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Tatsuki Tsugawa, [Kazuto Hatakeyama](https://orcid.org/0000-0002-3369-3371), Junya Kawasaki, [Agamoni Pathak](https://orcid.org/0009-0001-5088-5973), [Kazuhito Tsukagoshi](https://orcid.org/0000-0001-9710-2692), Shintaro Ida, [Takaaki Taniguchi](https://orcid.org/0000-0002-8460-5431)

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[Electrochemically Driven Tandem In‐Plane Reduction and FeCl<sub>3</sub>‐ Intercalation of Highly Crystalline Graphene Oxide Thin Films](https://mdr.nims.go.jp/datasets/fe194360-26b5-435f-b87b-e01339625457)

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Electrochemically Driven Tandem In‐Plane Reduction and FeCl3‐ Intercalation of Highly Crystalline Graphene Oxide Thin FilmsRESEARCH ARTICLEwww.afm-journal.deElectrochemically Driven Tandem In-Plane Reduction andFeCl3- Intercalation of Highly Crystalline Graphene OxideThin FilmsTatsuki Tsugawa, Kazuto Hatakeyama,* Junya Kawasaki, Agamoni Pathak,Kazuhito Tsukagoshi, Shintaro Ida,* and Takaaki Taniguchi*The current design and understanding of graphene oxide (GO) and reducedGO (rGO) materials rely largely on insights derived from structurally defectiveand chemically inhomogeneous GO synthesized by Hummers’ method. As aresult, this early-stage knowledge fails to fully exploit the potential benefits ofGO. This study explores electrochemical approaches using highly crystallineBrodie’s GO for fabricating reduced GO (rGO)-based transparent conductivefilms (TCFs) without using toxic reducing agents or heat treatment.The study first demonstrates an in-plane electrochemical reduction methodfor fabricating rGO nanocoatings on plastic substrates. During this process,rGO at the rGO/GO/electrolyte three-phase interface functions as a growingcathode, inducing tandem reduction parallel to the insulating substrate surface.The crystallinity of the GO precursor determines the activation or deactivationof the three-phase interface at nanometer-scale thickness. The electrochemicalreduction of epoxide groups leads to a well-extended 𝝅-electron networkwith suppressed local strains and carbon vacancies. The sheet resistivity ofrGO-based TCFs is further optimized through hole doping via electrochemicalFeCl3 intercalation, breaking the transparency–resistivity limit of availablerGO-TCFs. These discoveries will facilitate the development of next-generationgreen graphene processes based on precise synthetic chemistry.T. Tsugawa, J. KawasakiGraduate School of Science and TechnologyKumamoto UniversityKumamoto 850-8555, JapanK. Hatakeyama, S. IdaInstitute of Industrial Nanomaterials (IINa)Kumamoto UniversityKumamoto 860-8555, JapanE-mail: hatakeyama-k@kumamoto-u.ac.jp; ida-s@kumamoto-u.ac.jpA. Pathak, K. Tsukagoshi, T. TaniguchiResearchCenter forMaterialsNanoarchitectonics (MANA)National Institute forMaterials Science (NIMS)Tsukuba 305-0044, JapanE-mail: taniguchi.takaaki@nims.go.jpThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adfm.202510430© 2025 The Author(s). Advanced Functional Materials published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution License, which permits use, distributionand reproduction in any medium, provided the original work is properlycited.DOI: 10.1002/adfm.2025104301. IntroductionGraphene has remarkable structural flexi-bility, good chemical stability, high electri-cal and thermal conductivity, and uniformoptical transmission from visible to near-infrared wavelengths.[1–5] These character-istics provide opportunities to develop ul-trathin devices in a wide range of techni-cal fields, from nanoelectronics to indus-trial coatings. Chemical vapor deposition(CVD) has proven most effective for pro-ducing graphene thin films, allowing pre-cise control over domain size, layer num-bers, and carrier transport properties.[6–10]However, CVD presents considerable draw-backs, including high costs, substantialenergy consumption, and process toxic-ity. Moreover, the complex transfer pro-cess required to move graphene layers fromCu or Ni growth substrates to target sub-strates presents further challenges, limit-ing widespread practical applications.[11,12]The reduction of graphene oxide (GO),synthesizable in large quantities from nat-ural graphite, represents a viable approachfor industrial graphene production. Since Ruoff et al. demon-strated the chemical reduction of GO using hydrazine,[13] numer-ous reduction techniques have emerged,[14,15] leveraging chemi-cal and thermal driving forces both independently and in com-bination. These developments have also simultaneously high-lighted specific critical technical challenges in GO-to-grapheneconversion.[16,17] First, although chemical reduction initiatesat a low temperature, conventional reducing agents such asNaBH4,[18] hydriodic acid,[19] and liquid ammonia[20] exhibithigh toxicity and reactivity.[21,22] Moreover, the limited deoxy-genation capability of chemical reduction necessitates addi-tional heat treatment during or after the reaction to achievehigh conductivity.[23] This thermal deoxygenation, however, gen-erates CO and CO2 gases, creating carbon vacancies that re-duce carrier mobility.[24] High-temperature reduction at leastabove 1000 °C is required to repair the atomic vacancies.[13] Ad-dressing these limitations often necessitates the use of sophis-ticated, expensive equipment like laser and microwave irradi-ation systems,[25–30] which negate the industrial advantages ofGO processing. Another significant challenge arises from theAdv. Funct. Mater. 2025, e10430 e10430 (1 of 12) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbHhttp://www.afm-journal.demailto:hatakeyama-k@kumamoto-u.ac.jpmailto:ida-s@kumamoto-u.ac.jpmailto:taniguchi.takaaki@nims.go.jphttps://doi.org/10.1002/adfm.202510430http://creativecommons.org/licenses/by/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadfm.202510430&domain=pdf&date_stamp=2025-07-25www.advancedsciencenews.com www.afm-journal.depoor crystallinity of pristine GO, particularly when synthesizedusing the Hummers’ method—the most widely adopted GOsynthesis technique.