power source， hasgarnered significant interest due to its eco-friendly， costeffective，secure， and sustainable benefits. There are threeprimary phases in photocatalytic H2O2 production： initially，gathering light energy to stimulate charge carriers caused bylight; next， electrons stimulated by light transition from VB toCB， resulting in h+; and finally， the remaining charge fromphotogeneration interacts with surface-absorbed H2O and O2molecules to create H2O2. Viewed thermodynamically， adjustingthe energy bands of semiconductors serves as a thermodynamictactic to alter light absorption and redox characteristics.Furthermore， the photocatalytic mechanism encounters kinetichurdles as merely a handful of photogenerated carriers manageto access the active site for H2O2 formation， hindered by swiftrecombination， sluggish movement， and brief lifespan.

The researchers found that organic sacrificial agents acting aselectron donors can improve the efficiency of photocatalytichydrogen peroxide production， but this method is costly andrequires the elimination of by-products. Herein， Zhao et al.synthesized a Bi4O5Br2/g-C3N4 heterostructure without organicadditives for photocatalytic generation of hydrogen peroxide（Up to 124 μmol?L?1 of H2O2 in 60 min） 117. At the interface ofcontact， the photo-induced e? of Bi4O5Br2 binds to the h+ of g-C3N4. The unbound and h+ are able to involve in the reaction toproduce free radicals， resulting in more H2O2. Moreover， theresearchers developed novel methods to boost the efficiency oftraditional catalysts owing to severe aggregation and limitedlight uptake， improving the hydrogen peroxide yield. As shownin Fig. 17a， He and colleagues synthesized a floatable S-schemeTiO2/Bi2O3 configuration via hydrolysis， hydrothermal， andphotoreduction steps， and applied it to H2O2 production coupledwith FFA oxidation reactions 107. The floatable design increasedlight utilization and facilitated closer contact between reactantsand the catalyst， whereas the S-scheme heterostructureefficiently separated electrons and holes， enhancing redoxcapability for efficient H2O2 generation and FFA oxidationreactions. The optimal sample TBO40 exhibited a superior rateof H2O2 production， reaching 1.15 mm?h?1 within 12 h， showinga higher H2O2 yield in a gas-liquid-solid system compared totraditional suspended powder photocatalysts （Fig. 17b）.Additionally， TBO40 demonstrated a high catalytic FAproduction rate of 0.45 mm?h?1 within 12 h. Under sunlightirradiation， electrons moved from the oxygen lattice sites to Ti4+，forming a charge separation state （Fig. 17c）. When FFAmolecules were adsorbed， their α-hydrogen got separated， andthe alcohol group binds to Ti4+. Electrons transfer from theoxygen lattice to Ti4+， while holes transfer to the C5 position，removing the β-hydrogen in the alcohol group and generating acarbon radical. This process was confirmed by EPRspectroscopy， which detected an increase in carbon-centeredradicals. Subsequently， the holes interact with hydroxide，resulting in the creation of ·OH. The carbon radicals then detachfrom the Ti3+ sites and react with hydroxyl radicals， undergoingfurther dehydrogenation to ultimately form FA.

