We then give an overview of the optimization strategies for graphene-based photocatalysis and the latest advances in its application. Finally, we present challenges and perspectives for graphene-based applications in this field in light of recent developments.The global energy shortage and environmental degradation are two major issues of concern in today’s society. The production of renewable energy and the treatment of pollutants are currently the mainstream research directions in the field of photocatalysis. In addition, over the last decade or so, graphene (GR) has been widely used in photocatalysis due to its unique physical and chemical properties, such as its large light-absorption range, high adsorption capacity, large specific surface area, and excellent electronic conductivity. Here, we first introduce the unique properties of graphene, such as its high specific surface area, chemical stability, etc. Then, the basic principles of photocatalytic hydrolysis, pollutant degradation, and the photocatalytic reduction of CO The excessive development of fossil energy causes energy shortage and also promotes the excessive emission of CO2. Various environmental pollution problems around the world are becoming more and more serious . Th. Th93].  process ,95,96. TOverall water decomposition reaction:Complete water decomposition reaction:2, the CO2 that is generally reduced to CO can also be reduced to methane or ethane using special materials [During the catalytic splitting of water into Haterials .The photocatalytic reaction of metal oxides contains the above general photocatalytic reaction mechanism. The difference is that metal oxide photocatalysis mainly involves a catalyst containing transition metals. The whole cycle includes four main steps: oxidation, reduction, addition, and removal. The main mechanism is that the d-orbital electrons in the transition metal has the characteristics of easy separation or addition, which makes it easier to carry out redox reactions. At the same time, the requirement of d-orbital bonding and bond-angle matching is low and the bonding energy is not high, which is beneficial for the formation of low-orbital bonding of reactants. In some of the transition metal catalytic systems, after the introduction of platinum, palladium, rhodium, silver, and other precious metals, these transition metals can be used as photon acceptors to produce photoelectrons, which is conducive to the formation of active compounds, quantum light, and effective hole separation ,99.2-composite graphene nanomaterials have been proved to have better photoelectrochemical activity and photocatalytic activity [2. At the same time, a Ti–O–C bond is formed that introduces the TiO2 bandgap intermediate state to promote a visible light reaction. Unfortunately, TiO2 also has the limitation of a light wave frequency. It is only suitable for the ultraviolet band, while ultraviolet only accounts for 4% of the solar spectrum, and a large number of electromagnetic waves in the band cannot be used [2 photocatalytic derivatives of new materials have emerged ceaselessly and have been developed into a variety of composite types of metal oxides, organic photocatalytic semiconductors, etc. An organic photocatalytic semiconductor is a good alternative material. Its efficient photoelectric conversion efficiency and longer service life have a profound impact on solving the increasingly tense resource and environmental crisis and energy shortage. However, an unsatisfactory aspect is that there are fewer active sites in the material, the two carriers excited by the energy given by the light have a short interval, and the overall photocatalytic efficiency is not high, which make the development of this material still have great limitations [Graphene can be doped with B, Se, N, P, O, S, F, Cl, Br, I, etc. ,106,107.activity ,111,112. be used . In ordeitations ,115.In addition to metal-based materials, graphene-based composite conductive materials are also promising research directions. The main problem of this graphene-based material is that the free electrons and holes diffuse from the conduction band and valence band and then the free electrons and holes recombine quickly, losing the conductivity. This phenomenon affects photochemical stability and ultimately leads to a decrease in photocatalytic efficiency. The macroscopic performance shows that the material has high resistance. At present, there are a large number of strategies to solve the problem of the recombination of free electrons and holes excited by the optical quantum ,118,119.In addition to the above mentioned, graphene can be used as a photocatalytic material substrate and as a co-catalyst. In order to improve the photocatalytic effect of graphene materials, doped N has particularly prominent derivatives ,123. At Graphene’s ability to function as a photocatalytic substrate material and auxiliary material has a lot to do with its structure . It is wThe enhanced catalytic effect of high-dimensional graphene can be attributed to the fact that, compared with one-dimensional graphene, two-dimensional and three-dimensional graphene can reduce the Coulomb binding energy of electron holes and increase the specific surface area to solve the shortcomings of the loose substrate contact and fewer active sites on graphene. At the same time, fixing metals at specific sites can also avoid the aggregation of metal oxides . In part3N4) is a two-dimensional material that has been intensively studied in ozone oxidation and in composites with rhodium phosphide [3N4 material with heterostructural defects. They observed enhanced water-splitting hydrogen-production efficiency and photocatalytic H2-evolution activity by the heterojunction and the construction of defective ultrathin two-dimensional materials, respectively [3N4 and divided it into three groups: g-C2.52N4, g-C1.95N4 and g-C1.85N4. These three groups were also doped with Ag–Pd to form a two-dimensional composite material that strongly promoted photocatalysis. They controlled the ratio of carbon and nitrogen, and the removal rate of NO2− and NO3− was used as a reference for the degree of photocatalytic reaction. The XRD pattern showed that g-C1.95N4 had the best photocatalytic activity. After 3 h of reaction, the removal rate of NO3− was 87.4%, and the removal rate of NO2− was 61.8% [Graphitic Carbon Nitride  on TiO2-functionalized graphene (TiO2-FG) without adding precious metal co-catalysts. The porous HCP–TiO2 graphene composite structure was used as a photocatalytic material to achieve a CO2 absorption capacity of 12.87 wt% and a CH4 yield of 27.62 μmol g−1 h−1. The principle of effective CO2 absorption in this process is to introduce the material into the HCP layer  [2 into amorphous TiO2, Ti atoms are rearranged, GO is reduced, and unpaired electrons are formed. The unpaired electrons and Ti atoms reduce the conduction-band level of TiO2, thereby reducing the band gap and improving the conductivity and photocatalytic effect. Qiu’s team provided a new idea for the treatment of graphene heterostructures by self-doping Ti3+ to place GO and TiO2 on the surface of graphene. GO-cracking into small size graphene; the smaller the size of the graphene, the more binding surface with TiO2 there is in the structure to promote the efficiency of photocatalysis. Moreover, this process also generates Ti–O–C bonds to form shallow surface defects, and the band gap is slightly reduced and red-shifted to 2.98 eV [Lee et al. provided a heterostructural treatment of graphite carbon three-dimensional nanomaterials. The team used hydrothermal treatment and the simultaneous reduction of GO and TiO2.80 eV) . Through 2.98 eV .3N4/GO (p-CNG)) is also a good idea. Li’s team fixed Au, Pd, Pt, and other precious metals on the three-dimensional p-CNG skeleton. By monitoring the positive correlation between the metal content and the diffraction intensity of the three-dimensional porous-graphene-phase carbon nitride skeleton, it was determined that the three metals were successfully anchored on the three-dimensional porous-graphene-phase carbon nitride body. Subsequently, the performance, photostability and thermoelectrochemical performance of the hydrogen evolution reaction (HER) were tested. Because precious metals can be used as electron acceptor sites, the effect of electron-hole separation is significantly improved, and it has strong HER activity under simulated solar light (SSL). The test results show that the 3D p-CNG–Pt composite catalyst exhibits better HER activity than the 3D p-CNG–Au and 3D p-CNG–Pd composite catalysts under optimal conditions. This is due to the effective charge transfer of platinum and 3D p-CNG skeletons, which shows better HER kinetic curve linearity and lower catalyst dose consumption and results in better durability. In terms of thermoelectrochemistry, the 3D skeleton exhibits a wider absorption boundary (531 nm) than the 2D skeleton, and the anchored Au, Pd, and Pt significantly improve the photoreaction effect of the 3D p-CNG composite catalyst due to the improvement of the electron transition pathway [Many functional catalysts are used to enhance two-dimensional graphene phase carbonitrides ,141,142. pathway .2 absorption, and significantly improve the activity of the photocatalytic reduction reaction [Wang’s team provided a new approach to the photocatalytic charge transfer pathway. The team used the characteristics of the ordered structure of the metal organic layer to easily obtain active sites and prepared a 1.5 nm metal coordination layer with rGO as an electron mediator. The ultrathin two-dimensional metal–organic layers (MOLs) are distributed on the two-dimensional template constructed by rGO. The synergistic effect and electronic mediation of the two compensate for the gap between heterogeneous catalysts and homogeneous antenna molecules, improve the energy efficiency of photocatalysis and COreaction .2. This study undoubtedly provides ideas and future research directions for supramolecular organic photocatalysis at the level of quantum modification [Although metal-doped photocatalysts have been extensively studied, as described above, the efficiency and effectiveness of photocatalytic hydrogen production have been significantly improved. However, the use of these doped materials is still challenging in many aspects, especially with regard to their impact on the environment. For example, if the material is applied to too many metals, the cost of subsequent waste recycling will be increased. If not handled properly, this may lead to regional heavy metal pollution. This runs counter to the previous idea of improving global energy tensions and environmental crises through in-depth research on