[31,32] This method introduces substantialstructural and compositional disorders in the two-dimensionalGO lattice,[31] degrading the carrier transport properties of indi-vidual reduced GO (rGO) nanosheets. Therefore, although GO-related research has increasingly shifted toward practical applica-tions, we still need a deeper insight into the chemistry of GO, in-cluding oxidationmechanisms[33–35] and chemical reactivity[36–41]in order to pursue the green production of graphene-based func-tional nanomaterials.Recently, we demonstrated an electrochemical reductionmechanism for GO proceeding through the following two reac-tions in an aqueous solution at room temperature, as evidencedby the monitoring of functional groups and carbon radicals dur-ing repeated electrochemical oxidation–reduction cycles.[42]C−O−C + 2e− + 2H+ → C=C +H2O (1)C−O−C + 2e− + 2H+ → C−OH + C−H (2)Notably, this mechanism enables reduction without generat-ing carbon vacancies. Furthermore, in-plane X-ray diffractionanalysis revealed that GO synthesized using Brodie’s method(B-GO) exhibits superior uniformity in chemical compositionand higher crystallinity compared to GO synthesized usingHummers’ method (H-GO),[31] establishing B-GO as a preferredprecursor for low-temperature thermal reduction at 300 °C.In addition, individual B-GO nanosheets are virtually free ofnanopores normally found in H-GO nanosheets,[43] as recentlydemonstrated by vertical proton blocking properties of B-GOmembranes.[44] Therefore, although both H-GO and B-GO arecommonly referred to as GO, they should be recognized as dis-tinct materials due to their largely different crystallinities andphysical properties. Building on these findings, the present studyaims to explore a greener electrochemical reduction process forrGO-based transparent conductive films (TCFs) without toxicchemical additives or heat treatment. The results demonstratesuccessful fabrication of rGO-based TCFs directly on insulatingsubstrates through tandem in-plane electrochemical reduction ata three-phase interface comprising growing rGO (cathode), unre-duced GO (reactant), and an electrolyte. The efficacy of this pro-cess depends on the use of B-GO; substitution withH-GO signifi-cantly inhibits the tandem reduction at the three-phase interface,resulting in poorly conductive rGO thin films because of defec-tive rGO cathode formation.Further, intercalation chemistry is utilized to reduce the re-sistivity of rGO thin films. The interlayer spacing of graphiteallows the intercalation of various molecules and ions, form-ing graphite intercalation compounds with distinct electrical andmagnetic properties compared to pristine graphite. Specifically,the intercalation of metal chlorides, such as FeCl3, AuCl3, andMoCl5, has been demonstrated to effectively modulate the elec-trical properties of bilayer and few-layer graphene.[45–47] How-ever, chloride intercalation in rGO layers stacked without specificin-plane crystallographic orientation remains unexplored. Thisstudy demonstrates conductivity enhancement through hole dop-ing via room-temperature electrochemical FeCl3 intercalation,establishing an interface chemistry approach for refining GO-based nanomaterials. Additionally, optimization of GO film qual-ity through spin coating results in further resistivity reduc-tion at transparency levels exceeding 90%, yielding figures ofmerit that surpass corresponding values for previously reportedrGO-based TCFs. The environmentally friendly electrochemi-cal approach and deepened GO chemistry offer potential forfunctionalizing and industrializing diverse GO and rGO-basednanomaterials.2. Results and Discussion2.1. In-plane Electrochemical Reduction of GO Thin FilmsFigure 1a,b illustrates the electrochemical reduction mecha-nisms of GO films on conductive and insulating substrates,respectively. On conductive substrates, electrons transfer fromthe substrate surface to the adjacent GO layer. The elec-trolyte diffused into the interfaces between GO layers pro-vides protons for electrochemical reduction Equations (1) and(2). The freshly formed rGO layer functions as a cathode atthe rGO/GO/electrolyte three-phase interface, promoting reduc-tion perpendicular to the surface—a process designated as thethrough-plane reduction mode. Meanwhile, on insulating sub-strates, a conductive material in contact with part of the GO filmacts as a cathode and initiates reduction. Similar to the conduc-tive substrate case, the three-phase interface sequentially devel-ops under applied bias; however, the rGO region expands parallelto the substrate surface, a process termed as the in-plane reduc-tionmode. Among the numerous studies on GO electrochemicalreduction, to our knowledge, only two have investigated in-planereduction. The first study demonstrated effective in-plane reduc-tion in 7-μm-thick H-GO films, achieving a conductivity of ≈8.50× 103 S m−1.[48] Meanwhile, the second study showed that the re-duction of nanoscale-thickness H-GO thin films resulted in elec-trical resistivities as high as 1‒3 kΩ m−1, 108 times higher thanthat of commercial ITO substrates.[49] This observation indicatesa nanoscale effect inhibiting in-plane reduction in thin films. Wehypothesize that the nanoscale thickness of the rGO cathode po-tentially limits electron supply to reduction sites, resulting in in-complete reduction. If this is true, an effective solution wouldinvolve substituting conventional H-GO with highly crystallineB-GO, as the enhanced electrical conductivity of reduced B-GOmay facilitate nanoscale cathode activation.The atomic forcemicroscopy (AFM) image in Figure 1c revealsthat the monolayer B-GO nanosheets used in this study have anaverage thickness of ≈1 nm and a lateral sheet size of ≈ 2–5 μm(Figure S1, Supporting Information). These nanosheets weredeposited on polyethylene terephthalate (PET) substrates usinglayer-by-layer (LbL) assembly to fabricate GO thin films. Theprocess began with surface modification of the substrate by de-positing a polycationic layer of poly(diallyldimethylammoniumchloride) (PDDA) to impart positive charge, followed by its im-mersion in a B-GO dispersion to allow negatively charged B-GO nanosheets to attach electrostatically. Figure 1d illustratesthe reduction of a five-cycle LbL-processed B-GO film at −1.7 V(vs Ag/AgCl) in a 0.005 M H2SO4 electrolyte. The reductionprocess initiated at the glassy carbon (GC)/GO/electrolyte in-terface, with the darker-colored rGO region expanding laterallyover time. Prolonging the reduction time by 2400 s resulted inAdv. Funct. Mater. 