5.5 NOx removal

Nitrogen oxides， predominantly NO and NO2， enter theatmosphere through industrial production and vehicle exhaust.NO is considered a major pollutant due to its role in severalmedical and ecological issues such as reduced bloodoxygenation， air pollution， and contribution to global warming.Consequently， research has focused on employing photocatalyststo remove NO. Current studies on photocatalytic NO removalinclude oxidation of NO to NO3?， reduction of NO to N2， andreduction of NO to NH3. Three processes are involved in NOremoval photodecomposition， photoselective reduction， andphoto-oxidation. Researchers have found that BiOBr exhibitsexcellent photocatalytic activity for applications such aspollutant photodegradation and DeNOx reactions， attributed toits appropriate valence and conduction band positions.Enhancing light absorption and effectively inhibiting e?/h+recombination is crucial for further improving BiOBr’sdenitrification efficiency. Fig. 18a，b illustrated the developmentof a durable NiTi-LDH/BiOBr heterojunction by Oliva et al.，which was subsequently investigated for its photocatalytic rolein NOx pollution control 137. Fig. 18c，d demonstrate that theformation of a NiTi/BiOBr heterojunction results in electroniccontact with matched Fermi levels between the phases. The CBof BiOBr shifts from ?0.11 eV to ?0.41 eV， and the VB shiftsfrom 2.76 eV to 2.46 eV （with respect to the NHE）. The e–transfer from the BiOBr’s CB to the NiTi-LDH’s， along with theh+ from the NiTi-LDH’s VB to the BiOBr’s， delaying the e–/h+recombination and adhering to type II heterojunction， therebyenhancing the denitrification efficiency of the NiTi/BiOBrphotocatalyst. The photocatalyst surface promotes the formationof superoxide （O2 + e? → ·O2?）， thereby oxidizing NO toNO2?/NO3? through reaction with ·O2? or O?lattice.

5.6 Selective organic synthesis

The growing significance of photocatalysts in selectiveorganic synthesis arises from their ability to achieve highlyspecific chemical transformations through light-inducedreactions 26. Recently， polymer chemists have utilized catalystsfor the late-stage modification of polymeric materials， achievinghigh yields and effectively facilitating reversible de-activatedradical polymerization reactions and reactive polymerization.Catalysts have opened new avenues for advanced organictransformations， particularly in polymer synthesis andtransformation. For instance， Zhang et al. explore the utilizationof metal nodes in the excited states of MOFs such as Bi-TATBand Bi-BTC （Fig. 19a）. Fig. 19b demonstrates the redshiftphenomenon induced by Bi-TATB， indicating the presence oflong-lived triple excitons， with an extended duration of 4.77 ms（Fig. 19c）， confirming emission induced by a triple exciton. Fig.19d illustrates the shorter lifetime of H3BTC compared to Bi-BTC. TATB and BTC exhibit stronger fluorescence output，whereas Bi-TATB and Bi-BTC show enhanced phosphorylation，suggesting a more efficient systemic crossover between Bi-TATB and Bi-BTC. The presence of Bi3+ in Bi-MOFs facilitatesthe unilinear → trilinear crossover of organic ligands， inducinga conversion of 3O2 → 1O2， and leading to the selective oxidationof alcohols to aldehydes. Bi-TATB and Bi-BTC display differingselectivities and activities in the conversion of aromatic alcoholsinto aldehydes. For Bi-TATB， the aromatic alcohols can bepartially oxidized within 5 h， a 37.59% alteration. Electronwithdrawing substituents （ ―NO2 and ― Cl） exhibit greaterselectivity than electron donating substituents （ ― CH3）. Incontrast， Bi-BTC shows low conversion of aromatic alcohols butachieves 100% selectivity regardless of substituents on thearomatic ring， likely due to its significantly lower surface area.Bi-BTC exposes fewer active sites to 3O2， resulting in lessgeneration of 1O2. Moreover， Bi-TATB enhances photonabsorption and increases the exciton count in its excited state，extending the lifespan of triple excitons and thereby convertingmore 3O2 to 1O2. Based on these insights， both catalysts can beapplied in photo-oxidation reactions involving C6H5CHO 138.

6 In situ detection technology

The development of in situ characterization technology forphotocatalysis is crucial and urgent for advancing photocatalyticprocesses. Currently， there is limited review on the in situcharacterization of bismuth-based photocatalysts. Therefore， thischapter aims to comprehensively introduce relevant in situdetection technologies including in situ XPS， in situ FTIR， in situTEM， in situ UV-Vis DRS， and in situ EPR for the analysis ofbismuth-based photocatalysts.