photocatalytic technology. Therefore, research on promoting the photocatalytic effect by relying on the characteristics of different dimensions of graphene-based materials is still advancing. Researchers want to improve the photocatalytic effect of graphene composites through the dimension of materials and the introduction of non-metallic elements, such as carbon, hydrogen, oxygen and phosphorus, to reduce the content of doped metals as much as possible, and even not introduce the whole process. Based on the photocatalytic process of perylene bisimide (PBI) supramolecular materials, the addition of zero-dimensional  graphene quantum dots (GQDs) significantly improve the effect of visible light photocatalysis . The smafication .2 under visible light. They reported two ways to narrow the band gap of GQD and explained the mechanism. GQD-Anln-OCH3, GQD-Anln-OCF3 and GQD-Anln-OCCl3 have a Z-scheme structure, and charge separation can promote coupled photocatalysis  bifunctional electrocatalyst is formed by the nitrogen doping and nickel doping of 3D nanoporous graphene, in which nitrogen and nickel have two forms: single atom and cluster. A rechargeable all-solid-state zinc–air battery was prepared by using nitrogen and nickel co-doped np-graphene as a self-made flexible air cathode, PVA gel as an electrolyte, and Zn foil as an anode. As shown in xibility .4 and Fe-N4 Janus structure with distributed self-assembly, in which a planar configuration is formed to coordinate with four nitrogen atoms. The Janus structure on the inner and outer walls of the GHSs separates the ORR and OER active sites. The outer Fe-N4 site plays a major role in the ORR activity, and the inner Fe-N4 site plays a major role in the OER. When Ni-N4/GHSs/Fe-N4 is used as the air cathode, the specific capacity of Ni-N4/GHSs/Fe-N4 can reach 777.6 mAh gZn−1, and the energy density can reach 970.4 Wh kgZn−1, showing excellent bifunctional electrocatalytic activity. The Ni-N4/GHSs/Fe-N4-based zinc–air battery still maintains stable electrocatalytic performance after about 200 h of operation, which is considered excellent cycle stability [The single atoms of nickel and iron are respectively embedded in the inner and outer walls of graphene hollow nanospheres (GHSs) to form a new type of highly active Ni-Ntability .In recent years, antibiotics have been widely used in human treatment, animal husbandry, and aquaculture . HoweverAntibiotics have a negative impact on plants, aquatic organisms, and microbial communities, causing damage to the ecological environment and affecting human health. The common antibiotics contained in wastewater are tetracycline antibiotics, quinolone antibiotics, β-lactam antibiotics, macrolide antibiotics, sulfonamide antibiotics, etc. . As one 3O4 composites formed by adding GO are further improved compared with using Fe3O4 alone. The OFX removal rate can even reach 99.9% complete removal. Compared with the alkaline environment, the degradation rate and ability of OFX is more effective under acidic conditions [It has been found that the removal efficiency and kinetic constant of rGO-Fenditions . After 0nditions .2–Fe3O4 nanocomposite plasma has a good degradation effect. Compared with rGO–TiO2, the addition of Fe3O4 enlarged the specific surface area of the composites, accelerated the separation of electron-hole pairs, and increased the magnetic strength, which made the catalyst easy to separate from water. At the best doping amount of Fe3O4 of 20 wt%, the removal efficiency can reach 98.1%, which is the highest removal efficiency. Other influencing factors include the catalyst dosage, air-flow rate, peak voltage, and pH value, and their optimal values are 0.24 g/L, 4L/min, 18, and 3.2 kV, respectively. When the above conditions are reached, rGO–TiO2–Fe3O4 has the best removal performance. The rGO–TiO2–Fe3O4 still has high catalytic performance after four uses [Using citric acid-modified GO and carboxymethyl cellulose membrane (GO-CMC) to remove antibiotics, the adsorption capacity of OTC, quinic acid (OA), and trimethoprim (TMP) can reach 102.05, 252.68 and 370.93 mg/L, respectively. Antibiotics are deposited on the surface of GO by π-π interactions and cation-π bonds. The citric acid-modified GO-CMC membrane can be reused and maintains stable recyclability after 5 times of recycling, which can remove antibiotics in wastewater . For proour uses .2-based catalytic system is widely considered to remove pollutants in the environment without producing secondary pollutants, which is conducive to environmental protection [2/N-doped porous graphene nanocomposites (TiO2/NHG) is better than that of bare p25 TiO2, TiO2/GO and TiO2/porous graphene frameworks. The results showed that the oxidative degradation rate of TiO2/NHG was affected by the catalyst loading, the initial antibiotic concentration, and the presence of H2O2. The complete mineralization of 25 mg/L of antibiotics was observed within 90 min of irradiation, and the activity level of the TiO2/NHG catalyst did not decrease significantly within three repeated cycles. The complete mineralization of antibiotics can be achieved only by sunlight irradiation within a reasonable time span, which means that the oxidative degradation of antibiotics remains efficient even in the absence of H2O [Heterogeneous photocatalytic technology based on the TiOotection . For cepe of H2O .−1 at pH = 6.0. At the same time, after four absorption cycles, the adsorption capacity of MZs still did not decrease significantly and maintained a relatively stable adsorption activity. MZ materials have been proved to have the advantages of fast separation speed, strong adsorption capacity, reusability, and simple operation in use [3C2, a graphene-layer anchored TiO2/g−C3N4 (GTOCN) photocatalyst was formed by a one-step in situ calcification method. This method of synthesis can not only act on antibiotics but also on another persistent organic pollutant, namely dyes. Under visible light, such as sunlight in daily environments, GTOCN will have highly oxidizing active ·O2 and ·OH, hence offering high-efficiency degradation of tetracycline (TC) and ciprofloxacin (CIP) antibiotics and bisphenol A (BPA) and rhodamine B (RhB) dyes [Magnetic GO/ZnO nanocomposite (MZ) materials offer an excellent adsorption capacity with reusability for tetracycline antibiotics (TCs) in wastewater. The results show that the maximum adsorption capacity of MZ materials for TCs can reach 1590.28 mg gn in use . Based ohB) dyes .3–Fe3O4 nanocomposites. Compared with single WO3 and rGO–WO3 without Fe3O4, rGO–WO3–Fe3O4 has a larger specific surface area and higher transfer rate of photogenerated carriers. Fe3O4 doping does not make the higher the better. With the gradual increase from zero, the TAP removal curve first increased and then decreased. When the Fe3O4 doping amount was 24 wt%, the catalyst dosage of 0.23 g/L obtained the best catalytic performance, and the removal rate of TAP could reach 99.3%. Acidic conditions and the presence of O3, H2O2, and ·OH are more conducive to the catalytic degradation of TAP [The synergistic catalytic removal of thiamethoxam (TAP) induced by pulsed discharge plasma (PDP) was studied by using graphene–WOn of TAP .3N4-MoS2 loading. Perhaps due to the cleavage of functional groups such as amide bonds and peptide bonds, the removal rate of ampicillin by the photoanode reached 74.6% after the system was fully reacted in sewage for 2 h [The three-dimensional (3D) graphene can be designed as an anode to create an enhanced electron and mass-transfer photocatalytic circulation system, which can be used to remove ampicillin in wastewater and to enhance the antibacterial properties of water. It enhances electrons by using a three-dimensional graphene photoelectrode as an anode, a Pt/C air breathing electrode as a cathode, and C for 2 h .2 sponge modified by molybdenum disulfide nanospheres and GO adsorbs organic molecules and provides a multidimensional electron transport path, which has a positive effect on the degradation of advanced oxidation processes (AOPs), especially for aromatic organics. After pilot-scale experiments in 140 L wastewater, it still maintains efficient and stable activity for AOPs. Even after 16 days of continuous experiments, 3D-MoS2 can still maintain a degradation rate of 97.87% in wastewater containing 120 mg/L antibiotics. This is of great practical and economic interest for industrial applications; if the sponge could be produced industrially in large quantities, the cost of treating a ton of wastewater would be only USD 0.33 in the future [The 3D-MoSe future .Ion-doped GO nanocomposites can perform photon absorption, electron transfer, and the generation of active species under visible-light irradiation. For example, an iron oxide/hydroxide N-doped graphene-like nanocomposite has been synthesized by a laser-based method to remove antibiotics from wastewater. The method is shown in One of the main causes of surface water pollution is from the textile industry, where waste from the printing and dyeing process is directly discharged into the water environment without qualified treatment .The incr2), MG consumption may lead to many serious human health issues, e.g., cancer, etc. [−1. The hydrogel complex showed sustainable usability and could still adsorb 91% of MG after three consecutive dye adsorption–desorption cycles [A triphenylmethane cationic dye—Malachite green (MG)—is a common dye in the dyeing process of the textile industry. However, due to the presence of nitrogen  dye from aqueous solution with a maximum adsorption capacity of 270.27 mg g−1.When the reaction environment meets the conditions of temperature  mesoporous structure with a high surface area of 214.75 mng ratio .2·h. Compared with other compositions and doping materials, the nanocomposite membrane with 0.1% w/v GO is superior. When operating conditions reach 0.1 MPa operating pressure, the dye rejection rate can reach 98%. After several tests on the membrane, the flux recovery rate of almost all dyes can be stabilized at 94%, while the removal rate of individual dyes such as methyl orange can reach 95%, and that of Allura red can be as high as 98% [A polyvinylidene fluoride (PVDF)–polyaniline (PANI) and GO mixed membrane was prepared by incorporating PANI–GO as a nano-filler, which greatly improved the antifouling performance and solvent content. The pure water flux through the membrane increased from 112 to 454 mL/mh as 98% .A nanocomposite hydrogel (NCH) formed by chitosan (CS) and carboxymethyl cellulose (CMC) crosslinked-modified GO has a significant adsorption effect on methylene blue (MB) and methyl orange (MO). At pH 7, the adsorption rate of 0.4 g/L CS/CMC-NCH for 50 mg/L MB was about 99%. At pH 