2025, e10430 e10430 (2 of 12) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202510430 by National Institute For, Wiley Online Library on [30/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 1. Schematics illustrating (a) the through-plane electrochemical reduction of a GO film on a conducting substrate and (b) the in-plane electro-chemical reduction of a GO film on an insulating substrate. c) AFM image of B-GO nanosheets. d) Snapshots of the electrochemical in-plane reduction ofa five-cycle LbL-deposited B-GO film on a PET substrate. The corresponding movie is available in Figure S1 (Supporting Information). e) Cross-sectionalTEM image of a five-cycle LbL-deposited rGO film. For this observation, an SiO2/Si substrate was used to facilitate specimen fabrication via focused ionbeam processing. rGO films formed (f) flexible and (g) spring-shaped PET substrates. The diameter of the spring is 1 cm.the resistivity to 1.6 kΩ sq−1. The resistivity of the electrochem-ically reduced B-GO film surpassed that of a B-GO film pre-pared under the same LbL deposition conditions but subjected tohigh-temperature thermal reduction at 700 °C under an H2/Argas flow (2 kΩ sq−1).[31] Electrochemical reduction therefore rep-resents an effective process for converting GO to highly conduc-tive rGO at room temperature without using hazardous chem-icals or forming byproducts. Cross-sectional transmission elec-tron microscopy (TEM) observation revealed an ≈15-nm-thickrGO film under the current deposition and reduction condi-tions (Figure 1e). An rGO coating on a flexible PET substrate,prepared under the same deposition and reduction conditions,formed a bendable conducting sheet (Figure 1f). Furthermore,the proposed electrochemical reduction process enables conduc-tive coating on complex-shaped substrates—a feature difficult toachieve with conventional CVD graphene processes. For exam-ple, GO deposited on a plastic spring underwent uniform reduc-tion to form a continuous rGO layer (Figure 1g), enabling LEDillumination upon bias voltage application to the spring ends(Figure S2, Supporting Information).As a control experiment, electrochemical reduction was per-formed using H-GO, instead of B-GO. However, the reduc-tion of the H-GO film remained localized around the GC con-tact area throughout the reaction duration (2400 s) (Figure S3,Supporting Information). Furthermore, the resistivity of the re-duced area ranged from 59–145 MΩ sq−1, ≈108 times higherthan that of the reference ITO substrate (10 Ω sq−1). Notably,five-cycle LbL deposition of B-GO and H-GO should yield sim-ilar film thicknesses.[31] Therefore, differences in thickness canbe ruled out as a factor contributing to deactivation of therGO/GO/electrolyte interface. In our previous study, we demon-strated that the in-plane proton conductivity of pristineH-GO (2.9× 10−3 S cm−1 at 100% RH) is an order of magnitude higher thanthat of pristine B-GO.[44] Therefore, the supply of H+ across thetrilayer interface is unlikely to be the rate-limiting factor in thestrongly suppressed reduction of H-GO. In addition, we foundthat B-GO could achieve a low resistivity of 4 kΩsq−1 throughthermal reduction at 300 °C, whereas H-GO reduced under thesame conditions exhibited a ≈20-fold higher resistivity.[31] Thissignificant resistivity difference suggests that a higher activationenergy is required either to mitigrate structural disorder in H-GO and to promote the proposed in-plane reduction reactions.Consequently, the three-phase interface could deactivate at de-fective 1D rGO/GO boundaries. These findings demonstrate thatthe use of B-GO or highly crystalline GO is essential for fabricat-ing rGO-based TCFs via electrochemical reduction, as their crys-tallinity profoundly influences not only physical properties butalso chemical reactivity.2.2. Characterization of rGO-Based TCFs Obtained viaElectrochemical ReductionX-ray photoelectron spectroscopy (XPS) was performed to inves-tigate the carbon bonding states before and after electrochemi-cal reduction at −1.7 V (vs Ag/AgCl) in a 0.005 M H2SO4 elec-trolyte. As illustrated in the C 1s XPS spectra in Figure 2a,pristine GO displayed two primary peaks corresponding tocarbon–carbon (C─C) and epoxy (C─O─C) bonds at 285 and287.1 eV, respectively.[44,50] Upon reduction, the epoxy peak nearlyAdv. Funct. Mater. 2025, e10430 e10430 (3 of 12) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202510430 by National Institute For, Wiley Online Library on [30/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 2. a) C 1s XPS spectra and b) Raman spectra of pristine and electrochemically reduced B-GO films deposited via five-cycle LbL assembly onquartz glass. c,d) In situ detection of CO2 during thermal, photoinduced, and electrochemical reduction, presented with different Y-axis scales. d) Insitu detection of CO2 evolved during electrochemical reduction followed by photoinduced reduction.disappeared, while the C─C peak intensified owing to the forma-tion of C═C bonds, as described by Equation (1). Additionally,the relative intensity of the peak corresponding to the C─OHbond increased compared to that of the C─C peak, suggest-ing that Equation (2), which involves the formation of C─OHbonds (286.4 eV), possibly accompanied by C─H bond forma-tion, should be considered. Figure 2b presents the Raman spec-tra of these samples. Before reduction, broad D and G bands withhalf-maximum full-width (HMFWH) values of 114 and 122 cm−1were detected at 1332 and 1588 cm−1, respectively. After reduc-tion, both peaks narrowed to FWHM values of 32 and 52 cm−1,respectively, while the strong D band was retained. The thermalAdv. Funct. Mater. 