6.1 In Situ X-ray photoelectron spectroscopy （XPS）

XPS serves as a method for analyzing surface features，offering insights into the chemical states， elemental composition，and chemical characteristics. In situ XPS is employed to identifythe shifts of the electrons in the inner layers of a photocatalyst，which can provide information about the atomic bonding stateand electron distribution of the catalyst. In situ irradiation XPSinvolves XPS analysis of the sample in complete darkness. Thisis followed by in situ irradiation with the laser turned on. Finally，XPS analyze and spectra are taken while irradiating to identifythe electron transformation. In situ XPS technology can be usedto detect the photocatalytic reaction process. For instance， Wangand colleagues designed a PbBiO2Cl （PBOC） double unit cell（DUC） material with abundant oxygen vacancies （ROV） andutilized in situ FTIR spectroscopy and SI-XPS to deeply analysisthe mechanism of its photocatalytic CO2 reduction 139. The insitu FTIR spectroscopy results indicated that CO2 was graduallyadsorbed and activated， forming the intermediate COOH andultimately converting into CO， confirming the effectiveconversion of CO2 （Fig. 20a）. Depicted in Fig. 20b–e， the SIXPSuncovered the pathway of photoexcited electrons andconfirmed the formation of Bi（3?x）+ sites that act as electron traps，effectively reducing the reaction energy barrier for COOH andCO intermediates， thereby promoting the CO2 reductionreaction. Fig. 20f shows that the mechanism diagram furtherexplains this process. The ultra-thin oxygen-rich vacancystructure leads to local lattice distortion near the Bi site， resultingin a slim band gap and the introduction of doping energy levels，thereby facilitating the electron movement to the 5p orbital nearthe Bi sites， forming Bi（3?x）+ sites. Ultimately， the ROV PBOCmaterial exhibited excellent photocatalytic efficiency， with ahigher CO production rate compared to PBOC’s， withoutneeding sacrificial agents and photosensitizers. As a powerfulcharacterization technique， in situ XPS is crucial in the study ofbismuth-based photocatalysts. It empowers researchers toacquire deep insights into the reactive mechanisms of thesematerials， thereby providing crucial theoretical guidance fordesigning efficient and stable bismuth-based photocatalysts.

6.2 In Situ Fourier transform infrared （FTIR）spectroscopy

FTIR， the infrared spectrum， created by Fourier transform ofinfrared interferogram， is widely used to analyze the functionalgroups and structure information of compounds 140. However，conventional FTIR spectroscopy provides only partialinformation about the sample offline， which complicates thereal-time detection of photocatalyst reactions. With the rapiddevelopment of in situ FTIR technology， researchers have deeplyexplored the evolution of photocatalytic reaction processes. Forinstance， Xie et al. utilized in situ DRIFTS for analyzing theintermediates and reactant of the photoreduction process ofBiOBr-1 photocatalyst， revealing the photocatalytic mechanismsuch as the adsorption concentration of CO2 and activationprocess 141. As shown in Fig. 21a，b， m-CO3 2? absorption peaksare observed in the 1319，1473， and 1510 cm?1， whereas HCO3?peaks manifest at 1418 and 1437 cm?1. A novel peak emergesupon light exposure to the sample， intensifying with longerirradiation periods， a process associated with CO2 reduction toCO and CH4 intermediates. The peak of infrared absorption at2339 cm?1 is linked to an asymmetric stretching when exposedto CO2， where the strength gradually diminishes with lightexposure， suggesting a reducing CO level on the catalyst surface，thereby confirming the occurrence of the CO2 reduction process.Moreover， Wang et al. utilized an FTIR spectrometer and an insitu reflectance reactor to create an in situ infrared testingdevice 142. Fig. 21c demonstrates the in situ FTIR spectra of theNO adsorption on the Bi/BiOCl-200 surface in darkness and thephotocatalytic process under simulated sunlight. In darkness，nitrate displayed its absorption peak （1048 cm?1） following astate of adsorption balance. The reason for this phenomenonmight be ascribed to the activation of adsorbed O2 by OVs inBiOCl， initiating superoxide radicals （radical ·O2?）， leading to thetransformation of NO into nitrate 143. To determine theconversion of reduction NO， Li et al. collected in situ DRIFTSspectra of Bi-NPs and Bi-NPs@GO over time （Fig. 21d，e） 144，investigating how the carboxyl groups act as the absorption sitesin the photocatalytic oxidation of NO and its conversion into safenitrate.