3, the adsorption rate of 0.6 g/L CS/CMC-NCH for MO was about 82%. The adsorption capacity of CS/CMC-NCH for MO is 404.52 mgdye/gads, and the adsorption capacity for MB can reach 655.98 mgdye/gads. More importantly, the composite hydrogel has a stable adsorption performance for the dye after continuous use of 20 adsorption–desorption cycles. CS/CMC-NCH also has an excellent effect on the removal of anionic and cationic dyes . The novFor the removal of the crystal violet (CV) dye, it was found that two kinds of nanocomposite hydrogels, acrylamide-bonded sodium alginate (AM-SA) and acrylamide/GO sodium alginate (AM-GO-SA), can be synthesized by the free radical method, and both of them have good adsorption properties for CV. Compared with other influencing factors, the removal efficiency of AM-SA and AM-GO-SA was more dependent on pH, and the maximum single-layer adsorption capacity could reach 62.07 mg/g and 100.3 mg/g .A novel GO/poly (N-isopropylacrylamide) (GO/PNIPAM) composite system removes organic dyes in water by a similar extraction mechanism and undergoes a reversible sol–gel transition at a temperature higher than the lower critical solution temperature. PNIPAM is anchored on the surface of GO to prevent the reduction of GO and inhibit its aggregation, which greatly improves the stability of GO dispersions. Moreover, the dye can be effectively adsorbed and enriched in the gel phase, which is convenient for its separation from water during the extraction process .The rapid development of modern transportation, petrochemical, and marine engineering has accelerated the release of a large amount of oil into the sea and rivers, causing energy loss, destroying the local ecological environment, and seriously endangering the sustainable development of society .The urgeBased on polyimide (PI), a novel zeolitic imidazolate framework-8/thiolated graphene (ZIF-8/GSH) nanofiber membrane can be prepared by electrospinning and in situ hydrothermal synthesis. The membrane has superhydrophobicity/superoleophilicity and can effectively purify oily wastewater. For various oil/water mixtures and water-in-oil emulsions, the separation efficiency of oil and water can reach 99.9% through the action of fiber membranes. More importantly, the film maintains superhydrophobicity without requiring the environment to remain under harsh conditions. Under harsh reaction conditions, such as excessive acid and alkali, long-term contact with salt and corrosive organic solvents, high temperature irradiation, mechanical wear, ultrasonic treatment and other simulated environmental conditions, ZIF-8 @ GSH can still exhibit excellent photocatalytic degradation efficiency. This shows that the membrane has self-cleaning and active antibacterial abilities and can maintain its performance through the mechanical and chemical environment, which makes future industrial applications very promising .3O4/OA/GO-PU) was formed based on a 3D microstructure by linking GO and coating with functionalized oleic acid Fe3O4 nanoparticles. It can be water repellent, with a contact angle of 158°, and has high selectivity in contact with organic solutions and oils. Theoretically, the microstructure polyurethane (M-PU) sponge has a capacity of 80–160 g/g, and it can undergo 15 adsorption cycles. The oil can be selectively extracted from the wastewater, exhibiting excellent recyclability and the ability to be recovered in a static state using an external magnetic field. Compared with the absence of a magnetic field, the increase of M-PU adsorption capacity seems to be driven by magnetic field exposure due to the enhancement of surface-active sites of the M-PU sponge. The continuous collection of kerosene from the surface water while cleaning wastewater with Fe3O4 @ OA @ GO-PU is cost-effective, highly selective, and an excellent recyclable, environmentally friendly oil-spill cleanup option [With the release of a large amount of oil pollution into the water environment, the emergence of reusable superhydrophobic oil adsorption materials are required. A magnetic superhydrophobic polyurethane sponge  remained above 90%. Even without any alkaline and acidic cleaning, it exhibits good separation rate for oil-in-water emulsions and stable antifouling and antibacterial properties [Using phase inversion technology and the dip coating method, a nanocomposite film was composed of polybenzimidazole (PBI), graGO, rGO, and polydopamine (PDA) in a coated and uncoated manner as shown in operties .A novel poly (oxyethylene) graphene oxide-based nanofluid (P-GO-O) prepared as shown in the As shown in 3+ as a crosslinking agent and a spacer layer for insertion; moreover, it stabilizes the SFGO film selection layer and achieves selective molecular transport. The permeability of methanol is 2.9 times higher than that of large-flake graphene oxide (LFGO), and it has high selectivity for three organic dyes. More importantly, the SFGO-La3+ film exhibits at least 24 h of stable stability under hydrodynamic stress, which represents real OSN operating conditions. Through the interfacial polymerization (IP) of low-concentration resorcinol on the surface of the graphene quantum dot (GQD)-polyethylenimine (PEI)-modified polyimide substrate, a new nanocomposite (TFN) organic solvent nanofiltration membrane with a sandwich structure was prepared as shown in the −2h−1MPa−1, and the penetration rate of Rhodamine B from 87.4% to 98.7%. After long-term immersion in pure N, N-dimethylformamide (DMF), it showed superior solubility. After 81 days of storage at room temperature, it was stored at 80 °C for 45 days and then filtered with Bengal Rose (1017 Da) DMF solution at 25 °C for 5 days. There was no scar solute rejection, which proved the antifouling performance during long-term filtration [A small sheet of graphene oxide (SFGO) film for high-performance organic solvent nanofiltration (OSN) applications, using Laltration .Common heavy metals in sewage, such as Cd, Zn, Pb, Fe, Cu, Hg, Ni, Mn, Co, etc., generally exist in trace amounts. Even in very small amounts, heavy metals are considered to be the most harmful, toxic and most widely distributed components in wastewater due to the mobility of the ions .The disc2+ and Cd2+ has excellent adsorption capacity with great potential [In the past, traditional technologies were used, such as non-destructive processes using resins or adsorbents , non-desotential . Becauseotential .2S3-BiVO4 graphene aerogel (SVGA) requires only a simple hydrothermal method, which can provide effective assistance in both the photogenerated electron transfer and photocatalytic ability of SVGA. The results show that removal rates of Cr (VI) and bisphenol A (BPA) using SVGA materials can be infinitely close to 100% after 40 min of adsorption and 120 min of photocatalysis under visible-light irradiation at 420 nm. The harmful Cr (VI) is preserved as low-toxic Cr (III) after photocatalysis on SVGA, and BPA is degraded into CO2 and H2O [The preparation of a Bi and H2O .2SO4 aqueous solution. Reductive self-assembly (RSA), one of the main materials of MFHGs, is cheap and environmentally friendly, so the cost of MFHGs is not high. They have excellent magnetization and large coercivity and can work normally in high-salinity water [Due to the presence of oxygen-containing functional groups and high porosity, 3D magnetic fungal mycelia/graphene oxide nanofibers (MFHGs) can remove Co (II) and Ni (II) from high-salinity aqueous solutions, increasing the ion removal rate. The optimum reaction conditions were as follows: at 323 K and pH = 6.0, MFHGs could remove 97.44 and 104.34 mg/g of Ni (II) and Co (II), respectively, from 2 g/L Naty water .Under aerobic conditions, a novel graphene-like biochar supported trivalent iron (GB/nZVI) to remove Cd (II) and As (III). The main principle of removing As (III) is through oxidation and surface complexation, while the removal of Cd (II) mainly depends on surface complexation. At the same time, the strong synergy between GB and nZVI has a positive effect. The removal ability of GB/nZVI composites was significantly higher than that of pure GB and nZVI under both acidic and neutral conditions. The presence of As (II) significantly promoted the removal of Cd (III) when both ions were present in the same water environment. The maximum removal of As (III) can reach 181.5 mg/g when nZVI is 363 mg/g. The maximum removal capacity of Cd (II) can reach 46.4 mg/g when n ZVI is 92.8 mg/g. It is worth mentioning that the presence of phosphate and humic acid in the coexisting background ions has a reverse inhibitory effect on the removal of Cd (II) and As (III) .h) with different degrees of oxidation were combined with tridentate terpyridine ligand (Tpy) to form GO, GOh, and GO–Tpy as shown in h, and GO–Tpy, GOh–Tpy has the highest adsorption efficiency for heavy metal ions due to the synergistic effect of GO and Tpy components. The maximum adsorption capacity (qmax) of the GOh–Tpy system for Ni (II), Zn (II), and Co (II) reached 462, 421, and 336 mg g−1 at pH = 6, respectively, and showed excellent repair performance, which proved that the GOh–Tpy mixture had an ideal cycle stability, reusability, and easy separation operation [GO and highly oxidized graphene oxide (GOperation .2), benzene, and isopropanol are the three major pollutants that can be commonly found outdoors. Titanium dioxide/graphene hybrid nanomaterials were synthesized by the sol–gel method. Under UV-A irradiation, the presence of isopropanol and benzene will form different free radicals, which improves the removal efficiency under UV-A irradiation. The addition of at least 1.0 wt% of granolol to TiO2 can double the photocatalytic efficiency, and the system exhibits more stability under oxidizing conditions compared with pure TiO [Exposure to air pollution is one of the five major global human-health risk factors. Photocatalytic oxidation is a promising method for the treatment of environmental pollutants. Nitrogen oxides  is at a high concentration, the photodegradation rate of benzene and toluene can reach a 91% oxidation rate, while PIL @ TiO2/m-GO can reach 97% at a low concentration, and the percentage is obtained within 24 min [A multi-ionic liquid (PIL)/TiOn 24 min .A novel high-strength graphene aerogel was prepared by adding tetraethoxysilane (TEOS) to the precursor GO solution, as shown in the 2 began in 1972 and 1979 when Fujishima and Honda used TiO2 as a motor to photocatalyze the reduction of CO2 in water [2 is matched with the valence band and the conduction-band position of the semiconductor photocatalyst, but most of the semiconductors cannot satisfy this requirement; thus, to achieve CO2 reduction and emission reduction in photocatalytic reactions under sunlight, it