2025, e10430 e10430 (4 of 12) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202510430 by National Institute For, Wiley Online Library on [30/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 3. a) In-plane electrochemical reduction time and resulting sheet resistivity of 1 cm × 2.5 cm in size B-GO films deposited via five-cycle LbLassembly on a PET substrate, plotted as functions of (a) pH and (b) applied potential. c) Raman spectra of a five-cycle LbL-deposited rGO film electro-chemically reduced in 0.005 m H2SO4/water and 1 m LiClO4/acetonitrile electrolytes. The spectra measured on three different positions are displayedfor each sample. These samples were deposited on quartz substrate.reduction of B-GO by 700 °C in our previous study largely pre-served the width of the G and D bands,[31] highlighting a struc-tural distinction between thermally and electrochemically re-duced B-GO.To acquire reliable insights from the Raman spectra, thegases produced during thermal, photo-, and electrochemical re-duction were analyzed using gas chromatography. As depictedin Figure 2c,d, CO2 was detected immediately after the startof heat treatment to 300 °C for thermal reduction, indicatingthat oxygen atoms in the GO lattice were removed along withcarbon atoms as CO2. While photoreduction also generatedCO2, the detected amount was approximately 50 times lowerthan that observed during thermal reduction. In contrast, theamount of CO2 evolved during electrochemical reduction wasclose to the detection limit. However, CO2 was detected imme-diately after switching the reduction method from electrochem-ical to photoreduction (Figure 2e), confirming that CO2 gen-erated in the electrolyte was detectable using the experimen-tal setup. These results suggest that electrochemical GO reduc-tion proceeds with minimal CO2 formation and carbon vacancygeneration.Based on the presence and absence of carbon vacancy for-mation in the thermal and electrochemical reduction of B-GO, respectively, as revealed by the analysis of evolved gas,the Raman spectra can be interpreted as follows. First, theD band in pristine B-GO is activated owing to phonon con-finement by abundant epoxy groups. The epoxy-bridged C─Cbond expands, inducing local lattice distortion, which broad-ens the G and D bands. The persistence of a strong D bandafter electrochemical reduction suggests phonon confinementby C─OH and C─H bonds, mirroring the effect of epoxidegroups. However, these bonds create less pronounced lattice dis-tortion in the graphene framework compared to the originalepoxy-modified B-GO structure, explaining the observed sharp-ening of the G and D bands. In contrast, the persistent broadG and D bands observed after the thermal reduction of B-GOindicate that the relaxation of local lattice distortion followingepoxide removal is counterbalanced by new distortions origi-nating from carbon vacancy formation.[31] This interpretationof Raman spectra provides broader insights into the structureand properties of GO-related materials beyond the character-ization of rGO obtained via the electrochemical reduction ofB-GO.2.3. Kinetic Control of Electrochemical Reduction ReactionsThe above characterizations indicate that electrochemical reduc-tion proceeds primarily through reactions (1) and (2), with reac-tion kinetics expected to depend on proton concentration. To ex-amine this dependence, we conducted electrochemical reductionexperiments in 0.05, 0.005, 0.0005, and 0.00005 M H2SO4 elec-trolytes at pH 1, 2, 3, and 4, respectively. The results confirmedthat reduction kinetics generally accelerated as pH dropped(Figure 3a), while observed vigorous H2 evolution was likely as-sociated with the slowed reduction rate at pH 1. Additionally, atpH 2, the reduction reaction was accelerated at a negative re-duction potential of −1.3 to −2.1 V (vs Ag/AgCl) (Figure 3b).These findings are generally consistent with those reported byQuezada-Renteria et al. for the pH- and bias-driven electrochem-ical through-plane reduction of H-GO.[51] However, no linear cor-relation was observed between the reduction rate and electri-cal resistance in our experiments, where resistances were ≈ 1.6kΩ sq−1. Thus, while lowering pHor applying amore negative po-tential accelerates the overall reaction rates of Equations (1) and(2), Equation (1), which extends the 𝜋-conjugated network, is notselectively promoted. As an alternative approach, electrochemicalreduction was performed in a nonaqueous electrolyte consistingof 1 m LiClO4 dissolved in acetonitrile. As a result, applying abias voltage of −4 V induced the formation of a three-phase in-terface, yielding an rGO film with improved resistivity of 0.712kΩ sq−1. In the Raman spectra (Figure 3c), the G and D bandsbecame sharper after reduction, mirroring the trend observed forrGO electrochemically reduced in an aqueous electrolyte. Simul-taneously, the relative intensity of the G band to the D band in-creased, suggesting that the observed resistivity decrease was as-sociated with themore extended sp2 carbon network owing to theenhanced reaction rate of (1).Adv. Funct. Mater. 2025, e10430 e10430 (5 of 12) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202510430 by National Institute For, Wiley Online Library on [30/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deBased on our current findings and previous study on the ther-mal reduction of B-GO and H-GO, we have clarified the criticalrole of in-plane crystallinity in electronic transport. First, ther-mally reduced B-GO exhibits higher conductivity than thermallyreduced H-GO due to its initially lower carbon vacancy density.Then, electrochemical reduction of B-GO in H2SO4 results inhigher conductivity than thermally reduced rGO, as carbon va-cancy formation during deoxygenation is suppressed. Electro-chemical reduction in non-aqueous electrolytes further enhancesconductivity by promoting the restoration of conjugated C═Cbonds and suppressing the formation of C─OH groups poten-tially acting as electron scatters. In addition, electrochemicallyreduced B-GO in the non-aqueous system clearly exhibits