Our group is also very interested in in situ FTIR testing. Asshown in Fig. 22a， Bian et al. synthesized In2O3/Bi19Br3S27composites using hydrothermal and high temperature calcinationmethods， which consisted of a large number of sea-urchin-likeBi19Br3S27 nanoflowers attached to In2O3 layered nanospheres.As radiate duration extends， the obtained in situ DRIFTS spectraare shown in Fig. 22b， and the absorption regions of m-CO3 2? at1702 cm?1 and b-CO3 2? at 1559 cm?1 are thought to represent thewater and carbon dioxide interactions. The cycling test in Fig.22c shows a 21.5% reduction of the In2O3/Bi19Br3S27 catalyst，which is more stable than pure In2O3 （66.4%） and Bi19Br3S27（38.0%）. It is shown that In2O3/Bi19Br3S27 has superior catalyticfunction and stability. This outcome was ascribed to theheterojunction of In2O3/Bi19Br3S27 S-scheme to improve chargedivision effectiveness， maintain the photocatalyst’s redoxfunction， and boost the rate of CO2 reduction.

6.3 In situ transmission electron microscopy （TEM）

TEM is now a formidable method for analyzing in areas likechemistry， materials science， and other related disciplines due toits ultra-high temporal and spatial resolution. In situ TEM hasattracted widespread attention for its ability to observe changesin samples during experiments and analyze their morphology，structure， and grain characteristics. This is achieved bymodifying the sample rod under various conditions such asthermal， gas， liquid， applied voltage， and light stimulation.Visualization of crystals during growth has been fundamentallylimited to traditional characterization techniques， as results canonly be obtained from the final structure. However，technological advances in situ electron microscopy have made itpossible to visualize nanocrystal growth with nanoscaleevolution in real time， providing not only quantitativemeasurements but also a deeper understanding of the growthprocess. In addition， in situ TEM can observe the growth of aunit-cell Bi2O2CO3 layer from the surface of Bi2O4 nanocrystalsto synthesize efficient Bi2O4/Bi2O2CO3 heterostructurephotocatalysts 146. As shown in Fig. 23a， Li et al. used real-timeobservation via in situ TEM to reveal that the growth ofBi2O2CO3 monolayers on Bi2O4 nanocrystals’ surface， creates aheterojunction configuration. The in situ grown 2D ultra-thinBi2O2CO3 nanosheets exhibited covalent bonding characteristicsand well-matched lattice parameters， ensuring the epitaxialgrowth of abundant monolayers of Bi2O2CO3 on onedimensionalBi2O4 nanocrystals. This heterogeneousphotocatalyst demonstrated excellent photocatalyticperformance， effectively degrading organic pollutants such asphenol and ciprofloxacin. Compared with pure Bi2O4，Bi2O2CO3， and Bi2O4/Bi2O2CO3 composite materials， theBi2O4/Bi2O2CO3 composite material obtained after 2 h of lightirradiation exhibited superior catalytic efficiency in degradingCIP in aqueous solution （Fig. 23b）. During the reaction，Bi2O2CO3 facilitated the separation of electrons from Bi2O4’sconduction band， thereby enhancing the photocatalytic removalrates of CIP and phenol （Fig. 23c）. Moreover， Chang andcolleagues revealed the in situ methods for utilizing twodimensionalBiOCl photocatalysts using FEG-TEM to analyzethe growth of Bi NPs 147. The randomly generated bismuthnanoparticles in the BiOCl matrix became more and morecontrasty and numerous with the prolongation of electron beamirradiation， and the nanosheets finally turned into a porousstructure after 40 min of irradiation. With the continuousadvancement of in situ TEM technology， its application in Bibasedphotocatalysis research is becoming increasinglyimportant， providing scientists with powerful experimentaltools. This technology enables researchers to deeply observe thecomposite structure of bismuth-based photocatalysts andeffectively guide the development and optimization of efficientphotocatalysts.