is necessary to find semiconductors that meet the above conditions.Solar energy will be an important component of future energy improvements for new sustainable development. Research on the photocatalytic reduction of COin water ,215,216.2 has three main steps: (1) the generation of photogenerated electrons by bandgap engineering; namely, the generation of photogenerated charge carriers and the formation of electron-hole pairs; (2) the transfer of photogenerated electrons—this process can be called charge carrier separation and transport; and (3) the reduction of CO2 by photogenerated electrons on the surface.According to the basic reaction principle, the photocatalytic reduction of CO2 emission reduction. Graphene is used as electron acceptor/transporter due to its high power function and good conductivity.Given that electron-hole pairs combine easily and restrain charge separation to a great extent, effective charge separation is the key to achieve large-scale CO2 adsorption through π-π conjugation between graphene and CO2, (iii) activates CO2 molecules, (iv) improves corrosion resistance, (v) enhances surface area and light absorption, which subsequently results in higher photocatalytic activity [In the process of photocatalysis, graphene plays the following roles: (i) reduces photogenerated electrons and hole recombination, (ii) promotes COactivity .2 reduction, the catalyst surface activity is reduced, and the production efficiency is low [Since most catalysts are easy to recombine due to the influence of electron-hole pairs in the process of COy is low . Grapheny is low . Grapheny is low ,221,222.y is low ,226,227.Doping is the most feasible and convenient method to adjust the band structure of graphene from semi-metal to p-type or n-type materials ,229. Pri2O, O2, N2, NO2, PMMA, etc.) will lead to obvious p-type doping. If the heteroatoms are removed from the p-type doping, the p-type doped graphene will return to the state before the original doping [Graphene doping can be divided into n-type doping, p-type doping and p/n co-doping of single-layer or double-layer graphene. The chemical doping of graphene is realized by attaching heteroatoms to the surface of graphene or by replacing C atoms in graphene. For graphene, it is easier to produce p-type doping through surface adsorption . Exposurl doping .Chemical doping can effectively open the band gap of graphene . The Fer2O, N2, O2, and CO2. These small molecules will promote graphene to form p-type doping. We can also open the sp2 bond of graphene by replacing the position of carbon atoms or other atoms and bonding with carbon atoms to form doping. Because graphene can be prepared in large quantities by chemical vapor deposition (CVD), we can add different reaction sources to it at the end of the production, so that some carbon atoms in graphene can be replaced to form lattice doping. For example, under certain conditions, boron atoms can partially replace carbon atoms, forming p-type graphene. Common p-type doping molecules include fluoropolymer, water, N2, NO2, O2, oxidizing solution, B, Cl, and metal [2 on B-doped graphene and double B-doped graphite has strong chemisorption. The strong interaction between single- and double-B-doped graphene composites show that B-doped graphene and double B-doped graphene can catalyze or activate, which indicates that B-doped graphene and double B-doped graphene can be used as effective catalysts for CO2 reduction [As a doped atom, graphene can provide a large number of electrons, which is due to the existence of large π bonds. The electrons in its parallel p-orbitals enter the conduction band, leaving a large number of holes in the valence band, forming an electron-hole pair. The Fermi energy level is close to the bottom of the conduction band. This doping method is p-type doping. In general, graphene has a two-dimensional honeycomb structure, so its surface easily adsorbs some small molecules, such as Hnd metal . Khudaireduction .Individual metals are easy to dissolve in the reaction environment because of their low bulk-binding energies, which have been extensively calculated by Chen et al. In the ORR performance of the 10 metal-doped graphene (M-G) catalysts  they studied, the metal bound to the graphene had higher binding energy, which shows that the M-G catalyst can be more stable compared with samples without metal dopants .In the study by Min et al., different kinds of metal-doped graphene  were found to have different band gaps . The ban2 with a GrN700-CuC catalyst. At the same time, in order to verify that the reduction product methanol is produced by CO2, they replaced CO2 with N2 and found that the methanol content is small enough to be ignored. All the results show that the N-doped graphene catalyst has good chemical stability and can be further applied and developed.As shown in 2 reduction. However, in practical research, it is found that a single-metal material cannot achieve high photocatalytic efficiency, and a graphene heterostructure can effectively solve this problem.As a material with excellent photosensitivity and conductivity, graphene has a good application prospect in the field of photocatalysis and CO2, because the graphene heterojunction effectively reduces the energy band structure of CO2 and can more easily dissociate the C-O bond (750 kJ·mol−1) [2O6 heterostructure can reduce its energy band structure to 0.025 eV. This is because heterostructures in graphene can effectively migrate electrons from the interior to the surface. They are excited to graphene through the electrostatic field, promoting the separation of electron-hole pairs and inhibiting the recombination of carriers. In this way, a large number of electrons can be formed on the graphene surface, and a large number of holes can be accumulated on the heterostructure. Such a good combination can greatly promote charge transfer and photocatalytic efficiency [First of all, we found that the heterojunction can significantly improve the reaction efficiency in the reduction reaction of COJ·mol−1) . Doping ficiency  . In order to obtain materials with more photoelectrons, Tang et al. coated silver nanoparticles on the materials, according to the characteristics of silver nanoparticles with low Fermi energy, and finally obtained more photoelectrons in Cu2O. At the same time, because of the high specific surface area and two-dimensional honeycomb structure of GO after sp2 hybridization, it can adsorb gas better and enhance activation ability by coating a Ag/Cu2O surface. In the end, they concluded from a number of experiments that the ternary heterojunction can effectively reduce the CO2 content in photocatalysis and selectively produce CH4 [2O/graphene/TiO2 nanotube array (TNA) heterostructure as a separate oxidation and reduction catalyst, which uses anode TNA as the substrate, electrodeposits graphene and Cu2O in turn. The results of photoelectrochemical measurements show that the ternary heterogeneous materials display the advantages of each component, improve the photocatalytic performance significantly, and show excellent performance in light absorption and the reduction of electron-hole pair coincidence; in other words, ternary heterogeneous materials are excellent catalysts for photocatalytic reactions [Zhiling Tang et al. developed the ternary catalyst of rGO-coated Ag/Cuduce CH4 . Similareactions .2 and convert it into fuel because of its superior photocatalytic properties when combined with metal oxides.It is concluded from the above-mentioned optimization strategies that the inhibition of electron-hole-to-ground recombination is the key to enhancing photocatalytic efficiency. Graphene-based photocatalytic engineering is an effective way to reduce COGraphene’s broad absorption spectrum is one of the main reasons for its excellent photosensitivity, and its excellent performance in light absorption is an effective way to improve the efficiency of photocatalysis. Li and his colleagues reported that the addition of graphene to Pt-TiO enhanced the light absorption. In this process, finding the appropriate doping stoichiometry is the key to improving the optical absorption efficiency.3N4(PDI/G-CN) nanosheet and demonstrated the potential practicability of an S-type heterojunction in photocatalysis [A great deal of research has been conducted on the preparation of highly efficient heterojunction photocatalysts. Rui Sun et al. obtained the heterojunction of a perylene diimide/Graphene-g-Catalysis .2 reduction [Radovic et al. have found that the transfer of photogenerated electrons occurs through Valery Karpin sites in graphene after doping and that the surface of the doped graphene has more oxygen vacancies, and realignment occurs. The active sites on the surface provide more sites for COeduction .2-modified N-graphene heterostructure catalyst for CO2 reduction for highly selective methane production. Doped graphene itself has excellent photocatalytic performance, and TiO2 also plays an excellent role in synergistic catalysis. Properly designed plasma gold nanoparticles electrodeposited onto TiO2-modified N-doped graphene (ANGT-x) heterostructure catalysts exhibit significant CO2 reduction activity and high methane generation selectivity. Compared with the typical binary Au-TiO2 photocatalyst, the electron consumption rate (Repetron) value of the reduction product of ANGT2 is about 742.39 µ mol g−1h−1,the latter is 60 times more than the former. As far as we know, this is the highest PCO2R rate reported under comparable conditions. These remarkable improvements are attributed to good light selectivity and improved electron transfer dynamics. At the same time, due to the formation of the heterojunction by doping, the components form a seamless interface. Their contact greatly inhibits the recombination of electron-hole pairs and improves the efficiency of charge separation.Khaja Mohaideen Kamal, Rekha Narayan, Narendraraj Chandran, and others studied the synergistic enhancement of plasma gold nanoparticles on a TiO2 reduction. On the premise that the coupling of CO2 reduction and H2O oxidation forms a complete photoelectrochemical cycle, graphene can change the conduction-band potential of semiconductors to form graphene-based composites, which leads to the reduction of CO2. It has been reported that graphene/WO3 nanobelt composites can reduce CO2 to CH4, in which graphene improves the conduction band of WO3 in the composites, while single WO3 inherently limits the reduction of CO2.The combination of graphene and semiconductors as a synergistic heterogeneous composite can improve the efficiency of photocatalysis or electrochemical COThe main preparation methods of graphene matrix composites include physical