supe-rior conductivity compared to B-GO thermally reduced at 700 °C,where interlayer species could be thermally removed. This sug-gests that in-plane transport exerts a greater influence on theelectrical conductivity of the rGO films than interlayer transport.Note that in-plane reduction in the non-aqueous electrolyte wasslow, requiring ≈5000 s to fully reduce the GO layer depositedon a 1 cm × 2.5 cm quartz substrate. Additionally, PET was dis-solved in acetonitrile, rendering the non-aqueous approach un-suitable for fabricating rGO-based TCFs on plastic substrates.Therefore, we tentatively selected the aqueous electrolyte system.Subsequent experiments were conducted under pH 2 and a re-duction potential of −1.7 V as optimal conditions balancing re-duction rate with resistivity based on the experimental results.2.4. Electrochemical FeCl3 Intercalation into rGO-Based TCFsThe interfacial modification of thin films prepared by the deposi-tion of chemically exfoliated nanosheets has traditionally focusedon removing or reducing impurities between nanosheets.[52,53]Unlike such passive approaches, this study introduces an activeintercalation strategy to lower the resistivity of rGO-based TCFs.Among the various functional intercalants used for graphene lay-ers prepared via mechanical exfoliation and CVD growth, FeCl3,which induces p-type doping, is widely employed owing to itsavailability, stability, and ease of handling.[45,46] Although FeCl3intercalation has been previously achieved through thermal re-actions at 300‒360 °C, it can also be electrochemically interca-lated into bulk graphite at room temperature.[54] In this study,an electrochemical approach was adopted, as it enables the directfabrication of rGO-based TCFs on substrates with low heat resis-tance. The intercalation conditions namely the application of aconstant potential of+1.7 V (vs Ag/AgCl) for 400 s in a 9M FeCl3aqueous solution—were based on previously examined FeCl3 in-tercalation in bulk graphite. Consequently, the sheet resistivityof the rGO film obtained via five-cycle LbL deposition decreasedby ≈70% to 458.4 Ω sq−1. The rate of resistivity reduction wascomparable to that achieved via thermal FeCl3 intercalation inCVD-grown bilayer graphene films.[47] The sheet resistivity re-mained stable at this value for over six months under ambientconditions. In subsequent durability tests conducted at the ther-mal tolerance limit of PET (60 °C) under high humidity (90%RH), no significant degradation in sheet resistance (≈ 6%) wasobserved over a period of 240 h (Figure S4, Supporting Infor-mation), although the PET substrate exhibited slight bending.The robustness of FeCl3-intercalated rGO under these conditionsis in good agreement with the excellent environmental durabil-ity reported for thermally FeCl3-intercalated few-layer grapheneby Wehenkel et al.[55] Cross-sectional elemental mapping usingTEM‒energy-dispersive X-ray (EDX) spectroscopy confirmed theuniform distribution of Fe and Cl species throughout the rGOfilm (Figure 4a). Furthermore, high-resolution imaging revealedlattice fringes corresponding to the interlayer spacing of rGO,with no observable particulate impurities (Figure 4b). Electronenergy loss spectroscopy (EELS) on the film revealed Fe L2,3 edgesconsisting of the two peaks of Fe3+ species (Figure 4c).[56] Theseresults suggest that FeCl3 molecules not only adsorbed onto thesurface but also diffused into the layered structure. The C 1sXPS spectra showed two peaks from C─C and C─OH bonds(Figure 4d), excluding the electrochemical oxidation of rGO dur-ing the anodic intercalation reaction. The Fe 2p XPS spectra sup-ported a predominant trivalent iron state, where Cl/Fe and Fe/Celemental ratios were 2.08 and 0.026, respectively (Figure 4e).The Cl/Fe ratio of less than three suggests that a fraction of Fe3+ions formed compounds other than FeCl3. Moreover, the Fe/Cratio was lower than that obtained for electrochemically FeCl3-intercalated bulk graphite (0.05‒0.1). Accordingly, we suggestthat the disordered interlayer structure of rGO, characterized byrandom in-plane orientation and residual hydroxyl groups, actsas a steric barrier to FeCl3 insertion during electrochemical inter-calation. Raman spectroscopy showed a slight shift in the D andG band to higher frequencies, ≈ 5 cm−1, following electrochemi-cal FeCl3 intercalation (Figure 4f). This shift indicates that FeCl3intercalation induced hole doping,[46,57] contributing to the resis-tivity reduction of the rGO film. In the referenced study, electro-chemical FeCl3 intercalation in bulk graphite gradually reducedthe intensity of the original G band at≈1580 cm−1, while an addi-tional G band appeared at ≈1620 cm−1.[54] Only the latter band isobserved when the interlayer space is fully occupied with FeCl3.In our case, the 1620 cm−1 band was not detected, likely due tothe lower intercalation level, as confirmed by XPS analysis. Nev-ertheless, previous studies have shown that pristine graphenesubjected to thermal FeCl3 intercalation exhibited an efficient re-duction in sheet resistance even at low intercalation levels withG band shift of 10 cm−1,[58] while further doping did not leadto significant additional improvements. This behavior suggeststhat intercalants not only act as p-type dopants to increase carrierconcentration but also degrade carrier mobility due to enhancedscattering. Thereofore, even a modest degree of FeCl3 intercala-tion, as achieved in the present study, was sufficient to inducea reduction in sheet resistivity. Although challenges remain inprecisely controlling the location and density of the intercalatedspecies, these results demonstrate the successful intercalation ofmetal chlorides into rGO layers that are stacked without prefer-ential orientation to improve TCF performance.2.5. Transparency and Resistivity RelationshipTo examine the correlation between optical transmittance andelectrical resistivity, rGO films with and without FeCl3 interca-lation were additionally prepared using 1–4 cycles of LbL depo-sition. As illustrated in Figure 5a, the transparency of the filmsincreased progressively as the number of deposition cycles de-creased. The plot of optical transmittance at 550 nm (T550 (%))Adv. Funct. Mater. 