6.4 In situ diffuse reflectance UV-visible spectra

UV-Vis DRS is applicable for examining photocatalysts’characteristics， utilizing surface light reflection to derive insightsinto the electronic configuration of chemicals. For example，He’s team successfully prepared a flexible fiber compositematerial with a hierarchical structure， modified with graphenequantum dots using electrospinning and in situ reaction （GQDBiOI/PAN） 148. They studied the changes in the UV-visibleabsorption spectrum over time after adding GQD-BiOI/PAN toa phenol solution under visible light illumination. A rapidreduction in the distinctive peak’s intensity at 269 nm， indicatesa reduction in phenol concentration， confirming the effectivedegradation of phenol. This result demonstrates that GQDBiOI/PAN has excellent visible-light photocatalytic activity andperforms exceptionally well in phenol degradation.

Moreover， Wu and colleagues synthesized Er3+ and Yb3+ codopedBi2MoO6 flower-like microspheres using a hydrothermalmethod 149. Er3+， serving as an activator， can produce uniqueupconversion emissions under near-infrared excitation， whileYb3+， acting as a sensitizer， efficiently absorbs near-infraredlight， transferring energy to Er3+， enhancing the upconversionemission performance. Er3+/Yb3+-co-doped Bi2MoO6 caneffectively break down TC when exposed to UV-visible light，with samples deposited with metallic Bi showing higher TCremoval rates （Fig. 24a，b）. The involvement of the free radicals·O2?， ·OH and h+ in the reaction is known from the TC removalin the presence of different scavengers （Fig. 24c）. The findingsfrom the ESR reveal that the samples under study showed noDMPO-·O2? and DMPO-·OH signals in darkness， while thesignals appeared in the light （Fig. 24d）. By increasing theirradiation time， their intensity could increase， thus exposing thegeneration of ·O2? and ·OH throughout the entire process. Asshown in Fig. 24e， these flower-like microspheres effectivelyremoved TC under sunlight irradiation through the generatedelectrons， holes， and reactive species. These characteristicsendow Bi2MoO6：Er3+/0.03Yb3+@Biy with a higher TC removalcapability.

6.5 In situ electron paramagnetic resonance （EPR）

Understanding the mode of operation or reactivity ofphotocatalysts is essential for improved development andenhanced performance. Electron paramagnetic resonance （EPR）is capable of identifying unpaired electrons in atoms or mattermolecules while analyzing the configuration of theirenvironment. For example， Oxygen vacancies is crucial foradjusting the band configuration and charge transport process.Dong and colleagues developed Bi metal@defective BiOBr foruse as photocatalysts 150. Fig. 25a，b demonstrates EPR findingsthat the number of OVs increased simultaneously with theincrease in bismuth metal yield. These signals were furtherenhanced after 15 min of light irradiation， illustrating the OVs’sensitivity and the partial movement of electrons inherent in thedefects. When exposed to visible light， Bi/BiOBr-200 exhibits amore intense EPR signal compared to BiOBr-100， suggestingenhanced electron mobility of the Bi metal. Moreover， Wei et al.constructed diatomic layered BiOCl nanosheets rich in OVs andconducted EPR tests to investigate oxygen species activity asdynamic intermediaries （Fig. 25c，d） 151. The inference is drawnthat the surface OVs and poorly coordinated Bi locations help toaggregate oxygen molecules between air and the NS-OV surfacewith Pd nanoparticles as co-catalysts， significantly influencingthe photocatalytic transformation of BA. Furthermore， Wang andcolleagues prepared three-dimensional plasma Bimetal@defected BiOCl layered microspheres and used EPR todemonstrate the presence of OVs 142. Fig. 25e show that BiOCl-100 shows a pronounced signal peak at g = 1.996 duringdarkness， attributable to OVs. Meanwhile， as the dissolutiontemperature increases， the signal of Bi/BiOCl-X becomesstronger indicating more OV production. The generation of OVsin BiOCl was attributed to the reduction of ethylene glycol.When exposed to visible light， both BiOCl-100 and Bi/BiOCl-Xphotogenerated radicals improved the strength of the EPRsignals. The combined impact of OVs and metallic Bi effectivelyenhances the properties of Bi/BiOCl. Furthermore， Qin’s team，via EPR spectroscopy， confirmed the generation of Br vacanciesin BiOBr （BOB） catalysts under light irradiation， and exploredthe interaction between the ground and excited states of carbondioxide at the catalyst sites 152.