coating, CVD, electrophoresis, and electrodeposition. The physical coating method is to attach the available graphene or graphene-based composite materials to the target materials without changing their physical and chemical properties. The CVD method has been briefly explained in the above section on chemical doping. Electrophoretic deposition (EPD) is a general processing technology for depositing graphene with controllable thickness and uniform structure on a wide range of substrates ,248,249.2-rGO nanocomposites developed by Liu et al. They used GO and TiO2 nanoparticles as starting materials to realize the efficient photocatalysis of GO composites through simple and relatively general methods.Herein, we show you the preparation method for TiO2 efficiently simply by using components as photocatalytic materials. Metal–organic framework (MOF)/GO composites were prepared for CO2 capture under flue gas conditions by using the method of Mégane Muschi and Sabine Devautour-Vinot, and its performance greatly exceeds that of pure components. Based on the microporous water-stabilized MIL-91(Ti), a series of CO2-capture composites were prepared by in situ and post-synthesis methods. It was observed that 5 wt% GO in situ composites exhibited semi-conductive behavior, whereas the composite was insulating, even though the GO content was high (20 wt%). Therefore, compared with pure MOFs and post-synthesized materials, this composite material absorbs microwave radiation more effectively. Finally, Muschi and Devautour-Vinot reported that CO2 desorption under microwave irradiation is faster than direct electrical heating on MOF/GO in situ materials, paving the way for energy-efficient microwave-swing adsorption processes in the future [It is difficult to reduce COe future .2 and the cost-effective production of 3D graphene. In many reactions, in order to solve the problem of building up micro-scale 3D graphene materials, it is necessary to form alkali metals and CO2 graphene at the same time and use them as original templates. Etching graphene by CO2 is the key project to control the formation of porous surface structures, so it also provides a good scheme for constructing various excellent 3D graphene materials [Graphene itself is a two-dimensional honeycomb material. It is precise because of its structural characteristics of a high specific surface area, which greatly increases its reaction efficiency. Zhuxing Sun and Yun Hang Hu proposed several 3D structure graphene materials that can reduce the accumulation of graphene sheets and help improve the photocatalytic efficiency of graphene-based materials. They found that the exothermic reaction between an alkali metal and inorganic carbon compounds provides an ideal solution for the efficient utilization of COaterials .2 emissions. There are many types of materials in this field, but they are mainly based on graphene for the mixing of three or other components. Barbara Szczęśniak et al. have studied three-component GO/ordered mesoporous carbon/metal organic matrix composites to address the need for expensive organic binders in the field of composites, and they found that the three-component composite also contributes to the formation of MOF crystals during air raids on mesoporous networks and CO2 recovery [The current research on graphene-based composites is aimed at reducing COrecovery .2” hybridization, which greatly enhances its structural strength and flexibility. Based on this hybrid, graphene “sp2” hybrid-connected carbon atoms can be tightly stacked into a single-layer two-dimensional honeycomb lattice structure. Bandgap engineering can effectively adjust the bandgap of graphene, and it plays a role as a charge-transfer carrier to enhance the efficiency of electron transfer and to increase the reduction rate of CO2 [Graphene has a strong s bond between its two carbon atoms due to its “spe of CO2 .The translational symmetry of the original graphene is broken, that is, when two equivalent atoms in the original cell become unequal, a band gap will be generated. The experiment of introducing a band gap into the original graphene includes growing graphene on some substrates, applying an external bias voltage, generating graphene in the form of narrowband graphene, and perforating graphene to form a “graphite nanonet” ,254.Through a lot of research on graphene bandgap engineering ,261,262,It has been proved by experiments that if nanopores or molecularly modified graphene are introduced into the original graphene, the required translational symmetry can be removed, thus opening the band gap. Doping some elements into graphene can open the band gap. For example, the hydrogenation of graphene can effectively adjust the band gap of graphene. In addition, some graphite intercalation compounds (GIC) are outstanding in inducing superconductivity .2” and “sp3” hybridization. Therefore, the proportion of operating hybrid-bonding atoms can effectively adjust the broad band gap of GO, transforming it from an insulator to a semiconductor and into a metal similar to graphene. The oxidation functional groups around GO can operate through H3PO4. The amount of H3PO4 is the key to the operation affecting the oxidation functional groups, which leads to a tuned band gap that is well aligned with the CO2/CH3OH redox potential  and shows an improved activity of CO2 photoreduction to methanol. The conversion of CO2 to methanol requires stretching the band gap of GO so that there is a CH4 production orientation in the CO2 conversion. After much research, it has been found that there are many oxygen groups on the surface of GO, which can effectively stretch the band gap of GO. This stretching regulates the oxidation and reduction potential of CO2 to match the valence band and the conduction band and promotes the generation and excitation of photogenerated electrons, which can migrate to the surface more effectively and induce CO2 reduction.At the same time, because the volatile organic compounds adsorbed on the surface of the catalyst combine with the lattice oxygen on the surface of the catalyst, the catalyst surface generates oxygen holes and is reduced, and the catalyst is oxidized by filling the oxygen vacancies with the dissociated adsorbed oxygen. The resulting graphene derivatives, such as GO and rGO, have the characteristics of insulators and similar semiconductors in terms of photocatalysts, resulting in wide band gaps. GO has a limited band gap due to the two-dimensional grid generated by its “sp2 can separate the electron and hole more effectively and keep them in different positions. Tang and colleagues have also demonstrated that graphene-coated metals exhibit rapid CO2 reduction over a longer period of time [Sokal reported on the bandgap engineering of graphene. We know that photocatalysis produces photogenerated electrons, while doped graphene exhibits better charge separation properties. In his paper, Sokal pointed out that the graphene doped with TiO of time .In order to effectively adjust the band gap structure of graphene, Wu et al. calculated the energy band structure of polycrystalline graphene through the density functional theory (DFT). The results showed that even under the condition of external stress, the grain boundaries (GBs) with symmetrical polycrystalline graphene still have zero band gap, while some asymmetric GBs can also open the band gap, which can be adjusted by external stress . The disGraphene and its derived materials are functional materials that have emerged in recent years. They have also been involved in a series of breakthrough discoveries in the field of photocatalysis and pollutant degradation and have spawned the creation of many new photocatalytic materials and applications. So far, many of their advantages have been exploited and utilized by people, and there will be more excellent potential applications in the future. However, there is no denying that graphene also has some shortcomings to be overcome, mainly involving the following aspects:2 and to obtain chemical fuel–metal composite graphene materials similar to CH4 that can be used. However, as a new material, at the environmental level, the economic cost and environmental resource cost of using precious metals to improve the photocatalytic effect are still huge. If some metals are not properly treated, this may cause pollution to the environment and indirectly increase the cost of the subsequent treatment of materials. Metal–graphene composites with low cost and high photocatalytic energy efficiency remain to be found. TiO2 can efficiently utilize ultraviolet light. Although many TiO2 composite graphene materials can expand the range of the photocatalytic reaction light wave, the effect still has a large room for improvement. Making full use of the visible-light band will greatly improve the photocatalytic energy efficiency. In graphene bandgap engineering, the band gap can be opened by reducing the size of the graphene material [(1) Graphene semiconductor materials have been deeply studied in the field of photocatalysis, and they have shown good photocatalytic material properties when combined with many precious metals. They can reduce the bandgap energy and improve the electron-hole yield. They are a good choice to reduce the greenhouse gas COmaterial . This me2 absorption rate, water decomposition efficiency, etc. and significantly improve the photocatalytic effect. Furthermore, there will be no environmental disadvantages of noble-metal composite graphene photocatalytic materials. However, to be economically viable, researchers need to continue to push for lower material costs so that they can be produced on a large scale and use solar energy efficiently. At the same time, there are also some problems to be addressed in the construction of heterojunction structures by organic non-metallic graphene materials. For example, the existence time of two carriers excited by quantum light is short and the photocatalytic efficiency is low, and it is difficult to avoid agglomeration when graphene nanomaterials are introduced. The reaction site of photocatalysis is not positively correlated with the amount of material input. According to the semiconductor properties of graphene, the development and production of highly active graphene-based photocatalysts can be further studied. For different types of graphene-based materials , their electrical properties may have unexpected effects in their applications. At the same time, the application of graphene-based composites in photochromic or electrochromic fields is not mature. Metal oxide-based composite graphene materials or glass-fiber composite graphene may make full use of the semiconductor properties of graphene to participate in the redox process of electrochromic materials and to promote the development of products such as “smart glass”. Furthermore, in catalytic reactions, due to the easy recombination effect of electron-hole pairs, we need to introduce heteroatoms to adjust the band structure of graphene and to reduce the recombination rate of electron-hole pairs. For different types of doped graphene, we have made a detailed overview of metal-doped graphene and non-metal-doped graphene and have given certain conclusions in terms of their basic electronic structure, catalytic active site, and morphology. However, the structural characteristics and functions of doped graphene still need further discussion and research.