2025, e10430 e10430 (6 of 12) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202510430 by National Institute For, Wiley Online Library on [30/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 4. a) Cross-sectional images with EDX elemental mapping, and b) high magnification image, and c) Fe L2,3-edge energy loss spectra of FeCl3-intercalated rGO. For these TEM analyses, GO thin film was deposited via five-cycle LbL assembly on a SiO2/Si substrate followed by electrochemicalreduction and FeCl3 intercalation. Os coating was performed to protect the FeCl3-rGO layer during the TEM specimen fabrication. d) C 1s and e) Fe2p3/2 XPS spectra and f) Raman spectra of rGO and FeCl3-intercalated rGO taken on three different positions for each sample. These samples wereprepared on quartz substrates.Figure 5. Sheet resistivity and optical transmittance at 550 nm (T550 (%)) plotted against a) LbL deposition cycles (1–5) and b) GO concentration usedin spin coating (0.5–4 g L−1). c) Reduction rates in resistivity due to FeCl3 intercalation and T550 (%) for rGO TCFs prepared via LbL deposition andspin coating under different deposition conditions.Adv. Funct. Mater. 2025, e10430 e10430 (7 of 12) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202510430 by National Institute For, Wiley Online Library on [30/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 6. Comparison of GO-TCFs prepared by previously reported chemical and thermal reduction methods with GO-TCFs synthesized via electro-chemical reduction and subsequent FeCl3 intercalation in this study. a) Sheet resistance versus T550 (%). b) Process temperature versus 𝜎DC/𝜎OP foreach method.versus the number of deposition cycles revealed a linear relation-ship, indicating that the average film thickness was controlledaccording to the LbL deposition principle. Assuming a linearrelationship between thickness and absorption, the estimatedthicknesses of rGO films obtained from one, two, three, andfour deposition cycles were 3, 7, 10, and 13 nm, based on themeasured thickness of 15 nm for the rGO film prepared via five-cycle LbL deposition. A linear relationship was also observed be-tween the number of deposition cycles and the resistivity of rGO-based TCFs fabricated using 2–5 LbL deposition cycles (1.6‒9.1kΩ sq−1). However, the film obtained after a single LbL deposi-tion cycle exhibited higher resistance (29.8 kΩ sq−1), deviatingfrom the linear trend. This deviation could be attributed to theincomplete coverage of the GO layer after the first LbL deposi-tion cycle (Figure S5a, Supporting Information), which also likelyassociated with the particularly slow reduction rate observed dur-ing sample fabrication. To improve the quality of thin films withtransmittances above 90%, a simple spin-coatingmethod was de-veloped. Similar to the LbL approach, the substrate surface wasinitially positively charged with PDDA modification. A GO dis-persion was then deposited onto the substrate and spin-coated at2,000 rpm. The rGO film formed following a single spin-coatingattempt using a 1 g L−1 GO dispersion exhibited a transmittanceof 94% at 550 nm, comparable to that of the film formed aftera single LbL deposition cycle (95%) (Figure 5b). Scanning elec-tron microscopy (SEM) observations revealed that the rGO filmprepared via spin coating achieved nearly complete surface cov-erage (Figure S5b, Supporting Information). The film transmit-tance could be tuned by adjusting the dispersion concentration:A higher concentration of 4 g L−1 resulted in lower transmit-tance (84.1%), while a lower concentration of 0.5 g L−1 led tohigher transmittance (96.4%). Notably, films with transmittanceexceeding 90% exhibited improved resistivity, which should beattributed to the nearly uniform coverage of GO nanosheets onthe substrate. The resistivity of the samples decreased by 60‒80%following FeCl3 intercalation. Meanwhile, the increase in trans-mittance at 550 nm was limited to ≈3% (Figure 5c). In general,the associated resistivity reduction rates were independent of therGO film preparation conditions, highlighting the need for im-proved control over the FeCl3 intercalation process. This opti-mization, along with a deeper investigation into the underlyingmechanism, remains a key focus of our future research on chlo-ride intercalation into rGO layers.A literature review revealed that rGO-based TCFs preparedvia electrochemical reduction and intercalation exhibit trans-parency/resistivity ratios superior to those achieved throughchemical reduction and high-temperature thermal reduction pro-cesses (Figure 6a; Table S1, Supporting Information).[2,18,59–82] Anestimated transmittance–resistivity boundary, derived from liter-ature references, suggests that in the high-transmittance range(>90%), previously reported processes would likely struggle toachieve resistivity values comparable to those obtained in thepresent study. Compared to the previous room temperature pro-cess based on NaBH4 reduction and subsequent Au3+ doping,[18]our electrochemical approach can produce much better TCFs.The relationship between processing temperatures and TCF per-formances was further discussed based on the figure of merit(𝜎DC/𝜎OP, with 𝜎DC and 𝜎OP denoting direct current con-ductivity and optical conductivity, respectively), calculated us-ing Equation (3), as a function of process temperature for boththe current study and previous methods (Table S1, SupportingInformation).𝜎DC𝜎OP=Z02R(T− 12 − 1) (3)In this equation, Z0 is the impedance of free space (377 Ω).[83]R is sheet resistance. T is transmittance of 550 nm. As shownin Figure 6b, the plots reveal that thermal reduction at ≈1000°C is required to achieve high 𝜎DC/𝜎OP values. Chemical re-duction still requires thermal activation at 60–100 °C. In con-trast, electrochemical reduction at room temperature generallyyields 𝜎DC/𝜎OP values comparable to those obtained throughthermally accelerated chemical reduction, with a few excep-tions. The plots also indicate that the electrochemical reduction–intercalation method achieves the highest 𝜎DC/𝜎OP value (3.5),underscoring the potential of the electrochemical route forserving as environmentally friendly processing techniques forrGO-based TCFs.Adv. Funct. Mater. 