7 Conclusions and perspectives

The review methodically encapsulates recent advancements inbismuth-based photocatalysts， encompassing theircategorization， synthesis techniques， structural controlapproaches， uses， and on-site characterization methods. Theprimary deductions and forthcoming opportunities are outlinedas follows：

1. Photocatalysts based on bismuth have demonstratedremarkable capabilities in converting solar energy， attributed totheir distinct characteristics. Discussions have been held oncategorizing photocatalysts based on bismuth， such as layeredbismuth， bismuth metal， BiVO4， Bi2S3， and Bi2O3.

2. Typical techniques for creating bismuth-basedphotocatalysts encompass hydrothermal/solvothermal， chemicalprecipitation， and solid-state reactions. Selecting a synthesistechnique ought to be in harmony with the particular needs ofthe application.

3. Strategies for structural control， including defectengineering， heteroatom doping， morphology regulation， SPReffect， and heterojunction construction were used to improve theefficiency of bismuth-based catalysts.

4. Bi-based photocatalysts have shown promising capabilitiesin areas like reducing CO2， splitting water， fixing N2， producingH2O2， eliminating NOx， and in selective organic synthesis.

5. Techniques including in situ XPS， FTIR， TEM， UV-VisDRS， and EPR， used for in situ characterization， have beenexamined to understand the photocatalytic activities and reactivepatterns of bismuth-based photocatalysts.

Successfully employing bismuth-based catalysts incommercialization requires surmounting major hurdles.Although augmenting quantum efficiency and absorptioncapacity is essential for better photocatalyst efficiency，additional studies are required to ascertain whether suchadvancements can be realized without substantial hikes in costsand complexity. The role of stability is also a crucial aspect.Ensuring the catalyst’s longevity with multiple uses is essentialfor its effective application. Should the catalyst deteriorateswiftly， the sustainability of its environmental and economicadvantages in the future will be doubted. Moreover， whileaiming for mass production， it’s vital to assess if themanufacturing process is in compliance with sustainableguidelines and risks negative environmental repercussions.Regarding theoretical analysis and experimental representation，scientists must focus on both advancing methodologies andguaranteeing that these techniques yield precise， repeatable， andsignificant data for informing catalyst development andenhancement. Ultimately， interdisciplinary teamwork andtailored catalyst development for particular uses representeffective strategies in advancing the creation of bismuth-basedphotocatalysts. Nonetheless， this necessitates that scholarsconcentrate on both technological progress and the receptivenessof the technology， along with its market usage and societalrepercussions. To summarize， the creation and utilization ofbismuth-based photocatalysts heavily rely on the integration oftechnological， economic， environmental， and social aspects toguarantee their prolonged sustainability and beneficial impact onsociety. Upcoming areas of research include the creation ofinnovative bismuth-based photocatalysts known for their highefficiency and specificity. Additionally， research will exploreenvironmentally friendly and efficient manufacturing techniquesfor sustainable production. Furthermore， optimizing bismuthbasedphotocatalysts with complex heterogeneous structures andcomposite formations will rationalize and enhance theirperformance. New methods for in situ characterization will alsobe developed to gain deeper insights into the photocatalyticprocess. This review provides comprehensive guidance aimed atcreating effective and environmentally friendly bismuth-basedphotocatalysts that promote new ideas and advances for energyand environmental uses. Such actions are key to advancing greenchemistry， fostering sustainable development， and promotingfurther reforms in academic research and industrial applications.