(2) By constructing two-dimensional and three-dimensional graphene composites, or by constructing porous materials, the high surface-area ratio, optical properties, and mechanical structure stability of graphene can be maximized. These characteristics can increase the photocatalytic reaction area, CO(3) The use of graphene for pollutant degradation has given rise to more graphene-based composites for environmentally friendly cleaning purposes or to enhance the performance of graphene. However, most forms are just composite materials, composite membranes, composite hydrogels, etc., and few become a green and clean treatment system. They have shown good performance in the laboratory, and some can even achieve a 100% removal rate. They also show good recyclability after an adsorption cycle and will not cause secondary pollution to the environment. However, most of them only have good adsorption for individual pollutants, and the removal efficiency for most of the pollutants not mentioned in the discussion are still unknown. In addition, it is also necessary to consider whether the laboratory simulation environment or sampling can fully match the real environment. The environment of different regions is also under different conditions. How to carry out the next industrialization or commercialization step is still a question mark. The cost of material manufacturing or cleaning system construction is one of the issues we need to consider. Nonetheless, if there is a better application than the current choice, that would be a small success.(4) Some defects of graphene-based photocatalyst lead to its low recycling rate, which is rarely considered in most of the current literature. For example, the synthesis of N-doped graphene-based photocatalysts will produce vacancy defects at the same time, which significantly affects the activity of photocatalytic materials. For environmental protection and sustainable development, how to maintain the stability and service life of graphene-based photocatalysts is a direction worthy of study.2 reduction rate. These methods include adding graphene to the substrate, applying an external voltage, producing graphene in the form of narrowband graphene, and using “graphite nanogrids”. The specific graphene band gap provides enormous room for progress in photocatalytic reactions and will be further expanded and developed in the future field of photocatalysis. Single-layer pure graphene sheets lack hydrophilic functional groups, and the preparation of semiconductor/pure graphene composites is extremely challenging. At the same time, as a 2D material, it is still worth exploring how to achieve large-area interface dynamics with other non-2D materials or to load various components to achieve charge separation in space.(5) Graphene has become an important photocatalytic material in the 21st century. In this article, we discuss in detail the bandgap engineering for constructing graphene, which can fully utilize its role as a charge carrier and improve electron transfer efficiency and CO(6) Studies suggest that cytotoxicity mainly depends on the physical and chemical properties of nanomaterials. Surface-doped chemical or biological molecules will also affect the expression of properties to some extent. Nowadays, many graphene products exist in the market with smart devices and sensors, which also creates a problem. Graphene-based materials have toxic effects. When put into use, graphene nanoparticles have a potential risk of exposure to air, which will cause certain irritation to the human body and affect health. In order to give full play to the commercial value of graphene nanocomposites in practical market applications, more in-depth research still needs to be conducted in this area.


Large spin-orbit coupling is often thought to be critical in realizing magnetic order-locked charge transport such as the anomalous Hall effect (AHE). Recently, artificial stacks of two-dimensional materials, e.g., magic-angle twisted bilayer graphene on hexagonal boron-nitride heterostructures and dual-gated rhombohedral trilayer graphene, have become platforms for realizing AHE without spin-orbit coupling. However, these stacking arrangements are not energetically favorable, impeding experiments and further device engineering. Here we report an anomalous Hall effect in Bernal-stacked tetralayer graphene devices (BTG), the most stable configuration of four-layer graphene. BTG AHE is switched on by a displacement field and is most pronounced at low carrier densities. The onset of AHE occurs in tandem with a full metal to a broken isospin transition indicating an orbital origin of the itinerant ferromagnetism. At lowest densities, BTG exhibits an unconventional hysteresis with step-like anomalous Hall plateaus. Persisting to several tens of kelvin, AHE in BTG demonstrates the ubiquity and robustness of magnetic order in readily available and stable multilayer Bernal graphene stacks—a new venue for intrinsic non-reciprocal responses. Intrinsic anomalous Hall effect has been observed in twisted graphene multilayers, but these structures are typically not energetically favorable. This study extends these observations to Bernal-stacked tetralayer graphene, which is the most stable configuration of four-layer graphene. Recently, intrinsic valley access in the absence of external magnetic field or circularly polarized light irradiation was demonstrated via orbital ferromagnetic ordering in moiré materials4, and rhombohedral trilayer graphene6, giving rise to an anomalous Hall effect (AHE). In these, valley polarization can be directly identified by the sign of the anomalous Hall response at zero magnetic field and switched hysterically7. However, consistently achieving these stacking configurations for device characterization is challenging8. In twisted bilayer graphene, AHE has only been found for twist angles that are simultaneously close to magic angle as well as aligned to a hexagonal boron nitride substrate8, while rhombohedral stacking arrangements are not the thermodynamic ground state in naturally occurring graphene trilayers11.Electrical access to the valley degree of freedom in two-dimensional Dirac materials requires the simultaneous breaking of inversion and time-reversal symmetries. For instance, by irradiating MoS10, as illustrated in Fig. 12, as well as topological phase transitions and helical states at high magnetic field13. In contrast to these previous works, we focus on the low-density and moderate displacement field region in BTG that displays pronounced valley-locked orbital magnetic moments and prominent density of states close to van Hove singularities . These features interact to produce an AHE.Here we uncover an AHE in Bernal-stacked tetralayer graphene (BTG), the most energetically stable allotrope of four-layer grapheneRxx and Hall resistance Rxy. The carrier density n = [Ct(Vt-Vt0)  +  Cb(Vb-Vb0)]/e and perpendicular displacement field D  =  [Ct(Vt-Vt0)  −  Cb(Vb-Vb0)]/2ϵ0 can be independently tuned by applying voltages on top and bottom gates  and a multi-peak like structure as a function of carrier density; these peaks can be controlled by D and have been attributed to gate-tunable Lifshitz transitions in BTG12. Similarly, Rxx at the CNP first increases with D and then saturates at larger D ( > 0.2 V/nm). These behaviors are signatures of Bernal-stacking order of BTG15.Figure Rxx  at a small out-of-plane magnetic field of B = 1 T in Fig. Rxx  are immediately apparent. First, and perhaps the most prominent, are vertical stripes that occur at uniform spacing of filling factor δν = nϕ0/B = 4, where ϕ0 is the magnetic flux quantum. The degeneracy of 4 isospin flavors  is often associated with a “full” metal (FM) phase, see also Supplementary Fig. Rxx  density plot with filling fraction labels. Additionally, superposed on top of the vertical stripes are diagonal fringes at moderate to high density that originate from the intersection between collections of Landau levels.To investigate the fermiology of BTG, we measure D , an unusual stitch-like ripple pattern emerges. These ripples arise from a complex pattern of Landau levels and their intersections that disperse with D12. Strikingly, this region is marked conspicuously by the absence of the clear vertical lines with δν = 4 degeneracy spacing. To further investigate the features in this region, the period of quantum oscillations is examined in Fig. B that can reveal the k-space area of the enclosed orbits. Extracting the oscillation frequency, fB, in the standard method16 via Fourier analysis of Rxx (1/B)  as f = fB / nϕ0; 1/f indicates the degeneracy of the Fermi surface.In contrast, at low density and moderate f ≈ 1/4 in agreement with the 4 fold-degenerate vertical stripes discussed above. In contrast, the “VM” region possesses f ≈ 1/2 indicating a lowering of the isospin flavor degeneracy with an orbit area that is approximately half the total Fermi surface size. As a result, this behavior is reminiscent of a gate-tunable transition from a full metal to a half-metallic-like regime, similar to other isospin-polarized phase transitions reported in rhombohedral trilayer graphene6 and bilayer graphene recently17. While the combination of an applied magnetic field  and D field (broken inversion) are sufficient to lead to externally broken valley symmetry18 and hence half-metallicity at low densities, as we now discuss, time-reversal remains spontaneously broken at B = 0. We denote this spontaneous time-reversal broken phase a valley metal state.For clarity, we examine the quantum oscillations for several illustrative points in the “VM” region (“FM” region) denoted by the empty (solid) polygons respectively. In the “FM” region, we find Rxy as a function of applied B field at D = 0.3 V nm-1 at two different densities 0.1 × 1012 cm-2 (deep in the “VM” region) and 1.0 × 1012 cm-2 (outside the “VM” region). Rxy for 1.0 × 1012 cm-2 exhibits a conventional linear in B dependence with near identical forward (red) and backward (blue) B-field sweeps. In