2025, e10430 e10430 (8 of 12) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202510430 by National Institute For, Wiley Online Library on [30/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFrom this perspective, numerous opportunities are availablefor improving rGO-based TCF quality. First, the electrochemi-cal reduction process is influenced by multiple factors, includingsolvent composition, electrolyte type and concentration, reduc-tion bias, reaction time, and temperature. Our results revealedthat using a non-aqueous electrolyte facilitates the formation ofan extended 𝜋-electron network. Further regulation of the aboveparameters may yield more conductive rGO nanosheets by re-fining the reduction process. Although this study primarily fo-cused on room-temperature processes, post-annealing at temper-atures up to 500 °C is feasible for coatings on glass substrates.Under these circumstances, a thermochemically driven dehydra-tion reaction between C─OH and C─H groups to promote C═Cbond formation while minimizing COx evolution may be worthconsidering. Additionally, the considered temperature range en-ables the thermal intercalation of various metal chlorides, sug-gesting that identifying an optimal intercalant and intercalationconditions for rGO layers could further improve 𝜎DC/𝜎OP inrGO-based TCFs. A systematic investigation of metal chlorideintercalation into rGO is also expected to clarify how structuralheterogeneities such as random stacking, carbon vacancies, andoxygen-containing functional groups influence intercalation ki-netics and 2D molecular configurations within nanoscale in-terlayer spaces. These insights may result in the emergence offunctionalities, including magnetic[84] and plasmonic[85] proper-ties, that differ from those induced by intercalation into idealgraphene layers.According to a recent review on state-of-the-art solution-processed TCFs for thin film solar cells,[86] a dip-coated carbonnanotube films exhibits a 𝜎DC/𝜎OP of ≈35,[87] which mayserve as a feasible next benchmark through the strategiesoutlined above. However, achieving 𝜎DC/𝜎OP values com-parable to those of TCFs based on CVD-grown graphene(≈100),[88] poly(3,4-ethylenedioxythiophene):polystyrene sul-fonate (PEDOT:PSS) (≈120),[89] or silver nanowires (≈340)[90]remains a significant challenge. Therefore, it is still prema-ture to position rGO thin films as candidates for high-endTCF applications. Nonetheless, their inherent advantagesmake them highly attractive to develop multifunctional thinfilm devices that require electromagnetic shielding, gas andmoisture barrier performance, thermal management, or me-chanical flexibility. The approach demonstrated in this study,which involves controlled reduction and active interlayermodification based on highly crystalline GO, is expected toprovide a breakthrough toward improving the performanceand applicability of rGO-based thin films beyond current lim-itations. Moreover, electrochemical reduction is applicablenot only to thin films but also to GO-based bulk materials,porous structures, and composite assemblies.[49] Thus, thefindings from this study, along with future research on pro-cess optimization and intercalation strategies, are expected tocontribute to the advancement of a broad range of rGO-basednanomaterials.Finally, from an industrial perspective, although the presentstudy demonstrates the feasibility of tandem in-plane electro-chemical reduction of GO and subsequent FeCl3 intercalationusing B-GO as a precursor, a more detailed mechanistic under-standing would greatly benefit from the development of a quan-titative kinetic model. Developing such a model would requiresystematic investigation of several key parameters that criticallyaffect the reduction dynamics, including the size and geometryof the conductive support used to initiate in-plane reduction re-actions, the configuration of the counter electrode, and the thick-ness of the GO film. Therefore, comprehensive future studieswill be essential to fully elucidate the kitetic mechanism. In ad-dtion, importantly, both the layer-by-layer (LbL) deposition andelectrochemical processes employed in this study are compat-ible with roll-to-roll manufacturing techniques,[91,92] offering apathway to cost-effective, large-scale production. Accordingly, theintegration of these processes into roll-to-roll platforms repre-sents a key objective toward the practical implementation of thistechnology.3. ConclusionThis study demonstrated the feasibility of electrochemical rGO-based TCF fabrication through the controlled oxidation, re-duction, and intercalation of GO. First, in-plane reduction ofnanoscale GO thin films at room temperature was achieved us-ing a highly crystalline B-GO precursor. The sharpening of theG and D bands in the Raman spectra after electrochemical re-duction indicated that the resulting C─OH and C─H bonds pre-vented the introduction of local strain into the 2D lattice. This,in turn, facilitated the formation of an extended 𝜋-electron net-work. Subsequently, the resistivity of rGO-based TCFs was fur-ther reduced through electrochemical FeCl3 intercalation, whichinduced hole doping. This finding demonstrates the potential ofchloride intercalation chemistry to tailor the functionality of awide range of rGO-based nanomaterials. Finally, the combinationof these electrochemical methods helps surpass previous trans-parency/resistivity limitations for rGO-based TCFs.4. Experimental SectionSynthesis of B-GO: B-GO was synthesized using a modified Brodie’smethod.