contrast, Rxy for densities of 0.1 × 1012 cm-2 is hysteretic with a non-zero Hall resistance at B = 0 of several tens of Ohms  = ηB + RAH (M), where η = 1/ne is the Hall coefficient and M is the magnetization. Plotting RAH as a function of applied B in Fig. RAH increases rapidly before saturating at approximately 40–50 mT. RAH also highlights the hysteretic features of BTG AHE. In the small field window where hysteresis is most pronounced, the blue and red curves mirror each other to display the same stepped profile but displaced in the y-axis. This shift between forward and backward sweeps indicates that red (-) and blue (+) RAH curves track different ferromagnetic states  as they evolve with applied B.BTG AHE can be better visualized by delineating the total Hall resistance into a conventional component that scales as applied FK = Σp fK(p) ΩK(p), displays a rapid change as a function of chemical potential close to band edge; away from the band edge it saturates  is the electronic distribution function for valley K. When both valleys are equally populated , the net Berry flux Ftotal = FK + FK’ that tracks the strength of the AHE20 is zero. Crucially, close to the band edge in the VM region, BTG possess D tuned van Hove singularities with pronounced peaks in the density of states  ≠ fK’(p). Spontaneous valley polarization features a non-zero RAH and a non-zero Rxy at B = 0, as shown in Fig. AHE in BTG can be naturally understood by examining the Berry curvature distribution in the bands of gated BTG. In valley K, the Berry curvature distribution  ≠ fK’(p)] can be further tuned by B field  favouring one valley over the other, see also Fig. RAH with applied B. Importantly, because Berry curvature is concentrated at the band edge, dFK,K’/dμ is most pronounced at the band edge , RAH is flat with no appreciable hysteresis. As density is lowered, both the size of the hysteretic window and the saturated RAH increase, in agreement with the peaked dFK,K’/dμ close to the band edge. This density sensitivity allows BTG AHE to be switched “on” by tuning into the low electron density region where Berry flux changes rapidly. Similarly, since a finite D field turns on Berry curvature in the valleys of BTG, we find that AHE can also be switched “on” by D field  nature of the half-metallicity in rhombohedral trilayer graphene6. This arises due to the minimal spin-orbit coupling in graphene multilayers that yields a Berry curvature which has the same sign for opposite spins. We note that evidence for spin polarized half-metallic states have also been reported more generally in rhombohedral stacked few-layer graphene 27.It is interesting to compare BTG AHE discussed here with other AHE found in graphene stacks, e.g., in rhombohedral trilayer graphene4 where both spin- and valley- symmetry are broken. In sharp contrast, in BTG, we find that AHE occurs in the VM region where the frequency of quantum oscillations are consistent with a half metal while we found no signatures of a quarter metallic state. This indicates the valley polarized half metal origin of BTG AHE.Similarly, in twisted bilayer graphene heterostructures, AHE occurs near 3/4 filling of a moiré flat bandD field tuned van Hove singularities, which are naturally found in gated (even) layers of Bernal-stacked graphene. As a result, we anticipate that AHE and spontaneously valley-polarized states are likely ubiquitous in gapped graphene multilayers. Therefore our results imply that the energetically stable form of graphene multilayers can be used to probe and exploit graphene based AHE beyond the metastable configurations of moiré superlattices4 and rhombohedral trilayer graphene6. Perhaps the most exciting aspect is the ability to sustain valley polarization at zero magnetic field. Requiring both inversion symmetry breaking (controlled by D field) as well as spontaneous time-reversal symmetry breaking, such valley polarization mirrors the requirements for magneto-chiral responses28, which has implications for valley-mediated intrinsic non-reciprocity. Further, since BTG ferromagnetism is turned on by D field, they are ideal candidates to be readily combined with layered (out-of-plane) ferroelectrics30 to achieve stack- engineered multiferroic functionality.The key ingredients of BTG AHE are valley contrasting Berry curvature and the high density of states close to Tetralayer graphene, graphite and hexagonal boron nitride (hBN) flakes are exfoliated on silicon dioxide/silicon wafers. The whole heterostructure was assembled using dry-transfer technique using a PC (polycarbonate) film/PDMS stamp. We first pick up the top graphite and subsequently top hBN, BTG, bottom hBN and bottom graphite and put the whole stack on another wafer. The hBN layers are not intentionally aligned with BTG sample. Graphite gates are employed to reduce disorder and ensure high quality devices. Finally the device was etched into a Hall bar and contacted using edge-contact method with Cr (5 nm)/Au (70 nm).RAH are limited by the electrical noise in data acquisition.Transport measurements were conducted in a variable-temperature He-3 or He-4 Probe in Oxford Teslatron system. The lowest temperature reached is ~300 mK. Standard lock-in technique is used to measure the four-probe voltages using SR830. Low pass filters are used to suppress low frequency noises. Constant current source is applied with ranges from 10 to 100 nA. Gate voltages are applied using Keithley 2400 and 6430. Error bars in determining ΔSupplementary InformationPeer Review File


Thetopography of such graphene has two important properties. First, itssurface is comprised of folds that have different orientations, andsecond, it has several multilayer-graphene regions distributed overthe monolayer-graphene surface. On such folded graphene features,we vacuum evaporated pentacene and observed three-dimensional islandswith an average height of ∼15 nm. They are oriented eitherparallel or perpendicular to the folds, and they are also predominantlyoriented along the symmetry axes of graphene. Orientation of pentaceneislands on graphene evaporated at room temperature has a wide distribution.On the contrary, most of the pentacene islands evaporated at 60 °Care oriented at 30° with respect to the fold direction. We observedthat the folds act as a potential barrier for the surface transportof pentacene molecules. In addition, we interpret the 3D growth ofpentacene islands on graphene in terms of reduced polar componentsof the surface energy on graphene investigated with contact anglemeasurements.Using atomic force microscopy, we probed the growth ofpentacenemolecules on graphene that was fabricated by chemical vapor depositionand transferred onto 300 nm-thick SiO I2 Most often, the organicmolecules are attached to the graphene surface by the π–πinteraction, which leads to a non-negligible charge transfer at thegraphene/OS interface. The density and areal configuration of transferredcharges is crucially dependent on the molecular arrangement on thegraphene’s surface, i.e., order/disorder is one of the principalparameters characterizing the graphene/OS interface. Therefore, itis crucial to understand the adsorption of organic molecules on graphene.Pentaceneis one of the organic molecules that forms well-ordered polycrystallinelayers with a relatively high charge carrier mobility on a varietyof substrates. This stimulated a considerable interest in pentacene-basedorganic thin-film transistors.3 However, the growth of pentaceneis highly influenced by the topography of graphene and the substrateunderneath.6The principal drawbackof graphene, precluding it from being usedin a widespread manner as a staple material in electronic switchingdevices, is the absence of the electronic energy band gap and negligibledensity of states at the principal points of the Brillouin zone. Anincrease in the on/off ratio of graphene-based field-effect transistorscould in principle be achieved by integration of organic semiconductor(OS) molecules with graphene.7 Third, trapped molecules (hydrogen and hydrocarbon)between graphene and copper during the CVD growth process can be anotherreason for such unusual fold formations on graphene.8 The 6-fold symmetry of the graphene unit cell is brokenat folds, giving rise to the triangular pattern of atoms, while flatgraphene exhibits a honeycomb lattice.9 In a triangular pattern, out of six carbon atoms, only three areobserved in honeycomb rings. Nirmalraj et al.10 have highlighted the influence of folds on the growth of pentaceneand reported a large molecular energy gap for pentacene depositedon high folds (1–3 nm thick) as compared to flat graphene.They measured a large conductance gap of pentacene molecules depositedon such folds (3.0 ± 0.3 nm) in comparison to that on flat grapheneregions (1.3 ± 0.2 nm). This is due to the increased spatialdistance at folds that leads to significant screening from the underlyingCu substrate. When pentacene is deposited at such regions of surfaceexhibiting increased local curvatures (folds), it may acquire a differentgrowth configuration relative to the flat graphene. Moreover, thegrowth mode may depend on the orientation of folds itself with respectto the flat regions.While mechanically exfoliated graphene presentsitself as mostlydefect-free, the tens-of-micrometer-size flakes preclude its use ina large-scale production setting. Chemical vapor deposition (CVD)instead yields large-scale graphene foils, which are frequently plaguedby folds. There are several factors that contribute to the formationof folds. First, differences in thermal expansion coefficients ofcopper (or nickel) foil used as a template for graphene growth inducestrain in graphene layers. Second, folds may also emerge during thetransfer process of graphene to the desired substrate and may exhibitchemical reactivity superior to flat, defect-free graphene.2 and graphene/sapphireby performing contact-angle measurements.In light of the importance of the initialstages of growth of OSson graphene on the device performance, we elected to explore the growthdynamics of pentacene at low coverage on graphene. Graphene in ourstudies was composed of a high areal density of folds. It was mostlyof a single-layer type with a sizable fraction of a multiple-layertype. Our experimental results confirm that pentacene islands canbe aligned in the direction perpendicular to the fold regions andhighly depend on the characteristics of the folds. We show here thatfolds not only attract pentacene molecules but also determine thein-plane direction of the island nucleation and subsequential growth.We also observe that pentacene growth on folds is not hindered bythe poly(methyl methacrylate) (PMMA) residues on graphene in the submonolayerregime. Finally, we examined the cause of the Volmer–Webergrowth mode of pentacene layers by extracting the surface energy components(polar and nonpolar) of graphene/SiOII2 layers thermallygrown on Si(001) substrates using a wet chemical route.12 A 300 nm-thick layer of PMMA dissolved in chlorobenzene was dropcast onto graphene at 60 °C. After Cu foil was etched in ironchloride solution, the PMMA/graphene sample was left to dry and transferredonto the SiO2 substrate at 90 °C. Prior to transfer,the SiO2 substrates were immersed for 10 min in a sequenceof baths comprising acetone, isopropanol, and deionized water. Theywere further cleaned in piranha solution for 10 min. PMMA from graphenewas cleaned, as described in ref , Veeco CP-II operating in a non-contact mode using Si tips mountedon cantilevers with a resonance frequency of 325 kHz and a springconstant of 40 N/m. The relative humidity level was kept in the rangeof 25–30% during the AFM measurements. Gwyddion software wasused to perform the statistical analysis of the pentacene islands.Commerciallyavailable CVD graphene on Cu foil (from Graphenea)was transferred onto 300 nm-thick SiOSelected samples were examined by contact-angle measurements toassess the role of the substrate on the surface energy of graphene.It was determined by measuring the contact angle of deionized water,diiodomethane, and ethylene glycol. A μm-sized liquid drop wasreleased from a microsyringe onto the substrate surface. Images ofa liquid drop were obtained using a charge-coupled device camera.The liquid contact angle was determined by measuring the angle betweenthe tangent line and the droplet–substrate interface line.III2 recorded by AFM. The scan size is 2 × 2 μm2, and the bright tones represent the higher elevations; theheight scale (black-to-white) is 20 nm. Apart from atomically smoothregions, we see a network of bright lines of different widths andheights. These features are folds in graphene. The position of suchfolds is highly influenced by the topography of the underlying Cusubstrate that is preserved after transferring to the SiO2/Si substrate.13 Folds have variation in the heightand width. The mean height of the folds is in the range between 0.4and 2.5 nm, and their width was found to vary in the range between50 and 310 nm. So the fold height ranges from a simple wrinkle (0.4nm) to a multilayer graphene (MLG) (1.3 nm). Such a variance in morphologyis expected and is widely described in previous reports.15 Kim et al.15 performed electron diffraction upon scanning an electronbeam across the folds. Inside the folds, the intensity of the diffractionspot was twice as compared to the regions outside the fold. For example,in a trilayer fold, they observed a set of three hexagonal diffractionspots, and the total intensity of the diffraction spots was threetimes larger than that of a single spot diffracted off a flat monolayer.Another set of hexagonal spots appears from the middle layer of thefold. The intensity inside the folded region is twice that of monolayergraphene, and the spot shows splitting of two distinct diffractionspots with a small angle rotation of 0.4°. A simple folded geometryis the most frequent and results in moderately higher and wider featuresunder the AFM tip. If the amount of the folded graphene exceeds acritical value, such folds collapse and form bilayer or even trilayerfolds, which appear lower than simple folds but wider. Such a variancein the geometry of the folds is exemplified in 15 The interlayer interactions in folds and strain as a resultof the bending of the fold play crucial roles in the final shape ofthe fold. The height profiles 2 and 3 represent the fold along theline profiles 2 and 3, respectively. They are taller and narroweras compared to the fold on line profile 1. Such folds have a rangeof heights varying from 1 to 2.5 nm and widths from 60 to 90 nm. Suchhigher and narrower features are intermediate “standing collapsedwrinkle”.13 Their height is largeenough, so they collapse but remain standing below a certain heightof 8 nm based on energy calculations of interlayer interactions andelastic bending.2. Pentacene was depositedat a substrate temperature of 25 °C. Pentacene forms three-dimensional(3D) islands, which are represented in the image as white irregularlyshaped features and are located near or on the folds. From this, wespeculate that the onset of island nucleation occurs at or in closeproximity of the folds. Islands grow in close proximity to the folds.These islands are of various shapes from circular to elongated rod-likestructures. Some of them extend from the top of the fold away fromit, while some of them tend to align along the fold lines. Similarresults have been confirmed with other molecules also where foldsserve as a template for the growth of organic molecules.16 Khokhar et al.17 reportedthat folds play a critical role for the growth of 4,4′-bipheneyldicarboxylicacid (BDA) on graphene grown on Ir(111). Essentially, folds representpreferential sites for the nucleation of organic molecules. The localvariations in strain on or near the folds in graphene reduce surfacemobility of the molecules as compared to defect-free regions.18 Therefore, folds effectively immobilize theincoming molecules, thereby increasing the probability for islandnucleation and finally leading to a 3D island growth.Onto the graphene surface exemplified in 11–2, and the mean island area is 0.15 μm2. On the other hand, the island density on MLG is 22.5 μm–2, and the mean island area is 0.002 μm2. This indicates that indeed there is a strong drive for the pentacenemolecules to preferentially form islands. Such a 3D island formationcorresponds to the Volmer–Weber growth mode on graphene.In addition to the folds on graphene in 2 high-resolution AFM image of pentaceneislands on smaller folds, which are ubiquitous on graphene. The depositionconditions were kept the same as in 6 and was interpreted in terms ofthe reduced influence of the interface dipole field on the surfaceenergy of graphene, thereby lowering the molecular surface mobilityon MLG. This results in smaller islands on this region of graphene.We also compared the height distribution of pentacene islands onSLG and MLG regions, as shown in 2 AFM topography map of pentacene layer grown on graphene atsubstrate temperature Ts = 60 °C.The surface coverage is 19.5%. We see that the layer morphology ischaracterized by elongated islands (bright features) aligned in closeproximity to the fold lines. The island density is 22.8 ± 1.8μm–2, while the mean island area is 0.008μm2. The mean value of the length of the longer islandaxis is 84 ± 5 nm, and the average island height is 15 ±2 nm. The surface mobility of the molecules increases at a highersubstrate temperature during growth. On the other hand, a confiningeffect of the folds is still present and appears to be dominating,causing the molecules to attach to those already immobilized nearthe folds. This leads to the formation of elongated islands. The interactionof the molecules with graphene contained in folds is so strong thatit precludes the lateral growth of the islands, thereby favoring a3D island formation. Interestingly, this effect is much less evidenton the MLG regions .29 Such high values cannot be reconciled by dispersion forces. It isalmost three times higher than that at the interface of SiO2 and graphene (0.45 J/m2).30 We consider that our samples of graphene/sapphire have a slightlyhigher interfacial energy compared to that on graphene/SiO2.31 have examined the morphology of submonolayerthick pentacene films on polymeric substrates with a lower polar contributionto the surface energy, and pentacene islands appeared more compactand exhibited a strong tendency toward the 3D growth. On stronglypolar substrates such as SiO2, pentacene nucleates in alayer-by-layer mode, forming strongly dendritic islands.33So, far we have analyzed the role of folds on the pentacenemorphology.Folds dictate the arrangements of these 3D islands on graphene. Furthermore,we can relate the 3D formation of islands to the surface energy ofthe substrate. The shape and size of islands may vary depending onthe surface energy of different substrates. Rebernik Ribic et al.34 This approach considers thatthe free surface energy can be expressed as a sum of polar and dispersivesurface energy. The polar component of the surface energy resultsfrom the electrostatic intermolecular forces between permanent dipolesand multipoles of molecules and surfaces. In contrast, the dispersiveenergy is determined by the electrostatic interaction between fluctuatinginduced dipoles and multipoles. By measuring the contact angle ofdeionized water, diiodomethane, and ethylene glycol and using thevalues for the polar and the dispersive energy of the liquids fromrefs . We can conclude the following: (1)Graphene is nonpolar and the pentacene–graphene interactionis lower on a nonpolar surface in comparison to a polar surface. Graphenetransferred onto SiO2 and sapphire has a similar but verylow polar component of surface energy. The 3D island formation isa result of the weak molecule–substrate interaction. (2) Dueto the lower surface energy of graphene on sapphire (36.77 ±6.40 mJ/m2) as compared to graphene on SiO2 (42.38± 8.00 mJ/m2), we observe smaller islands on the formersurface; as a consequence of the reduced surface energy, the surfacediffusivity of pentacene molecules is lower on graphene on sapphirethan on graphene on SiO2.We have, therefore, examined the graphene surface from thestandpointof polar contribution to the surface energy and employed the Owens–Wendtapproach.romrefs , we have2) of surface energy could be a possible cause of the 3D growthof islands. The lower surface energy of graphene/sapphire (36.77 mJ/m2) results in smaller islands as compared to those on graphene/SiO2 (42.38 mJ/m2).Our systematic AFM study of the initialstages of the growth ofpentacene on CVD graphene reveals the 3D growth mode on the foldedregions of graphene and on the MLG patches. Pentacene islands cangrow either parallel or perpendicular to the folds. At room temperature,most of the islands have no preferred orientation. But evaporatingat 60 °C, the majority of them are aligned with 90 ± 5°,i.e., horizontally in the AFM image. In addition, we observe thatthe growth mode is also not influenced by the change of the substrateafter CVD graphene was transferred to sapphire. We obtained a similar3D growth on sapphire substrates confining most of the islands onfolds. We interpret the nucleation of pentacene on folds as a resultof the reduced mobility of pentacene due to the local strain presentat the folds. Also, with contact angle measurements on graphene, alow polar contribution (3.64 ± 1.70 and 2.83 ± 2.00 mJ/m