[93] First, 1 g of graphite powder (FUJIFILM Wako Pure Chemi-cal Corporation) was dispersed in 40 mL of fuming nitric acid (FUJIFILMWako Pure Chemical Corporation) and stirred in an ice bath. Subsequently,8 g of potassium chlorate (FUJIFILM Wako Pure Chemical Corporation)was gradually added, and the mixture was stirred at room temperaturefor 1 h. The resulting oxidized graphite was washed with pure water, col-lected via centrifugation at 3,000 rpm (Kubota; 3700), and dried in an ovenat 50 °C. Next, 500 mg of oxidized graphite was dispersed in 500 mL ofan ammonia solution (FUJIFILM Wako Pure Chemical Corporation) ad-justed to pH 12.5. The dispersion was shaken for at least five days, fol-lowing which it was transferred into 50 mL glass containers and sub-jected to ultrasonic treatment for 30 min to exfoliate graphite oxide intographene oxide. Following centrifugation at 3,000 rpm, exfoliated flakeswere removed, and the supernatant was collected at 8,000 rpm.Monolayernanosheets were subsequently precipitated through high-speed centrifu-gation at 15,000 rpm. The precipitate was washed three times with purewater to obtain B-GO.Synthesis of H-GO: H-GO was synthesized using a modified Hum-mers’ method.[94] First, 2 g of graphite powder, 2 g of NaNO3 (FUJIFILMWako Pure Chemical Corporation), and 92 mL of concentrated sulfuricacid (FUJIFILM Wako Pure Chemical Corporation) were combined with10 g of potassium permanganate (FUJIFILM Wako Pure Chemical Cor-poration), and the mixture was stirred in an ice bath. Subsequently, purewater was added gradually, followed by the addition of 5 mL of a hydro-gen peroxide solution (FUJIFILMWako Pure Chemical Corporation). AfterAdv. Funct. Mater. 2025, e10430 e10430 (9 of 12) © 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202510430 by National Institute For, Wiley Online Library on [30/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.decentrifugation at 3,000 rpm and removal of the supernatant, the mixturewas washed with a 5%HCl solution and pure water. The resulting oxidizedgraphite was dried in an oven maintained at 50 °C. The dried material wasthen dispersed in pure water at a 1:1 ratio and subjected to sonication for2 h. The supernatant was collected by centrifugation at 8,000 rpm, whilethe precipitate was washed three times with pure water at 15,000 rpm toobtain H-GO.Preparation of GO Thin Films—LbL Method: The substrates weretreated with ultraviolet light and ozone (UV/ozone) prior to depositionto render their surface hydrophilic. First, the substrates were immersedin an aqueous solution of PDDA (100 g L−1, pH = 9.1; FUJIFILM WakoPure Chemical Corporation) for 5 min to create a positively charged sur-face with polycations. After rinsing with Milli-Q water and drying under anN2 gas stream, the modified substrates were dipped into diluted GO so-lutions (6% (v/v) for B-GO and 6% (v/v) for H-GO, pH = 9.1) for 5 min,followed by the same rinsing and drying procedure. This alternating im-mersion process was repeated to achieve thin films with a controlled GOcoating thickness.Spin-Coating Method: As in the LbL method, the substrate was hy-drophilized using ozone/UV treatment, immersed in the cationic solutionfor 5min, rinsed withMilli-Q water, and dried under anN2 gas stream. GOdispersions were then dropped onto the substrate to ensure complete cov-erage. Spin-coating was performed at 2,000 rpm for 12 s following a 5 minwaiting period. Finally, the spin-coated film was rinsed with Milli-Q waterand dried under an N2 gas stream.Electrochemical Reduction: GO thin films were electrochemically re-duced using a three-electrode system comprising an Ag/AgCl referenceelectrode, a GC counter electrode, and a GO working electrode. After elec-trochemical reduction under a constant bias voltage, the samples wererinsed with Milli-Q water and dried under an N2 gas stream.Characterization: The morphology of GO nanosheets was analyzedusing AFM (Nanocute; Hitachi High-Technologies Corporation). Elemen-tal analysis was conducted using XPS (K-Alpha; Thermo Fisher Scientific).The C 1s XPS spectrum was deconvolved into three peaks: carbon bonds,C─O─C, and C─OH at 285.0, 286.4, and 287.1 eV, respectively. Cross-sectional TEM images of GO films were acquired using a JEM-ARM200FNEOARM (JEOL) at an acceleration voltage of 60 kV. The sheet resis-tance of the samples was measured using a Keithley 2450 source mea-surement unit. Measurements were taken at five points per sample, andthe recorded values were averaged. The durability tests of FeCl3–rGO TFCswere performed using a temperature-controlled incubator(SH-221; ES-PEC). The transmittance spectra of rGO films were recorded using a Shi-madzu SolidSpec-3700DUV. Raman spectra of the GO and rGO films wereobtained using a laser confocal microscope (Ramanmicroscopy, inVia Re-flex, from Renishaw). A 100× objective lens was used to focus a 532 nmexcitation laser on the sample surface. The Raman signals were detectedusing a CCD detection system (Andor) with an 1800 I mm−1 grating. Thespatial resolution was horizontal 0.25 μm. The wavenumber resolutionwas 0.3 cm−1. At least 3 different area was scanned for each substratesurface to examine the uniformity of the samples. The cells used for the insitu detection of CO2 during the thermal, photo, and electrochemical re-duction of GO films are illustrated in Figure S6 (Supporting Information).In this measurement system, a unidirectional flow of Ar carrier gas wasmaintained through a quartz cell at a rate of 12 ccm, carrying generatedCO2 from the GO film to the outlet, which was connected to a micro-GCdetection system. For thermal reduction, a GO film was placed on a hotplate and heated to 230 °C. For photoinduced reduction, UV light froma mercury lamp was directed vertically onto the GO film, with the lightsource positioned 12 cm above the film. For electrochemical reduction, anelectrochemical cell was installed within the gas detection cell. Addition-ally, CO2 from rGO obtained via electrochemical reduction was monitoredunder UV irradiation.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work was partly supported by the World Premier International Re-search Center Initiative (WPI), MEXT, Japan, JSPS KAKENHI Grant Num-bers 22K04875, 23KJ1771, 24K01304 Japan.Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available on requestfrom the corresponding author. 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